w a t e r r e s e a r c h 6 5 ( 2 0 1 4 ) 3 2 1 e3 4 9

Available online at www.sciencedirect.com

ScienceDirect

journal homepage: www.elsevier.com/locate/watres

Review

Anaerobic digestion of pulp and paper mill

wastewater and sludge

Torsten Meyer*, Elizabeth A. Edwards

Department of Chemical Engineering and Applied Chemistry, University of Toronto, 200 College St., ON,
Canada M5S3E5

article info abstract

Article history: Pulp and paper mills generate large amounts of waste organic matter that may be con-
Received 2 March 2014 verted to renewable energy in form of methane. The anaerobic treatment of mill waste-
Received in revised form water is widely accepted however, usually only applied to few selected streams. Chemical
20 June 2014 oxygen demand (COD) removal rates in full-scale reactors range between 30 and 90%, and
Accepted 12 July 2014 methane yields are 0.30e0.40 m3 kg
1 COD removed. Highest COD removal rates are ach-
Available online 24 July 2014 ieved with condensate streams from chemical pulping (75e90%) and paper mill effluents
(60e80%).
Keywords: Numerous laboratory and pilot-scale studies have shown that, contrary to common
Anaerobic treatment perception, most other mill effluents are also to some extent anaerobically treatable. Even
Anaerobic digestion for difficult-to-digest streams such as bleaching effluents COD removal rates range be-
Pulp tween 15 and 90%, depending on the extent of dilution prior to anaerobic treatment, and
Paper the applied experimental setting. Co-digestion of different streams containing diverse
Wastewater substrate can level out and diminish toxicity, and may lead to a more robust microbial
Sludge community. Furthermore, the microbial population has the ability to become acclimated
and adapted to adverse conditions. Stress situations such as toxic shock loads or tempo-
rary organic overloading may be tolerated by an adapted community, whereas they could
lead to process disturbance with an un-adapted community. Therefore, anaerobic treat-
ment of wastewater containing elevated levels of inhibitors or toxicants should be initiated
by an acclimation/adaptation period that can last between a few weeks and several
months. In order to gain more insight into the underlying processes of microbial accli-
mation/adaptation and co-digestion, future research should focus on the relationship be-
tween wastewater composition, reactor operation and microbial community dynamics.
The potential for engineering and managing the microbial resource is still largely
untapped.
Unlike in wastewater treatment, anaerobic digestion of mill biosludge (waste activated
sludge) and primary sludge is still in its infancy. Current research is mainly focused on
developing efficient pretreatment methods that enable fast hydrolysis of complex organic
matter, shorter sludge residence times and as a consequence, smaller sludge digesters.
Previous experimental studies indicate that the anaerobic digestibility of non-
pretreated biosludge from pulp and paper mills varies widely, with volatile solids (VS)
removal rates of 21e55% and specific methane yields ranging between 40 and 200 mL g
1

* Corresponding author. 200 College St. Toronto, ON, Canada M5S 3E5. Tel.: þ1 416 946 3690.
E-mail address: torsten.meyer@utoronto.ca (T. Meyer).
http://dx.doi.org/10.1016/j.watres.2014.07.022
0043-1354/© 2014 Elsevier Ltd. All rights reserved.
322 w a t e r r e s e a r c h 6 5 ( 2 0 1 4 ) 3 2 1 e3 4 9

VS fed. Pretreatment can increase the digestibility to some extent, however in almost all
reported cases, the specific methane yield of pretreated biosludge did not exceed
200 mL g
1 VS fed. Increases in specific methane yield mostly range between 0 and 90%
compared to non-pretreated biosludge, whereas larger improvements were usually ach-
ieved with more difficult-to-digest biosludge. Thermal treatment and microwave treat-
ment are two of the more effective methods. The heat required for the elevated
temperatures applied in both methods may be provided from surplus heat that is often
available at pulp and paper mills. Given the large variability in specific methane yield of
non-pretreated biosludge, future research should focus on the links between anaerobic
digestibility and sludge properties. Research should also involve mill-derived primary
sludge. Although biosludge has been the main target in previous studies, primary sludge
often constitutes the bulk of mill-generated sludge, and co-digestion of a mixture between
both types of sludge may become practical. The few laboratory studies that have included
mill primary sludge indicate that, similar to biosludge, the digestibility can range widely.
Long-term studies should be conducted to explore the potential of microbial adaptation to
lignocellulosic material which can constitute more than half of the organic matter in pulp
and paper mill sludge.
© 2014 Elsevier Ltd. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Benefits of anaerobic digestion in pulp and paper mills . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.1. Cost reduction by removing parts of the COD anaerobically . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.2. Waste sludge production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.3. Dewaterability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.4. Improving stability of the activated sludge process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.5. Nutrient recovery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.6. Reduced space requirement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3. Anaerobic treatment of wastewater . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1. Mechanical pulping effluents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.2. Chemical pulping effluents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.3. Semi-chemical pulping effluents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.4. Effluents from paper and board production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.5. Bleaching effluents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
4. Inhibition, microbial acclimation/adaptation and mitigation strategies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.1. Microbial acclimation, adaptation and community shifts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.2. Co-digestion of different in-mill streams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
4.3. Sulfur compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
4.4. Wood extractives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
4.5. Peroxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
4.6. Chlorinated compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
4.7. Diethylenetriaminepentaacetate (DTPA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
4.8. Suspended solids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
4.9. Future prospect in acclimation, adaptation and bioaugmentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
5. Anaerobic digestion of sludge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
5.1. Primary Sludge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
5.2. Anaerobic digestion of biosludge without pretreatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
5.3. Sludge pretreatment to enhance anaerobic digestibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
5.3.1. Ultrasound and microwave . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
5.3.2. Thermal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
5.3.3. Hydrodynamic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
5.3.4. Chemical . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
5.3.5. Biological . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
6. Biorefinery concepts involving anaerobic digestion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
6.1. Ethanol production and anaerobic digestion of stillage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
6.2. Agronomic use of digestate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
 w a t e r r e s e a r c h 6 5 ( 2 0 1 4 ) 3 2 1 e3 4 9 323

6.3. Co-digestion of mill sludge with organic waste from outside the mill . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
6.4. Combined generation of hydrogen and methane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
6.5. Other bio-based products as a result of anaerobic digestion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
7. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
8. Uncited references . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

1. Introduction components unique to the waste streams in pulp and paper

mill operation, such as lignocellulosic material. Other obsta-
The pulp and paper industry is facing increasing economic cles are large day-to-day variations of the wastewater
and environmental constraints due to globalized competition composition and the occurrence of anaerobic inhibitors such
and more stringent environmental legislations. The global as resin acids, sulfur and organochlorine compounds.
pulp and paper waste and wastewater treatment market is Paper production is a growing business with about 5000
expected to increase by 60% between 2012 and 2020 (Frost & pulp and paper mills worldwide (Mensink, 2007) producing
Sullivan, 2013). Anaerobic digestion of organic waste has the nearly 400 million tons of paper annually (Skogsindustrierna,
potential to address the economic and the environmental 2010). The water use in pulp and paper mills is 10e100 m3 per
pressure at the same time. While anaerobic digestion has ton of produced paper (Greenbaum, 2002; FPAC, 2009) and the
been widely applied in wastewater treatment of various in- sludge generation ranges between 0.2 and 0.6 wet tons per ton
dustries, agriculture, and in the municipal sector, only since pulp produced (CANMET, 2005). The chemical oxygen demand
the late 1980s it has gained increasing attention also in the (COD) concentrations of mill effluents typically range between
pulp and paper industry (Fig. 1). Whereas the number of 1 and 10 g L
1, (Hall and Cornacchio, 1988; Rintala and
anaerobic installations worldwide has more than doubled Puhakka, 1994). Thus, the amount of COD available for en-
within the last decade, the COD removal capacity has ergy conversion in pulp and paper mill wastewater alone
quadrupled, because the reactor capacity in terms of the daily ranges from 4 to 400 million tons annually. If only 25% of that
organic loading rate per reactor has also steadily increased COD load could be transformed into biogas, 1 to 100 TWh of
over time. Anaerobic reactors are commonly treating only a electricity could be generated. As a comparison, in 2011 all
few selected in-mill streams, such as paper mill effluents and biogas plants worldwide have generated ~40 TWh of elec-
evaporator condensates from chemical pulping, while many tricity (REF, 2012). Anaerobic digestion involves a series of
other effluents are excluded. Compared to mill wastewater processes in which microorganisms break down organic
treatment the application of anaerobic digestion to mill matter in the absence of oxygen. In a concerted action, various
derived sludge is lagging behind. While full-scale digestion of Bacteria and Archaea perform step-wise degradation of com-
sludge is still uncommon, a few projects on a pilot or plex organic matter, such as carbohydrates, proteins and fats,
demonstration scale have been implemented. to methane and CO2 (biogas) via hydrolysis, acidogenesis
Current obstacles to a more widespread use of anaerobic (fermentation), acetogenesis, and methanogenesis (Rittmann
treatment include the inherent difficulty to digest and McCarty, 2001). The resulting biogas consists of about
50e80% of the energy carrier methane, with the remainder
being mainly carbon dioxide. Each step is accomplished by
different groups of microbes, and their interactions often
enable reactions that would otherwise be energetically unfa-
vored by maintaining very low concentrations of interspecies
metabolites, such as hydrogen (Li et al., 2012). While these
syntrophic associations enable the break down of a wide
range of organic matter in the absence of oxygen, they also
contribute to the sensitivity of the process.
Inherent to anaerobic wastewater treatment are slow mi-
crobial substrate removal rates (Grau second order rates:
0.3e11 gCOD gVSS
1 day
1) (Rajagopal et al., 2013; refs.
therein) as well as slow biomass growth rates (0.02e0.04 gVSS
gCOD
1 removed) (Lee et al., 1989; Mermillod et al., 1992),
features that have been considered a disadvantage compared
to aerobic processes. In order to accommodate the slow mi-
Fig. 1 e Global cumulative anaerobic wastewater treatment crobial growth, long sludge retention times (SRTs) have to be
installations in pulp and paper mills (by Sept. 2012) maintained. This is often at odds with the need for short hy-
(Totzke, 2004, 2012; Various unnamed equipment draulic retention times (HRTs) to treat large volumes of
suppliers). wastewater quickly and economically. Therefore, the main
324 w a t e r r e s e a r c h 6 5 ( 2 0 1 4 ) 3 2 1 e3 4 9

feature that distinguishes between the varying types of tannins, terpenes, and organochlorines (Pokhrel and
anaerobic reactors is how SRT and HRT are decoupled to Viraraghavan, 2004).
attain very high SRTs while minimizing the HRT. The most In the following, the term biosludge refers to sludge
successful high-rate reactor types in this regard are the upflow generated during aerobic wastewater treatment (waste acti-
anaerobic sludge bed (UASB) reactor and advanced versions vated sludge), whereas the term digestate refers to residual
thereof, such as various types of expanded granular sludge biosolids as a result of anaerobic digestion of sludge. Most
bed (EGSB) reactors (Kato et al., 1994). The latter constitute the pulp and paper mills treat their wastewater with an aerobic
vast majority of anaerobic installations currently employed in activated sludge treatment system. In those cases biosludge is
pulp and paper mills. These reactors are characterized by generated at 5e10% of the finally produced pulp (on a dry
anaerobic granular sludge where the biomass forms large weight basis), which can increase to 20e40% in the case of
granules with diameters ranging between 0.5 mm and 3 mm. recycled paper mills (Scott and Smith, 1995). Currently, the
Well-functioning sludge granules exhibit excellent settle- biosludge is first dewatered and either put on landfills, or
ability, while enabling HRTs as short as 4e8 h. One of the incinerated for energy recovery. Either way of biosludge
primary concerns related to reactors with anaerobic granular handling is cost-intensive, and can comprise one half of the
sludge has been their susceptibility to various types of shocks, total effluent treatment costs (Kantardjieff and Jones, 2000).
such as inputs of oxygen, changes in feed loading rates and Landfilling is associated with high tipping fees and incinera-
composition, as well as transient occurrences of toxic and tion with high dewatering costs. Anaerobic digestion of bio-
inhibitory substances. Such adverse conditions can lead to sludge is an attractive alternative; however the concensus is
granule disintegration, resulting in excessive washout of that some form of pretreatment is required to enhance
anaerobic biomass. anaerobic digestibility. Hydrolysis of complex organic matter
Advanced EGSB-type reactors include the internal circula- is perceived to be the main bottleneck in the digestion process.
tion (IC) reactor BIOPAQ®IC marketed by Paques, the Biobed® The process is slow and requires long sludge residence times
EGSB reactor from Biothane (Veolia Water), the R2S reactor associated with large reactors and high investment costs.
from Voith, and the external circulation sludge bed Numerous pretreatment methods have been investigated,
(STP®ECSB) reactor from HydroThane. These high-rate re- that are based on biological, chemical, thermal, and me-
actors enable organic loading rates up to 30e40 kg COD per m3 chanical processes (Elliott and Mahmood, 2007). Most studies
reactor volume per day (kg COD m
3 day
1) by using tall re- investigate biosludge from municipal and varied industrial
actors, effluent recycling, and clever methods for 3 phase sources, but not typically from the pulp and paper industry.
separation of effluent, biogas, and granular sludge. EGSB-type The consistency of mill sludge is notably very different from
reactors enable the treatment of low-strength wastewater other types of biosludge mainly because it contains large
containing COD concentrations of 0.75e2 g L
1, which are amounts of lignocellulosic material. In average, 70% of all
common e.g. in various kraft pulp mill streams. Attempts sludge generated in mills consists of primary sludge (Elliott
have been made to combine two different reactor types into and Mahmood, 2005). However, anaerobic digestion of pri-
one. For example, UEM Group is marketing a UASB reactor mary sludge from pulp and paper mills is to-date virtually
that includes a packed bed within its upper part, and Water- unexplored.
leau (Biotim combines a UASB reactor with an anaerobic This review is an attempt to outline the benefits of anaer-
contact reactor. The potential of combining high-rate EGSB- obic wastewater treatment in combination with subsequent
type reactors with membrane technology for pulp and paper aerobic treatment (Section 2), as well as the current state and
wastewater treatment is also currently being investigated (e.g. the potential of anaerobic wastewater treatment for effluents
Kale and Singh, 2013). In cases where wastewater contains from various mill processes (Section 3). Earlier studies inves-
high concentrations of suspended solids, high-rate EGSB-type tigated the feasibility of anaerobic treatment of specific in-mill
reactors may not be suitable, and low-rate anaerobic contact effluents (refs in Rintala and Puhakka, 1994; refs in Bajpai,
reactors are used. Examples are the Anaerobic Aerobic 2000) (Table 1). As a result of those studies it appears that an
Methane production (ANAMET) system from Purac, and the effluent type related to a particular mill process cannot clearly
an-OPUR technology from Wabag. In both systems the be assigned to a certain degree of anaerobic digestibility.
biomass is separated and recovered from the effluent by Therefore, emphasis is also placed on characterizing the di-
means of lamella plates in the sedimentation stage, and gestibility in relationship to individual anaerobic inhibitors
recycled into the anaerobic reactor. Organic loading rates of that can be present in a variety of in-mill streams (Section 4).
those reactors are well below 10 kg COD m
3 day
1. Section 5 reviews previous research related to anaerobic
Mill wastewater streams can vary largely between different digestion of mill-derived sludge. Finally, Section 6 is devoted
mills, within a mill, and also on a day-to-day basis, depending to biorefinery concepts involving anaerobic digestion, that are
on the mill process complexity and the raw materials used. In either already implemented on a full-scale, or have the po-
general, effluents containing relatively high COD concentra- tential for future application.
tions (>1 g L
1) and relatively low concentrations of inhibitory
or toxic compounds are suitable for anaerobic treatment (Hall
and Cornacchio, 1988). Also, wastewater generated during 2. Benefits of anaerobic digestion in pulp and
hardwood processing seems to be more easily treatable, than paper mills
streams from softwood processing (Yang et al., 2010). Sub-
stances that have been shown to cause process instabilities The benefits of anaerobic wastewater treatment can be har-
include resin acids and fatty acids (RFAs), sulfuric compounds, nessed to the fullest only when combined with aerobic post-
Table 1 e Composition and anaerobic digestibility of various pulp and paper mill streams (digestibility is based on the treatment of diluted and undiluted streams).
Type of wastewater COD Organic matter Concentration of primary TSS COD Methane Reference
concentration composition inhibitors [mg L
1] concen-tration removal generation
[g L
1] [mg L
1] [mg L
1] rates [%] [m3 kg
1
COD removed]
Mechanical pulping effluents
TMP e composite 2.0e7.2 Carbohydrates (1230e2700), Sulfate (200e700), 40e810 50e70 0.30e0.40 Jurgensen et al.,
Acetic acid (235), Peroxide (0e100), 1985; Hall et al.,
Methanol (25) Resin acids (30e200) 1986; Cornacchio
1989; Habets and
de Vegt 1991; Hoel
and Aarsand 1995
TMP e chip washing 5.6 n.a. n.a. n.a. 83 0.32 Cornacchio 1989
TMP e whitewater 3.3e9.0 n.a. n.a. 36e1400 50e70 n.a. Mehner et al., 1988
CTMP 6.0e10.4 Acetic acid (1500), Sulfate (500e1500), 500e3200 45e66 0.18e0.31 Welander and
Carbohydrates (1000), Sulfite (50e200), Andersson 1985;

w a t e r r e s e a r c h 6 5 ( 2 0 1 4 ) 3 2 1 e3 4 9
Wood extractives (1000) Resin acids (50e550) Habets and de
Peroxide (0e500), DTPA (100) Vegt 1991;
Cornacchio 1989
BCTMP 9.3 Acetic acid (1360) Resin acids (36e40) 2450 80e90 n.a. Kennedy et al.,
1992; Yang et al.,
2010
Debarking effluent 0.5e4.1 Carbohydrates (200e1400), Resin (25e200), n.a. 44e70 n.a. Field et al., 1988,
Phenols (100e800) Tannins (200e1600) and refs therein
(estimated based on data
from wet and half-wet
debarking effluents)
Chemical pulping effluents
Kraft digester 13.3 Methanol (250e12,000), Sulfides (1e270), 17 n.a. n.a. Dufresne et al.,
condensates Ethanol (20e3200), Sulfite (8) 2001
Phenols (31e40),
Terpenes (0.1e25,000)
Kraft evaporator 0.6e6.5 Methanol (375e2500); Sulfides (1e690), 0.5e105 70e99 0.29e0.35 Blackwell et al.
condensates Ethanol (0e190); Sulfite (3e10), (1979); Qiu et al.,
2-propanol (0e18); Resin acids (28e230) 1988; Cornacchio,
Acetone (1.5e5.1); 1989; Driessen
Phenols (17e42); et al., 2000;
Terpenes (0.1e660) Dufresne et al.,
2001; Xie et al.,
2010
Kraft combined 0.7e4.0 Methanol (1300) Sulfides (210) 12 59e90 0.20e0.32 Cornacchio, 1989;
condensates Dufresne
et al., 2001
Kraft mill streams: n.a. n.a. n.a. Cornacchio, 1989
(1) Woodroom effluent (1) 2.1e4.0 (1) 92e100 (1) 0.35e0.40
(2) Contaminated hot water (2) 3.9 (2) 88 (2) 0.34
(3) Brown stock decker filtrate (3) 0.7 (3) 86 (3) 0.20

325
(continued on next page)
 326
Table 1 e (continued )
Type of wastewater COD Organic matter Concentration of primary TSS COD Methane Reference
concentration composition inhibitors [mg L
1] concen-tration removal generation
[g L
1] [mg L
1] [mg L
1] rates [%] [m3 kg
1
COD removed]
Kraft dissolving pulp 70e120 Carbohydrates n.a. 1200e2700 32e90 n.a. Debnath et al.,
pre-hydrolysis liquor (30,000e54,000), 2013; Kale and
Lignin (11,000e25,000), Singh 2013; Bajpai
Furfural (1140), 2000, and refs
Acetic acid therein
(7000e10,400), Sulfate
(200e450)
Sulfite evaporator 3.0e27 Acetic acid (2000), Sulfite (450e800), Resin n.a. 87e90 0.28e0.30 Frostell 1984;
condensate Methanol (0e250), acids (3.2e9.3) Salkinoja-Salonen
Furfural (0e250) et al., 1985; Walters
et al., 1988;

w a t e r r e s e a r c h 6 5 ( 2 0 1 4 ) 3 2 1 e3 4 9
Cornacchio, 1989;
Driessen et al., 2000
Spent sulfite liquor 40e115 n.a. RFA (40), Sulfate (5100), 320 24e52 0.0e0.31 Cornacchio, 1989;
Sulfite (4800) Schnell et al., 1992;
Jantsch et al., 2002
Sulfite pulping effluent 6.2e48 n.a. n.a. n.a. 29e38 0.14e0.30 Cornacchio, 1989
Semi-chemical pulping effluents
NSSC composite 1.8e19 Lignin (500), n.a. 120e940 50e80 0.16e0.40 Hall et al., 1986;
effluents Carbohydrates (610), Lee et al., 1989,
Acetic acid (54), and refs. therein;
Methanol (9) Cornacchio, 1989;
Smith et al., 1994;
Arshad and Hashim
2012
NSSC spent liquor 28e40 Carbohydrates (6210); n.a. 250 68e71 0.38e0.40 Hall et al., 1986;
Acetic acid (3200); Cornacchio 1989
Methanol (90);
Ethanol (5)
APMP effluent 10e31 n.a. Resin acids (8.5e220), n.a. 64e76 (sCOD ~0.35e0.40 Schnell et al., 1993
LCFAs (32e172), Peroxide removal)
(800e1000), Sulfate (80e220)
Effluents from paper and board production
Recycled paper mill 0.6e15 n.a. n.a. 300e800 58e86 0.24e0.40 Maat, 1990;
effluent Paasschens et al., 1991;
Mermillod et al., 1992;
Driessen et al., 1999
Recycled paper mill 32 n.a. Resin acids (0.002e1.8), n.a. 85e90 n.a. Alexandersson and
whitewater Fatty acids (0.3e5.2) Malmqvist 2005;
Latorre et al., 2007
Bleaching effluents
 w a t e r r e s e a r c h 6 5 ( 2 0 1 4 ) 3 2 1 e3 4 9 327

treatment (Buyukkamaci and Koken, 2010). This is because in

1994; Dorica and Elliott,

most cases anaerobic treatment alone does not provide an

1994; Vidal et al., 1997

Setiawan et al., 2008;

Larsson et al., 2013
effluent of sufficient quality for release into surface waters. In

Yu and Welander,
Cornacchio, 1989;

Cornacchio, 1989;
Vidal et al., 1997

Wasenius (1994)
addition to economic benefits that directly impact the mills,

Qiu et al., 1988;

Chaparro and

Driessen and
combined anaerobic-aerobic treatment can prevent substan-
Pires 2011

tial amounts of greenhouse gas emissions. Habets and

Driessen (2006) refer to a reduction in carbon dioxide emis-
sion of 25 kg per ton of pulp (air dried) produced. The energy
and associated benefits are itemized below.

2.1. Cost reduction by removing parts of the COD

anaerobically
0.0e0.38

0.0e0.14

~0.40
n.a.

n.a.

Paasschens et al. (1991) conducted a case study where they

compared the energy requirements for anaerobic and aerobic
treatment of paper mill wastewater. Accordingly, in a
wastewater treatment plant processing streams from three
45e55

20e67

15e90

mills, two full-scale UASB reactors remove 23 tons of COD

~50
75

while generating 4490 m3 biogas per day. Whereas the biogas

can be converted to 35 GJ (9.6 MWh) electricity, the average
power consumption of the reactor feed pumps is 4 GJ
(1.1 MWh) per day (Paasschens et al., 1991), resulting in a
7e2200

daily net energy gain of 31 GJ (8.5 MWh). The energy required

40e60

<100

for anaerobic treatment is mainly related to the pumping of

n.a.

n.a.

wastewater in and out of the anaerobic reactor. On the other

Chloride (417 ± 93), (696 ± 57);

hand, the energy requirement for aerobic wastewater treat-

Sulfate (600); peroxide (<100)

ment in pulp and paper mills is typically between 3.2 and

Total organic chlorine (76)

3.6 GJ (880e1000 kWh) per ton COD removed (Hagelqvist,

Chloride (1300e1600),

Chloride (1200e1400),
AOX (16 ± 5), (22 ± 2);

2013b). Therefore, the energy needed for aerobic COD

Phenols (208 ± 17),

Sulfate (170e250)
(635 ± 49) (N ¼ 8)

removal in the above case study would be on average 77 GJ

AOX (110e120),

AOX (2.6e200),

(21.5 MWh) per day.

Habets and Knelissen (1985) evaluated the annual costs
involved for an extension of an activated sludge based
n.a.

wastewater treatment plant in a paper mill in order to double

the treatment capacity. Two options for the extension were
compared, one involving only aerobic treatment, and the
other involving combined anaerobic-aerobic treatment for the
Methanol (40.0e75.6)

entire plant. By considering the investment costs, interest

payments, depreciation as well as all operation costs, the
Methanol (140);

annual costs for anaerobic-aerobic treatment were half that of

acetate (<10)

aerobic treatment alone.

2.2. Waste sludge production

n.a.

n.a.

n.a.

Aerobic wastewater treatment in pulp and paper mills gen-

erates between 0.4 and 1.0 tons biosludge per ton COD reduced
1.1e2.4

0.7e0.9

0.6e3.9

0.3e4.3

1.5e3.5

(Hagelqvist, 2013b). The amount of sludge produced during

anaerobic treatment of mill wastewater on a full-scale is only
about 0.02 tons per ton COD removed (Habets and de Vegt,
1991; Nilsson and Strand, 1994). Habets and Driessen (2006)
n.a. e information not available.
(Z, D, EOP, O2) from two mills

refer to a decrease in the overall sludge production by two

thirds if anaerobic treatment precedes the activated sludge
free bleaching effluents
Kraft elemental chlorine

process.
bleaching effluents

bleaching effluents
bleaching effluent
Chlorine bleaching
Total chlorine free

2.3. Dewaterability
TMP peroxide
Kraft alkaline
effluents

Sludge dewatering is usually one of the most cost-intensive

parts of the wastewater treatment system. By including
anaerobic digestion into the system, the sludge dewaterability
may potentially be improved in two ways. First, if parts of the
328 w a t e r r e s e a r c h 6 5 ( 2 0 1 4 ) 3 2 1 e3 4 9

biosludge were anaerobically digested, the mass ratio of pri- 2.6. Reduced space requirement
mary sludge to biosludge would increase, leading to improved
dewaterability of the combined sludges. This is because pri- Aerobic treatment requires a relatively large surface area
mary sludge is often more easily dewaterable than biosludge within a mill mainly due to the aeration tanks and settling
owing to high content of wood. Second, if anaerobic waste- basins. Previous studies refer to a space requirement for
water treatment is included, the biosludge produced in the combined anaerobic-aerobic treatment in pulp and paper
subsequent activated sludge process is usually easier to mills half that of aerobic treatment alone (Maat, 1990;
dewater. In the anaerobic digestion stage the more easy Mermillod et al., 1992). Modern EGSB-type reactors have an
biodegradable organic matter, such as carbohydrates origi- even smaller footprint due to higher applicable organic
nating from starch and hemicelluloses, is largely removed. loading rates and a larger height-to-diameter ratio, compared
Those compounds are often responsible for the excess growth to older reactors. Therefore, the space requirement for com-
of filamentous bacteria in the activated sludge process, which bined anaerobic-aerobic treatment ranges now between 25
in turn deteriorates biosludge dewaterability (Habets and and 50% of that for aerobic treatment (Habets and Driessen,
Driessen, 2006). 2006).
It is common practice in packaging mills to add the bio-
sludge to the pulp; however, this can cause problems with
paper dewatering speed. In cases where anaerobic treatment
is used, the addition of biosludge to the pulp seems to not 3. Anaerobic treatment of wastewater
cause any problems (Habets, 2012). Thus anaerobic treatment
may bring about a number of unexpected positive conse- The chemical composition of in-mill effluents varies widely
quences by virtue of different mechanisms. between individual mills as well as on a day-to-day basis
within one mill. Almost every pulp mill is unique in terms of
their wastewater. Its consistency depends on activities such
2.4. Improving stability of the activated sludge process as handling of raw material, pulping processes, chemical re-
covery, and bleaching (Habets, 2012) and the age of the mill.
The diminished growth of filamentous bacteria in activated Each of those activities involves numerous options. The types
sludge also improves the sludge settleability, e.g. sludge vol- of raw materials used are wood from various softwood and
ume indexes of <100 mL g
1 are attained. Accordingly, the hardwood species, recycle paper such as newsprint, mixed
stability of the aerobic treatment process is improved and the office waste, and old corrugated containers, as well as various
amount of sludge conditioners such as polymers can be types of non-wood materials. Various mechanical, chemical,
reduced (Habets, 2012). and semi-mechanical pulping processes generate wastewa-
ters that are process-specific. During chemical recovery a
number of different condensates are produced. Finally, mills
2.5. Nutrient recovery that bleach their pulp usually apply sequences each consist-
ing of various bleaching methods, which in turn generate
Macro-nutrients have become an increasingly important cost specific types of wastewater. Because of the large variability of
factor in wastewater treatment. If biosludge is anaerobically in-mill streams, the anaerobic digestibility varies also widely.
digested the remaining digestate contains large amounts of In a rare study, Hall and Cornacchio (1988) investigated 43 in-
released ammonia and phosphate that can be recycled into mill streams from 21 Canadian pulp and paper mills in terms
the activated sludge process. of their digestibility using anaerobic toxicity assays (ATA) and
Based on the results of their experiments with pretreated biochemical methane potential (BMP) assays. The results of
and non-pretreated pulp and paper mill biosludge, Elliott and this study should still be valid because the principal pulping
Mahmood (2012) concluded that 36e54% of the nitrogen and methods and the applied chemicals have not much changed
19e24% of the phosphorus necessary for in-mill wastewater since then. Remarkably, almost 50% of the effluents tested
treatment may be provided by means of anaerobically appeared to be suitable for anaerobic digestion, because they
digesting the biosludge and recovering the nutrients from the exhibited little inhibition to an un-adapted microbial com-
digestate. A medium-sized pulp mill may spend US $0.5 to $1 munity yet were sufficiently rich in COD. The highest di-
million per year for nitrogen and phosphate alone (Laudrum, gestibility was found in effluents from non-sulfur semi-
2011). In addition to nitrogen and phosphorus, the mineral- chemical pulping and chemical recovery processes, and
ized and digested sludge contains other compounds of high lowest digestibility from bleaching effluents (Hall and
agronomic value such as organic carbon, potassium and cal- Cornacchio, 1988). It should be noted that the inhibition
cium. Therefore, the digestate can be further processed to tests of this study used un-adapted microorganisms though
become commercial fertilizers. The prices of the latter have adaptation can dramatically improve the anaerobic di-
increased two- to three-fold in recent years (Elliott and gestibility (see Section 4.1). Also, there are today only very few
Mahmood, 2012). Since many mills use sulfuric compounds cases where low COD concentrations would be an obstacle.
in their pulping processes one of the recoverable compounds Modern EGSB reactors are able to treat streams containing
after anaerobic digestion is elemental sulfur, which can be COD concentrations as low as 0.75 g L
1, and low-strength
applied as a fertilizer and natural fungicide e.g. on vineyards effluents can be combined with high-strength effluents.
(Habets, 2012), or even reused in the pulping process (Buisman Therefore, apart from few exceptions such as debarking
et al., 1993). effluent and certain types of bleaching effluents, the majority
 w a t e r r e s e a r c h 6 5 ( 2 0 1 4 ) 3 2 1 e3 4 9 329

of in-mill streams could be suitable for anaerobic treatment, pulping (CTMP). The latter two methods may include a
even if only as a co-substrate, and after sufficient microbial bleaching stage leading to the production of BTMP or BCTMP
adaptation has occurred (see Section 4). pulp. The chemical and thermal pretreatment conditions
Often, in-mill streams are deficient in the nutrients that are (temperature, time, pH) are much less vigorous than in a fully
necessary for anaerobic treatment. Although the nutrient chemical pulping process because the goal is to make the fi-
demand for anaerobic treatment is lower than that for aerobic bers easier to refine mechanically, and not to remove most of
treatment (Rintala and Puhakka, 1994), in most cases macro- the lignin. Mechanical pulping effluents typically contain
nutrients and trace nutrients have to be added at the pre- moderate to high COD concentrations ranging between 2 and
acidification stage. Also, in-mill streams typically exhibit an 10 g L
1, and relatively large amounts of easily digestible
imbalanced substrate composition, which can lead to process carbohydrates and acetic acid (Table 1). Resin acid concen-
disturbance and diminished methane generation. Optimal trations can be quite high while ranging between 10 and
digestion occurs when the feed stream contains sufficient 10,000 mg L
1 (Liss et al., 1997 and refs therein), and often
macronutrients, trace elements, vitamins, and an appropriate exceed those in effluents from chemical pulping. Other
COD to nitrogen to phosphorus (COD:N:P) ratio. Maat (1990) notable anaerobic inhibitors are sulfate and sulfite (Table 1).
identified an optimum ratio of 350:5:1 in several full-scale Bleaching in mechanical pulping usually involves hydrogen
operations. Co-digestion of in-mill streams containing sub- peroxide or sodium dithionite, and in contrast to chemical
strate from various sources can help overcoming such an pulping, no chlorine-containing compounds. Hydrogen
imbalance (see Section 4.2). peroxide is also being applied in the APMP process, however
From the numerous types of effluents generated in mills, prior to the refining process. Peroxide can be efficiently
there are currently only a few selected streams used for full- removed prior to anaerobic treatment (see Section 4.5). Most
scale anaerobic treatment. In fact, approximately two thirds effluents generated during mechanical pulping are suitable
of all anaerobic reactors in the world treat effluents from for anaerobic treatment (Hall and Cornacchio, 1988; Rintala
recycled paper mills and one third from pulp mills (Habets and and Puhakka, 1994). Reported COD removal rates range be-
Driessen, 2006) (Fig. 2). The former often contain high con- tween 45 and 90%, and specific methane yields between 0.18
centrations of COD and low concentrations of toxic or inhib- and 0.40 m3 kg
1 COD removed (Table 1). An exception is
itory compounds. Furthermore, one of the main components debarking effluent, mainly because of high concentrations of
in wastewater from recycled paper processing is well digest- condensed and hydrolyzable tannins, monomeric phenols,
ible starch, as it is one of the most important binding additives and resin compounds (Field et al., 1988; Sierra-Alvarez et al.,
in papermaking. Of the full-scale reactors in pulp mills, most 1994).
of them are treating condensate effluents from chemical
pulping, especially sulfite pulping, as well as alkaline peroxide 3.2. Chemical pulping effluents
mechanical pulping (APMP) streams (Fig. 2).
In chemical pulping the majority of the lignin and hemicel-
3.1. Mechanical pulping effluents lulose is removed, resulting in a low yield (40e55%) wood pulp
consisting of a high degree of cellulose fibers. The principal
Mechanical pulping involves methods where wood chips are chemical pulping methods are kraft pulping and sulfite pulp-
physically ground up, often in combination with mild forms of ing. Whereas kraft pulping involves treatment with a mixture
thermal and/or chemical pretreatment. These high-yield of sodium hydroxide and sodium sulfide, in sulfite pulping,
pulping methods include refiner mechanical pulping (RMP), salts of sulfurous acid such as sulfites or bisulfites are used to
alkaline peroxide mechanical pulping (APMP), thermo me- extract the lignin from the wood chips. Digester and evapo-
chanical pulping (TMP), and chemi-thermo mechanical rator condensates are usually the only streams from chemical

Fig. 2 e Distribution of anaerobic wastewater treatment in paper mills and pulp mills worldwide (Habets and Driessen, 2006)
(N ¼ 218; areas in figure to scale; NSSC e neutral sulfite semi-chemical pulping; non-wood raw material is usually bagasse
or hemp). The individual pulping processes are briefly described in Sections 3.1e3.3.
330 w a t e r r e s e a r c h 6 5 ( 2 0 1 4 ) 3 2 1 e3 4 9

pulping that are treated in full-scale anaerobic typically low. Also, bleaching is often not applied, keeping
reactors (Habets and Driessen, 2007; Driessen et al., 2000). The chlorine compounds and peroxide out of composite effluents.
COD content of those condensates range widely with reported COD concentrations are typically higher than in streams from
concentrations of 0.7e13 g L
1 for kraft pulping condensates, mechanical and chemical pulping, and in many cases exceed
and 3e27 g L
1 for sulfite pulping condensates. Whereas kraft 10 g L
1. Effluents from semi-chemical pulping are usually
condensates contain high concentrations of methanol, acetic well digestible and reported COD removal rates range between
acid is one of the main organic components in sulfite 50 and 80%, with specific methane yields between 0.20 and
condensate (Table 1), both of which are well digestible. Pri- 0.35 m3 kg
1 COD removed (Table 1). If NSSC pulping effluents
mary inhibitors in condensates are sulfur compounds, with such as spent liquor and condensates are anaerobically
reported 60e700 mg L
1 sulfide in kraft condensate and treated the occasional occurrence of potential inhibitors such
450e800 mg L
1 sulfite in sulfite condensate (Table 1). Resin as tannins (spent liquor) as well as sulfur and ammonia
acid concentrations, on the other hand, are usually lower (condensate) require regular monitoring of reactor influent
(<0.05e1000 mg L
1) than what has been found in mechanical (Hall and Cornacchio, 1988). Anaerobic digestion of NSSC
pulping streams (Liss et al., 1997 and refs therein). Elevated pulping effluents is well researched (Arshad and Hashim,
concentrations of resin acids and tannins can often be found 2012; Bajpai, 2000, and refs therein) and practiced in full-
when softwood species are used for chemical pulping (Makris, scale operation with COD removal rates well above 50%
2003; Yang et al., 2010). The digestibility of condensates can be (Smith et al., 1994).
very high with COD removal rates ranging between 70 and 99%
and specific methane yields of 0.20e0.35 m3 kg
1 COD 3.4. Effluents from paper and board production
removed. Other streams from Kraft mills are reportedly also
suitable for anaerobic treatment, including woodroom If raw materials are used for pulping, the effluents during
effluent, brown stock decker filtrate, and contaminated hot papermaking often contain low COD concentrations (<0.5
water. COD removal rates for those streams were between 86 g L
1). In cases where pulp and papermaking is integrated
and 100% (Cornacchio, 1989). In the case of sulfite pulping, within one mill, effluents from both processes may be com-
clarifier effluent, combined sewer effluent, hardwood acid bined in order to elevate COD levels of blends (Habets, 2012).
condensate, hardwood/softwood pulp washing effluent, and Paper mills that use recycled material generate wastewater
final effluent are also digestible (Hall and Cornacchio, 1988), that is typically well digestible. Reported COD removal rates
with reported COD removal rates between 61 and 87% are 58e90%, and the specific methane yield ranges between
(Cornacchio, 1989). Streams that contain elevated levels of 0.24 and 0.40 m3 kg
1 COD removed (Table 1). However, cal-
inhibitors or toxicants require dilution with less polluted cium concentrations in the stream can be high (~10% that of
streams and/or extended periods of adaptation in order to be COD concentrations), leading to scaling problems due to pre-
suitable for anaerobic treatment. Most problematic are efflu- cipitation of calcium carbonate. The calcium in the waste-
ents generated during chlorine dioxide bleaching (Habets, water has its origin in the raw material. As well, calcium
2012) because of organochlorine compounds generated in carbonate is used as a coating agent for papermaking.
the process (Table 1) (see Section 3.5).
Dissolving pulp is a special grade of pulp produced by 3.5. Bleaching effluents
chemical pulping. The process involves a pre-hydrolysis step
to remove hemicelluloses, and the generation of pre- Various bleaching methods are commonly used in form of
hydrolysis liquor (PHL). PHL contains relatively large concen- consecutive bleaching sequences. Most common methods
trations of easily digestible carbohydrates, acetic acids and involve chlorine dioxide (D), extraction with sodium hydrox-
furfural (Debnath et al., 2013). However, only one mill ide (E), alkaline hydrogen peroxide (P), oxygen (O), sodium
currently treating PHL anaerobically was identified (Patrick, dithionite (sodium hydrosulfite) (Y), and ozone (Z). The
2000). The digestibility of PHL varies largely, and reported methods E, O and P are often combined into one (EOP). Several
COD removal rates are between 32 and 90%. PHL as a potential studies have investigated the anaerobic digestibility of efflu-
substrate will likely become more attractive in the near future ents from D bleaching as well as EOP bleaching (see Table 1),
because of the marked growth of the worldwide consumption both of which are part of the elemental chlorine free (ECF)
of dissolving pulp, which is conservatively expected to remain bleaching concept, that is commonly applied in chemical
steady at 3e5% per year until 2025 (Råmark, 2012). pulping processes. Because D and EOP bleaching are usually
combined in one sequence effluents from both types of
3.3. Semi-chemical pulping effluents bleaching can contain notable amounts of highly toxic chlo-
rinated organic compounds. The digestibility of bleaching ef-
Pulp yields using semi-chemical methods range between 60% fluents varies widely and reported COD removal rates are
and 80%, compared to 40%e55% for chemical pulping because between 15 and 90% with specific methane yields of
less lignin is removed in the former. Common methods are 0e0.40 m3 kg
1 COD removed (Table 1). In general, bleach
NSSC pulping and Soda pulping. In NSSC pulping, the plant effluents containing chlorinated compounds are not
impregnation of wood chips with sulfite and carbonate at a suitable for anaerobic treatment if undiluted (Cornacchio,
neutral pH is followed by mechanical refining. Soda pulping 1989). Dilution may involve mixing with effluent from aero-
involves sodium hydroxide as a cooking chemical. Both bic treatment or co-digestion with less toxic streams, whereas
methods are usually applied to hardwood species, therefore, the fraction of bleaching effluent contributing to the com-
effluent concentrations of inhibitory wood extractives are posite stream may range between 5 and 50% (Cornacchio,
Table 2 e Case studies of full-scale and pilot-scale anaerobic treatment of pulp and paper mill wastewater.
Composite stream COD Concentration TSS COD Methane Specific reactor operation & Reactor type Reference
concen-tration of potential concen-tration removal generation digestibility problems
[g L
1] inhibitors [mg L
1] [mg L
1] [%] [m3 kg
1 COD
removed]
NSSC e spent Spent liquor Tannins (2730) (Spent n.a. 45e50 0.31 At OLRs higher >19 kg Demonstration Habets et al.
sulfite (40), FCE (7) liquor), (340) (FCE) COD m
3 day
1, influent plant (1988)
liquor þ FCE dilution required due to
high toxicity
Paper mill 1.1e15 n.a. n.a. 60e80 n.a. n.a. Full-scale Lee et al. (1989)
effluents reactors and refs therein
in 9 mills
Recycled paper 0.14e2.8 (sCOD) n.a. n.a. 65 ~0.30 Temporary inhibition due Full-scale UASB Paasschens
mill effluents to high influent sulfate et al. (1991)
concentration

w a t e r r e s e a r c h 6 5 ( 2 0 1 4 ) 3 2 1 e3 4 9
(500 mg/L); problem fixed
after change of acid
sizing e sulfate
conc. lowered to 140 mg/L
TMP/CTMP 4.0e7.2 Resin acids(50e550), 300e400 30e40 n.a. Initial problems with Full-scale UASB Habets and
Sulfur (200e300), high TSS concentrations in de Vegt (1991)
Peroxide (0e100) the influent
CTMP 7.5e10.4 Sulfate (1220e1500), 2000e3200 45 0.18e0.31 Inhibition due to undefined Pilot-scale Habets and
Sulfite (50e200) wood extractives de Vegt (1991)
Recycled paper 3.3 n.a. 300 75 0.35 Temporary inhibition due to Full-scale UASB Mermillod
mill effluent high levels of cationic polymer in the et al. (1992)
influent, polymer replacement
fixed the problem
Peroxide bleached TMP (2.3), TMP: Sulfate (600), TMP: (<100), 45e60 ~0.33 Peroxide largely removed in Full-scale UASB Driessen and
TMP effluent þ KC KC (3.5) Peroxide (<100) KC (<10) PA-tank; a decline in Wasenius (1994)
methanogenic activity was
assumed to be a lack of trace
elements due to the presence
of DTPA e addition of iron
fixed the problem
Peroxide bleached 1.5e3.5 Peroxide (<100) <100 ~50 ~0.33 n.a. Full-scale UASB Driessen and
TMP effluent Wasenius (1994)
SEC þ CEL (1:1 ratio) 30 AOX (0e25), n.a. 70 0.29 Temporary low anaerobic Full-scale CSTR Dalentoft and
Sulfate (0e130) biomass growth & poor € nsson (1994)
Jo
sludge settling
SEC þ CEL (1:1 ratio) SEC (5.8), AOX (<25), n.a. 65 0.31 Slow, progressive addition Full-scale CSTR Nielsson and
CEL (8.4) Peroxide (<50) of CEL over the course of 2e3 Strand (1994)
months, and SO2 stripping
from SEC prior to digestion
was necessary

331
(continued on next page)
 332
Table 2 e (continued )
Composite stream COD Concentration TSS COD Methane Specific reactor operation & Reactor type Reference
concen-tration of potential concen-tration removal generation digestibility problems
[g L
1] inhibitors [mg L
1] [mg L
1] [%] [m3 kg
1 COD
removed]
NSSC 4e19 n.a. 120 50e60 n.a. Initial problems with high Full-scale UASB Smith et al.
TSS concentrations due to (1994)
poor upfront solids removal
TMP þ peroxide 3.1 Resin acids (28), Peroxide 150 55 0.40 Minor initial problems with Full-scale UASB Andersen and
bleaching (prior to PA tank) (150e600) limited sludge growth and Hallan (1995)
biomass washout; elevated
peroxide levels were
diminished in PA tank
KC 4e6.6 n.a. n.a. 74e86 n.a. Initial problems with high Pilot-scale Wiseman
concentration of salts and projects et al. (1998)
sulfides in KC

w a t e r r e s e a r c h 6 5 ( 2 0 1 4 ) 3 2 1 e3 4 9
PHL 70e80 Sulfate (200e450), 1200e1500 85 0.30 n.a. Pilot-scale Patrick (2000)
Chloride (200e250) and full-scale
KEC 2.5e6.5 n.a. n.a. 80e85 0.33 Temporary low COD removal Full-scale IC Driessen
due to lack of micronutrients et al. (2000)
SEC 3e7.5 n.a. n.a. 90 0.30 Sulfite reduction in steam Full-scale IC Driessen
stripper from 400 mg/L to et al. (2000)
50 mg/L prior to digestion
was necessary
Several types of KC 1.5e13.3 Phenols (29e35), <3.5e105 41e68 0.32 Fraction of foul evaporator Pilot-scale Dufresne
Terpenes (0.6), condensate in composite et al. (2001)
Sulfide (62e700), stream had to be limited
Sulfite (3e10) due to high sulfide
concentrations
Co-digestion of various 3e4.5 Sulfide (50) (KEC) 360e4050 50e65 0.22e0.34 KEC required stripping Pilot-scale Lin et al. (2013b)
mixtures incl. KEC, due to high sulfide packed bed
D- and EOP-bleaching content digester
streams, SPL

Abbreviations: SEC e sulfite evaporator condensate, KEC e kraft evaporator condensate, KC e kraft condensates, CEL e caustic extraction liquor from alkaline bleaching, PHL e pre-hydrolysis liquor
from dissolving pulping, SPL e screw press liquor, FCE e floatation clarifier effluent, n.a. e information not available.
 w a t e r r e s e a r c h 6 5 ( 2 0 1 4 ) 3 2 1 e3 4 9 333

1989; Nilsson and Strand, 1994). Microbial adaptation can was necessary for anaerobic biomass to tolerate resin acid
further improve the suitability for anaerobic digestion concentrations of 16 and 105 mg g
1 VSS. On the other hand,
(Nilsson and Strand, 1994) (see Section 4.6) (Table 2). Wu et al. (1993) and Nilsson and Strand (1994) suggest several
months of microbial adaptation in cases where organochlo-
rine compounds, typical for bleaching effluents, are included
4. Inhibition, microbial acclimation/ into anaerobic treatment. In both studies the concentrations
adaptation and mitigation strategies of inhibitors in the substrate were slowly and progressively
increased over time (Wu et al., 1993; Nilsson and Strand, 1994)
Pulp and paper mill effluents contain a myriad of compounds, (see Section 4.6). These varying reported “adaptation” times
often including inhibitors or toxicants such as wood extrac- betray very different underlying causes.
tives, sulfur compounds, and chlorinated compounds. The Even without a definitive or specific mechanistic explana-
effect that these compounds may have individually or syn- tion, reproducible approaches to increasing microbial process
ergistically on the anaerobic digestion process is not easy to robustness are very useful. Two studies have illustrated that
predict. Also, the component microbial communities may feed stress induced by intermittent feeding can develop a
respond differently to imposed stressors. This section is microbial community with a higher degree of functional sta-
devoted to anaerobic inhibitors and toxicants that are bility including a higher tolerance to stress situations such as
commonly present in pulp and paper mill wastewater, the high organic loadings (De Vrieze et al., 2013; Callejas et al.,
response of the microbial community to these adverse con- 2013). Kullavanijaya et al. (2013) investigated the anaerobic
ditions, as well as reactor operation strategies to mitigate treatability of glycerol distillated residue in a bench-scale
adverse effects. reactor. Whereas reactor operation failure occurred by
applying an un-adapted population, a slow and progressive
4.1. Microbial acclimation, adaptation and community increase in organic loading rate over the course of five months
shifts led to adaptation and stable reactor operation. In some cases,
the response to adverse conditions results in a shift in the
Microbial communities can become more tolerant (accli- dominant populations in the microbial community; for
mated) to certain chemical and physical stressors or even example from Methanosaeta to Methanosarcina as a result of
adapt to metabolize new substrates. Rigorously speaking, temperature shocks and organic overloading (Regueiro et al.,
“acclimation” relates phenotypic plasticity (e.g. Morgan-Kiss 2013). These findings have implications for the day-to-day
et al., 2006) where for example organisms modify their operation of anaerobic reactors. For example, it is common
membrane lipid content or metabolic rate to respond to a practice that in cases of anaerobic process disruption, reactors
stressor. These kinds of changes are generally not permanent are re-seeded with un-adapted sludge often coming from di-
and can be induced relatively rapidly. In contrast, “adapta- gesters that treat very different types of wastewater. There-
tion” is when individuals within a population acquire one or fore, a better solution might be to lower the organic loading
more genetic mutations that confer an advantage under the rate and allow the biomass to recover by itself (Habets et al.,
imposed conditions and these individuals are then selected 1988). As a result, a more resilient population may enable
over others over many generations (Morgan-Kiss et al., 2006). less interrupted operation in the future. More examples
Both acclimation and adaptation occur simultaneously. illustrating the power of microbial acclimation and adaptation
Overlaid on top of these organism-level physiological and are listed throughout the remainder of Section 4, although as
genetic changes are shifts in community structure, and indicated above, only a few recent studies involving careful
together these three effects result in what is observed overall experimental design and next generation sequencing tech-
as improved tolerance to an adverse condition. For example, nology can really begin to reveal the underlying mechanisms.
several studies have shown that bleaching effluents that are
toxic to un-adapted biomass, may not cause process distur- 4.2. Co-digestion of different in-mill streams
bance when the population has been adapted over the course
of months (see Section 4.6). Benjamin et al. (1984) suggested Another strategy to develop a dynamic microbial population
purposely adding toxicants in well defined doses to the reactor with a high resilience to operational stress is substrate
in order to adapt biomass that is resistant to future shock diversification by using several co-substrates (Mata-Alvarez
loads of those toxicants. Only in recent years, with the advent et al., 2013 and references therein). Since pulp and paper
of next generation sequencing and sophisticated molecular mills often generate a variety of diverse effluents, an
biology tools, has it been possible in some cases to identify improvement in digester stability may be achieved by anaer-
and distinguish the mechanisms of acclimation, adaptation obically treating composite effluents, or several streams
and community shifts. In most cases, an effect on changes in simultaneously. Co-digestion can also dilute anaerobic
overall microbial activity is reported without any data to toxicity in difficult-to-digest streams such as bleaching efflu-
determine the underlying cause. For example, early studies ents, a strategy which has been previously applied in pulp and
report highly varying times required for microbial acclima- paper mills (Table 2). In a Finnish pulp mill, kraft pulping
tion/adaptation from one week to several months. Benjamin condensate and TMP peroxide bleaching effluent were com-
et al. (1984) conducted batch experiments with sulfite evapo- bined and anaerobically treated, while COD removal rates of
rator condensate and noted that acclimation to high concen- 45e60% were achieved (Driessen and Wasenius, 1994). In a
trations of guaiacol (2200 mg L
1) lasted 10e15 days. Liver and few mills caustic extraction liquor from alkaline bleaching
Hall (1996) refer to an acclimation time of 7e13 days which and evaporator condensates from chemical pulping were
334 w a t e r r e s e a r c h 6 5 ( 2 0 1 4 ) 3 2 1 e3 4 9

blended prior to anaerobic digestion (Nilsson and Strand, of the purified biogas into the digester. The latter method was
1994; Dalentoft and Jo € nsson, 1994). By keeping the percent- suggested to be the most efficient and economic of all
age of the caustic extraction liquor in the composite stream (Stephenson et al., 1994).
below 50%, COD removal rates between 65 and 70% were In contrast to sulfide, less is published about the toxicity of
possible (Table 2). Sections 4.6 and 6.3 list more examples of sulfite specifically with respect to anaerobic treatment of pulp
co-digestion. and paper mill wastewater, although both compounds can be
similarly problematic (Habets, 2012). Sulfite has been used as a
4.3. Sulfur compounds microbial inhibitor in foods and food processing, owing to its
reactivity (Sapers, 1993). However, under anaerobic condi-
Pulp mill effluents, especially those from chemical pulping, tions, many microbes can reduce sulfite to sulfide. Habets
often contain notable concentrations of sulfur compounds (2012) suggests a threshold value of 50 mg L
1 of sulfite for
such as sulfate ðSO2
2
2

4 Þ, sulfite ðSO3 Þ, thiosulfate ðS2 O3 Þ, sul- un-inhibited operation of EGSB-type reactors (Table 3). Eis
fur dioxide (SO2), hydrogen sulfide (H2S and its dissolved et al. (1983) observed little process deterioration in BMP and
dissociated form HS-), as well as various organic sulfur com- ATA assays after biomass adaptation, even in the presence of
pounds, such as lignosulfonates (Eis et al., 1983). The latter are several hundred mg L
1 of sulfite.
generated by the reaction of bisulfite ðHSO
3 Þ with lignin. Much is still to be learned about the diversity of microbial
Sulfur removal mechanisms include volatilization of metabolism of sulfur compounds. For example, new studies
hydrogen sulfide (or other volatile compounds such as mer- are revealing microbes that thrive in the sulfidic waters
captans), precipitation of iron sulfide (FeS), formation of (>10 mM) of deep sea hydrothermal vents and sulfur springs
elemental sulfur, and incorporation into the biomass (Eis (Lloyd, 2006). These environments may offer clues for better
et al., 1983). Sulfur compounds are among the most impor- operation of engineered reactors as well.
tant anaerobic inhibitors in pulp mills. The highest toxicity
seems to be related to unionized hydrogen sulfide and sulfite, 4.4. Wood extractives
whereas thiosulfate and sulfate are much less toxic (Khan and
Trottier, 1978; Stephenson et al., 1994). Hydrogen sulfide is Wood extractives such as resin acids, long-chained fatty acids
toxic, corrosive and contributes to the COD content in the (LCFAs), volatile terpenes and tannins can be inhibiting to
effluent. Its toxicity is not the only adverse effect, a high anaerobic digestion depending on their wastewater concen-
concentration of partially or fully oxidized sulfur compounds trations (Sierra-Alvarez et al., 1994). Structural features which
in the wastewater leads to a decrease in methane yield enhance compound apolarity contribute to the methanogenic
because such compounds are electron acceptors for sulfate- or toxicity. Sierra-Alvarez and Lettinga (1991) found a linear
sulfur- reducing bacteria that can outcompete acetogenic relationship between octanolewater partitioning and toxicity
bacteria and methanogenic archaea for the utilization of vol- within one homolog group. On the other hand, extremely
atile fatty acids (Eis et al., 1983). Sulfate reducing bacteria are hydrophobic homologous groups such as triterpenes (log
thermodynamically advantaged compared to their competi- KOW > 7.5) were found to be less toxic (50% IC > 1000 mg L
1),
tors. For example, in an anaerobic reactor of a TMP mill perhaps because a minimum water solubility is required in
treating effluents from hydrosulfite bleaching, 66% of the COD order for a substance to be bioavailable (Sierra-Alvarez et al.,
substrate was used for the reduction of sulfur compounds 1994).
(Schnell et al., 1993), and anaerobic treatment of CTMP/CMP Resin acids are among the most cited anaerobic inhibitors
effluents containing high levels of sulfur compounds resulted present in pulp mill effluents (Rintala and Puhakka, 1994 and
in very low specific methane yields ranging between 0.1 and refs therein). Various threshold values above which anaerobic
0.25 m3 per kg removed COD (Schnell et al., 1993). Sulfide inhibition occurs have been reported. However, they vary
concentrations higher than 100 mg L
1 may cause anaerobic widely among the studies, with threshold values for resin
inhibition. Dufresne et al. (2001) observed that contaminated acids ranging between 20 and 600 mg L
1 (Table 3). On the
evaporator condensate with a sulfide concentration of other hand, after an adaptation period of 2.5 years McFarlane
65 mg L
1 was not inhibitory, however foul digester conden- and Clark (1988) were able to operate a bench-scale UASB
sate and foul evaporator condensate with sulfide concentra- reactor fed with screw press wastewater containing a resin
tions of 166 and 1161 mg L
1, respectively, could not be acids concentration of 1360 mg L
1. Nevertheless, operators of
anaerobically treated if undiluted. Stephenson et al. (1994) and full-scale EGSB-type reactors often aim at keeping influent
references therein refer to a threshold concentration of concentrations of resin acids below 100 mg L
1. Concentra-
200 mg∙L-1of unionized hydrogen sulfide, at which inhibition tions in the influent are commonly used to assess the poten-
would just start taking effect. tial of resin acid inhibition. However, it might be beneficial to
Sulfide inhibition is more likely to occur when wastewaters also monitor those resin acids that are sorbed to the anaerobic
have low COD concentrations and COD/SO2
4 ratios of less granular sludge. Sludge bed concentrations can exceed those
than 7.5. In those cases, the quantities of biogas produced may in the influent by one to two orders of magnitude (Richardson
be insufficient to strip the sulfide from the liquid as it is pro- et al., 1991; Meyer et al., in prep). The inhibitory effect of resin
duced. Measures to decrease sulfide toxicity include precipi- acids also depends on their degradability during anaerobic
tation with iron salts, pH increase to remove unionized treatment. Dehydroabietic acid seems to be one of the more
hydrogen sulfide, two-stage anaerobic digestion where toxic resin acids (Sierra-Alvarez and Lettinga, 1990), and at the
hydrogen sulfide is removed in the first (non-methanogenic) same time very little degradable. Whereas the concentration
stage, as well as hydrogen sulfide stripping and recirculation of the sum of resin acids decreases by roughly 50% during
Table 3 e Anaerobic inhibitors/toxicants, critical concentrations above which notable anaerobic inhibition has been reported to occur, and some mitigation strategies.
Compound Class Inhibitor Critical concentrations Most effective mitigation Affected effluents References
[mg L
1] strategies
Sulfuric compounds Hydrogen sulfide (unionized) 50e200 Sulfide stripping and recirculation Most effluents from Kroiss and Wabnegg (1983) and
of the purified biogas into the chemical pulping Stephenson et al. (1994);
digester
Sulfuric compounds Sulfite 50 Dilution Most effluents from Habets (2012);
chemical pulping

w a t e r r e s e a r c h 6 5 ( 2 0 1 4 ) 3 2 1 e3 4 9
Sulfuric compounds Sulfate 500, or COD/SO4 ratios of 5e7.5 After reduction to sulfide, stripping Most effluents from Habets (2012)
and recirculation chemical pulping
Wood extractives Resin acids ~20 to ~600 Upfront solids removal; dilution Most pulp mill effluents Field et al. (1988); McCarthy et al.
(higher with softwood) (1990); Patel et al. (1991); Sierra-
Alvarez and Lettinga (1990);
Kennedy et al. (1992)
Wood extractives Fatty acids 73e1670 (50% IC) Upfront solids removal; Most pulp mill effluents Koster and Cramer (1987); Hwu et
intermittent feeding al. (1996); Kim et al. (2004)
Wood extractives Volatile terpenes 42e330 (50% IC) Dilution Most pulp mill effluents Sierra-Alvarez and Lettinga (1990)
(higher with softwood)
Wood extractives Tannins 350e3000 (50% IC) Dilution Debarking effluents Field and Lettinga (1987); Field et al.
(1988)
Chlorinated compounds AOX 100 Upfront storage in PA tank; dilution Bleaching effluents (Ferguson, 1994)
Chlorinated compounds Pentachlorophenol 0.9e76 (50% IC) Upfront storage in PA tank; dilution Bleaching effluents Patel et al. (1991); Sierra-Alvarez
and Lettinga (1991); Piringer and
Bhattacharya (1999); Puyol et al.
(2012)
Others Hydrogen peroxide ~50 Upfront storage in PA tank Peroxide bleaching and Habets and de Vegt (1991); Nilsson
APMP effluents and Strand (1994)
Others DTPA Several 100s Dilution Peroxide bleaching and Habets and de Vegt (1991)
APMP effluents
Others Suspended solids 500, or 10e20% of COD Upfront settling; dilution Most non-condensate pulp Totzke (2012)
concentrations mill effluents

335
336 w a t e r r e s e a r c h 6 5 ( 2 0 1 4 ) 3 2 1 e3 4 9

anaerobic treatment, that of dehydroabietic acid decreases by that storage and neutralization in the PA tank or equalization
less than 10% (Andersen and Hallan, 1995; Meyer et al., in basin can remove almost half of the AOX levels (Ferguson,
prep). Measures to diminish the inhibitory effect of resin 1994).
acids include upfront solids settling prior to anaerobic treat- Chlorophenol concentrations are sometimes used to
ment or, in case of difficult to settle fibers, dissolved air floa- characterize the toxicity of bleach plant effluents. For the
tation (Habets and de Vegt, 1991). Upfront dilution with non- most toxic compound pentachlorophenol (PCP), 50% IC values
toxic, aerobic effluent can also significantly reduce resin acid of 0.9e76 mg L
1 have been reported (Table 3). Wu et al. (1993)
concentrations. accomplished adaptation of anaerobic granular biomass to
LCFAs are also often present in pulp mill effluents how- PCP concentrations of 40e60 mg L
1 in a lab-scale UASB
ever, typically at lower concentrations than resin acids. Re- reactor. The PCP influent concentrations were progressively
ported 50% IC values (concentrations at which 50% of the increased over the time period of 6e7 months. Removal stra-
activity of methanogenic microorganisms is inhibited) range tegies of chlorophenols include precipitation with divalent
between 73 and 1670 mg L
1 (Table 3). LCFAs are very hydro- cations, or storage in the PA tank (see Section 4.5).
phobic and most of them are sorbed to solids within the Chlorine dioxide (D) bleaching and alkaline (EOP) bleaching
reactor (Meyer et al., in prep). The main inhibiting effect of effluents are commonly perceived as being toxic or highly
LCFAs is illustrated by their accumulation within the sludge inhibitory to the anaerobic digestion process mainly due to
bed while encapsulating the biomass and creating a physical relatively high concentrations of organo-chlorine compounds.
barrier for the transport of substrate and products. This However, several BMP assays, as well as bench-scale and
encapsulating effect can be reversed by means of non-feeding pilot-scale experimental studies indicate that those streams
periods where LCFAs become the substrate for microorgan- may be at least partly included into anaerobic treatment, after
isms (Pereira et al., 2005). microbial adaptation has occurred and/or when those streams
Volatile terpenes are also among the more toxic wood ex- are sufficiently diluted by e.g. upfront mixing with purified
tractives with 50% IC values ranging between 42 and effluent from aerobic treatment (Salkinoja-Salonen et al.,
330 mg L
1 (Sierra-Alvarez and Lettinga, 1990). Most affected 1985; Qiu et al., 1988; Parker et al., 1993; Ferguson et al., 1990;
are effluents originating from softwood pulping (Fengel and Vidal et al., 1997; Yu and Welander, 1994; Setiawan et al.,
Wegener, 1984). While tannins have similar toxic character- 2008; Chaparro and Pires, 2011). Larsson et al. (2013) used
istics, their presence in pulp mill wastewater is mainly alkaline bleaching effluent from a kraft pulp mill as the sole
confined to debarking effluent. substrate to feed a lab-scale UASB reactor. Stable operation
conditions were achieved with total organic carbon removal
4.5. Peroxide rates of ~43% when softwood was processed, and ~60% when
hardwood was processed. In a full-scale anaerobic contact
Hydrogen peroxide (H2O2) can be present in pulp mill effluent reactor in a Swedish pulp mill, where caustic extraction liquor
at elevated concentrations due to peroxide reinforced alkaline (CEL) and sulfite evaporator condensate (SEC) were co-
bleaching or APMP pulping. Peroxide concentrations in the digested at a COD ratio 1:1 and at an OLR of 3 kg
influent of full-scale UASB reactors higher than 50 mg L
1 can COD m
3 day
1, COD reduction rates of 65% were achieved
deteriorate the anaerobic digestion process (Habets and de (Nilsson and Strand, 1994) (Table 2). In that study, AOX con-
Vegt, 1991; Nilsson and Strand, 1994). However, much higher centrations were as high as 25 mg L
1 and peroxide concen-
concentrations of approximately 1 g L
1 can be handled rela- trations were ~50 mg L
1. Although a start-up period of more
tively well by the presence of a pre-acidification (PA) tank prior than one year was necessary for microbial adaptation, and
to anaerobic treatment (Andersen and Hallan, 1995; Habets biomass growth, the authors suggest a minimum adaptation
and de Vegt, 1991), where peroxide and thus oxygen levels period of 2e3 months for similar CEL/SEC mixtures. The au-
are considerably decreased. Many microorganisms contain thors stressed that acclimation/adaptation to the first 20% CEL
enzymes (e.g. catalase) that readily decompose hydrogen was the most difficult part and has to be done slowly while
peroxide to oxygen and water, and the oxygen is utilized by being carefully monitored. In another Swedish pulp mill CEL
facultative bacteria as an electron acceptor in the conversion and SEC were also anaerobically digested at a ratio 1:1 how-
of VFAs present in the wastewater (Habets and de Vegt, 1991; ever, only on a trial basis. Over the course of several months
Fiorenza and Ward, 1997). COD reduction rates of 70% were attained (Dalentoft and
€ nsson, 1994) (Table 2). Qiu et al. (1988) pointed out that co-
Jo
4.6. Chlorinated compounds digestion of CEL and kraft evaporator condensate, the
former containing high amounts of alkalinity, has the benefit
In-mill streams that contain elevated concentrations of chlo- of saving costs for caustic addition to the anaerobic process.
rinated organic compounds are often related to bleaching
sequences involving D and EOP bleaching. As a result the 4.7. Diethylenetriaminepentaacetate (DTPA)
wastewater streams contain numerous different organochlo-
rine compounds that are often quantified using the lumped DTPA is widely used as a complexing agent to sequester metal
parameter “adsorbable organic halides” (AOX). In bench-scale ions that would otherwise decompose peroxide in various
laboratory experiments AOX concentrations of more than bleaching processes. The substances' inhibitory effect is
100 mg L
1 appeared to have a toxic effect on the anaerobic mainly expressed by its ability to build chelates with essential
microbial community, which was expressed by a diminished micronutrients while diminishing their bioavailability. Up to a
biogas production (Ferguson, 1994) (Table 3). It was observed few hundred milligrams DTPA per liter were shown to be
 w a t e r r e s e a r c h 6 5 ( 2 0 1 4 ) 3 2 1 e3 4 9 337

tolerated by anaerobic biomass (Habets and de Vegt, 1991). problematic substrates. However, such feeder reactors are not
However, concentrations of DTPA in Kraft and CTMP pulping unheard of, and further technological and microbiological
effluents are typically only around 100 mg L
1 (Welander and advances exploiting adaptation and microbial inoculation
Andersson, 1985; Alarco  n et al., 2005). Therefore, in the ma- (bioaugmentation) are likely to contribute to bioreactor un-
jority of cases DTPA should not notably inhibit the anaerobic derstanding, design and performance in the future.
process. In a pilot-scale anaerobic reactor treating TMP pulp
mill wastewater, notable reduction in DTPA concentration
could be achieved by adding iron (Driessen and Wasenius, 5. Anaerobic digestion of sludge
1994).
Whereas full-scale anaerobic treatment of pulp and paper mill
4.8. Suspended solids wastewater is established for some effluents, anaerobic
digestion of mill derived sludge is still in its infancy. This is
For EGSB reactors the concentrations of total suspended solids because the hydrolysis of lignocellulosic material, microbial
(TSS) in the influent, mainly consisting of lignocellulosic and cells (biosolids) and associated complex organics (extracel-
inorganic material, needs to be limited in order to prevent lular polymeric substances or EPS) is difficult, takes a long
solids accumulation in the sludge bed, and as a result deteri- time and thus constitutes the major bottleneck to anaerobic
oration of granule settleability and biomass washout digestion of sludge. Slow and incomplete hydrolysis requires
(Richardson et al., 1991). As a rule-of-thumb TSS concentra- high sludge retention times, large reactors, and ultimately
tions should not exceed 10e20% of the COD concentration in high investment costs (Elliott and Mahmood, 2007).
the influent of UASB or EGSB reactors (Totzke, 2012), or in case Numerous sludge pretreatment methods have been
of high-strength wastewater approximately 500 mg L
1 (Table investigated so far including various thermal, chemical, bio-
3). Upfront settling out of particles or co-digestion with solids- logical, and physical methods. Elliott and Mahmood (2007)
free evaporator condensates are effective measures to limit provide a comprehensive review on pretreatment of pulp
the amount of solids in the anaerobic reactor influent. and paper mill biosludge to enhance anaerobic digestibility.
Richardson et al. (1991) were able to operate a bench-scale Most references however, are related to non-mill sludge;
UASB reactor with TSS influent concentrations as high as experimental studies that have actually used biosludge from
2450 mg L
1. Although the accumulation of fines increased the pulp and paper mills are rare. The characteristics of mill bio-
sludge bed volume and deteriorated the sludge settleability, sludge and municipal biosludge for example are considerably
the microbial biomass became adapted to high TSS concen- different, mainly because of the large amount of lignocellu-
trations within the reactor after several months. losic material in the former (Table 4). Not only biosludge but
also primary sludge should be investigated for anaerobic di-
4.9. Future prospect in acclimation, adaptation and gestibility. In Canadian pulp and paper mills the average pri-
bioaugmentation mary sludge to biosludge ratio is estimated to be 70:30 (Elliott
and Mahmood, 2005). However, this ratio can be highly vari-
In the last two decades, owing to advances in “omics” tech- able among the individual mills (Stoica et al., 2009). Some mills
nologies, our understanding of microbial diversity, physiology even produce only waste primary sludge and no waste bio-
and metabolism has exploded (Simon and Daniel, 2011) sludge, and vice versa. In numerous mills primary sludge and/
opening new avenues of research and application using or biosludge is incorporated into the pulp. Also, aerobic sta-
specialized microbes or microbial consortia. A clear example bilization basins often produce only very little biosludge.
is the discovery of anaerobic organohalide respiring bacteria Mills often have several primary settlers and the charac-
that reductively dehalogenate organochlorine contaminants. teristics of the different types of sludge vary largely in terms of
This discovery has remarkably changed perspectives on the anaerobic digestibility and dewaterability. Therefore, it might
toxicity and recalcitrance of organohalide compounds. Orga- me more efficient to pretreat and co-digest difficult to dewater
nohalide respiring bacteria (in particular from the Firmicutes primary sludge together with biosludge, whereas well dew-
and Cloroflexi) naturally associate with acetogens and metha- aterable primary sludge may be used for energy recovery via
nogens in anaerobic communities (Maphosa et al., 2010). Such incineration in biomass boilers.
enrichment cultures are already in use for bioremediation and The case studies described in Section 5 and listed in Table 5
commercial scale anaerobic bioaugmentation of groundwater are mostly related to mesophilic anaerobic digestion. The few
contaminated with chlorinated solvents and other persistent cases involving thermophilic digestion are specifically deno-
€ ffler and Edwards, 2006). From the perspective of
pollutants (Lo ted as such.
pulp and paper wastewater, it has been shown that penta-
chlorophenol can be readily dechlorinated at high concen- 5.1. Primary Sludge
trations (80 mg L
1) in bioaugmented anaerobic reactors
(Tartakovsky et al., 1999; Villemur, 2013). Bioaugmentation is a Among the few experimental studies involving anaerobic
strategy for overcoming upsets or poor performance in digestion of pulp and paper mill sludge, almost all are related
anaerobic reactors (Tale et al., 2011), by adding specialized to biosludge, or sludge mixtures containing minor fractions of
microorganisms to alleviate a particular bottleneck in a primary sludge (Table 5). Only one study was identified that
sequential microbial process. One of the current challenges to has used primary sludge as the only substrate. Bayr and
this approach is that the bioaugmentation inoculum must be Rintala (2012) used semi-continuously fed CSTRs to digest
maintained in a separate reactor amended with the primary sludge and a mixture of primary and biosludge
338 w a t e r r e s e a r c h 6 5 ( 2 0 1 4 ) 3 2 1 e3 4 9

Table 4 e Comparison of municipal and pulp and paper sludge.

Parameter Municipal biosludge Primary pulp & paper sludge Pulp & paper biosludge References
Total dry solids (% TS) 0.8e1.2 1.5e6.5 1.0e2.0 1e3
Volatile solids (% TS) 59e68 51e80 65e97 1, 2
Ash content (% TS) 19e59 20e49 12e41 4, 5
N (% TS) 2.4e5.0 0.1e0.5 3.3e7.7 1, 2, 5
P (% TS) 0.5e0.7 No Data 0.5e2.8 1, 2
pH 6.5e8.0 5e11 6.0e7.6 1, 2, 6
Heating value (MJ/kg e dry basis) 19e23 14e20 22e25 1, 2, 7
Carbohydrates (%VS) 17 No Data 0e23 8, 9
Protein (%VS) 46e52 No Data 22e52 8, 9
Lipids (%TS) 5e12 No Data 2e10 10, 11
Cellulose (%TS) ~1 36e45 19e27 (ref. 5) 12, 5
Lignin (%TS) <0.1 20e24 36e50 12, 5
1
Tchobanoglous et al. (2003); 2Elliott and Mahmood (2007), 3Scott and Smith (1995); 4Khan et al. (1991) 5Migneault et al. (2011); 6Ochoa de Alda
(2008); 7Likon and Trebse (2012); 8Frølund et al. (1996); 9Kyllo
€ nen et al. (1988); 10Pokorna et al. (2009); 11Navia and Mittelbach (2012); 12Zorpas
et al. (2011). The volatile solids content of the primary pulp & paper sludge was calculated from the ash content in ref. 5.

generated in pulp and paper mills. Thermophilic anaerobic methane produced in the reactor was at a maximum. The
digestion of sole primary sludge resulted in higher methane latter exceeded the total amount of methane generated at a
yields (190e240 mL g
1 VS fed) than that of the mixture HRT of 20 days by the 2.5-fold. A potential strategy for large-
(150e170 mL g
1 VS fed), and was feasible at HRTs of 16e30 scale digestion might therefore be to anaerobically digest as
days. The study indicates that by providing pH stability, HRTs much organic matter as possible while maintaining low HRTs.
of 14e16 days and a volatile solids loading rate of 2 g L
1 day
1 The remaining solids fraction of the digestate may subse-
could be possible. The difference between volatile solids quently be pretreated with methods described in Section 5.3,
loading rate and COD based organic loading rate is that the followed by re-injection into the digester, or alternatively,
former involves, besides organic matter, also undefined dewatered and incinerated. In another longer-term study (9
amounts of bound water and losses due to decomposition and months) Karlsson et al. (2011) digested biosludge from a kraft
volatilization of some mineral salts. Volatile solids comprise pulp mill and a TMP pulp mill in two bench-scale reactors
the portion of a sample (dried at 104  C) that is lost in high- (CSTR). The loading rate was varying, however increased from
temperature combustion (500e550  C). Concentrations of approximately 2 to 4 gVS L
1 day
1, and a mean VS reduction
volatile solids in sludge, however, represent organic matter rate of 40% was achieved in both reactors. The specific
concentrations more closely than do COD concentrations. methane yield from the digestion of kraft mill biosludge
Pulp and paper mill biosludge contains between 1.6 and 1.9 g remained relatively constant and on average 120 mL g
1 VS
COD per g volatile solids (Table 5). fed, whereas that of the TMP mill biosludge was approxi-
mately 180 mL g
1 VS fed. In both studies (Puhakka et al., 1992;
Karlsson et al., 2011) the VS loading rate could steadily be
5.2. Anaerobic digestion of biosludge without
increased over the course of several months with a relatively
pretreatment
small impact on the specific methane yield.

A few, mostly batch experiments have been conducted to

anaerobically digest pulp and paper biosludge without pre- 5.3. Sludge pretreatment to enhance anaerobic
treatment. Accordingly, specific methane yields range widely digestibility
between 30 and 200 mL g
1 VS fed, and VS removal rates were
21e55% (Puhakka et al., 1992; Wood et al., 2010; Karlsson et al., In numerous previous studies a variety of pretreatment
2011; Saha et al., 2011; Park et al., 2012; Elliott and Mahmood, methods have been investigated with the goal to enhance the
2012; Bayr et al., 2013) (Table 5). There is no clear recognizable anaerobic digestibility of biosludge. Five approaches having
trend between the digestibility of the sludge, and the type of potential relevance for pulp and paper mill sludge were
mill where the sludge is generated. identified in the literature, and are briefly reviewed below.
Puhakka et al. (1992) conducted a pilot-scale experiment More detailed descriptions of the methods are provided in
with biosludge from a kraft pulp mill for the duration of 21 Elliott and Mahmood (2007).
months. After a startup and acclimation/adaptation period of
50 days, hydraulic retention times where varied between 24 5.3.1. Ultrasound and microwave
and 8 days while mostly maintaining steady operation. Along Sludge disintegration leading to an enhanced anaerobic di-
with a progressively increasing loading rate from 2.2 to 5.2 gestibility can be achieved by means of cavitation triggered by
gVS L
1 day
1, the specific methane yield first increased from high-frequency waves. Whereas ultrasound treatment uti-
130 to 200 mL g
1 VS fed, and then decreased to 100 mL g
1 VS lizes acoustic waves, microwave treatment applies electro-
fed, whereas the VS removal rates ranged between 37 and magnetic waves. In both cases the periodic formation and
55%. At a remarkably low HRT of 8 days, although the specific collapses of small gas bubbles causes strong temperature and
methane yield was at a minimum, the total amount of pressure gradients which in turn ruptures cell walls and
Table 5 e Results from previous experimental studies on anaerobic digestion of pulp and paper mill sludge (all units deviating from those in the first table row are denoted
as such).
Type of sludge COD TS VS VS Detention Type of VS (or VSS, COD) Specific methane Experimental Reference
concentration (or TSS) (or VSS) loading time (DT) pretreatment removal [%] yield [mL g
1 VS setup
[g L
1] [%] [%] rate [days] fed] (or mL g
1
[kg VS m
3 COD fed)
day
1]
Mixture PS/WAS from CTMP 18e53 1.4e4.0 1.3e3.8 2.5 15 No pretreatment 41 (VSS) 90 (mL biogas g¡1 Bench-scale Puhakka et al.
pulp mill (70e90% WAS) VSS fed) (1988)
Kraft pulp mill WAS 13e100 1.6e11 0.9e6.3 1.5e5.2 n.a. No pretreatment 40 123 Pilot-scale Puhakka et al.
(1992)
Mixture municipal sludge, 45 4.4 2.6 1.0 30 No pretreatment 27 185 Bench-scale Jokela et al.
PS and WAS from TMP (1997)
pulp and paper mill
Mixture PS and WAS from n.a. 31 20 Batch 40 (1) Caustic (NaOH, 80 (1) 21e24 (COD) (1) 320 (mL g¡1 VS Bench-scale Lin et al.
pulp and paper mill þ 3% g per 1 kg TS) removed) (2009)

w a t e r r e s e a r c h 6 5 ( 2 0 1 4 ) 3 2 1 e3 4 9
monosodium glutamate
waste liquor
Mixture PS and WAS from n.a. 31 20 Batch 40 (2) Untreated control (2) 21e24 (COD) (2) ~175 (mL g¡1 VS Bench-scale Lin et al.
pulp and paper mill þ 3% removed) (2009)
monosodium glutamate
waste liquor
Sulfite pulp mill WAS 12 0.9 (TSS) 0.7 (VSS) Batch 34 (1) Hydrothermal (170 (1) 65 (VSS) (1) 185 (mL g¡1 COD BMP assay Wood et al.

C, 1 h) fed) (2010)
Sulfite pulp mill WAS 12 0.9 (TSS) 0.7 (VSS) Batch 34 (2) Caustic (NaOH, (2) 62 (VSS) (2) 145 (mL g¡1 COD BMP assay Wood et al.
pH 12, 140  C, 1 h) fed) (2010)
Sulfite pulp mill WAS 12 0.9 (TSS) 0.7 (VSS) Batch 34 (3) Ultrasound (20 (3) 28 (VSS) (3) 120 (mL g¡1 COD BMP assay Wood et al.
kHz, 30 min) fed) (2010)
Sulfite pulp mill WAS 12 0.9 (TSS) 0.7 (VSS) Batch 34 (4) Untreated control n.a. (4) 120 (mL g¡1 COD BMP assay Wood et al.
fed) (2010)
Kraft pulp mill WAS 27 1.8 (TSS) 1.4 (VSS) Batch 34 (1) Hydrothermal (1) 31 (VSS) (1) 115 (mL g¡1 COD BMP assay Wood et al.
(170  C, 1 h) fed) (2010)
Kraft pulp mill WAS 27 1.8 (TSS) 1.4 (VSS) Batch 34 (2) Caustic (NaOH, (2) 28 (VSS) (2) 110 (mL g¡1 COD BMP assay Wood et al.
pH 12, 140  C, 1 h) fed) (2010)
Kraft pulp mill WAS 27 1.8 (TSS) 1.4 (VSS) Batch 34 (3) Ultrasound (20 (3) ~2 (VSS) (3) 40 (mL g¡1 COD BMP assay Wood et al.
kHz, 30 min) fed) (2010)
Kraft pulp mill WAS 27 1.8 (TSS) 1.4 (VSS) Batch 34 (4) Untreated control n.a. (4) 30 (mL g¡1 COD BMP assay Wood et al.
fed) (2010)
WAS from six mechanical & n.a. n.a. n.a. Batch 20 (1) Ultrasound n.a. (1) 96e148 BMP assay Karlsson et al.
chemical pulp and paper (2e30 Wh/L) (2011)
mills
WAS from six mechanical & n.a. n.a. n.a. Batch 20 (2) Enzymatic (mixture n.a. (2) 178 BMP assay Karlsson et al.
chemical pulp and paper of Hydrolases, 40 mg/g (2011)
mills TS)
(continued on next page)

339
 340
Table 5 e (continued )
Type of sludge COD TS VS VS Detention Type of VS (or VSS, COD) Specific methane Experimental Reference
concentration (or TSS) (or VSS) loading time (DT) pretreatment removal [%] yield [mL g
1 VS setup
[g L
1] [%] [%] rate [days] fed] (or mL g
1
[kg VS m
3 COD fed)
day
1]
WAS from six mechanical & n.a. n.a. n.a. Batch 20 (3) Ultrasound n.a. (3) 196 BMP assay Karlsson et al.
chemical pulp and paper (30 Wh/L) þ (2011)
mills Enzymatic
(40 mg/g TS)
WAS from six mechanical & n.a. n.a. n.a. Batch 20 (4) Untreated control n.a. (4) 43e155 BMP assay Karlsson et al.
chemical pulp and paper (2011)
mills
BCTMP pulp mill WAS 34e40 ~2.5 ~1.9 Batch 21 (1) Microwave (1) 23e34 (DT ¼ 43 (1) 50e95 (mL g
1 BMP assay Saha et al.
(50e175  C, 2450 days) COD fed) (2011)
MHz)
(2) 26e30 (DT ¼ 43 (2) 70e90 (mL g
1

w a t e r r e s e a r c h 6 5 ( 2 0 1 4 ) 3 2 1 e3 4 9
BCTMP pulp mill WAS 34e40 ~2.5 ~1.9 Batch 21 (2) Ultrasound (20 BMP assay Saha et al.
kHz, 15e90 min) days) COD fed) (2011)
BCTMP pulp mill WAS 34e40 ~2.5 ~1.9 Batch 21 (3) High-pressure (3) 26 (DT ¼ 43 days) (3) 90 (mL g
1 COD BMP assay Saha et al.
homogenization fed) (2011)
(NaOH 0.1% by
weight, 83 MPa)
BCTMP pulp mill WAS 34e40 ~2.5 ~1.9 Batch 21 (4) Untreated control (4) 23 (DT ¼ 43 days) (4) 50 (mL g
1 COD BMP assay Saha et al.
fed) (2011)
BCTMP pulp mill WAS þ PS 34 2.2 1.8 Batch 21 (1) Microwave (50 (1) 16e24 (DT ¼ 43 (1) 55e75 (mL g
1 BMP assay Saha et al.
(40:60% v/v) e175  C, 2450 MHz) days) COD fed) (2011)
BCTMP pulp mill WAS þ PS 34 2.2 1.8 Batch 21 (2) Ultrasound (20 (2) 15e23 (DT ¼ 43 (2) 60e70 (mL g
1 BMP assay Saha et al.
(40:60% v/v) kHz, 15e90 min) days) COD fed) (2011)
BCTMP pulp mill WAS þ PS 34 2.2 1.8 Batch 21 (4) Untreated control (4) 10 (DT ¼ 43 days) (4) 55 (mL g
1 COD BMP assay Saha et al.
(40:60% v/v) fed) (2011)
WAS from BCTMP pulp and n.a. 31 19 4.5 n.a. No pretreatment 57 270 Bench-scale Lin et al.
paper mill þ 10% (2013)
monosodium glutamate
waste liquor
Kraft pulp and paper mill PS 1.4e1.8 (soluble 2.7e3.8 2.2e3.2 1.0e1.4 16e32 No pretreatment 25e40 190e240 Bench-scale Bayr and
COD) (thermophilic) Rintala (2012)
Mixture kraft pulp and n.a. n.a. n.a. 1.0 25e31 No pretreatment 29e32 150e170 Bench-scale Bayr and
paper mill PS þ WAS (VS (thermophilic) Rintala (2012)
ratio 3:2)
WAS from BCTMP/TMP pulp 30 2.5 1.9 Batch 28 (1) Combined caustic (1) 30 (1) 67 BMP assay Park et al.
mill (raw) (NaOH at 0.21e0.26 (2012)
g/g TS) and Ultrasound
(40 kHz)
WAS from BCTMP/TMP pulp 30 2.5 1.9 Batch 28 (2) Untreated control (2) 21 (2) 85 BMP assay Park et al.
mill (raw) (2012)
WAS from BCTMP/TMP pulp 88 6.5 5.5 Batch 28 (1) Combined caustic (1) 27 (1) 96 BMP assay Park et al.
mill (thickened) (NaOH at 0.21e0.26 g/ (2012)
g TS) and Ultrasound
(40 kHz)
WAS from BCTMP/TMP pulp 88 6.5 5.5 Batch 28 (2) Untreated control (2) 23 (2) 88 BMP assay Park et al.
mill (thickened) (2012)
WAS from mechanical pulp ~47 3.1 (TSS) 2.8 (VSS) 1.4 (kg VSS m¡3 20 (1) Mechanical shear (1) 32 (VSS) (1) 73 (mL g
1 COD Bench-scale Elliott and
mill day¡1) (high-shear mixing at fed) reactor Mahmood
1500 rpm) (2012)
WAS from mechanical pulp ~47 3.1 (TSS) 2.8 (VSS) 1.4 (kg VSS m¡3 20 (2) Ultrasound (20 (2) 39 (VSS) (2) 90 (mL g
1 COD Bench-scale Elliott and
mill day¡1) kHz) fed) reactor Mahmood
(2012)
WAS from mechanical pulp ~47 3.1 (TSS) 2.8 (VSS) 1.4 (kg VSS m¡3 20 (3) High-pressure (3) 58 (VSS) (3) 91 (mL g
1 COD Bench-scale Elliott and
mill day¡1) homogenization fed) reactor Mahmood
(NaOH 0.1% by (2012)
weight, 83 MPa)
WAS from mechanical pulp ~47 3.1 (TSS) 2.8 (VSS) 1.4 (kg VSS m¡3 20 (4) Untreated control (4) 29 (VSS) (4) 77 (mL g
1 COD Bench-scale Elliott and
mill day¡1) fed) reactor Mahmood

w a t e r r e s e a r c h 6 5 ( 2 0 1 4 ) 3 2 1 e3 4 9
(2012)
Mixture WAS from TMP/ n.a. 5.1 3.5 Batch 19 No pretreatment 50 84 Batch Hagelqvist
CTMP pulp mill þ (2013a)
municipal WAS (VS ratio
1:1)
Pulp and paper mill WAS 1 (soluble COD) 4.7 3.9 Batch 20e23 (1) Hydrothermal (150 n.a. (1) 97 BMP assay Bayr et al.

C, 10 min) (thermophilic) (2013)
Pulp and paper mill WAS 1 (soluble COD) 4.7 3.9 Batch 20e23 (2) Enzymes (mixture n.a. (2) 66 BMP assay Bayr et al.
of Accelerases, 70 mg/ (thermophilic) (2013)
gVS)
Pulp and paper mill WAS 1 (soluble COD) 4.7 3.9 Batch 20e23 (3) Ultrasound (45 n.a. (3) 68 BMP assay Bayr et al.
kHz, 30 min) (thermophilic) (2013)
Pulp and paper mill WAS 1 (soluble COD) 4.7 3.9 Batch 20e23 (4) Caustic (NaOH, pH n.a. (4) 11 BMP assay Bayr et al.
12) (thermophilic) (2013)
Pulp and paper mill WAS 1 (soluble COD) 4.7 3.9 Batch 20e23 (5) Acid (HNO3, pH 3) n.a. (5)
3 BMP assay Bayr et al.
(thermophilic) (2013)
Pulp and paper mill WAS 1 (soluble COD) 4.7 3.9 Batch 20e23 (6) Untreated control n.a. (6) 67 BMP assay Bayr et al.
(thermophilic) (2013)

341
342 w a t e r r e s e a r c h 6 5 ( 2 0 1 4 ) 3 2 1 e3 4 9

releases intercellular matter. The latter increases the amount (Onyeche, 2004). The latter approach builds the basis for the
of soluble COD for subsequent anaerobic digestion. A few commercialized high-pressure (~83 MPa) homogenization
studies have been investigating the effect of ultrasound on system MicroSludge (Stephenson et al., 2005). The method
pulp and paper mill biosludge (Wood et al., 2010; Park et al., involves the addition of NaOH at 0.1% by weight to weaken the
2004; Elliott and Mahmood, 2012; Saha et al., 2011) and one cell membranes and decrease the viscosity, followed by cell
study was identified that applied microwave pretreatment disruption within the homogenizer (Elliott and Mahmood,
(Saha et al., 2011). Ultrasound pretreatment increased the 2012). Two experimental studies have investigated the effect
specific methane yield by up to 80%, and microwave pre- of high-pressure homogenization on mill biosludge di-
treatment by up to 90% (Saha et al., 2011) (Table 5). The largest gestibility. Saha et al. (2011) observed an improvement in
improvements were achieved in those cases where the non- specific methane yield by 80% after 21 days of digestion. Elliott
pretreated biosludge exhibited a relatively low anaerobic and Mahmood (2012) conducted bench-scale experiments
digestibility. with a continuously fed reactor. By using this method, the
methane yield at an HRT of only 3 days was nearly as high as
5.3.2. Thermal at an HRT of 20 days when digesting untreated biosludge.
Thermal pretreatment involves temperatures usually ranging
between 150  C and 200  C, and in some cases additional alkali 5.3.4. Chemical
treatment, leading to cell lysis and an increase in soluble COD The most commonly investigated chemical method is alkaline
(Elliott and Mahmood, 2007). Wood et al. (2010) conducted a (caustic) treatment, which has also often been used in com-
BMP assay with biosludge from a kraft pulp mill and a sulfite bination with other forms of pretreatment. Results of previous
pulp mill, and compared the effectiveness of three different studies are conflicting. In the study by Wood et al. (2010)
pretreatment methods (thermal, caustic, and sonication). alkaline pretreatment was found to be almost as effective as
Thermal treatment at 170  C appeared to be the most effective thermal pretreatment with a 20% and 270% increase in spe-
with a 55% and 280% increase in specific methane production cific methane yield compared to untreated mill sludge. How-
for kraft mill sludge and sulfite mill sludge, respectively. In ever, in another study the methane yield decreased by 80% as
another study where thermal pretreatment was applied at a a result of caustic pretreatment (Bayr et al., 2013) (Table 5). The
lower temperature (150  C) and followed by thermophilic reason of this large discrepancy is not clear. During trial
anaerobic digestion, an increase in specific methane yield of operation of a full-scale anaerobic reactor in a Swedish sulfite
45% was achieved (Bayr et al., 2013) (Table 5). Again, the largest pulp mill, alkaline pretreatment led to solubilisation rates of
improvements in terms of methane yield were accomplished 70e75% of the suspended solids (Dalentoft and Jo € nsson, 1994).
with difficult-to-digest biosludge. Acidification of sludge to enhance digestibility has been
There is currently only one full-scale anaerobic reactor that previously suggested, however this method seems impractical
was found to be treating mill biosludge (Kepp et al., 2000). This because of the necessary high acid usage (Elliott and
4,000 m3 large digester at a Norwegian pulp mill processes Mahmood, 2007). In other studies attempts have been made
about 4000 dry tones of biosludge per year (Panter and Kleiven, to solubilise the COD of municipal biosludge using highly
2005) while producing biogas with an energy content of 108 TJ oxidative conditions using ozone (Elliott and Mahmood, 2007).
(30 GWh) per year. Pretreatment of the thickened sludge Ozone application is likely too cost intensive in relation to the
(3e6% TS) is achieved using the Cambi Thermal Hydrolysis achieved improvement in anaerobic digestibility of mill
process where surplus pressurized steam is used to disinte- biosludge.
grate the sludge at a temperature of 165  C (Panter and
Kleiven, 2005). Using the above information and assuming 5.3.5. Biological
an average TS content of 4.5%, the calculated hydraulic Studies on enzyme pretreatment of biosludge from municipal
retention time would average at 16 days. Prior to using the wastewater treatment plants have shown notable improve-
Cambi process, the mill's biosludge was pretreated by means ments in anaerobic digestibility (Parawira, 2012 and refs
of alkaline hydrolysis with sodium hydroxide, which was therein). However, previous attempts with pulp and paper mill
more costly than thermal pretreatment (Cambi et al., 2013). biosludge were not very successful. While conducting batch
Thermal pretreatment may open up an opportunity specif- experiments, Karlsson et al. (2011) added a mixture of various
ically for pulp and paper mills in order to make anaerobic hydrolases at a concentration of 40 mg gTS
1 to pulp mill
digestion economically viable. Often mills have surplus steam biosludge, which led to an increase in specific methane yield
or excess heat available that, instead of venting or releasing it, by 35%. However, by digesting mill biosludge within a semi-
may be used for thermal sludge treatment. An additional continuously fed bench-scale reactor at enzyme concentra-
positive side-effect of thermal pretreatment is an improve- tions of up to 80 mg gTS
1, the methane yield did not improve
ment in dewaterability of the digestate (Panter and Kleiven, at all. Karlsson et al. (2011) assume that this may have been
2005). caused by an unfavorable sludge viscosity. Bayr et al. (2013)
conducted a batch experiment and added a commercially
5.3.3. Hydrodynamic available mixture of accelerases (70 mg gVS
1) to pulp and
Mechanical treatment in order to rupture cell walls and in- paper mill biosludge. Subsequent anaerobic digestion did not
crease the soluble COD content of the sludge has been applied lead to any improvement. Nevertheless, enzymatic pretreat-
in a variety of ways, such as forcing the sludge under high ment may still have the potential to increase the digestibility
pressure onto a collision plate (Nah et al., 2000), or through an of mill sludge. The range of enzymes and enzyme cocktails
impact ring leading to sudden pressure and velocity changes that may be applied is vast, and the reported anaerobic
 w a t e r r e s e a r c h 6 5 ( 2 0 1 4 ) 3 2 1 e3 4 9 343

digestibility of biosludge from the municipal sector varies be further processed to become high value fertilizer. While the
widely depending on the applied enzymes (Parawira, 2012 and digestate as-is has already a higher agronomic value than
refs therein). animal slurry due to the higher proportion of mineral nitrogen
In the past, the development of efficient enzymatic and less decomposable matter, it still has some undesirable
sludge treatment methods was slowed by high production characteristics mainly related to its odor and biological
costs and the limited repertoire of the available enzymes instability (Bustamante et al., 2012). The most promising
and their low activities. The enzyme discovery and produc- method to upgrade digestate in order to obtain valuable end
tion costs are expected to drop as more enzymes are products for agricultural use may be composting (Rehl and
discovered with the extraordinary advances in DNA- Müller, 2011; Bustamante et al., 2012).
sequencing-based technologies, which may lead to an Potential problems related to land application of mill
upswing in this field of research. sludge may involve heavy metals. Although heavy metal
concentrations in pulp and paper mill sludge is usually low, it
has been shown that land application of biosludge can occa-
6. Biorefinery concepts involving anaerobic sionally exceed permissible levels within the underlying soils
digestion (Rashid et al., 2006).

Future pulp and paper mills are poised to become integrated 6.3. Co-digestion of mill sludge with organic waste from
biorefineries where paper production is only one part of the outside the mill
product line. Anaerobic transformation of lignocellulosic
material may play a large role to yield marketable products Difficult to digest sludge from pulp and paper mills may be
beyond methane, such as ethanol and volatile fatty acids digested together with more easily digestible waste from
(VFAs). In the long run, even carbon dioxide may undergo a outside the mill. In an attempt to co-digest pulp and paper mill
transition from waste to resource, as it might be used in the biosludge with municipal biosludge, Hagelqvist (2013a) pro-
future to replace carbon sources from oil, while becoming a gressively increased the fraction of mill sludge that is added to
major chemical feedstock for a carbon dioxide based economy a municipal sludge digester. At an HRT of 19 days, up to 50% of
(Aresta, 2010). The following is a list of concepts that are the municipal sludge could be replaced with mill biosludge
already practicable or may be so in the near future, for adding without diminishing the specific methane yield. Other options
value to pulp and paper mill wastes where anaerobic digestion include co-digestion of mill biosludge with manure and grass
plays a large role. silage (Hagelqvist, 2013b). During a batch experiment the
addition of only 10% food waste to municipal biosludge (based
6.1. Ethanol production and anaerobic digestion of on COD amounts) led to an increase in specific methane yield
stillage (amount of methane produced per amount COD added) by
55%. This was attributed to the synergistic enhancement of
The spent liquor from chemical pulping contains cellulose and enzyme activities in the presence of small amounts of food
hemicelluloses that can be saccharified and subsequently waste (Yun et al., 2013).
anaerobically fermented to ethanol. In mills where this is being Lin et al. (2011) investigated the possibility of co-digesting
practiced, the ethanol plant may be integrated into the pulp mill biosludge with a minor fraction of monosodium
wastewater treatment plant, where evaporator residue com- glutamate waste liquor using a CSTR reactor. The volatile
ponents can be used as substrates for ethanol fermentation. solids loading rate was progressively increased from 1.5 to 5 g
Whole stillage, which is the residue after distilling off the volatile solids∙L
1 day
1, while the SRT was decreased from 29
ethanol, is commonly separated into a solid portion (distiller's to 9 days, until reactor failure occurred. Maximum methane
dry grain) and a liquid portion (thin stillage), and further pro- yield attained was 245 mL g
1 volatile solids fed with a VS
cessed to become livestock feed. About 20 L of whole stillage removal efficiency of approximately 57%.
with COD contents of 100 g L
1 are generated per liter ethanol Co-digestion of mill biosludge with other substrates has
produced (Wilkie et al., 2000), and subsequent drying and only just started to be explored, yet the results of the first
evaporation of stillage constitutes one of the major expenses in studies are promising. The possibility of co-digestion on a full-
a bioethanol plant (Drosg et al., 2013). A more economic and scale has to be evaluated on a case-by-case basis by consid-
viable solution may be anaerobic digestion of thin stillage or ering criteria such as substrate availability and transport
whole stillage (Eskicioglu et al., 2011; Drosg et al., 2013). The costs. Co-location of different production facilities is recom-
possible problem related to the high ammonia content of stil- mended to take advantage of such synergies.
lage may be addressed by means of co-digestion (Drosg et al.,
2013) with other in-mill streams that are deficient in the 6.4. Combined generation of hydrogen and methane
macro-nutrient nitrogen. An additional benefit would there-
fore be to save costs for urea or ammonia, which is commonly Since hydrogen has a notably higher market value than
added to anaerobic digesters in pulp and paper mills. methane, mill derived organic waste may be anaerobically
digested in two stages, while producing hydrogen and
6.2. Agronomic use of digestate methane simultaneously (Ueno et al., 2007; Gavala et al., 2005).
The generation of hydrogen, instead of methane, can be
If it will become economically feasible to anaerobically digest enforced by means of heat treatment or operation at a lower
mill-related sludge on a full-scale, the resulting digestate may pH in order to suppress methanogenesis (Pachiega et al., 2013).
344 w a t e r r e s e a r c h 6 5 ( 2 0 1 4 ) 3 2 1 e3 4 9

In an attempt to anaerobically digest substrate rich in ligno- Anaerobic inhibitors such as resin acids, sulfur and
cellulosic material, the overall energy yield increased by 38%, organochlorine compounds can be present in elevated con-
when, instead of a one-stage biogas reactor, a two-stage centrations which may deteriorate the reactor performance.
hydrogen-biogas system was used. In both cases an HRT of Whereas mechanical pulping streams tend to contain rela-
20 days was maintained (Massanet-Nicolau et al., 2013). In tively high concentrations of resin acids (10 and
another study, Lin et al. (2013a) produced hydrogen and 10,000 mg L
1), those in chemical pulping streams are often
methane in a two-stage process while using a mixture of lower (<0.05e1000 mg L
1). Primary inhibitors in chemical
primary sludge and biosludge from a pulp mill as substrate. pulping effluents are sulfur compounds with reported sulfide
While maintaining mesophilic conditions and pH values of concentrations up to 700 mg L
1 and sulfite concentrations as
4.8e6.4 in the hydrogen producing reactor, and thermophilic high as 4800 mg L
1. Primary inhibitors in bleaching effluents
conditions and pH values of 6.5e8.8 in the methane producing from chemical pulping are organochlorine compounds. Re-
reactor, sCOD removal rates of 71%e87% were achieved. ported concentrations of AOX are between 3 and 200 mg L
1.
Strategies to address elevated concentrations of anaerobic
6.5. Other bio-based products as a result of anaerobic inhibitors in the wastewater include co-digestion, microbial
digestion acclimation/adaptation, dilution, stripping of sulfur com-
pounds, and/or upfront solids removal.
Methane may not always be the most favorable end product of Microbial adaptation and diversification through co-
anaerobic digestion, and different product routes from waste digestion can develop a microbial population that is more
to resource are being researched. Since methanogens have a dynamic and resilient to adverse conditions such as toxic
very low growth rate and susceptible energy metabolism they shock loads or feed stress. Feed stress associated with large
may be selectively washed out by e.g. operating a reactor with day-to-day variations in substrate composition may also be
a low sludge retention time and/or a low pH (Angenent et al., addressed by slow adaptation and/or by adding a large
2004). In this way a product spectrum can be obtained which is equalization basin prior to anaerobic treatment. Smaller seed
mainly composed of volatile fatty acids and short alcohols reactors that are provided more constant feed may also pro-
(Tamis et al., 2013). Volatile fatty acids are potentially more vide inoculum to boost main reactors. Future research should
valuable than methane, and can be used as a platform for the involve co-digestion of a variety of in-mill streams over pro-
production of a variety of bio-based products such as bio- longed periods of times. Those tests may include targeted
plastics (polyhydroxyalkanoates or PHAs), medium chain exposures to mill related toxicants in order to entice growth of
length fatty acids (MCFA), and methyl esters. PHAs and MCFAs populations that are more resilient to future shock loads. Also,
can then be used as building blocks for the chemical industry future research is warranted regarding the links between
(Tamis et al., 2013). The main bottleneck of this approach is microbial community dynamics and reactor operation per-
related to the difficulty to efficiently recover the VFAs from the formance. Modern methods to characterize microbial pop-
fermentation broth (Singhania et al., 2013). One of the more ulations are becoming more comprehensive, detailed and
promising future recovery methods may involve membrane affordable and may be used to develop benchmarks that
filtration in the context of a submerged anaerobic membrane provide valuable information about the state of the biomass
reactor. within an anaerobic reactor.
Although anaerobic digestion of sludge has been widely
researched, few experimental studies have been conducted
7. Conclusions with respect to pulp and paper mill sludge. Of those, most are
related to biosludge, although in average a larger fraction of
Anaerobic digestion of pulp and paper mill waste at full-scale sludge generated in mills consists of primary sludge. Future
is currently confined to the treatment of a few selected types research should be aimed at anaerobic digestion of biosludge,
of effluent, and the resulting biogas is commonly burned to primary sludge and mixtures thereof.
produce steam and electricity. Full-scale digesters for mill- In previous studies that have investigated anaerobic
derived sludges (biosolids) are almost non-existent. digestion of non-pretreated mill biosludge, VS removal rates
Aside from bleaching effluents and a few other streams, were between 21 and 55%, and methane yields between 40 and
COD removal rates of the majority of pulp and paper mill 200 mL gVS
1 fed. It is commonly agreed upon that mill sludge
streams are well above 50%, and methane yields usually range requires some form of pretreatment prior to anaerobic
between 0.20 and 0.40 m3 kg
1 COD removed. digestion. Studies on pretreatment refer to maximum VS
Contrary to common perception, most in-mill effluents can removal rates of 65%, and increases in specific methane yield
be at least a component of the reactor feed in anaerobic between 0 and 90%. However, in almost all previous studies
treatment. Even alkaline extraction and chlorine dioxide the specific methane yield of pretreated biosludge did not
bleaching effluents, which are commonly perceived as being exceed 200 mL gVS
1 added, which is the same as the highest
toxic to anaerobic biomass, may be in parts anaerobically yield reported for non-pretreated biosludge.
treated when diluted or combined with other less toxic There is a clear trend indicating that the more a pretreat-
streams. Also, microbial adaptation over the course of several ment method improves the anaerobic digestibility, the higher
months can increase the digestibility of those streams. Re- are also the treatment costs. Highest methane yield increases
ported COD removal rates of bleaching effluents range be- were reported for thermal treatment, microwave treatment
tween 15 and 90% depending on the degree of dilution and the and high-pressure homogenization. Because thermal and
experimental setting. microwave pretreatment is being done at elevated
 w a t e r r e s e a r c h 6 5 ( 2 0 1 4 ) 3 2 1 e3 4 9 345

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