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Characterization and Potential Utilization of Recycled Paper Mill sludge


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To ensure the sustainability of the recycled paper industry, economically viable and ecologically sound alternatives must be found for the re-use of the waste residue. One such solution is the utilization of the residue in a value-add product. In this paper, we prepare recycled paper mill sludge with the AGES/KDS micronizer technology and fractionate the resulting material. The characteristics of the sludge fibre and strength properties of handsheets blended from thermo mechanical pulp and recovered sludge fibre are evaluated.
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Annual Meeting - 2005 - 91
Congrès Annuel ATPPC D201
S. Krigstin and M. Sain,
Natural Fiber and Bio-Composite Group, Faculty of Forestry
Earth Sciences, 33 Willcocks Street, Toronto, Ontario M5S
Conservation, reuse, recycling, and composting are the
solid waste management philosophies of the 21st century. The
largest recyclable component of the solid waste generated in
Canada is paper products [1]. In 2003, 3,346,000 tonnes of
paper was recovered from Canadian domestic paper
production, a recovery rate of 45.1% and the highest in our
country’s history [2]. The growth of the paper recycling
industry has been ecologically beneficial leading to an
extension of our fiber base, conservation of our forest
resources and reduction in landfill requirement. Ultimately
this will reduce the intensity of forest management required to
meet the demand for paper which may help to preserve
sensitive habitats, and direct limited resources to solid wood
products rather than fiber.
The evolution of solid waste management practices has
lead to the development of increasingly comprehensive and
sophisticated collections systems in many large North
American municipalities. The end result being a reliable and
economical supply of waste paper available for recycling
processes. Paper mills situated in large urban areas are eagerly
exploiting waste paper as an economical fibre source.
However the deinking operations, which turn the raw waste
paper into usable fibre, generate substantially greater quantities
of sludge than do virgin fibre operations [3]. On a wet weight
basis the sludge output may approximate the total paper
production capacity of a mill. Residue generated from tissue
mills utilizing only recovered paper is reported to be 40.6%
[4], while residue generated from a newsprint mill utilizing
70% ONP and 30% OMG is reported to be 20% [5].
Sludge from a typical recycled pulp and paper effluent
treatment facility undergoes dewatering operations, which
reduces the moisture content of the liquid sludge from 3-5% to
about 50% (wet weight basis). Chemical flocculation coupled
with mechanical force is used to remove water from the
mixture. The resultant material is a tightly held wet mass of
wood fibre, inorganic clays and filler, and contaminants. The
current practices of sludge disposal such as land spreading,
land filling and incineration are becoming increasingly
unfavorable due to ecological arguments or economic
considerations. New residue management approaches must be
sought that utilize this material in a value-added manner. This
initiative has been considered by many but has proved
unrealistic due to large quantities, high moisture content,
unpleasant odours, substantial variability and difficulty in
handling [5,6,7]. To ensure the sustainability of the recycled
paper industry, economically viable and ecologically sound
alternatives must be found for the re-use of the waste residue.
The focus of this work was to use AGES/KDS technology
to prepare recycled papermill sludge for further processing.
The AGES/KDS sludge drier can be easily located at the end of
the traditional sludge dewatering process. It renders wet
sludge into a dry, non-odourous, easy to handle material. The
specific aim of this investigation is qualify the available fibre
in the dried sludge material for re-use in the papermaking
Material & Methods
Raw sludge (approximately 50% dry material) was
collected from three recycled paper mills. The mills selected
represent a range of recycling operations in North America.
They included a recycled newsprint mill (NP), a combination
recycled newsprint and tissue mill (NP/TM) and a recycled
tissue mill. The materials were successfully treated in the KDS
Micronex to dryness as shown in Figure 1.
Figure 1. Moisture content (dry weight) of sludge from
three mills before and after KDS treatment.
Fractionation of the KDS dried material was carried out by
subjecting the material to a very brief treatment in a Laboratory
Wiley mill equipped with a 2 mm holed-screen. The short
treatment time released entangled, smaller particles leaving the
long fibre fraction retained on the screen. The portion, which
passed through the 2.0 mm holed-screen, was screened on a 40
mesh Endecotte test sieve, yielding 40 mesh (+) and 40 mesh (-
) fractions. A comparison of the characteristics for the NP/TM
sludge materials is given in Table 1.
Ash testing was done using a modified Tappi procedure
T211-om93 [8]. The sample was charred at 250
C for 2 hours
to prevent flaming, and then heated at 525
C for 4 hours,
samples were desiccated and weighted and then re-heated to
C for 1 hour to enable determination of calcium carbonate
content. Fibre characteristics were determined using an Optest
Fibre Quality Analyzer – Code LDA93.
Table 1. Characteristics of air-dried untreated sludge and
Moisture Content (% oven-dry)
After KDS Untreated Sludge
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Congrès Annuel ATPPC
KDS sludge.
The recycled NP/TM sludge was selected for the initial
investigation. Handsheets were prepared using mixtures of the
long fibre fraction of the KDS dried sludge and commercially
produced northern softwood TMP fibre. Considerations were
made to account for loss of fines from both materials. For the
TMP, the loss of fines during handsheet preparation was
measured to be 11% of the original dry weight. The amount of
fines in the TMP was 14% as measured by the FQA. The
amount of fines in the long fibre fraction was 2.26% as
measure on the FQA. An adjustment of 10% was made for
potential loss of material from the TMP and 4% for the long
fibre fraction when determining the mass proportions for the
Handsheets were made according to Tappi standard T205
sp95, using British pulp evaluation apparatus, to a target weigh
of 1.26 grams moisture free. After formation the handsheets
were conditioned for 24-36 hours according to T402 om-93,
omitting preconditioning step. Fourteen handsheets were made
for each of the mass combinations. The handsheet properties
were determined using standard Tappi method T220 sp-96 for
bulk, tear index and tensile index. Brightness and yellowness
were measured on an Elrephro brightness meter.
Results and Discussion
In this investigation the quality of the fibre for re-use in the
paper making process is of ultimate interest. The KDS drying
process did not appear to cause a major change in the
characteristics of the sludge,
Table 1. Characteristics of air-
dried untreated sludge and KDS sludge.1. The ash content and
the fines content were slightly lower after the KDS dryer
treatment. This may be due to loss of small size particles
during venting or delivery of the dried sludge. The average
fibre length remained constant through the drying process,
which was surprising considering the intense mechanical
action in the KDS chamber. The curl and kink indices did
increased after drying. A curl index of 0-0.05 represents a
nearly straight fibre, while a measure of 0.5 would represent a
very curly fibre [9]. The increased average curl is possibly a
result of the mechanical action or heat to which the fibres are
exposed during the drying process. It is common to see fibre
develop curl when subjected to incidental mechanical action.
The fractionation process, yielded three fractions; a long
fibre fraction, a 40 mesh (+) fraction and a 40 mesh (-) fraction.
The long fibre fraction had lower ash content and fines content
and a higher average fibre length (2.24 mm) than the original
sludge. The smaller particles contain a higher concentration of
the inorganic materials. The overall yield of the various
fractions in terms of inorganic versus organic content is
illustrated in Figure 2. The long fibre fraction, which was
isolated for this investigation, represents approximately 15%
by weight of the dried sample.
Figure 2. Wiley mill fractionation of Newsprint/Tissue mill
sludge by weight.
The long fibre fraction was selected for the handsheet
testing because of its superior properties. It had low ash
(22.36%) and fines (2.26%) contents, and high average fibre
length. Normally high ash content translated to high fines
content however, this is not the case. The inorganics in the
long fibre fraction may be smaller in size than the detection
limit of the FQA analyzer (0.07 mm) and may therefore not be
counted as a fine. Alternatively, the inorganics might be
physically bonded to the fibre, and not liberated when
dissolved in water. Future work will confirm the nature of the
relationship between the inorganic component and the fibre.
The long fibre fraction has coarseness similar to TMP. A high
coarseness suggests thicker cell walls and less fibre flexibility.
On the other hand, higher curl and kink indices suggest that
this fibre is more flexible than the TMP fibre and will promote
superior bondability.
A SEM of the long fibre compared to a TMP fibre can be
seen in
Figure 3. The TMP fibre shows typical fibrillation of
the fibre, with well-developed microfibrils attached to the
fibre. The long fibre however, has a relatively clean surface,
with small (1-5m) platy shaped particles attached. These fibres
also show a high degree of damage as can be seen at 4000 X
magnification, where various lamellae of the secondary wall
can be seen. Microscopic evaluation of the fibre show that
they maintain their length but are severely fractured along the
cell axis. There are also transverse cuts and numerous kinks
and twists in the fibre. This suggests that the inherent fibre
Sample % Ash
Mean Fibre
% Fines-
48.04 1.45 31.04 0.092 1.18 0.56
44.80 1.46 25.70 0.106 1.44 0.69
Long fibre 22.36 2.24 2.26 0.146 1.85 0.34
Fines 40+ 0.98 11.25 0.088 1.32 0.34
Fines 40- 0.62 64.39 0.067 0.72 1.02
TMP 1.19 2.09 14.14 0.096 1.05 0.39
Or g anic
60.59 11.76 10.09 44.07
36.79 3.59 2.83 25.05
KDS Long fibre 40 M + 40 M -
Fractionated material
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Congrès Annuel ATPPC D203
strength may be reduced. The lack of fibrillation suggests
lower bond area and consequently diminishes bondability. The
higher flexibility of the fibre may offset this affect.
Figure 3. SEM micrograph of commercially produced
northern softwood TMP and long fibre fraction.
Influence of recovered fibre on strength properties
The bulk of the paper decreased with increasing recovered
fibre content (Figure 4). The longer fibre fraction which
exhibits higher flexibility and collapsed than the TMP portion,
contributed to compaction of the sheet. High fibre curl is
known to increase bulk, however this effect was overshadowed
by the influence of the collapsed fibre.
Figure 4. Decreasing bulk of handsheets with increasing
recovered fibre content.
A higher strength sheet is normally achieved at lower bulk
because of the greater bond area between fibres in closer
contact with one another. Accordingly, one would expect that
increasing recovered fibre content would result in a stronger
paper. However, it is also well known that lower intrinsic fibre
strength will lead to a decrease in paper strength. The damage
to the fibres may well generate this result. The tensile and tear
indices are depicted in Figure 5. Tensile strength is negatively
affected by the addition of recovered fibre however tear
strength is positively affected to a maximum level.
Figure 5. Influence of increasing recovered fibre content
on Tensile Index and Tear Index.
Compactness or density of the sheet is strongly influenced
by the amount of recovered fibre. Due to its compressed or
collapsed nature one would think that the higher the recovered
fibre content the lower the bulk and the stronger the paper.
This investigation demonstrates that increasing density
(decreasing bulk) actually leads to a lower strength paper.
Several factors may be responsible for this observation. There
was found to be a strong positive relationship between the
sheet ash and the compactness of the sheet. In addition, the
tensile strength showed a strong negative relationship to
increasing ash content. Ultimately the ash may be interfering
with the fibre-to-fibre bonding effectively diminishing the
strength of the sheet.
The high curl of the fibre can decrease tensile strength
because the fibres essentially curl out of the plane in which the
tensile test is taking place. This reduces the effective fibre
length resulting in more stresses on the inter-fibre bonds as
opposed to the fibre. Curl has been shown to have a positive
affect of tear strength [9].
TMP Fibre
1 um
1 um
y = -0.0052x + 2.8983
= 0.8059
0 5 10 15 20 25 30
% recovered fiber
Bulk (cm^3/g)
y = -0.0305x
+ 0.5932x + 8.337
= 0.9588
0 5 10 15 20 25 30
% recovered fiber
Tear Index (mN-m^2/g)
y = -0.1921x + 24.301
= 0.954
0 5 10 15 20 25 30
% recovered fiber
Tensile Index (Nm/g)
D204 PAPTAC 91
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Congrès Annuel ATPPC
Figure 6. Sheet ash and its influence on Tensile strength.
Tear strength, as is well documented, is strongly influenced
by fibre length and fibre strength. The recovered fibre offers
the benefit of longer average fibre length then the TMP fibre as
it contains strong chemical fibre from the initial raw material
resource. As can be seen in Figure 5 there was an increase in
the tear index up to a substitution level of approximately 10%.
After this point there is a decline in the tear strength. The
reduction in tear strength is believed to be from interference of
the ash in the fibre-to-fibre bonding.
In a paper recycling mill materials rejected from the
screening, cleaning and floatation processes become the waste
stream treated at the mill’s wastewater treatment plant.
Operational efforts concentrate on minimizing rejected
material to reduce the requirement for fresh water make up and
decrease effluent flows. In addition, holding more fiber and
filler in the accepted stream helps to conserve waste paper and
decrease shrinkage. However, even with the high priority
given to the activity of preserving incoming fibre, ample
usable fibre escapes the process. The recovery of this lost fibre
from recycled paper mill sludge may now be feasible with the
KDS Micronex technology. The dry, odourless material can be
successfully fractionated to enhance desired characteristics.
Substitution of recovered fibre into TMP pulp, to a
maximum of 9.7%, improves the tear strength. Tensile
strength is negatively affected, with each percent substitution
causing a 1% decline in the tensile index. The important fibre
characteristics, which affect the tensile strength of paper, are
the bond area, which is affected by fibre flexibility and degree
of fibrillation, the intrinsic fibre strength, which is affected by
the original source of the fibre and degree of damage, and the
strength of the fibre-fibre bond. Recovered fibre has some
advantages over TMP fibre with respect to fibre flexibility and
possibly in fibre strength. These advantages may be
overshadowed by the degree of fibre damage, the lack of
surface fibrils, or the interference of ash with fibre-to-fibre
bonding. In order to elucidate the true value of this fibre the
intrinsic strength of the fibre needs to be determined.
Thorough removal of ash and/or mild refining may also prove
to enhance the tensile strength of this fibre paper.
Authors wish to thank Natural Science and Engineering
Research Council for financial support of this work and AGES
Inc. for sample preparation.
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y = -0.8747x + 25.373
= 0.9517
0.00 2.00 4.00 6.00 8.00
Sheet ash (%)
Tensile Index (mN/g)
y = 0.2202x + 1.2138
= 0.9996
0 5 10 15 20 25 30
% recovered fiber
Sheet ash (%)
... In paper recycling, inorganic materials (ink and fillers) are removed by deinking the recycled paper, generating a waste stream of deinking paper sludge. The side stream could be valorised as raw material after incineration at 850-900 °C [1,9], for road construction. The present study offers a new concept towards more valuable valorisation. ...
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... Also, the hydroxyl vibration at 3305 cm −1 is indicative for the presence of cellulose [37]. The peak at 2899 cm −1 is correlated to the stretching vibration of aliphatic groups of biopolymers (cellulose, hemicellulose and lignin) [9]. The peak at around 1644 cm −1 can be correlated to the carbonyl groups of lignin. ...
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... The surface becomes smoother and some micropores may also appear (Figure 6e) in the treated fibers (Bledzki et al. 2008;Tserki et al. 2005). As previously observed by (Krigstin and Sain 2006), the CFW fibers maintain their length but are severely fractured along the cell axis with transverse cuts and numerous kinks and twists. The degraded appearance of the different modified CFW may not be directly related to the chemical modification but could also be due to the chemical recovery process of the sludge to which it was subjected in the industry. ...
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Pulp and paper industry is one of the major sector in every country of the globe contributing not only to Gross Domestic Product but surprisingly to environmental pollution and health hazards also. Paper and paperboard based material is the one of the earliest and largest used packaging form for food products like milk and milk based products, beverages, dry powders, confectionary, bakery products etc. owing to its eco-friendly hallmark. Various toxic chemicals like printing inks, phthalates, surfactants, bleaching agents, hydrocarbons etc. are incorporated in the paper during its development process which leaches into the food chain during paper production, food consumption and recycling through water discharges. Recycling is considered the best option for replenishing the loss to environment but paper can be recycled maximum six to seven times and paper industry waste is very diverse in nature and composition. Various paper disposal methods like incineration, landfilling, pyrolysis and composting are available but their process optimization becomes a barrier. This review article aims at discussing in detail the use of paper and paper based packaging materials for food applications and painting a wide picture of various health and environmental issues related to the usage of paper and paper based packaging material in food industry. A brief comparison of the environmental aspects of paper production, recycling and its disposal options (incineration and land filling) had also been discussed.
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The reduced interfiber bonding capability and reduced conformability of recycled fibers compared to virgin wood pulp fibers is caused by the drying phase of the first papermaking cycle. Changes in the fiber result in stiffness. This effect is more pronounced in chemical pulps than in high lignin content mechanical pulps. This chapter describes methods for restoring some or all the interfiber bonding. In an attempt to develop a “dry” newspaper recycling process, the water-intensive repulping and paper-forming steps were replaced with dry-fiberizing, air-forming, gas-phase ozone and ammonia treatments, and pressdrying. The tensile strength of the dry-recycled paper approached that of the original newsprint.
A significant increase in the amount of deinking residuals (DIR) generated by the growing North American mills deinking capacity is a consequence not often considered. Without debating the environmental impacts of virgin vs. recycled-content paper, it is certain that the production of recycled paper product greatly affects the environment. DIR management plays a significant part on the costs of producing recycled paper products. It is following a path similar to the national trends for solid waste management and has two traditional management practices, landfill and onsite incineration. Mills are seeking cost-effective, environmentally sound management strategies that are limited only by mill's ingenuity and individual circumstances.
It has been generally recognized that the deterioration of recycled pulp is mainly caused by the loss of total bonding strength of recycled pulp while the loss of intrinsic fiber strength is only minor. The loss of total bonding strength may be attributed to one or both of the two fundamental properties of fibers: wet-fiber flexibility and surface condition. Through studies on a series of hemlock pulps with different chemical composition, the changes in these two properties were determined and the main factor which causes the strength loss of recycled pulp was identified. It is found that the dominant factor causing the strength loss of recycled pulp is wet-fiber flexibility. This conclusion is supported by the strength-density relation of recycled pulp and by the changes in pulp Water Retention Value (WRV) on recycling. The other fundamental property of fibers, specific fiber bonding strength, is studied through the application of Page's equation of tensile strength and is found to remain largely unchanged during recycling. The validity of this application is also addressed.
The curliness of fibres and the degree of microcompression in the fibre wall strongly influence the properties of pulp suspensions, wet-webs and dry sheets . In mill operation, curl and microcompression can be induced accidentally or intention--ally, by shearing at high consistency . Some pulps are highly susceptible to curling ; others are more resistant . Curl is not necessarily stable ; it is readily removed from some pulps but not from others . Curl can be stabilized by certain treatments, notably by heat treatment at high consistency . This can be deliberate, or it can occur accidentally during mill operation, when a pulp is stored at an elevated temperature . Both curl and microcompression are often disregarded because they cannot be easily measured . Yet in practice their effects often domi-nate the properties of pulp suspensions, wet webs and dry sheets . Ignoring these effects has led to costly surprise; both in research and mill operation . In this paper the literature is reviewed and new data are introduced, illustrating the importance of curl and micro-compression for mechanical, chemi-mechanical and chemical pulps .
Unbeaten, beaten, and recycled softwood pulps were used as substrates to assess the various substrate characteristics that may influence the enzymatic hydrolysis of cellulose. After 4 h of hydrolysis, about 30% of the beaten pulp was hydrolyzed to glucose, as compared with 20% of both unbeaten and recycled pulps, regardless of their similarities in initial fiber length and degree of polymerization. The fragmentation profile (change in fiber length distribution) of both unbeaten and recycled pulps during enzymatic hydrolysis also suggested that they were degraded in a similar fashion. It appeared that the primary action of refining was to enhance swelling of the fibers, resulting in a better fiber-to-fiber contact area, paper strength, and accessibility to cellulase enzymes. In contrast, scanning electron microscopy of partially hydrolyzed residues indicated that recycling caused the disappearance of external fibrilation in the fibers, restoring the fine structure of the pulp to their original unbeaten state. The structural reorganization triggered by recycling also appeared to decrease substrate accessibility to cellulases as well as the paper-making qualities of the fiber. Although substrate characteristics such as crystallinity, particle size, and the degree of polymerization all influence the efficiency of enzymatic hydrolysis, it is primarily the extent of the cellulose surface area available to the enzymes that dictates the rate and degree of hydrolysis of the substrate.
-last update, overview of the recycling industry [Homepage of Pulp and Paper Products Council
  • Paper Recycling Association
PAPER RECYCLING ASSOCIATION, July 28, 2004, 2004-last update, overview of the recycling industry [Homepage of Pulp and Paper Products Council], [Online]. Available: en/1_0/index.html [07/28, 2004].
Waste Management Industry Survey Business and Government Sectors
  • Statistics
  • Canada
STATISTICS CANADA, 2000. Waste Management Industry Survey Business and Government Sectors 2000. 16F0023XIE. Statistics Canada.
Landfilling of Sludge remains most viable, but new options on horizon
  • J J Glowacki
GLOWACKI, J.J., 1994. Landfilling of Sludge remains most viable, but new options on horizon. Pulp & Paper, 68, pp. 95-97.
Utilization of Mill Residue (Sludge Recycled Paper Technology: An Anthology of Published Papers
  • Ppr Staff
PPR STAFF, 1994. Utilization of Mill Residue (Sludge). In: M. DOSHI, ed, Recycled Paper Technology: An Anthology of Published Papers. Atlanta, Ga.: Tappi Press, pp. 228-234.
Prospects for the utilization of paper mill sludges in manufactured products. 95-T12-B003968
  • K Bellamy
BELLAMY, K., 1995. Prospects for the utilization of paper mill sludges in manufactured products. 95-T12-B003968. Mississauga, Ontario: ORTECH.