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Composting of Mechanically Segregated Fractions of Municipal Solid Waste – A Review

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Note this review is best viewed on line at http://www.compostinfo.info/ where updates and additional resources are available. The aim of this review is to collate the large body of existing, and apparently forgotten, information about composting mechanically separated fractions of municipal solid waste (MSW) including sampling and sample preparation issues; and then to present this information in a form that is easily accessible to the UK waste management industry, environmental consultants and researchers. The volume of material is enormous, and only a fraction of it can be referenced in a conventional review. Hence this review operates in conjunction with an on-line bibliography at (www.compostinfo.info), which currently provides access to a bibliography of around 1,600 references linked to mixed waste composting. The review is intended to provide a general grounding in the subject and to sign post readers to sources of further information. The review is not intended as a “design and build manual” nor does it provide definitive guidance on legal, regulatory, policy or health and safety issues. The review covers the following topics. • Composting: past and present: past and recent UK and European composting experience • Feedstocks and composition: the physical, chemical and biological characteristics of mechanically segregated MSW used for composting • Sampling and analysis: Methods for quantifying and assessing the performance of mechanical separation, composting and refining systems, in particular sample collection, assessment and preparation. I.e. sampling and sample handling, designing the sampling scheme, sample collection, sub-sampling, sample preparation, preservation and transport, interlaboratory comparisons, health and safety issues, physical methods, chemical methods and biological methods. • Biology of composting: the terms used, a process description and review of process optimisation. • Pre-processing methods: technologies used for compost feedstock preparation (separation technologies such as, hand picking, size separation, density based separation, use of electric or magnetic fields; size reduction approaches; process integration; other conditioning approaches; and materials handling issues). • Composting techniques: turned windrow approaches, open aerated systems, and contained systems • Refining and packaging: separation processes used in refining, fine milling and pelleting, mixing and bagging, other techniques • Health and safety, emissions and emissions control: considering in particular: leachate, odour and volatile organic compounds, dust, bioaerosols and other health risks, vermin / birds / insects and fire risks • Product quality and environmental impacts: The quality of the composts produced by from mechanically segregated fractions of MSW, including: major chemical properties, trace elements, organic pollutants, inerts, microbial and pathogen issues, maturity and stability • End-uses: for composts produced by from mechanically segregated fractions MSW considering: landfill applications, land restoration, soil improvement, mulches, growing media, and composting as a pre-treatment for landfill • Operational and Strategic Issues: the role MSW composting can play in sustainable development, regulations standards and guidelines for compost products and the composting process, and compost marketing. This review has been compiled to provide generic guidance only. r3 environmental technology limited, AEA Technology PLC and the SITA Environmental Trust accept no responsibility whatsoever for any loss or prosecution resulting from acting on the information contained herein. Adherence to any recommendations or information does not necessarily imply endorsement by r3 environmental technology limited, AEA Technology PLC and the SITA Environmental Trust; neither does it necessarily ensure compliance with the respective regulatory requirements. It is strongly suggested that specialist advice be sought where appropriate.
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Composting of Mechanically Segregated Fractions of Municipal Solid Waste – A Review
r3 environmental technology limited 04/11/2004 Page 1
Composting of Mechanically
Segregated Fractions of Municipal
Solid Waste – A Review
Paul Bardos
r3 Environmental Technology Limited
www.r3environmental.com
Composting of Mechanically Segregated Fractions of Municipal Solid Waste – A Review
SITA Environmental Trust
SITA Environmental Trust distributes funding through the Government's Landfill Tax Credit
Scheme. The Trust has funded a wide range of sustainable waste management research projects.
Related research projects funded by SITA Environmental Trust include:
MBT Study
The Trust is funding a major research project into the Mechanical /
Biological Treatment of Waste - for more information click on
www.juniper.co.uk
Integrated Composting Project
The Trust is funding one of the UK's largest studies into composting
practices - for more information click on www.integratedcomposting.org
For more information about other research projects funded by the Trust, click on
www.sitaenvtrust.org.uk/research
SITA Environmental Trust has allocated over £40M funding since 1997. Since changes to the
Landfill Tax Credit Scheme in 1997, its focus is now on community and environmental
improvement projects around active landfill sites owned by the waste management firm, SITA
UK.
Acknowledgements
The author would like to acknowledge the contributions of Dr Elena Vanguelova, and Dr
Denise Lambkin who provided a large amount of the bibliographical work from r3, and the
input and support of all at SET, especially Paul Sison, Anthony Durston and Andrew
Saunders. The author would also like to acknowledge the valuable technical contributions of
Mr Pat Wheeler and Miss Louise Oldman of AEA Technology, and the editorial, web site and
technical comments provided by:
o Mr John Barton, University of Leeds
o Mr Mark Shelton, Cambridge County Council
o Members of the Chartered Institution of Wastes Management, the Biological
Treatment Special Interest Group, via Miss Tina Benfield.
The author is grateful for the interest and funding by SITA Environmental trust, and support
of the third party contributors: Viridor, Suffolk County Council, Norfolk County Council and
Cambridgeshire County Council. Finally, the author would like to thank David Coupe for
helping the whole thing get started.
r3 environmental technology limited 04/11/2004 Page 2
Composting of Mechanically Segregated Fractions of Municipal Solid Waste – A Review
Executive Summary
Over recent years there has been a resurgence of interest in composting of Municipal Solid
Waste (MSW). A large amount of source segregated wastes are now composted across
Europe, and the compost is used routinely by many users from domestic users to commercial
users.
Source segregation leaves behind residual organic materials. Composting combined with
mechanical separation processes may provide a means of recovering lower grade composts
and other recyclates both from the residual wastes, and from general waste collections, where
for economic, social or other reasons composting of source segregated materials is not carried
out. This combination of mechanical and biological treatments has come to be known as
“MBT”, and this technique is seeing an increasing number of applications across Europe.
However, while "MBT" is "new", mixed waste composting is not, and a large amount of
information has been collected about the performance of composting, sampling and separation
systems for mixed waste composting. Sita Environmental Trust have been supporting a
project which aims to collate the large body of existing information about composting
mechanically separated fractions of MSW including sampling and sample preparation issues;
and then to present this information in a form that is easily accessible to the UK waste
management industry, environmental consultants and researchers.
The volume of material is enormous, and only a faction of it can be referenced in a
conventional review. Hence this review operates in conjunction with an on-line bibliography
at (www.compostinfo.info), which currently provides access to a bibliography of 1,600
references linked to mixed waste composting. The review is intended to provide a general
grounding in the subject and to sign post readers to sources of further information. The
review is not intended as a “design and build manual” nor does it provide definitive guidance
on legal, regulatory, policy or health and safety issues. Among many findings, the review
identified the following key points:
Composting - past and present: past and recent UK and European composting experience
shows a cycle of interest and then disinterest in composting of MSW. At present, while it is
generally agreed that composts made from source segregated materials are likely to make
higher quality composts, there is increasing interest in composting mechanically segregated
MSW feedstocks as part of an “MBT” process. MBT, or mechanical biological treatment,
allows a range of secondary materials to be recovered, including compost, albeit of a lower
grade.
Feedstocks and composition: the physical, chemical and biological characteristics of
mechanically segregated MSW are highly variable. Contamination of the compostable
fraction by trace elements and “inerts” – i.e. non-compostables - appears to be an intractable
problem, with residual inerts and elevated trace element contents remaining in the refined
compost. The “best” composts made from mechanically segregated MSW are similar in trace
element content to the poorest composts produced from source segregated materials.
Sampling and analysis: MSW is a highly heterogeneous and variable material. Specialist
approaches are needed for its sampling, sample preparation and analysis.
r3 environmental technology limited 04/11/2004 Page 3
Composting of Mechanically Segregated Fractions of Municipal Solid Waste – A Review
Biology of composting: the key biological effects are decomposition including a period of
decomposition at elevated (Thermophilic) temperatures. The compost is sanitised by a
correctly optimised composting process. The dominant process variables are aeration,
temperature and moisture, and it can be difficult to sufficiently aerate the composting mass to
control temperatures and so maximise processing rates, without over-drying it.
Pre-processing methods: a wide variety of technologies for compost feedstock preparation
(separation technologies such as, hand picking, size separation, density based separation, use
of electric or magnetic fields) have been developed over the past 50 years or more. Size
reduction plays an important role in pre-processing before composting, with size reduction by
screening without shredding largely preferred.
Composting techniques: the principal techniques used in MSW composting are turned
windrow approaches, open aerated systems, and contained systems (vertical and horizontal
reactors and agitated systems). In the past rotating drum reactors followed by aerated piles or
turned windrows was the dominant composting approach. Each approach has advantages and
disadvantages. However, rotary compost reactors are rarely used for long enough to do more
than mix and condition the feedstock, and initiate the thermophilic stage of composting.
Operating problems appear to be most frequently reported for vertical continuous or silo type
reactors.
Refining and packaging: refining uses similar separations to pre-processes, residual content
of inerts may remain a problem. This may be masked by fine milling or pelleting.
Health and safety, emissions and emissions control: the principal emissions and health and
safety issues are leachate, odour and volatile organic compounds, dust, bioaerosols and other
health risks, vermin / birds / insects and fire risks. These can all be effectively controlled in a
well managed and planned composting operation.
Product quality and environmental impacts: The dominant benefit of composts arises from
their organic matter content, although they do contain useful amounts of plant nutrients and
may have a significant liming effect. Concerns about contents of trace elements and inerts
have limited the use of composts made from mechanically segregated fractions of MSW in
the past. An emerging concern is exists with elevated levels of toxic organic compounds
reported where tests have been carried out, although the significance of these is still being
debated.
End-uses: composts produced from mechanically segregated fractions of MSW are likely to
incur some form of ongoing regulation; possibilities might include soil improvement and soil
forming for restoration, daily cover in landfill management, as a pre-treatment prior to landfill
and perhaps as a pre-treatment for energy recovery.
Operational and Strategic Issues: MSW composting could play a role in sustainable waste
management. However, regulations standards and guidelines for compost exclude products
made from mechanically segregated fractions of MSW from “premium grade” markets in the
UK. The possible lower grade uses for compost, mentioned above, are currently subject to
regulatory uncertainty. This regulatory uncertainty is perhaps the most critical issue affecting
the implementation of MBT systems in the UK, and the provision of clear benchmarks and
guidance should be undertaken as a matter of some urgency by the regulators and policy
departments concerned.
r3 environmental technology limited 04/11/2004 Page 4
Composting of Mechanically Segregated Fractions of Municipal Solid Waste – A Review
Contents
1. Introduction 7
1.1 Aims 7
1.2 Context 9
1.3 Approach 10
1.4 Project Team 10
2. Composting: Past and Present 10
3. Feedstocks and composition 14
3.1 Physical characteristics 15
3.2 Chemical characteristics 17
3.3 Biological characteristics 19
4. Sampling and analysis 20
4.1 Sampling and Sample Handling 23
4.1.1 Designing the sampling scheme 23
4.1.2 Sample Collection 24
4.1.3 Sub-sampling, Sample Preparation, Preservation and Transport 25
4.1.4 Interlaboratory Comparisons 27
4.1.5 Health and Safety Issues 27
4.2 Physical Methods 28
4.3 Chemical Methods 31
4.4 Biological Methods 33
5. Biology of Composting 36
5.1 Terms and Definitions 36
5.2 Process Description 37
5.3 Process Optimisation 39
6. Pre-Processing Methods 44
6.1 Separation Technologies 46
6.1.1 Hand Picking 46
6.1.2 Size Separation 47
6.1.3 Density Based Separation 48
6.1.4 Use of Electric or Magnetic Fields 49
6.2 Size Reduction Approaches 50
6.3 Process Integration 51
6.4 Other Conditioning Approaches 54
6.5 Materials Handling Issues 54
7. Composting Techniques 55
7.1 Turned Windrow Approaches 57
7.2 Open Aerated Systems 58
7.3 Contained Systems 59
7.3.1 Horizontal Units 59
7.3.2 Mechanically Agitated Systems 60
7.3.3 Vertical Units 60
7.3.4 Rotating Drums 61
8. Refining and Packaging 61
8.1 Separation Processes Used in Refining 62
8.2 Fine Milling and Pelleting 63
8.3 Mixing and Bagging 63
8.4 Other Techniques 64
9. Health and Safety, Emissions and Emissions Control 64
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Composting of Mechanically Segregated Fractions of Municipal Solid Waste – A Review
9.1 Leachate 65
9.2 Odour and Volatile Organic Compounds 66
9.3 Dust 67
9.4 Bioaerosols and Other Health Risks 67
9.5 Vermin / Birds / Insects 69
9.6 Fire Risks 69
10. Product Quality and Environmental Impacts 70
10.1 Major Chemical Properties 71
10.2 Trace Elements 73
10.3 Organic Pollutants 75
10.4 Inerts 77
10.5 Microbial and Pathogen Issues 77
10.6 Maturity and Stability 78
11. End-uses 79
11.1 Soil Improvement 81
11.2 Growing Media 82
11.3 Mulches 83
11.4 Restoration 84
11.5 Landfill Applications 84
11.6 Other 85
11.7 Pre-treatment For Landfill 85
12. Operational and Strategic Issues 86
12.1 MSW Composting and Sustainable Development 86
12.2 Regulations Standards and Guidelines for Compost Products 87
12.3 Regulations Standards and Guidelines for the Compost Process 90
12.4 Marketing 92
13. Conclusions 93
14. References 95
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Composting of Mechanically Segregated Fractions of Municipal Solid Waste – A Review
1. Introduction
Over recent years there has been a resurgence of interest in composting of Municipal Solid
Waste (MSW). A large amount of source segregated wastes are now composted across
Europe, and the compost is used routinely by many users from domestic users to commercial
users.
Source segregation leaves behind residual organic materials. Composting combined with
mechanical separation processes may provide a means of recovering lower grade composts
and other recyclates both from the residual wastes, and from general waste collections, where
for economic, social or other reasons composting of source segregated materials is not carried
out. This combination of mechanical and biological treatments has come to be known as
“MBT”, and this technique is seeing an increasing number of applications across Europe.
However, while "MBT" is "new", mixed waste composting is not, and a large amount of
information has been collected about the performance of composting, sampling and separation
systems for mixed waste composting. It appears that not all of this information is being
exploited by MBT developers, who may therefore be at risk of repeating research that has
already been done, or perhaps even repeating mistakes from the past, or not carrying out
adequate sampling and analysis.
SITA Environmental Trust have been supporting a project which aims to collate the large
body of existing, and apparently forgotten, information about composting mechanically
separated fractions of MSW including sampling and sample preparation issues; and then to
present this information in a form that is easily accessible to the UK waste management
industry, environmental consultants and researchers.
1.1 Aims
The aim of this review is to collate the large body of existing, and apparently forgotten,
information about composting mechanically separated fractions of municipal solid waste
(MSW) including sampling and sample preparation issues; and then to present this
information in a form that is easily accessible to the UK waste management industry,
environmental consultants and researchers.
The volume of material is enormous, and only a fraction of it can be referenced in a
conventional review. Hence this review operates in conjunction with an on-line bibliography
at (www.compostinfo.info), which currently provides access to a bibliography of around
1,600 references linked to mixed waste composting. The review is intended to provide a
general grounding in the subject and to sign post readers to sources of further information.
The review is not intended as a “design and build manual” nor does it provide definitive
guidance on legal, regulatory, policy or health and safety issues.
The review covers the following topics.
Composting: past and present: past and recent UK and European composting
experience
r3 environmental technology limited 04/11/2004 Page 7
Composting of Mechanically Segregated Fractions of Municipal Solid Waste – A Review
Feedstocks and composition: the physical, chemical and biological characteristics
of mechanically segregated MSW used for composting
Sampling and analysis: Methods for quantifying and assessing the performance
of mechanical separation, composting and refining systems, in particular sample
collection, assessment and preparation. I.e. sampling and sample handling,
designing the sampling scheme, sample collection, sub-sampling, sample
preparation, preservation and transport, interlaboratory comparisons, health and
safety issues, physical methods, chemical methods and biological methods.
Biology of composting: the terms used, a process description and review of
process optimisation.
Pre-processing methods: technologies used for compost feedstock preparation
(separation technologies such as, hand picking, size separation, density based
separation, use of electric or magnetic fields; size reduction approaches; process
integration; other conditioning approaches; and materials handling issues).
Composting techniques: turned windrow approaches, open aerated systems, and
contained systems
Refining and packaging: separation processes used in refining, fine milling and
pelleting, mixing and bagging, other techniques
Health and safety, emissions and emissions control: considering in particular:
leachate, odour and volatile organic compounds, dust, bioaerosols and other health
risks, vermin / birds / insects and fire risks
Product quality and environmental impacts: The quality of the composts
produced by from mechanically segregated fractions of MSW, including: major
chemical properties, trace elements, organic pollutants, inerts, microbial and
pathogen issues, maturity and stability
End-uses: for composts produced by from mechanically segregated fractions
MSW considering: landfill applications, land restoration, soil improvement,
mulches, growing media, and composting as a pre-treatment for landfill
Operational and Strategic Issues: the role MSW composting can play in
sustainable development, regulations standards and guidelines for compost
products and the composting process, and compost marketing.
This review has been compiled to provide generic guidance only. r3 environmental technology
limited, AEA Technology PLC and the SITA Environmental Trust accept no responsibility
whatsoever for any loss or prosecution resulting from acting on the information contained
herein. Adherence to any recommendations or information does not necessarily imply
endorsement by r3 environmental technology limited, AEA Technology PLC and the SITA
Environmental Trust; neither does it necessarily ensure compliance with the respective
regulatory requirements. It is strongly suggested that specialist advice be sought where
appropriate.
r3 environmental technology limited 04/11/2004 Page 8
Composting of Mechanically Segregated Fractions of Municipal Solid Waste – A Review
1.2 Context
Composting as a waste management technique for MSW is growing in importance. However,
it is not clear that the lessons and knowledge of the past are informing some current projects
and future project proposals. By far the most frequent application of composting to MSW,
over the past ten years, has been for the treatment of separately collected wastes - mainly from
civic amenity sites. This constitutes, by volume, the bulk of MSW material composted in the
UK.
Recently a number of projects in the UK have focused on composting mechanically separated
fractions of mixed MSW, and some of these have run into difficulties about the acceptability
of their products, both to regulators and those managing Recycling Credits. It also appears
that the WRAP /BSI guidance on compost standards is not appropriate for compost
production from mechanically segregated MSW; for example it provides little guidance on the
principles of sampling, sample assessment and sample preparation (for analyses) of
heterogeneous MSW streams.
A large number of mixed waste composting projects (MBT) projects are “in the pipeline” and
may be commissioned in the next few years. There are a number of drivers for this. These
are, in no strict order of priority:
The advent of the Landfill Directive: composting separately collected wastes may
reduce waste to landfill by, say, 20%, but it may not deal with the vast majority of
biodegradable wastes in MSW - can composting offer a wider opportunity?
The "organic crunch": not only is there going to be a large volume of biodegradable
MSW looking for a home, but also controls on sewage sludge, agricultural wastes and
industrial wastes (for example the ending of sea disposal and stricter controls on re-
use in agriculture) mean that there will be even larger volumes of biodegradable
wastes potentially looking for beneficial re-use.
Dereliction: An increasing desire to restore land, in particular restoring large areas of
land for softer end-uses, and the potential combination of compost re-use with non-
food production such as biomass.
These developments have lead to the discovery of a number of new and exciting, and often
unique, composting approaches based on mixed waste separation, which nonetheless bear an
uncanny resemblance to techniques that have been used in the past and were often well
understood.
A large amount of information exists about compost feedstock preparation, product refining,
use of mixed MSW fractions and appropriate sampling, sample handling and sample
preparation. Much of this experience came from the UK, for example from:
the work of Warren Spring Laboratory (WSL), subsequently AEA Technologies for the
National Household Waste Analysis Programme and past mixed MSW composting work
for the Department of the Environment, and
others such as Leeds University, Luton University, MEL, Sheffield University, Enviros
Aspinwalls (now part of Enviros), HLC Henley Burrowes.
This experience appears not to be widely available, as consultancy and other reports can lack
due consideration of the difficulties of MSW analyses. Indeed the value of some of the
reporting carried out is open to question. This is, probably, in part because organisations from
many sectors have entered the MSW composting arena over the 1990s. Those without
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Composting of Mechanically Segregated Fractions of Municipal Solid Waste – A Review
previous MSW background may have found it difficult to either review existing literature or
to access what is largely an unpublished state of the art on MSW sampling and analysis.
Making this information more widely available would enhance the technical state of the art as
practised in composting in the UK.
The intentions of the project proposed here are that:
future composting initiatives benefit from the existing platform of knowledge being
made widely and easily accessible,
these future initiatives advance this state of the art rather than repeating it,
the wider consulting community has easy access to the state of the art for MSW
sampling and analysis and refining and handling of MSW process streams.
so (1) if mixed waste composting truly does have the potential to generate a beneficial
re-use in particular areas, its chances of reaching this potential are maximised and (2)
there is “technology transfer” to those carrying out composting of separately collected
feedstocks.
1.3 Approach
The work carried out comprised
Task 1: Inventory of existing document holdings
Task 2: Identification and collection of further documents
Task 3: Preparation of an annotated bibliography
Task 4: Preparation of a review report
Task 5: Publication, dissemination and promotion
Task 6: Project Management and Progress Reports
1.4 Project Team
This work has being carried out by:
r3 environmental technology limited – www.r3environmental.com
AEA Technology PLC – www.aeat.co.uk.
The project team was lead by Paul Bardos (r3) and Pat Wheeler (AEA Technology). The
review author is Paul Bardos (r3).
WSL in Stevenage was instrumental in MSW recycling and composting research until 1993,
after which time its work passed on to AEA Technology. Paul Bardos (r3) and Pat Wheeler
(AEA) were both involved with this composting work and carried it on in their subsequent
organisations.
2. Composting: Past and Present
Municipal Solid Waste (MSW) poses a difficult and complex problem for society. Some of
the difficulties arise because the MSW stream is quantitatively large and qualitatively
heterogeneous, reflecting the myriad consumer products manufactured in modern industrial
society. Inconveniently, MSW is largely generated in densely populated areas where its
management is most constrained. Thus the problem cuts across a very wide range of human
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Composting of Mechanically Segregated Fractions of Municipal Solid Waste – A Review
activities and interests. At the same time, MSW represents a uniquely familiar environmental
problem, in that everyone contributes to it palpably in the course of daily living. (Finstein
1992)
The application of composting to municipal solid wastes in mechanised treatment plants has
been recorded in the technical literature going back 50 years of more. The recorded use of
“refuse derived fuel” (RDF) is even older (Alter 1984). The earliest recorded use of
municipal solid waste in its discarded form as a fuel to generate steam during the last quarter
of the nineteenth century, apparently in England. The technology was quickly adopted in the
United States, Germany and Japan. In New York City, in the 1890's solid waste was
handpicked to remove useful materials and the residue became another form of RDF which
was burned to generate electricity. Jeris and Regan (1973) describe composting plants of the
1920s and 1930s. In 1961 Brunt described the Engineering and Economics of Composting
Plant, reviewing plants in Scotland against “old fashioned” processes in the USA, Italy and
Denmark. Interestingly this paper has one of the first mentions of 60 degrees C as a minimum
composting temperature. Gothard (1959) describes a composting plant in Jersey and
suggests process temperatures should be greater than 65oC to ensure sanitisation of materials.
Harrison (1965) describes the composting plant in Leatherhead. Hoortenstein and Rothwell
(1973) review the use of composted municipal refuse as a “soil amendment” going back to
1944. de Haan 1981 and Obeng et al. 1987 briefly review the use of composting by the
Netherlands, another country with a long history of applying composting to waste. The
composting plant at Wijster was opened in 1929, and by the end of 1960 fifteen composting
plants were operating in the Netherlands, some at very large scales (Teensma 1961) and a
number of composting plants operated in the USA through the 1960s (US EPA 1971). Indeed
the first issues of the journal “Compost Science” date back to 1961. The year the authors of
this critical review were born.
By 1971 composting in the UK had declined to 0.3% of the annual MSW arising. Composting
plants existed at Worthing and Chesterfield. A Working Party on Refuse Disposal report to
the Department of the Environment (1971) described the state of the art in composting in
some detail, and much of what it says about composts (then produced from mechanically
segregated and ground refuse) might seem very familiar to today’s experts. The compost was
seen as a soil conditioner rather than a fertiliser, given its contamination with “undesirable”
inorganic materials. The Working Party concluded that it is evident that to date municipal
compost has had little or no attraction in agriculture or horticulture in Britain, nor do we
think its attraction to be much greater as a humus or soil conditioner in private gardens. In
these circumstances there seemed to be no justification for installing composting plants on the
basis of an expected sale of compost unless governmental subsidies were made available for
the agricultural use of compost. Composting a fill material for landfill was seen as having few
advantages over using pulverised refuse. Net composting production costs, allowing for sale
of compost, were estimated at £3 per tonne, which is probably higher in real terms than net
processing costs today (typical gate fees £15 to £25 per tonne, depending on throughput).
The research into composting by Biddlestone et al. at the University of Birmingham
stimulated renewed interest in composting (e.g. Gray et al. 1973). Their work investigated
and documented the key composting process control parameters: aeration and temperature and
to a lesser extent pH, referring back as far as the work of Waksman in the 1930s (e.g.
Waksman and Cordon 1939). Gray et al. 1973 listed composting plants around the world.
Three composting process approaches were identified in the UK: DANO, NUSOIL, and
RENOVA., all based on mechanically segregated fractions of MSW. Operating plants in the
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UK were located at: Blyth (2 tonnes per day throughput), Chesterfield (40-50 tpd),
Cowdenbeath (DANO 10-13 tpd), Locherbie (DANO 30 tpd), Dum. Kirkconnel (10 tpd),
Jersey (vertical compost reactor - up to 80 tpd), Leatherhead (DANO 45 tpd), Leicester
(DANO 70 tpd), Newark, Paisley (80 tpd), Radcliff (DANO 20-25 tpd), Wetherby (up to 66
tpd), Worthing (up to 45 tpd). However, Gray et al. also listed a number of composting plants
which were closed down between 1971 and 1973, located at: Bristol, Cheadle and Gatley,
Edinburgh, Kilmarnock, Manchester, Middlesbrough, Twickenham. A plant at Caister was
the only facility built after 1971 (Gray and Biddlestone 1980). Biddlestone and Gray reported
retrospectively on their work in 1980. While there was clearly concern about the content of
trace elements in the composts made from MSW, acute toxicity in crop plants was rarely
observed and boron appeared to be the chief culprit. Stead and Irwin (1980) described a
composting facility near Chichester.
The most well known of the composting plants in the UK was the DANO plant at Leicester
(Wanlip) which produced a composted product called “Lescost”. This even merited an item
on the children’s TV show Blue Peter, which mentioned that the compost could be used in
parks, but was not suitable for growing food. Ultimately the Lescost plant shut down
(Hughes 1977). The Wanslip plant was originally built in 1966, damaged by fire in 1968 and
recommissioned in 1969. The plant operated till the mid-1970s and shut down because it
could not find markets for its composts. Hughes (1977) reports that the compost stockpile
was sold on quite easily, although Clark (1973) reports that the compost quality was poor and
could not easily be sold while the plant was operating. The plant (and others) is listed in the
case studies section of this review. Wanlip was the last major composting plant processing
mechanically segregated MSW for some time in the UK.
In the late 1970s through to the late 1980s a large programme of work was carried out by the
Department of the Environment, and subsequently ETSU, to investigate recent advances in
refuse processing technology for producing refuse derived fuel (e.g. Barton and Poll 1983).
This centred on two new plants, one built at Byker based on what was seen as a more
established approach based on the processing of shredded refuse, and one built at Doncaster
based on a more technically risky approach of trommel screening refuse before processing to
RDF. The trommel screening approach was found to be more reliable and produce a better
quality fuel. The Doncaster and Byker plants implemented much of what is regarded today as
“MBT” technology, but even they were based on earlier technologies improved over time.
Research at Warren Spring Laboratory considered both composting (Ege and New 1988) and
anaerobic digestion (Le Roux 1979) as possible recycling routes for the organic rich rejects
from the RDF process. These were seen as a potential opportunity for organic matter
recycling (Bardos et al. 1991, Poll 1994). It became clear that trommel screening rather than
shredding as the “front end” for MSW processing also resulted in better quality composts.
However by the early 1990s the work at Warren Spring had concluded that even with
advanced separation and refining techniques the quality of compost produced from
mechanically segregated composts was fundamentally limited by the nature of the feedstock,
with particular concerns over inerts and heavy metal contamination levels, matching similar
findings across Europe (Favioni 2002). Quality of composts from source segregated materials
was found to be much better (Newport et al. 1993) in line with findings from many other
investigations, (e.g. Richard 1991). However, review work indicated that the heavy metal
contamination levels in some composts produced from source segregated materials was no
better than that of the better composts from mechanically segregated feedstocks (Wheeler and
Bardos 1992).
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In the early 1990s research work was proposed to the Department of Trade and Industry and
the then Department of the Environment to develop a programme for developing composting
approaches for source segregated wastes, particularly from civic amenity sites, which in
preliminary studies had shown great promise for producing a step change in compost quality.
However, this work was not carried out as funding was ended.
Since then interest in composts derived from source segregated materials has been
unstoppable (Border 1999, DETR 2000, Gale and Walker 1997, The Composting Association
2003), although some interest in composting from mechanically separated wastes continued.
A composting plant. based on mechanically segregated MSW was built at Castle Bromwich
and then shut down in the early 1990s. Composting plant. based on mechanically segregated
MSW was proposed at Reading in Berkshire, but could not be financed. Very recently
composts produced from mechanically segregated wastes have been applied to land in Greater
Manchester and in Norfolk. In both cases the poor quality of the compost has lead to major
controversy. Composting plants based on mechanically segregated MSW have recently been
commissioned in Neath, Wales and in Aberdeenshire (Pringle and MacDonald 1999, Pringle
and Svoboda 2002). The Neath Plant also produces “green waste composts” from separately
collected materials. It is still developing ideas for end-uses for the mechanically segregated
waste compost, but anticipates no revenue from them. The Aberdeen compost is intended for
landfill restoration. (Note: - the feasibility of converting mixed-MSW composting plants to
source segregated feedstocks is discussed by Kranert and Horst 1990.)
The Neath plant is perhaps in the vanguard of the so-called “mechanical biological treatment”
plants which seek to apply mechanical segregation and biological processing to mixed refuse,
ideally residual waste left after source segregated materials have been removed (Crowe et al.
2002), an approach known in the Warren Spring days as “Integrated waste management”. A
large number of MBT plants have been proposed in the UK, and they are seen by many,
including Greenpeace, as an alternative to thermal conversion of residual wastes left after
source segregation of materials including compostables (Greenpeace 2001). The actual scale
of MBT processing in the UK appears, as yet to be relatively small, with 85,000 tonnes
reportedly processed in 2001 (The Composting Association 2003). However, major
uncertainties remain about how the compost (or digestate) products of MBT will be used.
Currently envisaged applications are:
Applications perceived as less sensitive by producers, such as restoration (Godley
et al. 2002)
Simply as a landfill pre-treatment (Bockreis and Steonberg 2004)
As a feedstock for energy from waste conversion (Efstathios. and Stentiford 2004)
So the circle of composting continues to turn in the UK and elsewhere (each country seems to
be making similar voyages of discovery and rediscovery, e.g. Ernst 1989, European
Commission 1997).
The purpose of this review is that the “cycles” of the past can be recycled to inform the
present cycle of interest in composting and mechanical segregation, which is most commonly
expressed as “MBT”. The aim is for decision-makers and developers to have the opportunity
to benefit from lessons learned in the past.
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In 2003 the House of Commons Environment, Food and Rural Affairs Committee found that
“Biodegradable (organic) waste is important because it represents a high proportion of
household waste and because when disposed of in landfill it produces the greenhouse gas
methane. Conversely, when managed well, biodegradable waste can be used to make valuable
high quality compost, which in turn can reduce our reliance on peat-based composts and can
be used as a soil improver.”
3. Feedstocks and composition
MSW is one of many feedstocks that have been or are composted. In fact the dominant
compostable wastes are agricultural wastes (Bardos et al. 1991).
Composts have been produced from unprocessed MSW and from MSW that has been
processed in some way to increase its relative content of biodegradable material, and/or
render the refuse more quickly degradable (typically by wetting and/or size reduction). When
these materials are used as the input source for a composting process they are often referred to
as “feedstocks”.
The aims of applying composting to MSW encompass one or more of the following:
producing a “product” that can be put to some kind of use, reducing the mass of MSW,
improving the qualities of the MSW for subsequent disposal or processing – for example as a
pre-treatment for landfill.
The principal effects of the composting process are biodegradation, drying, increasing bulk
density and physical attrition. The waste components that are most changed are those that are
biodegradable. Composting is of relatively short duration – weeks to months depending on
the processing route, hence rapidly biodegradable materials are those most affected. More
slowly biodegradable components may persist through the composting process, and even as
the compost is matured. This persistence can be a particular problem for various types of
“biodegradable” plastic (Colyer 2004), but also for paper, card and wood – including woody
components of garden waste, and notoriously for MSW composts: cigarette ends (filters).
The possible effects that composting has on non-biodegradable components such as glass or
many plastics is that of physical attrition, drying and the removal of adhering organic matter.
These components are often referred to as “inerts” since they are not affected by
biodegradation. Mechanical pre-processing and compost refining processes (discussed
elsewhere in this review) seek to remove these inert components as concentrates, to leave a
more organic rich “compost” product. Inert components are detrimental to compost quality
either as visible contaminating components, or as sources of potentially toxic substances in
compost, or both. They may also pose physical risks to grazing animals, or to people using
MSW compost simply by virtue of being sharp.
Hence the quality of any compost produced from MSW is constrained by the proportion of so
called “inerts” in the feedstock, and the effectiveness of processes to remove them before and
after composting. The “inert components” are not necessarily chemically inert, for example
metal ions may leach from batteries.
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In many cases MSW fractions are one component of a compost feedstock, and other
compostable materials may be added, most commonly sewage sludge. In the UK, sewage
effluent, from both domestic and industrial premises, is treated at wastewater treatment plants.
There are three standard stages of treatment:
Primary sludge is the settled solids from wastewater entering the treatment works
Secondary sludge is the solids arising from biological treatment (untreated sludge has
an approximate dry solid content of 2-7%).
Tertiary sludge is formed when the remaining solids are precipitated out to produce a
clear effluent for discharge.
The composition of MSW feedstocks can be considered in three ways, its physical
characteristics, its biological characteristics and its chemical characteristics. The composition
of MSW is very variable. Some of this variation is related to seasonal trends, the approach to
waste collection, and the locales waste is collected from. However, even within a given
locale and time of year composition is variable. This makes extrapolations of conclusions
from one area to another highly problematic. Regional comparisons are further complicated
by differences in analytical approach, and a standardised methodology for solid waste
analysis could enable greater comparability and accuracy of waste data within the European
Union (Dobson 2003).
MSW can contain hazardous components, and its degradation can cause hazards. Health and
safety issues for composting plants are outlined in the Critical Review Section, Health and
Safety, Emissions and Emissions Control. However, this is not a comprehensive treatise on
the subject and plant managers should seek professional advise on risk assessment and
compliance with health and safety regulations.
3.1 Physical characteristics
There are four basic ways in which the composting of MSW may be approached:
composting whole mixed MSW;
composting a mechanically concentrated organic fraction;
composting separately collected materials (e.g. via collections from Civic Amenity
sites or kerbside collection of wastes segregated by householders);
encouraging composting by individual waste producers (e.g. home composting).
This review focuses on composting from mixed MSW collections.
Composts are not made from whole MSW streams in the UK because of the relatively small
content of compostable material, and because some MSW materials are better suited for other
forms of recovery (for example metals, paper and plastic). Suggestions for the composition of
the organic fraction from an ‘average’ householder range from 21% to 35% for food and
garden waste, and the content of paper and card is estimated as 35% - % by mass (CIWM
2002).
Estimates from the Warren Spring Laboratory (Bardos et al. 1991, Newport 1990, Newport et
al. 1993) suggest the proportion of compostable materials in UK MSW is 35% by mass. The
overall biodegradable content of MSW in Wales has been estimated as 61% - “organics” 36%
and paper and card 25% (Welsh Assembly Government 2003).
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Often, surveys of waste composition are reported for “bin waste” only, i.e. from refuse
collection vehicles. However, MSW includes CA site waste, fly-tipped, street sweepings etc.
so the use of these terms must be specific. These different sources would also be affected
differently by the implementation of source segregation schemes.
The Warren Spring estimate is based on the content of materials falling into three categories
during analyses of (bin-waste) refuse by hand sorting. The physical classification of the
components of MSW is typically on the basis of size distributions and categories (Poll 1988) -
see Critical Review Section, Sampling and Analysis – Physical The three categories are:
putrescibles - plant, kitchen and garden wastes;
miscellaneous combustibles - disposable nappies, sanitary towels, leather goods,
wood;
fines - materials less than 10 mm in size that are too fine to sort by hand (such as
household dust or soil).
These materials may contain or entrain a proportion of non-compostable material and,
furthermore, may incorporate non-compostable categories such as glass, paper and plastics.
For example, woody wastes, plastics and some fibres used in disposable nappies and sanitary
towels persist through composting. Paper and card are not included as (a) these tend to be
diverted for recycling or energy recovery, and (b) paper is only slowly degraded during
composting (Bardos and Lopez-Real 1989).
Experience in the UK and overseas strongly indicates that composting of whole refuse is
unlikely to produce a usable product - - see Critical Review Section, Composting Past and
Present, although unsorted MSW has been composted in the past (e.g. Atchley and Clark
1979, de Haan 1981). Mechanical segregation processes can concentrate the compostables
present in refuse as well as producing other fractions suitable for energy recovery, metals
recycling etc. This integrated approach to MSW management has re-emerged in recent years
as “Mechanical Biological Treatment”. The separation employed at these plants can be
divided into two main strategies: those where all the incoming refuse is shredded prior to
sorting, and those where the first sorting stage is screening with a rotary trommel screen.
The separated undersize stream, from the trommel screening, is the compost process
feedstock, and typically includes fines; putrescibles; broken glass and ceramics etc; small
pieces of wood, plastic, paper and card; metallic items including batteries (New and Papworth
1988, Wheeler 1990 and 1993).
Composts produced from the compostable-rich fractions liberated by these two strategies
differ in their ease of refinement and their composition. Compost products derived from
screening pulverised refuse tend to be richer in fine particles of paper, plastic and glass than
composts produced from the screening of unpulverised refuse. As a result, composts
produced from pulverised refuse fractions are harder to refine than composts produced from
unpulverised refuse screenings (Wheeler 1990). The overall organic content of pulverised
refuse fractions may also be higher, because of the increased paper content. Despite the
potential liberation of metal contaminants during pulverisation, the technical literature
indicates that composts produced from pulverised refuse fractions tend to have lower heavy
metal contents than composts produced from unpulverised refuse fractions (Wheeler and
Bardos 1992). The reduction in metal concentration may be due to the dilution effect of the
higher content of paper and shredded inerts in the compost, or may be a feature of the
composition of different household waste inputs.
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Further pre-treatment processes may be applied before composting, in particular density based
separations (such as ballistic separation) and also separation of metallic components (e.g.
magnetic or using eddy current systems). Pre-processing techniques are discussed in more
detail in the Critical Review Section, Pre-processing Methods.
Many commentators believe that Mechanical-Biological (MBT) treatments are best operated
in parallel with schemes separating materials at source, for example garden and kitchen
wastes, “dry recyclables” (paper, plastic, metals), glass, and schemes encouraging waste re-
use at source (for example home composting). The rationale for this combined approach is
that the quality of products recovered from separately collected materials tends to be higher.
However, separation at source will dent but not eliminate the municipal waste stream, and a
significant amount of “residual” or “grey” waste will remain (Gould and Meckert 1994).
MBT is seen as a means of recovering, perhaps lower grade, materials from this residual
MSW and/or energy, and in reducing the content of biodegradable materials eventually being
landfilled. (Chertow 1989, Damiecki and Kettern 1993, Greenpeace 2000 & 2001, Jager et al.
1998, Koller and Thran 1997, Lechner et al. 2004).
For compost production, mechanically segregated MSW, has constraints in terms of its levels
of contamination by “inerts” (i.e. non-biodegradable components) and trace elements (see
Critical Review Section, Feedstocks and composition - Chemical characteristics. The
Critical Review Sections: Pre-processing Methods and Refining discuss the approaches that
have been employed to limit the impact of these inerts in compost product. Ultimately there
is an inverse relationship between product yield and product quality – the greater the removal
of inerts, the lower the compost yield, as organic material entrained with the inerts is removed
(New and Papworth 1988). The usefulness of composts produced from mechanically
segregated composts is being hotly debated, with positions ranging from their not having
much use at all (Hammer 1992), to a range of possible “lower grade” uses (Godley et al.
2002). Compost uses are discussed in more detail in the Critical Review Section, End-Uses.
It should not be assumed that materials separated at source will be free of contamination.
Plastic, glass and rubble can be significant contaminants in composts produced from “green
wastes” (Wragg 2004), and wide ranging contamination may occur in separately collected
kitchen wastes. Dealing with this inerts contamination may require similar pre-processing
and refining techniques to those used for mechanically segregated MSW streams.
3.2 Chemical characteristics
The key chemical properties of MSW fractions as a compost feedstock are:
its content of potentially useful substances such as the major plant nutrients (NPK)
and other plant nutrients such as magnesium, and calcium (also important for their
potential “liming” effect
its content of potentially harmful substances such as toxic organics and trace
elements.
Source segregated materials are now generally seen as being of “higher quality” for compost
production than mechanically segregated feedstocks. See the Critical Review Section,
Composting Past and Present.
Content of trace elements has been a particularly contentious issue. A review of literature
available in the early 1990s concluded that composts produced from mechanically segregated
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MSW tended to have higher contents of most trace elements than composts produced from
materials separated at source. However, the “best” composts produced from mechanically
segregated MSW have lower levels of trace elements than the “worst” composts produced
from materials separated at source (Wheeler and Bardos 1992).
There is a high degree of contentiousness (a) about how far some trace elements should be
seen as “trace nutrients” versus potential soil pollutants, and (b) whether compost quality
should be appraised on the basis of “total” as opposed to “bio-available” levels of trace
elements (European Commission 2002, Petruzzelli and Pezzarossa 2002). This is discussed
further in the sections on “product quality and environmental impacts” and “end-uses”.
The sources of heavy metals in composts are many, for example from metallic components in
refuse; household dust; wine bottle tops; compounds added to plastics, paints and inks;
cosmetics and medicines; and household pesticides (Culboard et al. 1988, Eder 1986,
Hagenmaier and Krauss 1982, Krauss 1985, Rousseaux et al. 1989 Rugg et al. 1992, van
Roosmalen et al. 1987). The trace elements in mechanically segregated MSW fractions
appear to be an intractable problem. Contamination levels tend to show a net increase over
composting, in part as dry matter is lost to biodegradation (Anid 1986, Hernando et al. 1989,
van Roosmalen et al. 1987) and indeed trace elements appear to be concentrated by common
refining techniques (Bardos 1989). Perhaps this concentration is the result of components of
low metal content such as glass fragments.
It has been observed that trace elements tend to be concentrated in the finer fractions
(Petruzzelli et al. 1989, van Roosmalen et al. 1987), removal of this fine fraction prior to
composting may not eliminate a sufficient amount of trace elements to make a “step change”
in compost quality, and also greatly reduces compost yield. A particular problem appears to
be that finely divided materials high in trace elements stick to putrescible materials, for
example dust coating wet materials, and metal items such as copper staples penetrating larger
putrescible or other organic materials (Krauss et al. 1987). There may be alternative pre-
processing and refining strategies that might, at least in part, produce composts from mixed
MSW with lower levels of trace elements (see the Critical Review Sections: Pre-processing
methods - Process Integration and Refining). There is also some evidence that the toxic
elements in finished composts may be less leachable than those in raw materials, but
conflicting reports also exist – see the Critical Review Section, Product Quality and
Environmental Impacts - Trace Elements. It has also been reported that adding sewage sludge
can lead to elevated trace element contents in MSW-derived composts (Hagenmaier and
Krauss 1982).
Some operators combine (or used to combine) green waste from source segregated sources
with mechanically segregated MSW before composting (e.g. Catto 1999). One possible
reason for doing this might be to reduce the content of trace elements and inerts in the
compost, compared with that which would have resulted from composting of mechanically
segregated MSW alone. Ultimately the “dilution” achieved may still be insufficient to make a
step change in compost quality, and a potential “quality” product stream from composting the
green waste alone is lost.
An emerging concern has been over the significance of toxic organic substances in composts
derived from source segregated or mechanically segregated MSW feedstocks. These arise
from a variety of sources, including plastics, coatings on papers, pesticides, soot (PAHs),
various household chemicals, ash and products of incomplete combustion (de Haan 1981,
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Hagenmaier et al. 1986, Harms and Sauerbeck 1983, Malloy et al. 1992). Some information
on contents of toxic organics in MSW feedstocks is also available for analyses carried out to
assess incinerator performance (e.g. Tosine et al. 1985, Wenborn et al. 1999). Some toxic
organics, for example many PAHs will degrade in time in composts, or are sorbed into humic
materials (Hagenmaier et al. 1986, Harms and Sauerbeck 1983). At present toxic organic
compounds are not seen as a major problem for sewage sludge or MSW composts (European
Commission 2002, Smith 2000). However, available information is limited, and analyses are
difficult and expensive. Several Member States have suggested limit values for some organic
compounds (for example PAHs) in forthcoming revisions of the EC sewage sludge Directive.
These limitations would severely curtail the use of sewage sludge in agriculture (Smith 2001),
and, given the EC policy linkage between biowastes and sewage sludge, would also limit
compost use in agriculture. However, no final decisions have yet been taken (European
Commission 2001, 2002 & 2003).
Composts produced from mechanically segregated MSW tend to be relatively low in nitrogen
(1% total N), but high in potassium content. Compositional information is discussed further
in the Critical Review Section, Product Quality and Environmental Impacts.
3.3 Biological characteristics
The components of MSW vary in their biodegradability (Bardos and Lopez Real 1989;
CIWM 2002) for example - and only as a “rule of thumb”:
rapidly degradable (putrescible) materials such as food scraps
slowly degradable organic materials such as egg board, tissue paper, leaves
gradually degradable organic materials such as wood and paper
nondegradable materials such as glass, metals and the main classes of
thermoplastics: polythene, polypropylene, polystyrene and polyvinyl chloride
(Evans 1974).
Biodegradability is an intrinsic property of the material. It is linked to the ease with which
materials can be subjected to enzymic attack, and the range of enzymes (and hence
organisms) able to react effectively with the material as a substrate. The most biodegradable
materials are typically those which yield energy to micro-organisms or nutrition, or in the
case of anaerobic systems can supply oxygen / act as terminal electron acceptors (as oxygen
does in aerobic respiration). In some cases substrates may be degraded co-incidentally
because they can substitute for a common substrate, for example chloro-ethane is oxidised by
the same enzyme that oxidises methane. These rapidly degradable materials fuel the rapid
temperature increases characteristic of the thermophilic stage of composting (discussed later
in this review), and can be used by a wide range of organisms.
More slowly degradable materials are typically carbon rich and nitrogen poor, and the carbon
is in a less readily usable form, for example as cellulose rather than starches. Relatively fewer
organisms degrade these materials, and the rate of degradation is slower.
Gradually degradable organics include wood, card and paper. Paper and card, although
cellulose rich, are rendered only gradually biodegradable both by their physical nature and
because the cellulose is somewhat denatured by the paper/card production process, reducing
the ease of enzyme attachment. Wood tends to be degraded by specialist fungi which employ
the production of oxygen and hydroxyl radicals and non-specific lignase enzymes. This
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fungal degradation system can also degrade a number of organic pollutants such as
pentachlorophenol and PAHs. There is an extensive technical literature on the degradation of
cellulose and lignin (Alfani and Cantarella 1987, Tuomela et al. 2000).
Biodegradation may be impeded also if materials are rendered unavailable or inaccessible to
microbial attack, for example in disposable nappies (Line 1998) or the cardboard in a
packaging laminate would be rendered in accessible by plastic and foil coatings. Physical
attrition may abrade these coatings, and render the card accessible to biodegradation.
However, the films themselves will remain as an “inert” contaminant in the compost
(Encarnacion-Rodriguez et al. 1995).
Many items disposed of to the waste stream contain a mixture of materials of differing
degradability like disposable nappies (Line 1998). Some of these materials, for example the
absorbent gel in the nappy, may be dispersed by the composting process, but not necessarily
fully degraded (Stegmann et al. 1993). It is a contentious issue whether or not such dispersed
materials should be considered composted. This argument is particularly contentious for
some classes of so-called “degradable” plastics, where the end result is that the polymer is
broken down to such an extent that it is no longer visible, but is still present. This is
particularly pertinent given the current interest in the use of “degradable” plastic bags for
waste collection (Cole and Leonas 1991). There are two difficulties, the first is that the
degradation of the material may be slow so that remnants remain visible as a contaminant in a
compost product at point of use (Colyer 2004). The second, is whether it is appropriate to
release a material containing undegraded polymer back into the environment, even if it is
fragmented, in a compost product (Klemchuk 1990, Satkovsky 2002). There is some
evidence that finely divided polythene is slowly biodegradable (Lee et al. 1991).
Inorganic compounds may also be attacked by micro-organisms, either to liberate energy to
drive their metabolic activity (for example the oxidation of sulphur) or indirectly through the
release of ligands and/or acids. Under anaerobic conditions biological processes may
mobilise inorganic contaminants. Arsenic and some heavy metals may be converted into
volatile and highly toxic methylated forms by microbial activity (Atlas and Bartha 1987).
MSW fractions also contain micro-organisms, both those that might promote composting, and
those that are potential pathogens. Typically, MSW fractions will compost spontaneously,
and so need no biological inoculation. Pathogen issues are considered in the Critical Review
Sections: Biology of Composting - Process Optimisation, Health and Safety, Emissions and
Emissions Control - Bioaerosols & Other Health Risks, and Product Quality and
Environmental Impacts - Microbial and Pathogen Issues.
4. Sampling and analysis
The characteristics of samples collected from a lot are used to make estimates of the
characteristics of that lot. Thus, samples are used to infer properties about the lot in order to
make correct decisions concerning that lot. Therefore, for sampling to be meaningful, it is
imperative that a sample is as representative as possible of the lot, and more generally, each
subsample must be as representative as possible of the parent sample from which it is derived.
Subsampling errors propagate down the chain from the largest primary sample to the
smallest laboratory analytical subsample. If a collection of samples does not represent the
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population from which they are drawn, then the statistical analyses of the generated data may
lead to misinformed conclusions and perhaps costly decisions.
It is quite a “lot” to ask of the tiny (on the order of a few grams, and often much lower)
laboratory analytical subsample to be representative of each of the larger and larger (parent)
samples in the chain from which it was derived, up to the entire lot (which could be many
tons). Therefore, it is imperative that each subsample is as representative as possible of the
parent sample from which it is derived. Any subsampling error is only going to propagate
down the chain from the largest sample to the smallest laboratory analytical subsample.
The primary reason that samples are being taken is to make some determination about the lot
(e.g., a contaminated site). The study goals and objectives determine the acceptable statistical
characteristics for the study. If a decision depends on the analytical results, then the first
issue is to determine what type of measurements are needed and how accurate and precise
they should be. These goals are referred to as Data Quality Objectives -DQOs (extracts from
US EPA 2003).
MSW is a complex material stream. It is particulate, and contains particles which vary
substantially in terms of:
size
shape
density
hardness
stiffness / flexibility
surface properties
composition.
A lot of particles are almost two-dimensional in nature, for example papers, while others may
wrap themselves around other particles, for example textiles. Some particles may be
composites of different materials – for example packaging. Often particles of different kinds
are contained in several layers of bags.
This complexity is also true for processed fractions of MSW, including mechanically
segregated fractions, composts and refined compost product (Barton 1983, Barton and
Wheeler 1988). The potential range of variability may, of course, be reduced by processing,
but perhaps not to the degree that one might expect. For example, after trommel screening at
50 mm, the undersize should be mostly below 50 mm in size and the oversize mostly greater
than 50 mm in size. However, it is quite possible for the undersize fraction to contain
materials larger than 50 mm in one dimension and the oversize materials smaller than 50 mm,
depending on how the material fell onto the screen. The oversize may still contain smaller
particles entrained or contained in or on larger particles. The undersize may contain larger
particles which were deformed and forced through the trommel screen. Particles may also
break and fall through trommel or flat bed screens, often this is intentional in waste
processing, however where screening is used in sample appraisal it is a potential source of
error.
There is a link between “information” required by a user and sampling and analysis. The
nature of this linkage is often overlooked, but it is critical to determining the approach to
sampling and analysis that should be undertaken. The critical factors relate to the type of
information needed and the “quality” of information necessary, which in turn are determined
by what the information will be used for. These linkages are well explored in other
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environmental business sectors, for example contaminated land management (Crumbling et
al. 2001), but do not appear to be widely considered in the appraisal of waste composting.
Most commonly information from sampling and analysis is used in the composting sector
as part of a quality monitoring process (including compliance with guidelines and
regulations)
for the evaluation of safety, health and environment (SHE) impacts
for predictions about likely process performance, especially at the planning and
commissioning stages
for predictions of likely quality and SHE impacts.
Sampling and analysis information may also have a range of uses in research activities
beyond the day to day operations of a composting facility.
Most practioners understand that this information is subject to errors, but not all understand
the range of potential sources of error and relative importance of these errors to information
for decision-making. A very basic distinction is between systematic errors and random errors.
A systematic error is one which is a function of the sampling or analysis approach, for
example digestion of a compost sample with aqua regia will not liberate all trace elements
into solution, hence estimates of “total metals” for example will always be systematically
under-estimated. Random errors are unpredictable errors that are a fundamental property of
what is being measured – its intrinsic variability. Statistical techniques can be used to
compare measurements to determine the probability that they are different given known
random error. Often the techniques employed assume that random errors follow the Normal
distribution. However, this is not always true, for example distributions may be skewed away
from Normal, for example, skewed distributions are often observed for “heavy metals” in
organic materials. An EC project (HORIZONTAL) has been investigating the distribution of
trace elements and micro-organic pollutants in soils, sewage sludges and composts (including
from MSW), and has collated and reviewed the various guidelines available for the sampling
and analysis of these materials (Lambkin et al. 2004).
Errors can arise at various stages of the sampling and analysis process:
during sample collection
during sample preparation, preservation and storage
during subsampling
during analysis.
Understanding the significance of errors can be compounded by a statistically inadequate
sampling regime, that prevents an adequate understanding of the variability of the
measurement being made.
It is also important to understand the cost of the information being collected versus its utility
to the decision maker. A common mistake is to invest a lot of money in few measurements
with high analytical precision, when the intrinsic variability of the material being sampled
renders a limited set of data points useless or even misleading in cases of compliance with
regulatory or guideline standards. In these circumstances it may be better to make many
analyses at, say, 20% precision, rather than few at 1% precision. Increasing use is being made
of sensors and field based measuring techniques as a means of collecting a large volume of
indicative data (e.g. see the EC Project SENSPOL:
http://www.cranfield.ac.uk/biotech/senspol/).
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Sampling is discussed in a little more detail in a subsection of this Critical Review Section.
The other subsections deal with biological, chemical and physical techniques, beginning with
physical techniques. For MSW fractions and products it is usually advisable for physical pre-
treatment to take place before measurement of chemical parameters, for fraction of size
greater than 10 mm, for composts as well as feedstocks (Brunner and Earnst 1986, Wheeler
1993)
4.1 Sampling and Sample Handling
4.1.1 Designing the sampling scheme
The statistical theory underpinning sampling of streams in waste management processing is
developed from the work of Gy for the mining and metallurgical processing industries (Gy
1970, 1976, Morvan 1988, US EPA 2003). This work has been used to determine sampling
rates and approaches for MSW fractions and products (Barton 1983, Poll 1988). While
standardised approaches do not yet exist for MSW fractions and products, methods used at
Warren Spring Laboratory for assessing the performance of refuse derived fuel plant and
subsequently in the Environment Agency’s National Household Waste Analysis Programme
(NHWAP) and by the EC-SWA-Tool project, offer approaches with useful “track record” or
previous use (Barton 1983, 1984; Barton and Poll 1983; Barton and Wheeler 1988; Barton et
al. 1988, Dobson et al. 2003, Environment Agency 1996; Johnson et al. 1993, Martin et al.
1995, Welsh Assembly Government 2003). Methods have also been elaborated by the United
States Environmental Protection Agency (USEPA 1973 and 1989). In 2004 Defra published
comprehensive guidance for waste composition analysis for local authorities (Defra 2004).
Guidance is available from the Defra 2004 guidance mentioned above for MSW composition
analysis. British Standards Institution have published standards for the sampling of soil
improvers and growing media (BS EN 12579:2000), which is based on the work of the
European Centre for Normalisation (CEN, http://www.cenorm.be) Technical Committee 223.
Sampling. Guidance is also given the WRAP/BSI PAS 100 guidance “Specification for
Compost” (2002, http://www.wrap.org) and in the British Standard for Topsoil (BS EN 3882:
1994). A wider review of available methodologies has been compiled by the EC Horizontal
Project (Lambkin et al 2004.), and detailed recommendations made. British Standards are
available from: http://www.bsonline.techindex.co.uk. A comprehensive review of MSW
sampling, particularly from a management point of view, has been written by Lewin et al.
(2004).
CEN TC 292, on Characterization of Waste is carrying out standards development work in
progress on waste stream sampling. (Web link:
http://www.cenorm.be/CENORM/BusinessDomains/TechnicalCommitteesWorkshops/CENT
echnicalCommittees/CENTechnicalCommittees.asp?param=6273&title=CEN%2FTC+292).
Sampling design is a complex subject. Compliance of sampling design with a standard does
just that, i.e. complies with the standard. It does not offer any particular guarantee that
sampling is statistically or technically rigorous. If in doubt, professional help should be
sought.
Sample recording is an important part of designing the overall sampling strategy. Samples
need to be described in such a way that provenance of analytical data is always clear (taking
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into account sample origin and date, sample handling and analytical methods). It is also
important that information can be readily stored and recalled from storage in some kind of
data management system.
4.1.2 Sample Collection
Sample collection for compost feedstocks and products is likely to be either from stockpiles,
or from process streams. Sample collection may also be necessary for soils to receive, or
which have received, composts, and for the assessment of process emissions, such as leachate,
dusts, bioaerosols, odour, volatile organic compounds and also the assessment of nuisance
problems such as flies and vermin.
Process sampling is where samples are taken from the material as it enters or exits a
processing step, for example from across a conveyor belt or the outputs of a screen. Process
sampling offers major advantages over stockpile sampling for compost and feedstock
appraisal (Barton 1983, Barton and Wheeler 1988, Poll 1988). The advantages are:
1) Samples can be collected from incremental process samples, which allows the whole
of the process stream to be assessed, compared with stock piles where bias may be
possible, for example related to the proximity to surface, and because of the
differential settlement of materials in stockpiles
2) Samples can be more easily logged and recorded
3) Quality control issues can be more easily specified and executed
4) Process performance can be more clearly assessed, for example undersize and
oversize from a screening process.
To avoid bias, it is important in process sampling that the whole stream is collected, for
example material carried at the edges of conveyor belts does not spill over.
Methods for stockpile sampling are offered in the British Standard for the sampling of soil
improvers and growing media (BS EN 12579:2000). Sampling guidance is also given the
WRAP/BSI PAS 100 guidance “Specification for Compost” (2002, http://www.wrap.org)
and in the British Standard for Topsoil (BS EN 3882: 1994). A wider review of available
methodologies has been compiled by the EC Horizontal Project (Lambkin et al. 2004).
British Standards area available from: http://www.bsonline.techindex.co.uk.
Methods for sample collection for soils are described in BS 3882:1994. However, more
extensive guidance is available from techniques developed for site investigation (see
http://www.eugris.org).
Methods for microbial, leachate, odour, dusts, vermin, flies, volatile organic compounds and
bio-aerosols appraisal are reviewed in Alvarez et al. 1972, Burge and Millner 1980, Federal
Environment Agency – Austria 1998, Gotaas 1956, Gilbert et al. 1999, Kim et al. 1995, Ulen
1997, Warde-Jones 1996, Wheeler et al. 2001. Further information is available in this Critical
Review in the sections on: Sampling and analysis - Biological methods, Health and Safety,
Emissions and Emissions Control, and Product quality and environmental impacts -
Microbial and pathogen issues. Further links to method references and data are provided by
the Defra Review of Environmental and Health Effects of Waste Management: Municipal
Solid Waste and Similar Wastes (Defra 2004).
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4.1.3 Sub-sampling, Sample Preparation, Preservation and Transport
A series of processes take place between sample collection and analysis:
aggregation of samples
sub-sampling prior to on site analyses / dispatch
sample packaging and preservation
transportation
off site sub-sampling and analyses
dispatch of samples for specialised analyses.
Incremental process samples (or samples from different stockpile locations) are typically
bulked before onward treatment. These then need to be thoroughly mixed, to prevent a bias
towards any particular individual component sample, although guidance on what constitutes
thorough mixing is usually not specified. For dealing with MSW fractions, guidance is
available from Poll 1988, Defra 2004, Dobson et al. 2003, Environment Agency 1996, Welsh
Assembly Government 2003 and SEPA 2004.
Where particle sizes are likely to be greater than 10 mm it is advisable to screen the
aggregated sample before subsampling takes place. This screening may form part of an on
site size and category analysis procedure (see Critical Review Section, Sampling and Analysis
– Physical). The samples being screened may be several 10s of kilograms, the size of the
sample being screened depends on its particle size. Samples should be screened at declining
screen sizes, typically: 10, 20, 40, 80 and 160 mm. It is important that the split of the sample
mass across the size ranges is recorded. Sub-samples of the screened fractions, usually taken
by coning and quartering, can then be taken for:
moisture content determination
organic matter estimation by ashing (followed by glass content assessment for <10
mm fraction)
category analysis by handsorting
air drying (at low temperatures) prior to sample preparation for chemical analyses
(mass loss on drying must be recorded). Chemical analyses (and several physical
measurements) for the whole sample can be estimated by combining the results
reported for each fraction, in proportion to the proportion of mass each screened
faction represents of the total sample. This screening step is very important to
prevent sample bias towards particle size (Poll 1988).
A few measurements may take place on unscreened samples, including assessments for bulk
density and pH. The Animal By-Product regulations and standards such as BSI/WRAP PAS
100 require microbiological assessments. It would seem best to apply these to samples which
have not been screened, because (a) the screening process may change the biological
properties of the material, (b) the screening process will carry over cross-contamination.
Some guidance on taking samples for microbiological purposes is provided by BSI/WRAP
PAS 100 and the Animal By-Products Regulations 2003 (SI 2003/1482).
In some situations it may be possible for a number of these operations to be carried out at the
waste facility where samples are being collected from. However, it may be necessary to carry
many of these operations out off site. It is generally advisable to carry out all of the stages
described thus far within 24 hours, unless refrigeration of the bulk samples is possible. The
samples are biologically active, and may degrade substantially otherwise. Bulk samples are
generally stored in heavy duty polythene bags.
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For MSW fractions and products size reduction is a necessary step prior to chemical analyses
of air dried samples. This size reduction tends to proceed in several steps, a bulk size
reduction in a knife mill, hammermill or similar unit down to 2 mm size, followed by further
sub-sampling, followed by milling to a fine powder. This size reduction process is a
potential source of major errors and biases. Significant issues for the first step of size
reduction down to 2 mm include the following:
some materials such as stones may be manually removed – which leads to a bias in
the analytical data – the removal of items such as button cells prior to milling is a
complicated issue. The milling of the button cell would cause a massive toxic
element spike in the sample, and possibly cross-contaminate future samples
(Brunner and Earnst 1986). On the other hand the button cell is a part of the metal
load of the compost.
the mill is typically steel, hardened with another metal such as nickel or
manganese – ensure that the hardening agents are not elements to be analysed in
the samples being milled
the mill is a source of sample cross-contamination if it is not cleaned after each
operation, for example by running a sand “blank”.
These knife and hammermills typically manage throughputs of up to several 10s of kilograms
per hour, adequate to mill or grind an entire of air-dried sub-sample.
The way that further sub-samples are taken can be a major source of bias or error in
subsequent data. Where it is at all possible mechanical sample splitting using a spinning riffle
(also known as a “sectorial” splitter) is advisable (Morvan 1988, US EPA 2003). Spinning
rifles with a capacity of several kilograms are available, down to small units for use in
analytical laboratories. Gerlach et al. (2002) evaluated five soil sample splitting methods
(riffle splitting, paper cone riffle splitting, fractional shovelling, coning and quartering, and
grab sampling) with synthetic samples. Individually prepared samples consisting of layers of
sand, sodium chloride and magnetite were left layered until splitting to simulate stratification
from transport or density effects. Method performance rankings were in qualitative
agreement with expectations from Gy sampling theory. Riffle splitting performed the best,
with approximate 99% confidence levels of less than 2%, followed by paper cone riffle
splitting. Coning and quartering and fractional shovelling were associated with significantly
higher variability and also took much longer to perform. Common grab sampling was the
poorest performer, with approximate 99% confidence levels of 100%-150% and biases of
15%-20%. Gerlach found that, for these synthetic samples, sampling accuracy was at least
two orders of magnitude worse than the accuracy of the analytical method. The synthetic
samples he tested seem rather homogeneous compared with composts and mixed waste
fractions, even after they have been hammermilled down to < 2 mm.
Very often it is the <2 mm fraction which would be sent for chemical analysis, and again very
often this is off site. Milling of the <2 mm fraction often takes place in TEMA mills, or
similar equipment. Where the mills are made of steel, it is important to know what the iron is
alloyed with. For example using a mill with a steel hardened with nickel will render
subsequent analytical data for nickel meaningless (unless of course the amount of nickel
abraded by the milling process can be exactly known). It is also important to specify how
sub-samples of both the <2 mm for further milling, and sub-samples of the subsequent
powder for analysis are taken. Grab samples will introduce a lot more error than using a
spinning riffle or similar mechanical device. Requirements for sample dispatch and
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packaging are written into the standard methods for chemical techniques, and should be
specified by the analytical service provider.
Problems of sample preparation are far greater where analysis is for organic compounds
(Langenkamp and Luca 2001, US EPA 1989). These may be destroyed by heat or lost due to
volatilisation during conventional sample drying and milling processes. Sample collection,
preservation and transportation requires special measures, particularly where volatile, semi-
volatile of biodegradable organic components are to be assessed.
British Standard BS EN 13040:2000 provides guidance on “Soil improvers and growing
media. Sample preparation for chemical and physical tests, determination of dry matter
content, moisture content and laboratory compacted bulk density”. British Standards are
available from: http://www.bsonline.techindex.co.uk.
The US EPA has published an excellent, and easily accessible, guidance on obtaining
representative laboratory analytical subsamples from particulate laboratory samples (US EPA
2003).
4.1.4 Interlaboratory Comparisons
Sampling and sample handling (recording preparation, preservation, transport) can be the
weak link in the sampling and analysis information gathering process for chemical analytical
data. However, while analytical variability within a single laboratory tends to be low, inter-
laboratory comparisons indicate that there can be substantial variation in between the
chemical analytical results laboratories report back for a single reference sample, perhaps by a
factor of ten (or more) in some comparisons (Bourque et al. 1999, Holmes et al. 1998, Kreft
and Bidlingmaier 1996). Little is known about how data reported for physical and biological
properties might vary between laboratories. For physical composition data, the lack of
standard methodologies can make any comparisons between data from different sources rather
unreliable (Bampatsis and Dobson 2004, Fischer and Crowe 2000).
4.1.5 Health and Safety Issues
Sample collection and processing are potentially hazardous operations. A few examples of
hazards (by no means an exhaustive list) include: being struck by vehicles or machinery;
being trapped in machinery, being struck by flying objects, noise and dust. Sample collection
and processing operations should therefore only take place with the advice of recognised
health and safety officers, both for the site where work is being carried out, and for the
employer of the operatives, and must be compliance with appropriate health and safety law
and regulations.
SEPA (2004) state in their guidance: Suitable and sufficient risk assessments of all associated
work activities should be carried out by [organisations], in accordance with their own
protocols and procedures, prior to conducting any MSW analysis. From these, safe systems of
work must be drawn up, to include details of the correct waste handling methods, personal
protective equipment requirements and appropriate hygiene procedures. Staff working on the
analysis must be made aware of both documents. All staff carrying out MSW analysis must be
trained and competent to carry out their appointed tasks safely.
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4.2 Physical Methods
Physical analyses of composts and compost feedstocks may be carried out for a variety of
purposes:
to determine quantities, size and category composition, moisture contents of
feedstock and compost components, which are also a precursor for further analyses
because of the complexity of the materials being assessed
to determine bulk densities, materials handling properties
as part of product quality and product performance (e.g. as a soil improver or
growing medium), assessment the most common assessments include air volume,
water volume, shrinkage value and total pore space, these may be assessed to fulfil
compliance needs for guidelines and standards;
prediction / monitoring of performance
The physical classification of the components of MSW is typically on the basis of size
distributions and categories (see Critical Review Section, Sampling and Analysis – Physical).
Categories used by the Warren Spring Laboratory and subsequently by AEA technology PLC
include: “putrescibles”, “paper and card”, “glass”, “ferrous metals”, “non-ferrous metals”,
“textiles”, “miscellaneous combustibles”, “miscellaneous non-combustibles”, “wood”, “dense
plastic”, “film plastic” (see Table A). Classifications are made by sorting by hand. Materials
tend to be sorted into size ranges before classification to prevent a bias towards larger items in
the hand-sorting. Sizing is usually on a logarithmically decreasing scale e.g., screening at
160, 80, 40, 20 and 10 mm. Typically materials below 10 mm in size are regarded as too
small to hand-sort and are referred to as “fines” (Poll 1988). These were developed under the
Environment Agency’s National Household Waste Analysis Programme (e.g. Environment
Agency 1996, Parfitt 1997), in Wales (Welsh Assembly Government 2003) and in 2004 the
Scottish Environmental Protection Agency released a slightly revised set of categories (SEPA
2004). Approaches may be varied when assessing feedstocks collected from civic amenity
sites, as opposed to households (Poll et al. 1990). In 2004 Defra published comprehensive
guidance for local authorities on waste composition analysis (Defra 2004).
Table A Waste Composition Categories Suggested by Defra 2004
Primary Secondary
Newspapers
Magazines
Other Recyclable Paper
Paper Packaging
Paper
Non-recyclable Paper
Liquid Cartons
Board Packaging
Card Packaging
Card
Other Card
Plastic Bottles
Other Dense Plastic Packaging
Dense Plastic Other Dense Plastic
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Other Plastic Film
Plastic Film Packaging Plastic Film
Textiles
Textiles Shoes
Glass Bottles
Glass Jars
Glass Other Glass
Treated Wood
Untreated Wood
Furniture
Disposable Nappies
Other Miscellaneous Combustibles
Miscellaneous Combustibles
Carpet and Underlay
Construction and Demolition
Miscellaneous Non-combustibles Other Miscellaneous Non-combustibles
Ferrous Food
Ferrous Beverage Cans
Ferrous Metal Other Ferrous Metal
Non-ferrous Food
Non-ferrous Beverage Cans
Non-ferrous Metal Other Non-Ferrous Metal
White Goods
Large Electronic Goods
TV’s and Monitors
WEEE (waste electrical and electronic
equipment) Other WEEE
Household Batteries
Car Batteries
Engine Oil
Other Potentially Hazardous
Hazardous
Identifiable Clinical Waste
Garden Waste
Soil
Organic Non-catering Other Organic
Home Compostable Kitchen Waste
Organic Catering Non-home Compostable Kitchen Waste
Fines Fines
These are not the only classification approaches. Overall principles of sizing and
classification into categories are widely accepted (e.g. Martin et al. 1995), however, the
nature of the sizing and the categorisation varies. Efforts are underway in an EC funded
project, SWA-Tool, to produce a standardised approach in Europe (Bampatsis and Dobson
2004, Dobson et al. 2003), who suggest a set of “primary” category classes: organic
(biowaste); wood, paper and cardboard, plastics, glass, textiles, metals, hazardous household
waste, complex (composite) products, inert, other and fines (<10 mm fraction). Further
differentiation is possible via a series of subclasses. The EC funded AWAST project (EC
Project 2004) is also developing standard approaches for the appraisal of MSW composition.
However, their suggestions are different to those of SWA-Tool. The cross-referencing of
even general waste arising statistics at a European level is unreliable, owing to the differing
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range of waste types considered as “household” or “municipal” waste in different countries
(Fischer and Crowe 2000). Guidance on information capture for essential waste statistics and
local management reporting in the UK is available from http://www.wastedataflow.org/,
which also serves as an overarching web-based data management system, and from Defra via:
http://lasupport.defra.gov.uk/Default.aspx?Menu=Menu&Module=Article&ArticleID=103.
At this web link can be found a beta testing version of the Materials Captures Toolkit. This
includes the following functions (among many more decision-support modules):
Module 1 – Waste Arising; analysing and projecting waste arisings
Module 2 – Current Services; entering baseline data on current schemes
Module 3 – Waste Composition; analysing the composition of different waste
streams.
While category analysis is not usually carried out on fractions below 10 mm, it is often
important to make an assessment of glass content, as this is a visible compost contaminant.
This assessment can be carried out by identifying the glass content in a sample that has been
weighed and then ashed (New and Papworth 1988). Other methods of determination of the
inerts fraction in compost based on density and other separations have been tested (e.g.
Morvan 1992). However, these are not in common use in the UK.
As well as category and size analyses, a range of other physical parameters are important in
compost processing, and may have a bearing on the design of the composting system, such as:
temperature (in particular), hydrological characteristics (moisture content, water holding
capacity and water permeability), bulk density, particle size distribution, porosity and air
flow resistance, mechanical, thermal and electrical properties. Agnew and Leonard (2003)
give a series of typical values for a number of physical parameters, including particle size
distribution, porosity, mechanical and electrical properties, along with empirical formulae
for bulk density particle density, free air space and specific heat capacity. Some methods are
also given by Poll (1988). Again there are no generally accepted “standard” international
methods.
Information about size and category distributions is used to assess the likely compostability of
the waste stream in question, the likely “quality of any composted product, what other
materials may be recovered from it, and to inform the design of pre-processing and refining
steps.
A series of models have been developed by various authors to enable outline predictions to be
made of the possible performance of different waste collection and processing methods -
screening, shredding and sorting methods (e.g. Billecoq 1981, EC Project 2004, Poll 1989,
Wheeler et al. 1989, Wheeler 1992). Information on size and category analysis has also been
used to try and predict likely levels of contamination of composts with heavy metals,
following observations that “fines” tend to carry the largest “load” of trace elements (van
Roosmalen et al. 1987). Size and category data has also been linked with information about
the economic circumstances of areas where materials are collected from (Barton and Poll
1983), and this kind of analysis appears as early as 1969 (Galier and Partridge 1969). This
kind of linkage could be used to “target” materials from different locales in urban areas for
particular waste recycling interventions.
These models, ultimately, are only as good as the information collected. Combining the use
of process models with generic data is not the best way of making reliable predictions of
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likely compost process performance. The collection of local waste size and category analyses
is always to be recommended.
The availability of standard protocols for physical measurements is higher for composted
products. However, it must be borne in mind that these protocols do not take into account the
likely range of particle sizes in composted fractions of MSW. As a “rule of thumb” it may
advisable to limit their use to products that have been screened 10 mm or less. For product
streams with larger particle sizes, it may be advisable for these measurements to be made on
sized fractions (as suggested in the Critical Review Section, Sampling and analysis –
Sampling).
Guidance is available from the British Standards Institution, based on the work of the
European Centre for Normalisation (CEN, http://www.cenorm.be) Technical Committee 223.
Available standards are:
BS EN 12580:2000 - Soil improvers and growing media. Determination of a
quantity
BS EN 13040:2000 - Soil improvers and growing media. Sample preparation for
chemical and physical tests, determination of dry matter content, moisture content
and laboratory compacted bulk density
BS EN 13041:2000 - Soil improvers and growing media. Determination of
physical properties. Dry bulk density, air volume, water volume, shrinkage value
and total pore space
Further methods are under development by CEN TC 223. The current status of this work is
given on:
http://www.cenorm.be/CENORM/BusinessDomains/TechnicalCommitteesWorkshops/CENT
echnicalCommittees/WP.asp?param=6204&title=CEN/TC%20223
Some methods are also given in the WRAP/BSI PAS 100 guidance “Specification for
Compost” (2002, http://www.wrap.org) and in the British Standard for Topsoil (BS EN 3882:
1994). British Standards are available from: http://www.bsonline.techindex.co.uk.
Physical and chemical analytical methods have also been elaborated by the United States
Environmental Protection Agency (USEPA 1973 and 1989).
4.3 Chemical Methods
Chemical analyses of composts and compost feedstocks may be carried out for a variety of
purposes:
environmental impact assessment, the most common analyses are those for trace
elements and nitrogen compounds and also odour, but also may encompass toxic
organic compounds and possibly measurements of redox potential (although
oxygen demand is more commonly assessed as part of an assessment of
“stability”) – see the Critical Review Sections: Sampling and analysis –
Biological Methods and Product quality and environmental impacts --Maturity &
stability.
product quality, the most common assessments are of pH, conductivity, organic
matter content, nitrogen, phosphorous and potassium (total / extractable), but
assessments for calcium, magnesium and other plant nutrients may be carried out.
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A range of chemical techniques have been applied to assess compost “maturity”
(Avnimelech et al. 1996 , Chanyasak and Kubota 1981 & 1982, Chefetz et al.
1996). However, this application is usually research orientated and does not form
part of day to day operations at composting plants. Compost maturity assessments
are discussed further in the Critical Review Section, Product quality and
environmental impacts - Maturity & stability.
prediction / monitoring of performance, most usually oxygen availability (e.g. Van
der Gheyns et al. 1997), but potentially also pH, nitrogen content - see the Critical
Review Section, Biology of Composting - Optimisation.
A range of chemical assessments may also be applied to assess the impact of
composts on soils, for example impacts on cation exchange capacity, pH and
organic matter transformations - see the Critical Review Section, Product quality
and environmental impacts.
These purposes may result from compliance needs for guidelines, standards and regulations,
or for research and development processes. For day to day composting operations these
measurements are mostly applied to final products, except where they form part of process
prediction or monitoring. On occasion chemical compositions for feedstocks or interim
process materials may be necessary, for example where a problem identified with a product is
being tracked back through the process. Research interests in feedstock chemical data relate
to fate of compound studies, environmental burden assessments and modelling of compound
flows through waste management processes. Recent advances in chemical analytical
approaches may allow finger-printing of contamination problems and subsequent
identification of sources (for example see NICOLE 2004, US EPA 2004). Combined
bioassay and chemical extraction techniques (Toxicity Identification Evaluation - TIE) offer
the possibility of identifying both the specific nature of generally observed toxicity problems,
and identifying the contamination source (NICOLE 2004).
A number of standard protocols for physical measurements exist for composted products.
However, it must be borne in mind that these protocols do not take into account the likely
range of particle sizes in composted fractions of MSW. As a “rule of thumb” it may advisable
to limit their use to products that have been screened 10 mm or less. For product streams with
larger particle sizes, it may be advisable for these measurements to be made on sized fractions
(as suggested in the Critical Review Section, Sampling and analysis – Sampling).
Those presenting or using chemical analytical data should always bear in mind the possible
impact of sampling errors and that different laboratories can report different analytical data
for the same samples (Bourque et al. 1999). These problems may make both the
interpretation of data difficult and prevent comparisons of different composts or products
being made. This issue is discussed further in the Critical Review Section, Sampling and
analysis – Sampling.
Guidance is available from the British Standards Institution, based on the work of the
European Centre for Normalisation (CEN, http://www.cenorm.be) Technical Committee 223.
Available standards are:
BS EN 13037:2000 - Soil improvers and growing media. Determination of pH
BS EN 13038:2000 - Soil improvers and growing media. Determination of
electrical conductivity
BS EN 13039:2000 - Soil improvers and growing media. Determination of organic
matter content and ash
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BS EN 13650:2001 - Soil improvers and growing media. Extraction of aqua regia
soluble elements
BS EN 13651:2001 - Soil improvers and growing media. Extraction of calcium
chloride/DTPA (CAT) soluble elements
BS EN 13652:2001 - Soil improvers and growing media. Extraction of water
soluble nutrients and elements
BS EN 13654-1:2001 - Soil improvers and growing media. Determination of
nitrogen. Modified Kjeldahl method
BS EN 13654-2:2001 - Soil improvers and growing media. Determination of
nitrogen. Dumas method
Further methods are under development by CEN TC 223. The current status of this work is
given on:
http://www.cenorm.be/CENORM/BusinessDomains/TechnicalCommitteesWorkshops/CENT
echnicalCommittees/WP.asp?param=6204&title=CEN/TC%20223
Some methods are also given in the WRAP/BSI PAS 100 guidance “Specification for
Compost” (2002, http://www.wrap.org) and in the British Standard for Topsoil (BS EN 3882:
1994). British Standards area available from: http://www.bsonline.techindex.co.uk.
Odour assessment has been reviewed by Agency guidance published in 2002 (Environment
Agency 2002).
Physical and chemical analytical methods have also been elaborated by the United States
Environmental Protection Agency (USEPA 1973 and 1989).
No British Standard methods are available for toxic organic compounds. CEN TC 292, on
Characterization of Waste, has a large programme of standards development work for the
sampling and chemical analysis of waste streams, including for a number of organic
compounds. (Web link:
http://www.cenorm.be/CENORM/BusinessDomains/TechnicalCommitteesWorkshops/CENT
echnicalCommittees/CENTechnicalCommittees.asp?param=6273&title=CEN%2FTC+292).
Its scope of work encompasses: Standardization of procedures to determine the
characteristics of waste and waste behaviour, especially leaching properties and
standardization of subsequent terminology. Its work specifically excludes the setting of limit
values and the setting of specifications for products and processes. Available standards
largely relate to chemical analyses, in particular for leaching tests. Work in progress includes
sampling protocols and further chemical determinations.
Lagenkamp and Luca (2001) review prospects for harmonised techniques for soil and sewage
sludge. A series of guidelines for contaminated site assessment produced by the Dutch
Ministry of Housing, Spatial Planning and Environment (VROM) include guideline values
and measurement techniques for a wide range of organic substances (VROM 2000).
4.4 Biological Methods
Biological analyses of composts and compost feedstocks may be carried out for a variety of
purposes:
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environmental impact assessment, the most common analyses are those for
potential pathogens and allergens – see also the Critical Review Sections: Product
quality and environmental impacts - Microbial and pathogen issues and Health
and Safety, Emissions and Emissions Control - Bioaerosols & other health risks;
product quality, the most common assessments are of human and animal
pathogens, content of weed propagules, stability, maturity and phytotoxicity, and
on occasion of parasites (for example Ascaris and Toxicara) and plant pathogens
see also the Critical Review Sections: Product quality and environmental impacts
- Microbial and pathogen issues and Product quality and environmental impacts -
Maturity & stability;
prediction / monitoring of performance, - see the Critical Review Section,
Biology of Composting - Optimisation. Evaluation of biodegradability has become
an important appraisal method, in particular for evaluating the use and fate of
“biodegradable” plastics - see the Critical Review Section, Feedstocks and
composition - Biological characteristics.
These purposes may result from compliance needs for guidelines, standards and regulations,
in particular for compliance with Animal By-product regulations, or for research and
development processes. Composting research work has included a number of experiments
where composting is simulated, ranging from very small scale simulations to larger
simulations using 50 to 100 kg of material. Another area of composting research making
intensive use of microbial analyses is appraisals of microbial population dynamics and
microbial activity (e.g. Potter and Harrman 1997, Peters et al. 2000, Vallini et al. 1989 and, of
course, Waksman et al. 1939).
Centralised composting processes result in the release of micro-organisms into the
surrounding atmosphere. Conditions currently being set in the waste management licences
specify that facility operators must sample for these micro-organisms around the site. The
Composting Association has developed a protocol to provide guidance on meeting regulatory
conditions and carrying out the necessary assessments (Gilbert et al. 1999).
The most usual product quality biological assessments of are of human and animal pathogens,
content of weed propagules, stability, maturity and phytotoxicity. Standard methods for
pathogen appraisal have been specified in the Animal By-Products Regulations 2003 (SI
2003/1482), which enact the EC Animal By-Products Regulation (EC 1774/2002). Some
methods are also given in the WRAP/BSI PAS 100 guidance Specification for Compost
(http://www.wrap.org). Methods are reviewed by Jones and Martin (2003). The use of
indicator organisms is discussed by (Jones and Smith 2004). Plant pathogen assessment is
reviewed by (Noble and Roberts 2003).
Compost maturity, stability and phytotoxicity are inter-related properties (Anid 1986, Inbar et
al. 1990).
Stability refers to the degree of biological decomposition. Composts stimulate high
microbial activity in soil, where their content of readily degradable carbon is high
they may cause oxygen deficiency and a variety of indirect toxicity problems to
plant roots. Such composts can also be odorous because they remain biologically
active, if oxygen is limited will begin to degrade anaerobically, which can be
exacerbated if the compost is packed in bags.
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Maturity refers to the ability of a compost to support plant growth. Over
composting relatively easily degradable materials are mineralised or converted into
slowly degradable “humified” forms. In young composts intermediate breakdown
products and degradable materials remain such as fatty acids and ammonia
compounds. These compounds are odorous, and may also be inhibitory to plant
growth. High concentrations of soluble nutrients present in immature composts
support growth of Salmonellae and/or other pathogens which depend on free
nutrients to grow (Inbar et al. 1990). Some stages of plant growth can be
sensitive to conductivity, depending on the plant species. “Young” composts may
have higher conductivity than older composts which have been subject to leaching
processes, for example through exposure to rainfall.
Phytotoxicity refers to the potential for detrimental effects of compost on plant
growth. Composts may have phytotoxic effects because they contain high levels
of certain trace elements or organic pollutants. This effect is unrelated to compost
stability or maturity. Young composts may contain substances inhibitory to plant
growth related to the breakdown and degradation processes still taking place (as
described above) or because naturally occurring inhibitory substances such as
phenolics from certain woody materials have not yet had time to degrade.
Compost stability and maturity assessments include chemical analyses, microbiological
assays and higher plant bioassays. Compost respiration rates (assessed on the basis of oxygen
uptake) as an estimate of microbial activity is the most common approach to compost stability
assessments and also rates of composting (Hoitink and Frost 2002, Lasaridi and Stentiford
1999, Pressel and Bidlingmaier 1981, Richard et al. 1993, Swannell et al. 1993, Zimmerman
and Richard 1992). Bio-assays based on effects on germination are the most common
techniques used for assessing compost maturity and phytotoxicity based on the work of
Zucconi et al. (1981). Grundy et al. (1998) proposed methods to determine weed seed
contamination in composts based on sieving and flotation. The Composting Association has
developed a standard test that combines a compost maturity/phytotoxicity assessment with a
very simple and easy assessment of the content of weed propagules in the compost: A method
to assess contamination by weed propagules and phyto-toxins in composted organic
materials. CATM/2000/01 (TCA 2000). Stability and maturity assessment methods are
included in the WRAP/BSI PAS 100 guidance Specification for Compost (2002,
http://www.wrap.org), and standard methods for the UK have been proposed by ADAS
Consulting 2003.
A wide range of test protocols have been developed for examining biodegradability in
composting systems (e.g. European Commission Project 1996, Itavaara et al. 1997, Pagga
1999, Satkofsky 2002). A European Quality Standard has been produced EN 13423 –
Packaging requirements for packaging recoverable through composting and biodegradation.
A certification scheme in the UK is operated by The Composting Association
(http://www.compost.org).
Process simulations are used to try and model the biological activity of the composting
process at a manageable scale that can be replicated. Replication is important to be able to
distinguish experimental from random effects. Unfortunately, because of the thermogenic
nature of composting, and the variability of compost feedstocks laboratory scale simulations
can be unreliable, and great caution is required to interpret their results. Larger scale
simulations (> 50 kg scale) are seen as more reliable, although more expensive and complex
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to operate. optimising the compost process for complex solid material such as municipal
solid waste (MSW), is fraught with difficulties. The research must be carried out at a scale
large enough for the sampled material to represent the waste accurately, but small enough to
allow the process conditions to be easily replicated in different reactors (Petiot and de
Guardia 2004, Magelhaes et al. 1993, Swannell et al. 1993).
5. Biology of Composting
5.1 Terms and Definitions
The terms “compost” and “composting” have a wide colloquial usage. Composting may be
used to describe any process of biodegradation of organic materials into a product of some
kind, whether carried out in the presence or absence of air. Compost may be used to describe
many different types of growing media, soil improver or mulch.
Compost and composting may be used in this sense in some regulations (for example the
Waste Management Licensing Regulations 1994). The Composting Association (TCA)
reports the following Defra definition for “compost” as biodegradable municipal waste
which has been aerobically processed to form a stable, granular material containing valuable
organic matter and plant nutrients which, when applied to land, can improve the soil
structure of soil and enhance its biological activity” (TCA 2001).
WRAP summarises the current definitions of compost used in the UK as follows (WRAP
2002): There are no obvious legal definitions of compost in the UK. The definition used in the
TCA standards is: ‘Material that has been subjected to controlled, self-heating
biodegradation under aerobic conditions and stabilised such that it is not attractive to
vermin, does not have an obnoxious odour and does not support the regrowth of pathogens
and their indicator species. Compost that has been subject to a screening process may be
classified in terms of its particle size grade, from fine to coarse.’
WRAP continue: The DETR Report of the Composting Development Group on the
Development and Expansion of Markets for Compost defines compost as: ‘Biodegradable
municipal waste which has been aerobically processed to form a stable, granular material
containing valuable organic matter and plant nutrients which, when applied to land, can
improve the soil structure, enrich the nutrient content of soil and enhance its biological
activity.’
From a scientific and technical standpoint “composting” and “compost” have a narrower
meaning. A number of technical definitions and descriptions have been proposed (CIWM
2002, European Commission 2001, Zucconi and de Bertoldi 1986, Zucconi et al. 1987). The
key features of these definitions are as follows:
Compost is the product of composting - and not other processes such as anaerobic
digestion or mixing.
Composting is a biological process in which complex solid organic feedstocks are
oxidised to a biologically stable residue, with the liberation of water, carbon
dioxide, inorganic ions and heat. It is aerobic and is characterised by a period of
elevated temperature caused by heat generated by the biological process.
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The term stable, used in this context, refers to a product in which all of the readily degradable
organic material has been fully decomposed. The compost product is not completely resistant
to any further microbial breakdown; further decomposition will occur when, for example, the
compost is applied to the soil, although at a far slower rate, depending on the maturity of the
compost. Stability and maturity are defined in the Critical Review Section, Sampling and
analysis - Biological methods.
5.2 Process Description
Composting microbiology has been the subject of much investigation over the past 80 years
or more (e.g. Waksman et al. 1939). There have been a large number of reviews of the
composting process, its microbiology and optimisation, including: Anon 1991, Bardos and
Lopez Real 1989, Biddlestone et al. 1981, Brunt et al. 1985, De Bertoldi et al. 1983 & 1988,
CIWM 2002, Finstein and Morris 1975, Finstein et al. 1986, Golueke 1972, Gotaas 1956,
Lacey 2002, Newport 1990, Palmisano and Barlaz 1996).
The composting process can be considered as taking part in three distinct phases, which are
delineated by the different temperatures at which the process takes place:
an initial phase taking place at temperatures close to ambient (mesophilic, up to
40oC)
a phase at elevated temperatures, where biological activity causes heating to
thermophilic temperatures - 50oC or more
a maturation phase, following thermophilic activity where more complex
substrates are degraded at a slower rate (hence a slower rate of heat generation).
There are three different type of decomposer organisms (Dindahl 1978):
first level consumers: true decomposer or primary organisms that feed and digest
directly from the waste debris
second level (secondary) consumers that feed on the initial composer and
third level consumer (tertiary) which prey on the second group and upon each
other.
A range of organisms are included, in particular: bacteria, actinomycetes, fungi, protozoa,
annelids, arthropods. The thermophilic phase is dominated by bacteria, actinomycetes, and a
few fungi (Finstein and Morris 1975), when temperatures as high as 70 to 80oC may be
reached if an uncontrolled build-up of heat within the composting material is allowed to
continue (depending on the size of the composting mass).
Composting is mediated by a diverse community of micro-organisms, many of which are not
individually capable of fully mineralising the compostable materials (mineralisation refers to
the process of full decomposition of organic materials to carbon dioxide, water and ions).
Degradation during composting may proceed via a series of intermediate compounds
degraded by different sets of organisms. These intermediate compounds (as well as the
feedstocks themselves) may be phytotoxic and/or odorous. These intermediate organic
products may either serve as substrates for other micro-organisms or may remain, for a period
of time, in the compost residue. Organic intermediate breakdown products which are known
to be toxic to plants and which have been identified in immature composts include tannins,
polyphenols, ethylene, ethylene oxide, aliphatic acids, various aromatic compounds and
sulphides. The thermophilic stage is largely “fuelled” by readily degradable substrates such
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as proteins, starches, and later cellulose (Biddlestone and Gray 1982, Forsyth and Webley
1948, Jeris and Regan 1973), and is mediated by a relatively small range of micro-organisms,
compared with that carrying out degradation at mesophilic temperatures (Peters et al. 2000,
Strom 1985). Some organisms are active at both mesophilic and thermophilic temperatures
(Waksman et al. 1939).
The thermophilic stage of the composting process comes to an end as the readily degradable
substrates are exhausted, and the temperature of the composting material falls to ambient
levels. Further composting, (maturation or curing) takes place close to ambient temperatures.
As temperatures fall from thermophilic ranges fungal activity resumes (Anid 1986). During
this stage the majority of degradation of complex polymers such as lignin, and lingo-cellulose
takes place (mainly through the activities of basidiomycete fungi), phytotoxicity abates and
nitrogen in the form of biomass, compost residue and ammonia begins to be oxidised to
nitrate, nitrification (de Bertoldi et al. 1983, Zach et al. 2000). As temperatures fall further
the compost will be invaded by a range of animals, not able to tolerate the higher
temperatures of the thermophilic stage (Bechmann and Schriefer 1988).
Humification is assisted by the activity of soil mesofauna (invertebrates and other animals),
which assist decomposition by reducing in size any agglomerations of organic matter,
increasing the surface area available to microbial attack and promoting the processes by
which the organic matter is incorporated fully into the soil (Beachman and Schriefer 1988,
Dindal 1978).
Temperatures may fall as microbial activity reduces owing to a result of lack of available
moisture or oxygen. In these circumstances if oxygen becomes available again (for example
as a result of turning), or moisture (as a result of wetting), rapid composting will recommence.
This can result in a series of periods of thermophilic activity, before maturation proper takes
place. If composts are used when the composting activity has abated because of lack of
moisture or oxygen, the compost is generally unstable / immature, and can cause damage to
plants and is likely to be odorous or capable of generating odour.
A stable, mature compost is ready for general use. The final product of the composting
process is a mixture of the recalcitrant organic residues which persist after the initial rapid
stage of decomposition has subsided. Most of the readily degradable organic material will
have been converted into carbon dioxide and water, and much of this water and the water
already present in the feedstock, or added during composting, will have been driven off
during the thermophilic stage. Although composted material can be considered as stabilised,
it will continue to degrade further, although this process may take many years to complete.
Once the compost has been applied to soil, further breakdown of the organic residues occurs
and they become assimilated into the soil structure (Morel et al. 1986). This process is known
as humification. Stehouwer (2004) wrote a good introductory review about soil biological
processes.
The organisms necessary to carry-out composting are already present in mixed-MSW. There
has been a large number of studies of inoculation of wastes with bacterial cultures of one sort
or another, or with finished composts, to promote more rapid composting. Many report that
these have not been able to demonstrate any substantial process benefit (Finstein and Morris
1975, Finstein et al. 1986, Golueke 1954), although there are some reports of benefits of
using finished compost as an “inoculum” (Jeris and Regan 1973).
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5.3 Process Optimisation
For MSW streams, the composting process is principally affected by:
availability of substrate
availability of oxygen - the minimum oxygen requirement for rapid composting is
quoted as around 10%
availability of moisture - at levels below 25% biological activity within the
compost is severely retarded and at 10% or less it effectively stops
temperature - thermophilic stage.
Other conditions, such as pH, conductivity and the presence of toxic compounds are not likely
to significantly impact the composting of mechanically segregated (or source segregated)
fractions of MSW. The optimum pH for composting is pH 6 to 8 (CIWM 2002, Nakasaki et
al. 1993) which is the usual pH encountered in MSW being composted.
Given that the feedstocks contain readily biodegradable matter the key process controls for
composting MSW are aeration (for cooling and oxygen supply), moisture and temperature.
These factors tend to be inter-related, and are also affected by how the physical nature of the
feedstock affects the free flow of fluids (water and gases). These affects are likely to change
as the feedstock degrades and so its physical nature changes. In addition the nature of the
composting process has an important bearing on air supply, temperature and moisture content.
C:N ratios are often discussed as an important process control parameter. For example, a
nitrogen rich waste might be added to a waste that is carbon rich and low nitrogen such as
straw. However, for the practical composting of putrescible rich mechanically segregated
MSW of source segregated MSW, C:N ratio do not generally require any intervention.
However, if the MSW stream is very rich in paper and card, it may be advisable to add a
nitrogen rich amendment such as sewage sludge.
CN Ratio
This is the ratio of carbon to nitrogen. A simplified picture is that organisms use "fixed"
carbon to provide energy, and nitrogen to build cellular components made of proteins. If
fixed carbon and nitrogen are in balance then the organism has enough energy to make use of
all the nitrogen available to grow and reproduce. Where there is not enough carbon, nitrogen
will be released during decomposition, typically as ammonia. Where there is an excess of
carbon, organisms will absorb nitrogen from the environment to support the extra growth and
reproduction that the energy from the carbon affords. In reality the situation is far more
complex. For example, some organisms are able to fix molecular nitrogen from air, and
others degrade nitrogen sources to release gaseous nitrogen or nitrogen oxides that are lost to
atmosphere. The carbon (and indeed nitrogen) will vary in its availability to micro-
organisms, particularly as some substances are more rapidly degradable than others.
Measurements of "total" carbon and nitrogen used to calculate it may bear little resemblance
to what is actually available to micro-organisms. Use of CN ratios is at best a rule of thumb
.
Far more important in terms of feedstock composition are moisture content, and the physical
structure of the material. This should be free draining to allow easy movement of air and
water. Particle size should be relatively small (e.g. <50 mm), as large items will take time to
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degrade and may be anaerobic at their core. It is also important that the material contains a
good proportion of slowly compostable materials, as these will help maintain an open
structure in the composting mass over time. Conversely, rapidly degradable materials are
important for stimulating a rapid increase in process temperatures and the onset of
thermophilic temperatures. Once these temperatures have been reached, they can be
maintained by the lower rates of activity from the degradation of the more slowly degradable
materials because of the relatively high thermal inertia of composting materials. This is due to
the high heat capacity of water and low thermal conductivity of organic materials.
Temperature is a key process control parameter. Higher temperatures are associated with
greater decomposition (Richards et al. 1993). Increasing temperatures (providing moisture
and oxygen availability are not limiting) accelerates biochemical (e.g. degradation) processes,
doubling every 10oC rise. It also changes the microbial composition of the community of
organisms carrying out the composting. As a rule of thumb organisms operating up to 45oC
are called mesophilic, and those beyond this temperature are called thermophilic, as described
above.
A principal aim of process control for composting is to maintain a steady thermophilic phase
until all readily degradable materials are exhausted. There are two reasons for this; firstly to
ensure the most rapid and extensive degradation, and secondly to sanitise the compost.
Compost feedstocks may contain a variety of pathogenic organisms (Epstein 1998).
Sanitisation describes the processes in composting that destroy harmful micro-organisms, in
particular those that are pathogenic to plants or animals, including man.
Several processes combine to assist sanitisation, including: thermal inactivation, microbial
antagonisms, sorption and predation by other organisms (Burman 1961), de Bertoldi et al.
1988, Knoll 1959 and 1963). Thermal activation is a function of both temperature and the
length of exposure to the elevated temperature. Similar processes (thermal inactivation and
decay) act to eliminate viable weed propagules, seeds and root fragments (Grundy et al.
1998).
Of these, thermal inactivation is judged the most important from a regulatory standpoint,
largely because it is the easiest to observe, although microbial antagonisms and competition
may be the dominant sanitising effect (de Bertoldi et al. 1983), although the direct evidence
of antibiotic production during composting is limited (Kuester et al. 1981). Control of
process temperatures plays a key role in the composting of mechanically segregated fractions
of MSW following the recent implementation of controls for animal pathogens.
Current legislation classifies biodegradable waste in the household waste stream as “Catering
Waste” and as such, requires it to be composted to a specific set of conditions that comply
with the Animal By-Products Order. This is a statutory order introduced in the wake of the
BSE and foot-and-mouth Epidemics. The aim of the order is to prevent the re-occurrence and
spread of these diseases by preventing the distribution of pathogens that can be carried in
compost that has been improperly stored or processed, and which is subsequently spread on
grazing land.
These regulations apply to mechanically segregated MSW, if composts are to be applied to
land where livestock including birds have access, which effectively means that these
regulations apply unless the composting is a pre-treatment before landfill or thermal
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treatment. The requirements for composting mechanically segregated MSW within these
regulations are:
enclosure of the waste reception and first stage composting process
that the composting process divided in to two distinct process stages
that each of these stages will be required to meet specified time-temperature
conditions
separation of the clean and dirty sides of the process requiring separate equipment or
rigid sterilisation procedures for equipment used either side of the composting process
There is a difficult trade-off between composting rate and sanitisation in that temperatures
above 65OC can only be tolerated by a limited range of organisms. Most (but not all)
evidence suggests that decomposition rates are highest at 60oC or lower (Bardos & Lopez-
Real 1989, de Bertoldi et al. 1983, Forsyth and Webley 1948, Gray et al. 1971, Jeris and
Regan 1973, Miller et al. 1989, Niese Neumeyer,-Seekatz 1979, Smith et al. 1987, Strom
1985, Stutzenberger et al. 1970 and 1971, Suhler and Finstein 1977, Tansey and Brock 1978,
Waksman et al. 1939, Webley 1948).
However, a number of investigations of thermal inactivation of pathogens in compost – but
not all - suggest temperatures a little higher than 60oC are required, including for inactivation
of parasites such as Ascaris (Andrews et al. 1994, Banse and Strauch 1966, de Bertoldi et al.
1988, Gotaas 1956, Gray et al. 1971, Knoll 1963, Krogstad and Gudding 1975, Lofgren 1979,
Morgan and MacDonald 1969, Stentiford et al. 1985, US EPA 1971, Wiley 1962, Wiley and
Westerberg 1969). Finstein et al. (1987) suggest that maintaining a temperature between 55
and 60oC for at least three days throughout the compost volume is likely to maximise rates of
decomposition, while still achieving an acceptable degree of thermal inactivation of
pathogens. This suggestion is supported by other workers in the field (Biddlestone and Gray
1982). Temperatures of 55oC also appear adequate to control weed propagation. However,
weed seeds may remain viable in the edges of compost piles, or may be carried onto finished
compost by wind (Grundy et al. 1998).
Most, but not all, plant pathogens are also eliminated by the composting process. There is
also some evidence that composts may protect plants against some plant pathogens, and that
water extracts of compost, “compost tea” have a similar protective effect (Scheuerell and
Mahaffee 2002). See Critical Review Section, Product quality and environmental impacts -
Microbial and pathogen issues for further information.
A further complication in compost sanitisation is that many organisms that mediate the
composting process, particularly actinomycetes and the fungal species Aspergillus fumigatus
produce spores which are allergenic (Clark et al. 1983, Lacey 1997 and 2002), and there may
also be risks from airborne bacteria, including from the endotoxins in the cell walls of Gram-
negative types (Lacey et al. 1990. Consequently composting creates a potential health and
safety issue (TCA 2004), discussed further in the Critical Review Section, Health and Safety,
Emissions and Emissions Control - Bioaerosols & other health risks.
To be active, composting micro-organisms require both air and water, whose availability
therefore affects the rate of decomposition. Typically air is provided to the composting
materials in one of more of the following ways:
via passive diffusion through the composting mass, which may be assisted to some
degree by convection through the composting pile
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by regularly turning the compost, so that it is physically broken up cooled and new
air is incorporated into the composting mass
by forced aeration, which may be negative or reversed (sucking air through
compost), positive (blowing air through compost), or occasionally a combination
(EC Project 1990, Lofgren 1979, Sesay et al. 1998).
Passive diffusion will not supply adequate oxygen to support controlled composting.
Aeration by turning does support a controlled composting process, but the oxygen supply is
rate limiting for the compost. Forced aeration can, in theory, supply abundant oxygen.
However, even in aerated systems methane generation has been noted indicating some areas
of anaerobic degradation (Swannell et al. 1993). Aeration also allows control of process
temperatures by evaporative cooling (Bach et al. 1987) which allows better process
temperature control than turning alone and which may also help dry the compost (Finstein et
al. 1986). Aeration to control temperature provides adequate oxygen for microbiological
processes. Forced aeration also helps distribute heat through the composting materials (de
Bertoldi et al. 1983). Air may also be re-circulated to reduce heat loss from the composting
materials (Koenig and Bari 1998). Maintenance of aerobic conditions is also a vital part of
odour control (Noble and Dobrovin-Pennington 2002).
Temperature feedback control systems have been used to control aeration, switching fans on
above a set temperature (say 60oC) and off at lower temperatures - say 55oC (Eccles and
Stentiford 1987). The effectiveness of this temperature feedback control is limited by the
high degree of variability in temperature in composting MSW materials (Atchley and Clark
1979). Variation in temperature is not only related to obvious factors such as depth in the
pile, (for example surfaces are cooler - Avnimelech et al. 2004), but also includes a large,
apparently random, element. The effect of this is that temperature feedback control is at best
approximate, as indeed is temperature logging for the purposes of compliance with standards
and regulatory controls, for example those relating to the Animal By-Products Order.
Aeration on an intermittent basis without temperature feedback control can result in very high
temperatures being reached, greater than 80oC for composting sewage sludge and wood chips
according to Finstein et al. (1987).
In practice fans are on almost constantly during the first week or two of composting, and still
compost temperatures climb well above set points (e.g. de Bertoldi et al. 1982, Sikora and
Sowers 1985). An alternative process control loop has been to use feedback based on oxygen
levels in the composting mass. Indeed limiting oxygen availability has been proposed as a
means of temperature control (Citterio 1987, EC Project 1991), however, the effectiveness of
this approach - given the thermal inertia of the composting materials and the high volumes of
air input - seems questionable (Finstein et al. 1986, Haug 1986). Oxygen feedback control
might be combined with temperature feedback in systems where air can be re-circulated (for
example to maintain process temperatures for a longer period of time), although thermal
inertia may make this kind of sophisticated process control hard to achieve in practice. Other
process control parameters that may be monitored using sensors include humidity and carbon
dioxide levels (EC project 1990).
Often the technical literature will refer to the “Rutgers” and “Beltsville” approaches to
composting. These refer to aeration approaches. The Beltsville approach uses intermittent
aeration on the basis of a timer only (Epstein 1997, Willson 1987). The “Rutgers” approach
is that of Finstein at al (1987) which applies temperature feedback control with a set point of
55oC or thereabouts, as well as intermittent aeration.
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Forced aeration must be undertaken with great care. Forced aeration can create fissures in the
compost mass through which the majority of the air will pass rather than permeate the
composting materials. This effect is known as channelling (Finstein et al. 1986). There are
problems in aeration related to the height of the pile being aerated. In part this is because to
supply air to the composting mass an ever increasing volume of air must be pushed (or
sucked) through the base of the pile, so the for tall piles an excessive volume of air must be
blown through a relatively small volume of material at the base. The problem of aeration is
also in part related to the relative compaction of material towards the bottom of tall compost
piles, which reduces the interstitial spaces between compost particles and so the ability of air
and water to pass through. This reduced porosity leads to a greater risk of anaerobic zones
developing within the composting material, and an increased risk of channelling. The risk of
channelling is increased by increasing air pressure to achieve high air flow volumes from the
base of silo type reactors, and the high flow rate may also dry the materials out sufficiently to
prevent composting. Maximum heights of 2 to 3 m have been recommended (de Bertoldi et
al. 1988, Finstein et al. 1987). Modelling of compost aeration has been reviewed by Mathsen
(2004).
Negative aeration is often used so that the air drawn through the composting mass can be
captured for odour treatment. However, its performance in controlling the composting
process for MSW fractions is not as good as positive aeration (Stentiford 1992 and 1993,
Stentiford et al. 1985). Negative aeration delivers less air supply than positive aeration, for
the same power consumption, because the movement of air also drives moisture in the same
direction. this can lead to condensation and water-logging in the vicinity of the vents the air
is being withdrawn from. The effect of this is not only to reduce oxygen supply in the local
area, but through out the composting mass. Positive aeration is able to deliver a greater air
supply as moisture is driven away from the vent to the edges of the pile with the movement of
the air. Using techniques such as covering piles with finished composts means that odour
problems can be contained. Not all researchers favour positive aeration over negative
aeration, e.g. Willson (1987) expresses a contrary point of view.
Positive aeration systems are often associated with drying out of the compost (EC Project
1991, Finstein et al. 1986, Lofgren 1979), particularly in the vicinity of vents, to such an
extent that composting ceases before the material is stabilised. There are MBT process
designs that exploit this drying effect to assist refining for one of two reasons, either the dried
organic material is treated to remove inerts such as batteries and then remoistened and
allowed to continue composting, or the refined dried organic matter is used as an alternative
fuel. However, another consequence of this drying can be that the compost can almost set
solid, making its handling and processing (for example downstream refining) more
problematic.
For MSW optimal starting moisture content appears to be 50 to 65% by mass (Biddlestone et
al. 1981, Finstein and Morris 1975, Jeris and Regan 1973, Wiley and Pearce 1955, Schulze
1961). Although mixed MSW feedstocks are typically already at an optimum moisture
content (around 60% by mass) at the start of processing, moisture control during the
composting can be quite difficult. Furthermore, very dry compost is hard to rewet (Finstein et
al. 1986). Water may be irrigated on to the compost surface, although this may not be
advisable for reverse aeration systems, or may be injected into the compost. The use of active
compost piles to treat on site drainage water or even landfill leachate may be possible. Too
much water is as bad as too little. If moisture levels during composting are too high the
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interstices between the particles of the composting material can fill with water, excluding air
and bringing about anaerobic conditions. Moisture contents greater than 70% have been
found to be sub-optimal for shredded MSW (Biddlestone et al. 1981, Wiley and Pearce 1955).
The key issue from a microbiological point of view is the microbial availability of water
(Miller 1989). Hence, for fractions with a high glass and inerts content, a relatively low
moisture content may still support composting. It may be more appropriate to consider
moisture content in the compostable components of the MSW fraction being composted.
However, little information is available to provide a process control benchmark. A further
problem is that moisture content within piles of composting materials is highly variable (Rynk
2000).
While many authors (e.g. de Bertoldi et al. 1983) believe that turning is inadequate for
oxygen supply to composting, windrow turning alone remains commonly used, albeit perhaps
with longer treatment times than for systems employing forced aeration.
However, for forced aeration systems there may be advantages in intermittent turning (Gray et
al. 1971, Illmer and Schinner 1997), not only to allow a better application of water and
homogenisation of moisture content, but also to ensure that pile edges are mixed in for the
next leg of the composting process, to expose fresh surfaces to microbial attack (Biddlestone
and Gray 1982) and to reduce the impact of channelling. In practice there are composting
systems such as some in-vessel approaches and aerated static piles where there is no turning.
In these systems the maturation of the compost becomes very important to allow a mixing
step, and rewetting for example by rainfall. The two stage composting required for
mechanically segregated MSW under the animal by-product regime has been applied using a
static first stage, and then windrow turning for the second stage composting (although so far
only for separately collected wastes). If no turning is employed then edges of compost piles
can be covered with previously made compost to provide thermal insulation or incorporated
into subsequent composting piles (Finstein et al. 1987).
Loss on degradation over composting of mechanically segregated MSW streams can be as 30
to 40% by mass on a dry matter basis (Bardos 1989, Hagenmaier and Krauss 1982).
However, if composts are matured outside the loss of mass will be less as they absorb rainfall.
6. Pre-Processing Methods
Pre-processing is applied during MSW composting for one or more of the following reasons.
To increase the proportion of compostable materials in the feedstock – Glenn
1991 – MSW contains a high proportion of non-compostable or poorly
compostable material, hence composting the whole MSW stream is inefficient and
leads to a very low grade of compost. See also the Critical Review Section,
Feedstocks and composition - Physical characteristics.
To improve feedstocks for other recovery options Removal of compostable
materials (and materials removed in parallel such as glass) can improve the
efficiency of down stream sorting processes to recover energy and dry recyclables
in mechanised plant (Anon 1991, Barton 1983, 1984 and 1986, Barton and Poll
1983, Barton and Wheeler 1988 )
To reduce levels of contamination by inerts and trace elements. MSW
contains items which may be hazardous in a finished compost, for example glass
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which may be sharp or plastics which can injure grazing animals, and also give the
compost an unsatisfactory appearance. It also contains levels of trace elements
which could restrict, or eliminate, the usefulness of the finished compost. Pre-
processing (often combined with post processing, or refining) is used to control the
levels of hazardous items and substances in composts, for example exploiting
known differences in contaminant distribution, such as the tendency of fines (<5-
10 mm) to be enriched with trace elements. See also the Critical Review Sections:
Feedstocks and composition - Physical characteristics, Refining and Product
quality and environmental impacts
To recover other recyclable or re-usable materials (such as ferrous metal).
Many of the non-compostable fractions of refuse may potentially be recovered for
other purposes, for example recycling or energy recovery. Poorly compostable
materials such as paper and card may also be better recycled or combusted for
energy recovery. As a rule MSW processing takes place in a plant that, as far as
possible, integrates several processing routes to divert fractions of the waste to the
most appropriate recovery approach. See the Critical Review Sections: Feedstocks
and composition - Physical characteristics and Composting: Past and Present.
To condition the feedstock to make it more easily compostable. Large items
will only degrade slowly, and may also degrade anaerobically beneath their
surface. Compostable materials may be embedded in non-compostable items such
as plastic bags. Conditioning liberates the compostable material and controls the
size of particles to support a more efficient biological processing step. See the
Critical Review Section, Biology of Composting - Process Optimisation. Pre-
processing with chemical amendments, for example to control pH or change CN
ratios has been applied for MSW feedstocks, but is uncommon.
To mix materials, ensuring even and thorough distribution of the moisture,
nutrients and substrates.
To reduce contents of pathogens and parasites, for example by the suggested
use of autoclaving. Pre-processing using microwave irradiation or autoclaving to
kill pathogens has been trialled, mainly at pilot scale, but is not in widespread use,
nor considered necessary Some of these processes are described in the
Environment Agency Waste Technology Data Centre at
http://www.environment-agency.gov.uk/wtd/.
Most of the pre-processing techniques that are applied are mechanical in nature. There are
two broad categories: shredding and separation. Separation technologies exploit differences
in properties between components of interest. For example MSW fractions that pass through
a 50 mm screen tend to be enriched in putrescibles. Separation is achieved by exploiting one
or more differences in size, shape, density or electro-magnetic properties. Shredding or
pulverisation is typically achieved by attrition, usually in knife mills or hammermills.
However, water based systems have been applied, most frequently the combined shredding
and screening approach “wet pulverisation”, but various maceration techniques have also
been applied, rarely at practical scales. For MSW streams “debagging” is also necessary, as
wastes are often contained in one or more plastic or paper bags. This may be achieved by
pulverisation or shredding, or using spikes in rotating trommel screens that pierce and rip
sacks.
In many cases an MSW processing plant, such as an MBT facility, will include several of
these processes arranged in various “circuits”. There are two broad families of approach that
depend on the initial step taken to deal with the input MSW:
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size reduction by milling or shredding
trommel screening to achieve separation into size ranges followed by subsequent
separations.
This chapter discusses the following pre-processing methods in more detail:
Separation technologies (handpicking, size and density based techniques, use of
electric / magnetic fields)
Size reduction approaches
How size reduction and separation are combined
Other conditioning approaches
Materials handling issues.
This chapter is not intended as a comprehensive review of mechanical and other waste
separation technologies, which have been discussed elsewhere. A review of MBT plants is
the subject of another major SET project (see http://www.sitaenvtrust.co.uk/).
6.1 Separation Technologies
Separation of MSW prior to composting typically includes a combination of techniques,
drawn from handpicking, size and density based techniques, and using electric and/or
magnetic fields.
6.1.1 Hand Picking
Handpicking of refuse is perhaps the earliest and most prevalent handling process. In MSW
composting plants handpicking encompasses activities such as the removal of large or
unsuitable items from process streams (for example mattresses, large dead animals, stones),
clearing materials handling blockages and also dedicated handpicking lines (Cross 1991,
Ernst 1988, Manios and Syminis 1988, Sabater and Penuelas 1986).
Hand picking lines are usually installed after some size separation and magnetic screening of
the refuse has taken place and is applied larger size fractions (such as plastics, paper and card,
metals). In a typical hand picking line materials are transferred in a wide slow moving
conveyor past individual “stations” where operatives stand and remove items of value
according to a particular scheme. These items are then dropped down chutes where they are
collected in skips or other receptacles before baling and sale. In the UK items which are
handpicked are those of relatively high value to the plant operator, for example various
plastics and possibly paper and card.
There are some who feel that handpicking is a somewhat unsavoury aspect of MSW
processing, carrying problems of health and safety (Powell 1992) – for example from
discarded hypodermic needles - and low self esteem. Others have the point of view that it is
the a cost effective means of recovering resources from mixed MSW streams and is a source
of unskilled or even sheltered employment. However, where it is used, hand picking is such
an important part of the process that the use of poorly motivated labour can have a significant
detrimental effect on the overall process.
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6.1.2 Size Separation
Size separation is usually carried out by screening. Screens can be made from bars, mesh,
wires or plates slotted with holes, or flexible plastic “stars”. They may be flat, or curved for
example, into a drum or trommel screen (Glaub et al. 1984, Harrison 1965).
Screens made from bars are usually used for screening jobs that require a robust design,
particularly for sorting larger heavier items such as rubble. The bars are arranged in a three
dimensional array. Large items roll off, and the undersize passes through the array. The
other types of screen are effectively two dimensional and material passing through the
apertures is “undersize” and material that does not is “oversize”. The area of apertures may
be referred to as a “bed”.
Flat bed screens are typically inclined from the horizontal so that oversize rolls off. Mostly
screens shake or vibrate to agitate the materials being screened and so assist the pass through
of undersize and the motion off the screen of oversize. The angle of inclination affects the
performance of the screening. A smaller angle means that the amount of time larger particles
spend on the screen (residence time) is longer. The advantage of a long residence time is that
a higher proportion of aggregates will be broken up into their constituent particles; the
advantage for a shorter residence time is higher throughput. However, a large angle of
inclination also affects the effective screen size, as the aperture becomes more oblique to
material falling on to it. The throughput is a function of both the angle of inclination and the
rate of agitation and how the agitation takes place. Flat bed screens can be more economical
in terms of use of space than trommel screens.
The screening action of trommel screens tends to be more effective as the rotation allows
multiple falls of material, and are less susceptible to “blinding” – see the Critical Review
Section, Pre-processing methods - Materials handling issues. Trommels are also inclined so
that oversize materials pass along them. Trommel screens may include a series of “screen
plates” of different apertures so that different size fractions can be removed. They may also
include spikes to act as bag bursters and so liberate the individual MSW components. (Other
debagging approaches are reviewed by Ballister-Howells – 1992 & 1993) Internal flights or
vanes lift material up the sides of the rotating drum from where they fall by gravity.
Throughput and screening efficiency are related to: screen sizes, the nature of the screen
plates, angle of inclination and speed of rotation. A further effect of trommel screens is that
the speed of rotation and the fall of materials will break brittle materials such as glass and
ceramics. This effect may be exploited for the removal of non-combustible glass and
ceramics from the “oversize” which can then be more easily used in energy recovery.
Trommel screen principles are described in detail by Barton (1983), Barton and Wheeler
(1988) and Wheeler et al. (1989).
The shape of individual MSW items has an important bearing on both their separation and
their effect on the separation process. For example for a 50 mm screen a 150 mm long rod of
20 mm diameter may or may not pass through depending on whether it falls to the aperture
side on or end on. If a material is very pliable a large item may be pushed through the screen
by the force with which it strikes the screen. Paper of plastic film may be carried through a
screen by a denser object falling through. Some materials, for example textiles may cause
problems in screens by becoming entrained in apertures or on spikes, see the Critical Review
Section, Pre-processing methods - Materials handling issues.
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A number of mechanistic models have been devised for estimating likely screen performance,
mainly based on empirical observations of existing plant. These relate estimated feedstock
compositions observed from size / category analyses – see the Critical Review Section,
Sampling and analysis - Physical methods - to projected screening performance.
6.1.3 Density Based Separation
Density based separations of MSW fall into three basic categories:
ballistic separators
systems where items fall in air
systems where items fall in water
Separation is achieved by the effect of frictional forces on the momentum of moving particles
in the MSW stream being treated. This effect is a function of both shape and density.
Consequently, separation on density is more effective where the size range of the materials to
be separated is controlled. Hence density separation tends to follow screening and/or size
reduction steps.
Ballistic separators work by imparting kinetic energy to the items in the MSW stream being
treated. In effect the MSW items are flung into the air. Those that carry furthest tend to be
denser (US EPA 1971, Wiley 1963). Often the ballistic separation is based on a fast moving
conveyor belt which flings items into the air. A “splitter plate” separates two recovery chutes
from the end of the conveyor, and is positioned where the degree of separation between
“lights” and “heavies” is greatest.