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Resources, Conservation and Recycling 54 (2010) 1303–1311
Contents lists available at ScienceDirect
Resources, Conservation and Recycling
journal homepage: www.elsevier.com/locate/resconrec
Recycling pulp mill sludge to improve soil fertility using GIS tools
Paulo Ribeiroa, António Albuquerquea,∗, Luis Quinta-Novab, Victor Cavaleiroa
aDepartment of Civil Engineering and Architecture, University of Beira Interior, Edificio 2 das Engenharias, Calcada Fonte do Lameiro, 6201-001 Covilha, Portugal
bEscola Superior Agrária, Polytechnic Institute of Castelo Branco, Quinta da Senhora de Mércules, Apartado 119, 6001-909 Castelo Branco, Portugal
article info
Article history:
Received 26 November 2009
Received in revised form 9 May 2010
Accepted 15 May 2010
Keywords:
Pulp mill sludge
Land application
GIS
Suitability maps
abstract
Pulp mill sludge produced in the Cova da Beira region (Portugal) contains organic matter (11–47%),
nitrogen (38–2560 mg N/kg) and phosphorus (167–370 mg P/kg), which may be valuable for increasing
soil productivity. The levels of heavy metals are below the limits recommended by legislation and the
amount of nitrogen and phosphorous to be introduced in soils does not present a risk for nutrient leaching.
After identifying the environmental and technical restrictions on its application, an area of 1650 ha was
identified where the sludge can be applied in forage crops, fruit trees, olive groves and vineyards. A
suitable area was also found for a biosolids storage centre. The use of GIS allowed to define a sludge
application index and to produce land-use suitability maps, which can be useful for sludge management.
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
The Cova da Beira region is located in the interior centre of Portu-
gal and is influenced by the moderate Mediterranean climate. It has
an area of 1375 km2, the annual average temperature is 14.5 ◦C and
the average rainfall is 820 mm. The majority of the soil in this region
has low organic matter content (Ribeiro, 2000; LQARS, 2000), which
could be a disadvantage for its use, taking into account the agri-
cultural productivity expected under the Irrigation Cova da Beira
Irrigation System and the maintenance of tourism projects (e.g. golf
fields).
Organic matter plays a very important role in soil conservation,
due to the beneficial effect it has on its physical, chemical and bio-
logical properties, and protection against some forms of pollution
and degradation. Some types of organic waste generated in urban
areas and industries, including municipal solid waste and sludge
from wastewater treatment plants (WWTP), may after treatment be
a source of organic matter and nutrients (e.g. nitrogen, phosphorus
and calcium) to incorporate in poor soils or under fast degradation
processes.
Pulp mill sludge from the paper industry seems to have contents
of both organic matter and nutrients (Nkana et al., 1999; Foley and
Cooperband, 2002; Jordan and Rodriguez, 2004) which may be con-
sidered suitable for organic and nutrient correction of poor soil such
as that covered by the Cova da Beira Irrigation System. This kind of
application, besides improving soil fertility and waste reuse, may
reduce treatment and disposal costs.
∗Corresponding author. Tel.: +351 275 329981; fax: +351 275 329969.
E-mail address: ajca@ubi.pt (A. Albuquerque).
However, the use of organic residues in soil requires good appli-
cation practices and periodic monitoring of the quality of the
soils, residues and water resources near the application area. The
presence of phosphorus and nitrogen compounds, heavy metals,
refractory organic compounds and pathogenic agents may pose a
risk to water quality (e.g. risk of eutrophication and groundwater
contamination) and soil (e.g. toxicity of soils and plants) and pub-
lic health. Restrictions on sludge application, based on its nutrient
content and plant needs, are less rigorous but the recycling prac-
tice must ensure no conflicts with good agricultural practices set
in codes and guides of good agriculture practices, both at national
(MADRP, 1997; MADRP, 2000) and international (EPA, 1994; ESD,
1999; DELG, 2008a) levels.
The application of pulp mill sludge (produced during the treat-
ment of effluents from pulp and paper industry) should therefore
be treated with caution and according to safety regulations as pre-
sented in Jordan and Rodriguez (2004) and IFC (2007). In Europe,
this practice is subject to regulation by the European Commu-
nity through Directives 86/278/EEC (Sewage Sludge Directive) and
91/692/EEC (Standardizing and Rationalizing Reports on the Imple-
mentation of Certain Directives Relating to the Environment),
namely in terms of annual maximum permitted application rates
and annual maximum concentrations of metals and nutrients to
be incorporated in soils. These acts seek to discipline and supervise
the use of waste through regulations which ensure that its use does
not contribute to soil contamination by heavy metals or to diffuse
sources of pollution, especially for nitrogen compounds (Directive
91/676/EEC - Nitrates Directive). The legal framework sets appli-
cation limits (Directive 86/278/EEC and Portuguese Decree-Law
No. 118/06 – Agricultural Application of Sludge), according to the
characteristics identified for waste, soils and spatial limitations.
0921-3449/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.resconrec.2010.05.009
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1304 P. Ribeiro et al. / Resources, Conservation and Recycling 54 (2010) 1303–1311
Celtejo’s plant (Vila Velha de Ródão, Portugal) produces around
1200 ton/year of dewatered primary sludge and 5000 ton/year of
dewatered secondary sludge. Since the costs associated with its
treatment, transportation and final disposal have risen significantly
in the last years, mainly due to landfill limitations introduced by
Directive 99/31/EC (a reduction to 35% of the total biodegradable
waste going to landfills must be achieved by 2016 in order to reduce
carbon dioxide and methane emissions), the company has con-
sidered several alternative solutions for the residues, namely land
spreading in Cova da Beira soils. This option may represent a natural
reuse and recycling of waste and it can be a cost-effective solution,
by reducing the dependence on landfilling while minimizing neg-
ative environmental impacts and it is a practice recommended by
authors such as O’Brien et al. (2003) and Ochoa de Alda (2008).
Due to the considerable distance between the source of sludge
production (Celtejo’s plant) and the potential application areas
(agricultural fields in the Block of Covilhã, covered by the Cova da
Beira Irrigation System), the location of a centre for biosolids stor-
age will be important to allow for a better control of its application.
The definition of a methodology for sludge application on agri-
cultural land requires the collection, processing and analysis of
complex information (e.g. land use, environmental and legal restric-
tions, characteristics of the sludge and agricultural practices, and
road accesses). The use of Geographic Information Systems (GIS)
allows the georeferencing, organization, processing and analysis of
such complex information, as well as the creation of a database
for sludge application management. GIS tools may also be used to
study a location for sludge storage.
The main objective of this work was to evaluate the potential
agricultural area in the Block of Covilhã (covered by the Cova da
Beira Irrigation System) for land application of the pulp mill sludge
annually produced at the Celtejo plant. GIS tools were used for
this purpose, as well as for selecting an area for a biosolids storage
centre.
2. Materials and methods
The study included the following four main stages:
1. Pulp mill sludge characterization.
2. Definition and characterization of the study area.
3. Identification of the agricultural areas with potential sludge
application.
4. Location of an area for a biosolids storage centre.
The first stage included the characterization of the industry, the
production process, the type of waste generated, the WWTP and the
pulp mill sludge (primary and secondary). The main study sources
were the materials provided by Celtejo Ltd., including publications
and reports on their activities and physical and chemical data from
the sludge characterization, complemented with visits to the plant.
The second stage included the identification and evaluation of
the agricultural areas with potential for receiving sludge, taking
into account several constraints (e.g. areas having both a protec-
tion status and technical restrictions such as water resources, urban
areas and high-slope areas). The study was confined to the Block of
Covilhã and made use of several and varied information in digital
form, including:
- Extract of Portuguese Military Maps No. 235 and No. 236 (1/25000
scale, raster).
- Map of the Perimeter Irrigation Block of Covilhã (1/25000 scale,
vector).
- Extract of the National Agricultural Reserve for the civil parishes
of Ferro, Peraboa and Caria (1/25000 scale, raster).
- Extract of the National Natural Reserve for the civil parishes of
Ferro, Peraboa and Caria (1/25000 scale, raster).
- Altimetry data (1/25000 scale, vector).
- Orthophotomaps (photogrammetric flights of 2002, 2003 and
2004; 1/5000 scale, raster).
In the third stage, several agricultural areas, suitable for sludge
application, were selected. The elements with technical restrictions
were located and georeferred (e.g. hydrographic network, water
transport pipelines, water supply and water irrigation systems,
areas for biological agriculture, roads, urban housing areas, isolated
residential areas and land slopes), including as well the collection
and interpretation of data on soil characteristics collected from 57
agricultural parcels.
Based on the collected information, some of which was con-
firmed in field visits, the following tasks were developed:
- Conversion of some information from analogue to digital form.
- Editing and processing of digital information.
- Construction of a geographic model.
- Construction of new thematic maps.
- Spatial analysis of the thematic maps.
- Structuring of alphanumeric and cartographic information.
- Construction of a database.
The fourth stage involved studying the location for a biosolids
storage centre in a strategic point, which must allow the optimiza-
tion of its distribution through the agricultural parcels identified
in the third stage. The search for a location was carried out after
producing a suitability map, generated from eight thematic maps
(National Agricultural Reserve (RAN), National Ecological Reserve
(REN), biological agriculture (organic agriculture), urban perime-
ter, raw water supply sources, water streams network, roads and
slopes), which were developed in the third stage. A suitability index
was developed for that purpose.
The information analysis was carried out using the software
ArcGIS 9.1 (ESRI, USA) and the ArcCatalog and ArcMap applications,
namely for the following main tasks:
- Integration and management of spatial and non-spatial data
(Raster or Vector).
- Editing of both data and geographical entities.
- Overlaying thematic information topics.
- Spatial analysis (Spatial Analyst).
- Design of slope maps (3D Analyst).
- Definition of a protection zone on the border of a geographical
entity, using the buffer application.
- Query of databases according to predefined criteria.
- Georeferring of elements or entities.
- Geoprocessing of the information for mapping information in the
selected study area.
- Determination of the locations with higher suitability for a
biosolids centre using map algebra (Raster Calculator).
Based on the information and cartography collected in either
raster or vector format the following tasks were carried out:
(1) Survey of agricultural areas with potential for pulp mill sludge
application.
(2) Establishment of a suitability map for reuse of pulp mill sludge.
(3) Selection of a location for biosolids storage.
(4) Development of a database for the selected agricultural parcels,
assuming the biosolids storage centre as the epicentre for
sludge distribution.
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P. Ribeiro et al. / Resources, Conservation and Recycling 54 (2010) 1303–1311 1305
Table 1
Pulp mill sludge characteristics from the primary and secondary treatment.
Parameters Primary sludge Secondary sludge Maximum permissible valuea
Dry matter (DM, %) 25 10 –
Organic matter (OM, %) 47 11 –
pH 7.2 7.8 –
Total nitrogen (TN, mg/kg) 38 2560 –
Ammonia nitrogen (NH4-N, mg/kg) 4 1090 –
Phosphorus (P, mg/kg) 167 370 –
Cadmium (Cd, mg/kg) 1.4 0.34 20
Copper (Cu, mg/kg) 13.0 2.8 1000
Nickel (Ni, mg/kg) 10.5 1.44 300
Lead (Pb, mg/kg) 13.2 1.1 750
Zinc (Zn, mg/kg) 83.0 12.9 2500
Chromium (Cr, mg/kg) 19.0 1.9 1000
aAccording to the Directive 86/278/EEC and the Portuguese Decree-Law 118/06.
3. Results and discussion
3.1. Pulp mill sludge characterization
The WWTP of Celtejo Ltd. includes primary treatment with sed-
imentation tanks and secondary treatment with activated sludge
tanks (described in Ribeiro, 2008), resulting in primary sludge
(about 3.5 ton/day) and secondary sludge (about 14 ton/day), both
of which have mechanical dehydration through centrifugation.
Table 1 presents a characterization of each type of sludge accord-
ing to the analysis carried out in Celtejo Ltd., as well as the limits
of heavy metals acceptable for soil application.
The pH of both sludge is proper for land application since the
limit set in several manuals and codes of good practice is 5 (EPA,
1994; EPA, 1997; ESD, 1999; MADRP, 1997; MADRP, 2000; DELG,
2008b). On the other hand, the pH of soils detected in the 57 agri-
cultural parcels ranged from 4.5 to 6.6, which means that most of
the soils of that region are acid (according to the classification pre-
sented in EPA, 1994; LQARS, 2000; DELG, 2008b). Therefore, the
addition of pulp mill sludge would contribute to increase the pH of
those soils.
The primary sludge presents contents of both organic mat-
ter and dry matter higher than the secondary one, because it is
less mineralized. It also has a higher content of heavy metals,
which should be associated with its adsorption on sedimentation
solids. The secondary sludge (biological sludge) has organic matter
content lower than the primary one because it is more mineral-
ized. However, the concentrations of nitrogen and phosphorus are
higher due to the high nutrient removal efficiency provided by the
activated sludge process.
The organic matter content detected in the primary sludge is,
however, lower than the values presented in other Portuguese stud-
ies, varying between 80% and 87% according to Ribeiro (2000) and
Nunes and Cabral (2000). The secondary sludge also presents val-
ues of organic matter lower than the ones detected in other studies
(between 49% and 89% according to Nkana et al., 1999 and Nunes
and Cabral, 2000). In the latter, besides the organic matter removal
being higher in the secondary treatment (in the liquid phase), the
sludge organic content is low due to the subsequent mineraliza-
tion during sludge storage and treatment. Nevertheless, the organic
matter content found in both sludge may benefit the soils of the
Cova da Beira region since values below 1% were detected in all the
57 agricultural parcels.
Nutrients (nitrogen and phosphorus) in both types of sludge
present lower concentrations when compared to the values indi-
cated by several authors (Nkana et al., 1999; Nunes and Cabral,
2000; Foley and Cooperband, 2002; Curnoe et al., 2006; Shipitalo
and Bonta, 2008), who pointed to values over 650 mg TP/kg,
5000 mg NH4-N/kg and over 10,000mg TN/kg. However, they are
still appropriate to be used in soils for fertilizer purposes, especially
the secondary sludge.
The concentrations of heavy metals in both types of sludge
are below the limits allowed by the 86/278/EEC Directive, and by
national and international guides (EPA, 1994; EPA, 1997; MADRP,
1997; DELG, 2008a). High concentrations are not expected in soil
after sludge incorporation. The values are also lower than the ones
found by Lacorte et al. (2003) except for Cd, but higher than the
ones reported by O’Brien et al. (2003). Therefore, as also reported
by Cabral et al. (2008), metals do not seem to be a limiting factor
for sludge application to land.
Therefore, it seems reasonable to assume that the sludge pro-
duced at Celtejo may be applied as soil fertilizer, soil organic
corrective, acidity corrective or source of nutrients for plants.
Shipitalo and Bonta (2008) have observed that paper mill sludge
can improve reclamation of surface-coal mines where low pH and
organic-carbon levels in the spoil cover material can inhibit reveg-
etation.
3.2. Definition and characterization of the study area
The area selected to carry out the study corresponds to the Block
of Covilhã, covered by the Irrigation Plan of Cova da Beira, located
in the South side of the “Serra da Estrela” mountain, a lowered area,
where altitudes range between 400 and 500 m.
The Block of Covilhã was delimited from the digitalisation of the
Military Maps No. 235 and 236 and the georeferencing of recent
structures (e.g. A23 motorway and new residential areas), having
been necessary to overlap cartographic elements. The total area
assessed was 1616 ha (Fig. 1).
3.3. Identification of the agricultural areas suitable for sludge
application
The first step included the identification of the agricultural areas
with deep soils classified as classes A and B according to the Por-
tuguese Decree-Law No. 73/2009 (Agricultural Reserve) and to the
World Reference Base for Soil Resources Deckers et al. (1998),
which belong to the RAN as shown in Fig. 1.
The second step included the identification and delimitation of
areas with spatial restrictions according to the Portuguese Decree-
Law No. 118/06, the Directive 86/278/EEC and to suggestions from
several guides and codes of good agricultural practices (EPA, 1994,
1997; USDA-SCS, 1994; ESD, 1999; MADRP, 1997; DELG, 2008a),
namely the ones located at least:
- 50 m away from water sources for irrigation;
- 100 m away from water supply sources for human consumption;
- 200 m away from urban residential areas;
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Fig. 1. Location of the Block of Covilhã (Cova da Beira Irrigation System).
Fig. 2. Buffer areas for 50 m and 200m.
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P. Ribeiro et al. / Resources, Conservation and Recycling 54 (2010) 1303–1311 1307
Fig. 3. Suitability map for pulp mill sludge application.
- 30 m away from borders of navigable waters;
- 10 m away from borders of non-navigable waters.
The areas for biological agriculture were also skipped since
sludge application is not permitted in soils with such activity as
referred in the Council Regulation (EEC) No. 2092/91 (Organic Pro-
duction of Agricultural Products).
The third step included the definition of two buffer zones: 50 m
for water sources (streams, wells, lakes and dams) and 200 m for
residential areas in order to allow the protection of these areas
against contamination and odours (Fig. 2).
The delimitation of all the restrictions resulted in a total suitable
area (without restrictions) of 253 ha (Fig. 3) for pulp mill sludge
application, which represents 15.7% of the total area of the Block of
Covilhã. In order to evaluate if this available area was enough for the
total application of the sludge produced at Celtejo (6200 ton/year),
a frequency of land application was established. The suitability of
the crops and agricultural parcels for sludge application was then
evaluated.
The allowed dry sludge application rate was 6 ton/(ha year)
(limit value defined in the Directive 86/278/EEC and used by most
Member States of UE as reported in IEEP, 2009). Higher values could
be used as long as the amounts of heavy metals incorporated into
the soil do not exceed the limit values shown in Table 1. Consider-
ing the typical dry matter percentages for this kind of sludge (25%
for primary sludge, 10% for secondary sludge), the annual available
quantity of sludge in dry matter is 800 ton (details on calculations
are presented in Ribeiro, 2008).
Taking into account that most of the agricultural parcels present
a moderate land slope and the information from the Portuguese
Guide for Good Agricultural Practices (MADRP, 1997) on the pre-
vention of both leaching risk and surface runoff associated to
organic waste application, a single annual application was consid-
ered. Therefore, the area required to apply all the sludge annually
produced at Celtejo is approximately 133 ha, 52.6% of the avail-
able area. As the company intends to increase production, the
suitable area available in the Block of Covilhã may absorb up to
11677 ton/year of pulp mill sludge (considering an average per-
centage of dry matter of 13%).
The loads of Cd, Cr, Ni, Pb, Zn and Cr, which could be intro-
duced in the soil after sludge application, are presented in Table 2
and were calculated considering a dry sludge application rate of
6 ton/(hayear) and the concentration of heavy metals in the sludge
(Table 1) following Eq. (1).
M=LS ·C(1)
where Mis the load of metals to be introduced in soil (kg/(ha year)),
LS the load of dry sludge to be applied (kg/(ha year)) and Cis the
concentration of heavy metal in the sludge (kg/kg).
As shown in Table 2, the quantities of heavy metals either in
both types of sludge or to be introduced in the soil are much
lower than the limits defined in both the Directive 86/278/EEC
and the Portuguese Decree-Law No. 118/06. The values are still
lower than the more restrictive goals set by the Irish Code of Good
Practice for the Use of Biosolids in Agriculture (DELG, 2008a). There-
fore, it may reasonable to assume that a dry sludge application rate
higher than 6 ton/(ha year) can be used in the soils of the Block of
Covilhã. ESD (1999) suggests application rates of pulp mill sludge
up to 12.5 ton/(ha year). However, the risks for leaching and sur-
face runoff should be previously evaluated. Even when considering
more restricted limits for pulp mill sludge application in agricul-
ture, such as those presented in the Canadian regulation (3 mg
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Table 2
Amount of heavy metals associated to pulp mill sludge produced at Celtejo.
Parameters Heavy metals
Cd Cu Ni Pb Zn Cr
Amount in the primary sludge (mg/kg) 1.4 13 10.5 13.2 83 19
Amount in the secondary sludge (mg/kg) 0.34 2.8 1.5 1.1 12.9 1.9
Amount to be incorporated in soils (kg/(ha year)) by the primary sludgea0.008 0.08 0.06 0.08 0.5 0.15
Amount to be incorporated in soil (kg/(ha year)) by the secondary sludgea0.002 0.017 0.009 0.007 0.080 0.011
Limit values for amounts that may be added to agricultural soils (kg/(ha year))b0.15 12 3 15 30 4.5
Limit values in sludge for use in agriculture (mg/kg)b20 1000 300 750 2500 1000
aFor an application rate of 6 ton/(hayear).
bDirective 86/278/EEC and Portuguese Decree-Law 118/06.
Cd/kg, 100 mg Cu/kg, 62 mg Ni/kg, 150mg Pb/kg, 500 mg Zn/kg and
210 mg Cr/kg; ESD, 1999), the values of Table 2 are still lower.
Organic N is also of concern since it converts (mineralizes) into
plant-available inorganic forms (ammonia nitrogen (NH4-N) and
nitrate nitrogen (NO3-N)). N availability during the year of applica-
tion depends on the N form. NH4-N and NO3-N are readily available
for plant uptake. However, if the sludge is applied on the soil surface
and not quickly incorporated, considerable NH4-N may be lost to
the air as ammonia gas (volatilization). The excess of ammonia not
volatilized may therefore be oxidized to nitrate through nitrifica-
tion, which may be subject to leaching loss. Nitrate is known to be
a precursor of negative environmental impacts (e.g. water stream
eutrophication) and public health problems (e.g. methemoglobine-
mia and gastric cancer), and therefore it must be controlled.
The amount of N accumulated in soil depends on the plant
uptake and the type of culture to be used. In general, about 10–50%
of the organic N may become available in the year of application,
5–20% will be available in the second year and smaller amounts
will be available in subsequent years (EPA, 1997; MADRP, 2000;
DELG, 2008a). According to Portuguese and international guides
for good agricultural practices (MADRP, 1997, 2000; DELG, 2008a,b)
the application rate of total N should not exceed 210 kg N/(hayear)
in any sensitive areas and 170 kg N/(ha year) in areas sensitive to
nitrate leaching according to the Directive 91/676/EEC (Nitrates
Directive).
Phosphorus is found in most soils as mineral forms, which tend
to be retained by mineral colloids or to form phosphates (calcium
phosphates, aluminum and iron) with low solubility. The use of
pulp mill sludge may improve the content of assimilative phospho-
rus (Nunes and Cabral, 2000; LQARS, 2000). The research carried
out in Portugal has observed that the application of an average
rate of approximately 75 kg P2O5/(ha year) is enough to increase
by 10 mg/kg the content of assimilative P in soil (LQARS, 2000).
Table 3 presents the loads of nitrogen and phosphorus to be
introduced in the soil for the two types of sludge considering a dry
sludge application rate of 6 ton/(hayear).
Results show that the amount of nitrogen to be introduced in
the soils of the study area (15.6 kg/(ha year)) is much lower than
the limits recommended by Portuguese and international guides
(MADRP, 2000; DELG, 2008a,b) even for areas sensitive to nitrate
leaching. Therefore, it seems reasonable to assume that sludge
application rates higher than 6 ton/(ha year) can be applied if the
amount of N in soil does not exceed 210 kg N/(ha year) and if both
the quality of water sources and the risk of nitrate percolation in the
soil were properly assessed. Limited data shows that EPA drinking
water standards were not exceeded with paper mill sludge appli-
cation rates having total N equivalents of less than 300 kg/(ha year)
(Shields et al., 1986; Kraske and Fernandez, 1993). If high C/N ratios
are applied, N equivalent application rates for paper mill sludge can
likely be higher because N is immobilized.
The C/N ratio may be adjusted to avoid depressed N uptake
through sludge application before planting. This practice will allow
sufficient time for N mineralization to occur (Cabral et al., 1998).
If immediate planting is desired, additional N inputs (biosolids or
fertilizer) would help the microorganisms to break down the pulp
residual without reducing N availability.
The amount of phosphorus to be incorporated in the soils is
much lower (3.2 kg/(hayear)) than the recommended values, used
as reference by some Phosphorus Index for assessing the vulner-
ability of a land (USDA-SCS, 1994; McFarland et al., 1998), and
therefore may be classified of very low risk for phosphorus move-
ment. That value is still compatible with the requirement of DELG
(2008b), which sets up a maximum farm phosphorus balance of
no more than 10 kg P/(hayear). Therefore, sludge application rates
higher than 6 ton/(hayear) can be applied if the amount of P in soil
does not exceed 157 kg P/(hayear).
3.3.1. Sludge application opportunities
The agricultural parcels have several types of crops, which may
present different susceptibility to pulp mill sludge application. The
criteria for selecting the types of crops with suitability for sludge
application was based on the restrictions presented in the Directive
86/278/EEC, Decree-Law No. 118/06 and Portuguese Standard NP
4434:2005 (Reuse of wastewater in irrigation; IPQ, 2005), includ-
ing:
(1) Total restriction on grassland or forage crops that are used for
cattle feeding.
Table 3
Amount of nitrogen and phosphorus associated to pulp mill sludge produced at Celtejo.
Parameters Total nitrogen Total phosphorus
Amount in the primary sludge (mg/kg) 38 167
Amount in the secondary sludge (mg/kg) 2560 370
Amount to be incorporated in soils by primary sludge (kg/(ha year))a0.23 1
Amount to be incorporated in soil by secondary sludge (kg/(ha year))a15.40 2.2
Recommended limit value to be incorporated in agricultural soils in no sensitive areas (kg/(ha year)) 210b157c
Recommended limit value to be incorporated in agricultural soils in sensitive areas (kg/(ha year)) 170b–
aFor an application rate of 6 ton/(hayear).
bDELG (2008a,b),MADRP (2000).
cUSDA-SCS (1994),McFarland et al. (1998).
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P. Ribeiro et al. / Resources, Conservation and Recycling 54 (2010) 1303–1311 1309
Fig. 4. Information associated to agricultural parcels.
(2) Total restriction on vegetable crops and fruit crops with direct
contact during the growing period.
(3) Total restriction on soils for vegetable or fruit cultures with
direct contact, for a period of 10 months before the harvest.
The accumulation of heavy metals present in sludge occurs more
rapidly in vegetables than in extensive cereal crops. Therefore, the
agricultural parcels selected for receiving pulp mill sludge were:
annual crops (fodder and cereal crops) and grassland not used for
livestock feeding, fruit crops, olive groves and vineyards.
The overlay between the suitability map for pulp mill sludge
application and the respective orthophotomaps resulted in a map
with all the selected agricultural parcels. A database was cre-
ated with information for sludge application management in each
agricultural parcel, namely the parcel code, location and parcel
coordinates, parcel photo, property-owner, type of culture, soil
characteristics, characteristics of sludge already applied, fertiliza-
tion plan, maximum amount of applied sludge, date of application
and history of applications (Fig. 4).
Although the total suitable area evaluated in the Block of Cov-
ilhã is sufficient for the total application of the sludge produced at
Celtejo, at parcel level it is necessary to guarantee its safe applica-
tion in proper conditions. Periodical data updating for each parcel
is needed for that purpose, namely the type of culture or crops and
the soil characteristics.
Besides allowing the introduction of new agricultural parcel
data and the modification of existing parcel data, the database
created allows the online consultation of historical data through
remote access.
These records perform a useful management tool, since they
enable the management and control of agricultural parcel activities
(for example, the risk of excessive accumulation of heavy metals
and nutrients may be avoided by consulting historical data and by
defining rotation procedures).
3.4. Localization of sludge storage centre
A GIS in raster format was used to create the suitability map
and the results were processed according to the algebra of maps
(maps overlapping for the different variables). A weighted linear
Table 4
Weights attributed to each variable.
Criteria Map Description P
Environmental RAN Area with RAN 0
Area without RAN 1
Environmental REN Area with REN 0
Area without REN 1
Technical BA Area with biological agriculture 0
Area without biological agriculture 1
Technical UPa200 m from urban areas 0
200–300 m from urban areas 1
300–400 m from urban areas 2
400–500 m from urban areas 3
500–600 m from urban areas 4
Over 600 m from urban areas 5
Environmental WA 50 m from water abstraction sources 0
50–100 m from water abstraction sources 1
100–200 m from water abstraction sources 2
200–300 m from water abstraction sources 3
300–400 m from water abstraction sources 4
Over 500 m from water abstraction sources 5
Environmental WS 50 m from water streams 0
50–100 m from water streams 1
100–200 m from water streams 2
200–300 m from water streams 3
300–400 m from water streams 4
Over 500 m from water streams 5
Technical R Over 200 m from roads 1
150–200 m from roads 2
100–150 m from roads 3
50–100 m from roads 4
Less than 50 m from roads 5
Technical S Over 10% 0
8–10% 1
6–8% 2
4–6% 3
2–4% 4
0–2% 5
aThe range of values and weights were defined according to the location of houses
or equipments.
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1310 P. Ribeiro et al. / Resources, Conservation and Recycling 54 (2010) 1303–1311
Fig. 5. Suitability map for the sludge storage centre location.
combination model was used for delineating and ranking suitable
sites for sludge storage. After the exclusion of restrictive areas, the
available area was divided into cells of 100 m2, each constituting an
alternative for the centre location. For producing the suitability map
eight thematic maps (“RAN”, “REN”, “biological agriculture (BA)”,
“urban perimeter (UP)”, “water abstraction sources (WA)”, “water
streams (WS)”, “roads (R)” and “slopes (S)”) were used. A similar
approach was used by Basnet et al. (2001) for land application of
animal waste, by Zhao et al. (2009) to locate wastewater treatment
in areas with several environmental restrictions and by Kallali et
al. (2007) to select sites for groundwater recharge in the North East
of Tunisia.
The inclusion of the thematic maps WA, WS and S is impor-
tant for the site selection process in order to minimize the leaching
of nutrients lost from the storage centre to water resources. The
UP map will minimize the presence of odours and risks for public
health, whilst the R map is important for searching places that can
satisfy the transport of sludge to all agricultural parcels.
The computation procedure involved the overlap between the
exclusion areas of each of the eight thematic maps and the suit-
able areas for pulp mill sludge application (Fig. 3) through algebraic
operations of maps as presented in Eq. (2). A Suitability Index for
Biosolids Centre storage (ISBC) was therefore developed.
ISBCi=(ak
ij)mn ×P=
tm
k=1
⎛
⎜
⎜
⎜
⎝
⎛
⎜
⎜
⎜
⎝
ak
11 ak
12 ··· ak
1n
ak
21 ak
22 ··· ak
2n
··· ··· ··· ···
ak
m1ak
m2··· ak
mn
⎞
⎟
⎟
⎟
⎠
×Pk
⎞
⎟
⎟
⎟
⎠
(2)
where (ak
ij) is the vector of cell values from each thematic map
which is in line iin row j,mand nare the dimensions of the thematic
grid map, kis the thematic map, tm is the number of thematic maps
and Pis the vector of weights associated to each thematic map.
The value of each cell of the suitability map (which corresponds
to the value of ISBCi) resulted from the sum of the products between
the weight attributed to each thematic map (Table 4) and the value
stored in each cell of each thematic map (cells with 10 m ×10 m
size) through algebraic operations of maps.
The Eq. (1) was therefore introduced in the Raster Calculator
function for the final suitability map to be calculated as express
Eq. (3), which is presented in Fig. 5.
ISBCi=(bij)mn =
⎛
⎜
⎜
⎜
⎝
b11 b12 ··· b1n
b21 b22 ··· b2n
··· ··· ··· ···
bm1bm2··· bmn
⎞
⎟
⎟
⎟
⎠
(3)
where (bij) is the vector of cell values for the suitability map which
is in line iin row j,mand nare the dimensions of the suitability
grid map.
The ISBC ranges between 0 and 3125 (maximum value for a
combination of p= 5 for the 5 last thematic maps, i.e. 55).
Taking into account the weight considered for each variable and
the combination of multiplicative factors, three suitability classes
were defined for the suitability map: 0 (“without suitability for
storage centre construction”), 1–1024 (“conditioned suitability for
storage centre construction”) and 1024–3125 (“good suitability for
storage centre construction”). This procedure is similar to the ones
followed by Basnet et al. (2001), for the animal waste application,
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P. Ribeiro et al. / Resources, Conservation and Recycling 54 (2010) 1303–1311 1311
and by Zhao et al. (2009), for the location of wastewater infrastruc-
tures.
Considering two sludge applications per year, the quantity of
sludge annually produced at Celtejo and the height of the stor-
age piles (5–10 m), an area of approximately 2.1 ha for the storage
centre would be necessary (without taking into account the area
for additional equipment such as a supporting building, washing
equipment and a lagoon for leachate storage). After a visit to the
areas classified as “good suitability for storage centre construction”,
three areas with enough space were identified and are shown in
Fig. 5 (referred as LA, LB and LC). From these, the LA area appears
to have better suitability since it has a better access.
4. Conclusions
The Cova da Beira region soils are, in general, acid and poor in
organic matter. The intensive land exploitation designed to support
agricultural, industrial and touristic activities may, if protective
measures are not taken, lead to the degradation of the land. The
annual amount of pulp mill sludge produced at Celtejo may, after
treatment, be used in less than 50% of the available suitable agri-
cultural land as an organic corrective, acidity corrective and as a
fertilizer for agricultural crops. This practice, besides the agricul-
tural sludge valorisation, would reduce the treatment and final
destination costs and contribute for soil protection, particularly in
intensive season exploitation. This practice will bring benefits to
the environment and the local economy, by presenting a good level
of social acceptance without needing many technical resources and
by contributing to a sustainable development of the region.
The maximum amount of heavy metals and nutrients that could
be introduced into the soil would not exceed the limits defined
by European Directives, Portuguese Decree-Laws and national and
international guides. The results allow assuming that, if necessary,
an application rate higher than 6 ton/(ha year) could be considered.
The use of GIS allowed to store, manipulate, analyze and geo-
reference complex information from soils, sludge, land use and
environmental and technical restrictions. A suitability map for pulp
mill sludge application was generated and a database for sludge
management of each suitable agricultural parcel was created. A
multicriteria analysis was carried out to identify potential sites for
the location of a sludge storage centre taking into account technical,
environmental and social criteria. A suitability index was gener-
ated, which allowed the production of a final suitability map and
the location of alternative sites.
Acknowledgements
The authors would like to thank the support provided by the
Celtejo AS industry and by José Riscado (STIG) during this study.
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