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Thermal Pretreatment of Sewage Sludge
to Enhance Anaerobic Digestion: A
Review
Sridhar Pillia, Song Yana, R. D. Tyagia & R. Y. Surampallib
a Institut National de la Recherche Scientifique - Eau, Terre et
Environnement (INRS-ETE), Québec, Canada
b Department of Civil Engineering, University of Nebraska-Lincoln,
Lincoln, Nebraska, USA
Accepted author version posted online: 31 Jul 2014.Published
online: 22 Dec 2014.
To cite this article: Sridhar Pilli, Song Yan, R. D. Tyagi & R. Y. Surampalli (2015) Thermal
Pretreatment of Sewage Sludge to Enhance Anaerobic Digestion: A Review, Critical Reviews in
Environmental Science and Technology, 45:6, 669-702, DOI: 10.1080/10643389.2013.876527
To link to this article: http://dx.doi.org/10.1080/10643389.2013.876527
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Critical Reviews in Environmental Science and Technology, 45:669–702, 2015
Copyright © Taylor & Francis Group, LLC
ISSN: 1064-3389 print / 1547-6537 online
DOI: 10.1080/10643389.2013.876527
Thermal Pretreatment of Sewage Sludge to
Enhance Anaerobic Digestion: A Review
SRIDHAR PILLI,1SONG YAN,1R. D. TYAGI,1and R. Y. SURAMPALLI2
1Institut National de la Recherche Scientifique - Eau, Terre et Environnement (INRS-ETE),
Qu´
ebec, Canada
2Department of Civil Engineering, University of Nebraska-Lincoln, Lincoln, Nebraska, USA
This review summarizes the effect of high- and low-temperature
thermal pretreatment (TPT) on sludge dewaterability, anaerobic
digestion (AD), and biogas production efficiencies. The AD of the
TPT sludge has demonstrated the following observations: reduced
sludge retention time, increased biogas generation, higher organics
degradation, improved dewaterability, and lower digester volume
than the conventional AD of sludge. Energy balance of the TPT pro-
cess was energetically self-sustainable and produced surplus energy
at solids concentration greater than 3%. Net energy and energy ra-
tio of TPT and AD revealed that net energy and energy ratio are
increased with increase in total solids concentration.
KEY WORDS: anaerobic digestion, biological wastewater treat-
ment, thermal pretreatment (TPT), waste-activated sludge
1. INTRODUCTION
An unmanageable quantity of sludge is generated in the 21st century from
wastewater treatment plants (WWTPs) due to high water demand by an in-
creasing population, industrialization, and urbanization, and also due to the
higher level of wastewater treatment. In Canada, in the province of Quebec,
230,000 mega grams per year (Mg/yr) of dry sludge is generated from the
existing 700 municipal WWTPs (Perron and Hebert, 2007; LeBlanc et al.,
Address correspondence to R. D. Tyagi, Institut National de la Recherche Scientifique -
Eau, Terre et Environnement (INRS-ETE), 490, rue de la Couronne, Qu´
ebec, Canada, G1K
9A9. E-mail: tyagi@ete.inrs.ca
Color versions of one or more of the figures in the article can be found online at
www.tandfonline.com/best.
669
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670 S. Pilli et al.
2009) and the production rate is expected to increase in the future. Sludge
management (treatment and disposal) is responsible for 60% of the total
WWTP operation cost (Pilli et al., 2011). Sludge is also a major problem
for environmental engineers as it is a heterogeneous medium largely con-
sisting of water (>90%) and solids (<10%). Furthermore, sludge disposal
laws are becoming increasingly strict. The common sludge disposal tech-
niques such as incineration, landfill, and land application used over the
years are neither economical nor sustainable, and generate huge amounts
of greenhouse gases (GHGs). Therefore, sludge generators are constrained
to reevaluate their sludge management strategies, resulting in a need to
achieve cost-effective and sustainable technologies for sludge treatment and
disposal. From the extensive research on sludge pretreatment technologies,
many researchers concluded that anaerobic digestion (AD) is efficient and a
sustainable technology for sludge treatment/disposal (Pilli et al., 2011). The
benefits associated with AD technology are high, which include mass reduc-
tion, odor removal, pathogen reduction, less energy use, flexibility toward
waste composition, and, more significantly, the energy recovery in the form
of biogas (Metcalf and Eddy, 2003; Appels et al., 2008, 2010).
AD of the sludge is a microbiological process that converts degradable
organic compounds to methane (CH4) and carbon dioxide (CO2)intheab-
sence of elemental oxygen. Mainly, the process occurs in four stages as
described in Appels et al. (2008). The microbiological conversion of sludge
to CH4and CO2by three groups of microorganisms is a slow process and
requires high retention time and larger digester volume. In particular, solu-
bilization of intracellular biopolymers and their conversion to lower molec-
ular weight compounds of degradable organics through hydrolysis is a rate-
limiting step in the AD process (Appels et al., 2008; Pilli et al., 2011). The
lower digestion rate (i.e., first-order digestion rate constant of 0.15 day−1for
sludge, Shimizu et al., 1993) due to nonavailability of the readily biodegrad-
able organic matter for AD necessitates pretreatment of the sludge. Pretreat-
ment of sludge enhances the biodegradable organic carbon by rupturing
the cell wall and releasing the intercellular matter in aqueous phase, which
enhances the digestion rates, reduces the retention time, and increases the
biogas production (Khanal et al., 2007; Pilli et al., 2011). Various pretreatment
techniques such as thermal, chemical, mechanical, biological, physical, and
several combinations such as thermochemical, physicochemical, biological-
physicochemical, and mechanical–chemical have been studied by several
researchers. However, economic constraints have limited the scale-up and
commercialization of these technologies.
To establish an economically feasible pretreatment technology for en-
hancing the sludge degradability, extensive research has been carried out
around the world. The thermal pretreatment (TPT) technology is a well-
established and commercially implemented technology. The several inher-
ent merits of TPT technology are higher volatile solids (VS) reduction (235%
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Thermal Pretreatment of Sewage Sludge 671
>control) during AD or enhanced biodegradability (Jeong et al., 2007;
Bougrier et al., 2008; Carr`
ere et al., 2008), increase in dewaterability (60–80%
improvement, Odeby et al., 1996), 60% increase in methane production
(Valo et al., 2004; Bougrier et al., 2008; Carr`
ere et al., 2008), destruction
of pathogens (Potts, 2007), the most effective treatment as per energy con-
siderations (Kepp et al., 2000; Perez-Elvira et al., 2008), reduces the auxiliary
fuel consumption for sludge dewatering, low transportation due to lower
residual sludge after AD, and produces class A biosolids (Climent et al 2007;
Camacho et al., 2008). Owing to inherent advantages and commercial appli-
cability of sludge TPT, this article attempted to present a systematic review
of the extensive research on TPT of sludge and consequent impact on AD
process including biogas generation and energy balance of the process (TPT
+AD).
2. TPT AT DIFFERENT TEMPERATURES
TPT (heat pretreatment) is a process where the temperature of sludge is
raised to a desired temperature to significantly increase the disintegration
and solubilization of sludge solids. Thermal treatment of sludge is usually
used for improving dewaterability; furthermore, thermal treatment is consid-
ered as a pretreatment process to enhance biogas production during the AD
process. The thermal energy required to raise the temperature of the sludge
is generally achieved through direct injection of steam or passing the steam
through heat exchangers. The hot pretreated sludge is cooled to anaerobic
digester temperature and the heat is recovered during the cooling process,
which can be used to preheat the feed (fresh) sludge. The heat recovery sig-
nificantly increases the energy efficiency and significantly reduces the cost
of pretreatment. Most of the research on pretreatment has been carried out
using a wide range of temperatures from 60 to 270◦C (Climent et al., 2007).
Based on the hydrolysis temperature, TPT has been classified into two differ-
ent categories, that is, the hydrolysis (pretreatment) temperature above 100◦C
(considered as high temperature TPT) and the temperature applied below
100◦C (considered as low-temperature TPT) (Climent et al., 2007). The effect
of thermal hydrolysis on AD of the pretreated sludge at temperatures greater
than 100◦C was reviewed by Carr`
ere et al. (2010) (i.e., pretreatment methods
to improve sludge anaerobic degradability), but the effect of TPT on the phys-
ical, chemical, and biological properties, and energy balance of the process
(integrated TPT and AD) are not considered. Therefore, detailed research
findings on these aspects of sludge TPT at different temperatures (high tem-
perature and low temperature) are presented and discussed in this review.
2.1. High-Temperature TPT Process
During the 1960s and the 1970s, sludge heat treatment was the major em-
phasis. Porteus and Zimpro, two main processes operating at temperatures
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672 S. Pilli et al.
typically between 200 and 250◦C, were installed (Camacho et al., 2008). The
problems encountered in these processes include odor generation, high-
strength liquors production, and corrosion in heat exchangers. Porteus and
Zimpro projects were closed during the late 1960s and/or the early 1970s.
With modified operating conditions and pretreatment at lower temperature,
the Zimpro process is still in use to enhance the dewaterability of sludge
(Camacho et al., 2008). Furthermore, various combinations of thermal hy-
drolysis with acid- and alkaline-based technologies emerged during the mid-
1980s to produce pasteurized sludge, but none of these processes were com-
mercialized because they were not economical and could not improve the
biodegradability. Synox and Protox are the best illustrations of the combined
thermal process to improve the dewaterability, which are not successfully
commercialized (Neyens and Baeyens, 2003).
Furthermore, the CAMBI process was developed for efficient thermal
hydrolysis during the early 1990s. Veolia Water Solution and Technologies
developed thermal hydrolysis processes known as BIOTHELYS R, to enhance
the biodegradability and dewaterability of the sludge. BIOTHELYS Rprocess
is the combination of thermal hydrolysis (ThelysTM hydrolysis process) and
AD. Both CAMBI and BIOTHELYS Rare commercial high-temperature TPT
processes. The process configurations of both models, which are widely
established, are explained below.
2.1.1. CAMBI THERMAL HYDROLYSIS PROCESS
CAMBI thermal hydrolysis is a three-step process and mainly consists of three
units. The first unit is a preheated tank, which eliminates the problems of
pumping under pressure and corrosion in heat exchangers. The second unit
is a steam reactor, where steam is introduced under pressure and the third
unit is a flash tank, where the reactor pressure is rapidly released (Figure 1).
The sludge is predewatered to around 14–18% dry solids (DS) content before
feeding into the preheated tank. The predewatered sludge is continuously
fed into the preheated tank where the temperature of the sludge is raised to
nearly 100◦C. Temperature in the preheated tank is maintained by directing
the return steam from the second reactor and the flash tank. In the steam
reactor, the sludge is heated by direct steam addition for 20–30 min (steam
pressure 12 bars) in batch mode to achieve a temperature of 150–160◦C
and a pressure of about 8–9 bars. The pressure in the reactor is reduced
to 2 bars before discharging the heated sludge into the flash tank and the
released steam is recirculated to the preheated tank. Releasing the pressure
rapidly from the steam reactor and flashing the sludge to the flash tank
(third unit) enable the cells to rupture. The sludge in the flash tank cools
down to about 100◦C. Furthermore, the hydrolyzed sludge is cooled down to
anaerobic digester temperature (35◦C). Technical information of the CAMBI
process is available in the European patent number EP0784504 (method for
hydrolysis of organic materials). The advantages of CAMBI process are the
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Thermal Pretreatment of Sewage Sludge 673
FIGURE 1. Thermal hydrolysis process flow diagram (Cambi Process).
sludge retention time in the anaerobic digester is reduced from 15–30 days
(conventional) to 10–12 days, 50% digester volume is saved in comparison
with conventional digestion, high biogas production, improved dewaterabil-
ity (30–40% DS content can be achieved), and pasteurized sludge (ready to
use in agriculture) (Camacho et al., 2008). In addition, the process was suc-
cessful in eliminating corrosion, scaling problems, and difficult-to-degrade
filtrate chemical oxygen demand (COD). The performance of CAMBI pro-
cess installed at various WWTPs is given on the web site http://www.cambi.
no.
2.1.2. BIOTHELYS RPROCESS
The thermal hydrolysis (ThelysTM hydrolysis process) followed by anaer-
obic biological treatment is the BIOTHELYS R, which is the result of a
decade’s development work by Veolia Water Solutions and Technologies.
The BIOTHELYS Rprocess configuration is shown in Figure 2. The main
objective of this process is to enhance the biodegradability of sludge by
solubilizing the organic matter, and enhancing the biogas production in the
anaerobic digester. The process mainly contains a hydrolysis reactor where
the dewatered sludge (15–16% DS) is added into the hydrolysis reactor. The
sludge is heated through direct steam injection for 30–60 min, the temper-
ature inside the reactor reaches around 150–180◦C. The hydrolyzed sludge
is withdrawn using the residual pressure in the reactor and is cooled down
to anaerobic digester temperature (35◦C). To recover the energy, two or
more batch hydrolysis reactors are parallelly operated in batch mode; the
released flash steam is used for preheating another hydrolysis reactor. The
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674 S. Pilli et al.
FIGURE 2. Thermal hydrolysis process flow diagram (BIOTHELYS R).
hydrolyzed sludge is less viscous and easier to pump with enhanced de-
waterability. The advantages of the BIOTHELYS Rprocess include: the AD
of the hydrolyzed sludge reduces the quantity of sludge up to 80% higher
as compared to conventional AD (without hydrolysis), increases biogas pro-
duction, decreases the retention time, requires low digester volume, can be
operated continuously, and is a secure process (http://www.veoliawaterst.
com/biothelys/en/applications.htm).
2.2. Low-Temperature TPT Process
Studies on low-temperature thermal sludge pretreatments are very limited.
To overcome the drawbacks of high energy requirements, toxic refractory
compounds formation (Amadori and melanoidins compounds), and low
biodegradability of sludge at high (>180◦C) thermal treatments, Wang et al.
(1997b) concluded that TPT of the sludge at lower temperature (60–100◦C)
enhances organic matter degradation and methane production significantly
during AD. The treatment time at lower temperature lasted from hours to
several days (Gavala et al., 2003; Ferrer et al., 2008; Lu et al., 2008; Borges
and Chernicharo, 2009; Nges and Liu, 2009; Appels et al., 2010).
3. EFFECT OF TPT ON SLUDGE CHARACTERISTICS
During TPT, sludge particles and cells are subjected to rapid increase in tem-
perature, which causes them to rupture and release the water-soluble cell
components. At high temperature, the released soluble organic contents will
undergo chemical and physical reactions. Therefore, many researchers eval-
uated the effect of TPT with respect to the changes in physical (particle size
distribution, turbidity, settleability, mass composition, and microscopic ex-
amination), chemical (increase in soluble chemical oxygen demand (SCOD),
nitrate nitrogen, NH3, protein, polysaccharides content of the supernatant),
and biological (destruction of pathogens, heterotrophic count, and change
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Thermal Pretreatment of Sewage Sludge 675
in specific oxygen uptake rate) properties. These changes substantially in-
fluence the AD, methane production during AD, and therefore are discussed
in this section.
3.1. Effect of High-Temperature TPT on Sludge Characteristics
3.1.1. PHYSICAL CHANGES
Physical changes during TPT of sludge can be used as a qualitative measure-
ment for evaluating the efficiency of thermal hydrolysis. Sludge dewater-
ability, sludge settleability, mass composition, and turbidity are some of the
techniques used to evaluate the effect of thermal hydrolysis. The extracellu-
lar polymers (ECP) are considered to influence the dewaterability of sludge
(Houghton et al., 2000, 2001; Neyens and Baeyens, 2003). Increased levels
of ECP in the activated sludge make the sludge more difficult to dewater.
Extracellular polymers (containing up to 98% water; either associated with
the bacterial cell wall or in suspension) are extremely hydrated and prevent
desiccation of the bacterial cell under environmental conditions (Houghton
et al., 2000). An excellent review on thermal sludge pretreatment processes
and the possible role of ECP to improve dewaterability is presented by
Neyens and Baeyens (2003).
Bound water and intracellular water in the sludge are difficult to dewa-
ter. Chemical conditioning can release the bound water, but the intracellular
water is very difficult to release. Thermal treatment releases both bound
and intracellular water due to several physical and chemical reactions that
take place during thermal hydrolysis. Furthermore, a rapid temperature in-
crease causes the sludge particles and microorganisms to rupture and then
release the intracellular water. The increased molecular activity at high tem-
perature causes the particles to collide with each other, resulting in a break-
down of the gel-like structure that releases the bound water (this process is
the so-called syneresis). With the release of bound water and intracellular
water, sludge dewaterability is enhanced (Chu et al., 1999). In this context,
Fisher and Swanwick (1971) and Anderson et al. (2002) observed that thermal
treatment at temperatures greater than 150◦C enhanced the dewaterability.
The results of various researches on enhancement of dewaterability with TPT
are summarized in Table 1.
CAMBI thermal hydrolysis enhances the dewaterability and DS up to
40% w/w can be obtained. Similarly, dewaterability could be enhanced by
BIOTHELYS Rprocess, that is, 28–35% DS after centrifugation and 35–50%
with filter press. Full-scale installation of CAMBI process at WWTPs con-
cluded that dewaterability of sludge was enhanced remarkably (10–12%
higher than the conventional) (Norli, 2006). Fernandez-Polanco et al. (2008)
observed that the centrifugation characteristics of hydrolyzed sludge (di-
rect steam injection) were much better than those of fresh sludge. They
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676 S. Pilli et al.
TABLE 1. Impact of thermal pretreatment on sludge dewaterability
Sludge type
Temperature and
treatment time Increase in dewaterability References
Sewage sludge 200–250◦Cand
0.5hr
52% dry solids in the sludge cake Lumb (1951)
WAS 100–225◦Cand
0.5 hr
Dewaterability increased with
increase in temperature,
dewaterability at 225◦Cwas
significantly higher than lower
temperature
Haug et al.
(1978)
Dewatered
sewage
sludge
175◦C and 1 hr Thermochemical liquidized
dewatered sludge was centrifuged
to 42.3% w/w precipitate.
Sawayama
et al. (1995)
Anaerobically
digested
sludge
175◦C and 1 hr Thermochemical liquidized
dewatered sludge was centrifuged
to 52.3% w/w precipitate.
Sawayama
et al. (1996)
WAS 190◦C and 0.24–1
hr
Capillary suction time (CST) of the
sludge increased until 130◦Cand
started to decrease after that. CST
of the sludge decreased from
1330 s to 31 s at 190◦C
pretreatment.
Bougrier et al.
(2008)
WAS High temperature Dry matter up to 50% was obtained
after dewatering
Jung and
Jungjohann
(1996)
WAS 70 and 90◦Cand
1hr
At 90◦C the specific filtration
resistance (R) reduced from 470
×1012 m/kg to 190 ×1012 m/kg
and to 19 ×1012 m/kg with
15 mM H2O2concentration. At
70◦C R decreased to 375 ×1012
m/kg and 125 ×1012 m/kg with
15 mM H2O2concentration
Mustranta and
Vikari (1993)
Sewage sludge 80◦C CST reduced from 48 to 27 s with
thermal pretreatment and 14 s
with thermal pretreatment and
polymer addition.
Lin and Shien
(2001)
Surplus sludge 90 and 121◦Cand
1hr
At 90◦C dewatering performance
was higher than 121◦C pretreated
sludge, that is, the change of
suction time at 90◦Cwas−7%
and that at 121◦C was 10%
Barjenbruch
and
Kopplow
(2003)
Mixed sludge 170◦C for 30 min The dewaterability was measured
using centrifugation (270 g for
1 min) and filtration tests
(Millipore, with 2 bar pressure,
volume filtered with respect to
time). The quantity of water
removed was 84% (thermally
pretreated sludge), which was
nearly twice that of fresh mixed
sludge (43%).
Perez-Elvira
et al. (2010)
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Thermal Pretreatment of Sewage Sludge 677
also observed that solids disintegration and sludge filterability increased
with pretreatment temperature but the biodegradability of the sludge de-
creased at high temperature. The optimum pretreatment conditions for better
solid–liquid separation during centrifugation and for greater methane pro-
ductivity were 170◦C and 30 min. Moreover, the temperature and treatment
time are important parameters for enhancing the dewaterability; temperature
greater than 150◦C and treatment time of 30 min showed better dewat-
erability. From the above discussion, it clear that sludge dewaterability is
deteriorated when the treatment temperature is less than 150◦C (this is due
to the sludge solids solubilization and production of small particles) and the
dewaterability is improved with treatment temperature greater than 150◦C
(due to the modification of sludge structure and release of linked water).
3.1.2. CHEMICAL CHANGES
Sludge is a complex material containing various types of microorganisms
with general approximate composition (w/w) of 10% carbohydrates, 50%
protein, 10% lipid, and 30% others including ribonucleic acid (RNA) and
fiber (Li and Noike 1987, 1992). The cell wall strength of these microorgan-
isms varies from each other, and the temperature required to break this cell
wall to release the intracellular compounds will vary from microorganism
to microorganism. The rapid rise of temperature (cooking of cells) causes
physical–chemical reactions, which release the bound water, intracellular
water, and also solubilizes organic particulates (carbohydrates, lipids, and
proteins or lower molecular weight compounds are solubilized). The chem-
ical changes during TPT at high- and low-temperature TPT reported in the
literature are summarized in Tables 2 and 3.
3.1.2.1. COD Solubilization. The soluble COD increased with in-
crease in temperature and treatment time (Valo et al., 2004; Carr`
ere et al.,
2008; Bougrier et al., 2006). The COD solubilization was linearly related to
treatment temperature (60–170◦C) (Carr`
ere et al., 2008). The thermal treat-
ment time has very little impact on sludge solubilization (Bougrier et al.,
2006). The solubilization of COD and VS increased with increase in temper-
ature in the range of 110–220◦C, COD (Mottet et al., 2009). Perez-Elvira et al.
(2010) evaluated the performance of thermal hydrolysis of mixed sludge (at
170◦C and 30 min) at pilot-plant and observed four times higher soluble
COD in the supernatant (which increased from 7.8% to 29%).
Bougrier et al. (2008) evaluated the effect of TPT temperature (95–210◦C)
on five different sludge samples (generated from urban, industrial, and
slaughterhouse wastewaters) and concluded that the solids solubilization
level (measured by evaluating the total suspended solids (TSS)/total solids
(TS) ratio and volatile suspended solids (VSS/TSS ratio)) increased with
increase in temperature. Takashima (2008) examined the combination of
different process configurations for treatment and AD of sewage sludge
(1: control or digestion with no thermal treatment, 2: pretreatment followed
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TABLE 2. Chemical changes during the high-temperature thermal treatment
Sludge Treatment SCOD (mg/L) Protein (mg/L) Carbohydrate (mg/L)
type TS Time (min) Temp (◦C) Time-temp Before After PT SCOD % S∗Before After PT protein % S∗Before After PT carbohydrate % S∗References
WAS 2.97% 30 100 3000 959.2 7810 6850.817.9 NA NA NA NA NA NA NA NA Haug et al. (1978)
WAS 30 135 4050 959.2 9840 8880.822.6NA NA NA NANA NA NA NA
WAS 30 175 5250 959.2 18,600 17,640.842.7NA NA NA NANA NA NA NA
WAS 30 200 6000 959.2 18,500 17,540.853.9NA NA NA NANA NA NA NA
WAS 30 225 at pH 12 6750 959.2 22,000 21,040.854.7NA NA NA NANA NA NA NA
WAS 120 175 at pH 1.2 21,000 959.2 21,100 20,140.848.4NA NA NA NANA NA NA NA
WAS NA 30 120 5250 1192.1 5694.1 4502 43.7 282.24 2039.9 1757.68 34.4 42.78 361.988 319.208 41.8 Li and Noike (1992)
WAS 30 150 5250 1192.1 6166.4 4974.29 41.0 282.24 2333.8 2051.59 41.9 42.78 443.82 401.04 48.4
WAS 60 170 3600 1192.1 7932.1 6740.01 49.7 282.24 2467.5 2185.27 44.3 42.78 332.25 289.47 46.6
WAS 30 175 4500 1192.1 8793 7600.89 55.2 282.24 2649.6 2367.36 48.0 42.78 418.08 375.350.8
WAS 17.8 (g/L) 30 130 at pH 10 10,200 700 10,600 9900 63 NA NA NA NA NA NA NA NA Bougrier et al. (2006)
WAS 30 170 5250 700 9200 8500 63 NA NA NA NA NA NA NA NA
WAS 17.7 (g/L) 30 150 3900 1100 4900 3800 39 NA NA NA NA NA NA NA NA
WAS 30 170 5100 1100 6300 5200 56 NA NA NA NA NA NA NA NA
WAS 17.1 (g/L) 60 130 7800 396.9 3719 3322.125.3 NA NA NA NA NA NA NA NA Valo et al. (2004)
WAS 60 150 9000 396.9 6453.3 6056.443.9NA NA NA NANA NA NA NA
WAS 60 170 10,200 396.9 10,353 9956.159.5NA NA NA NANA NA NA NA
WAS NA 30 120 3600 302.52 2499.3 2196.74 24.2 124.31 1416.8 1292.49 23.1 47.16 275.08 227.92 35 Jeong et al. (2007)
WAS NA 30 150 4500 302.52 1794.9 1492.43 17.8 124.31 1180.9 1056.59 19 47.16 163.47 116.31 20.8
WAS NA 30 170 5100 302.52 1815.1 1512.6 18 124.31 1230.7 1106.36 19.8 47.16 188.63 141.47 24
WAS 6.00% 30 180 5400 302.52 1764.7 1462.17 17.5 124.31 1243.1 1118.79 20 47.16 161.9 114.74 20.6
PS 120 130 15,600 6200 9300 3100 12.17 48 500 452 3.31 NA NA NA NA Wilson and Novak
(2009)
PS 120 150 18,000 6200 11,500 5300 15.05 48 2280 2232 15.10 NA NA NA NA
PS 120 170 20,400 6200 13,000 6800 17.02 48 2400 2352 15.89 NA NA NA NA
PS 120 190 22,800 6200 14,200 8000 18.59 48 2750 2702 18.21 NA NA NA NA
PS 120 220 26,400 6200 17,000 10,800 22.25 48 2750 2702 18.21 NA NA NA NA
WAS 120 130 15,600 830 6800 5970 9.71 429 460 31 1.42 NA NA NA NA
WAS 120 150 18,000 830 10,200 9370 14.57 429 2300 1871 7.08 NA NA NA NA
WAS 120 170 20,400 830 11,850 11,020 16.93 429 2800 2371 8.62 NA NA NA NA
WAS 120 190 22,800 830 14,050 13,220 20.07 429 2900 2471 8.92 NA NA NA NA
WAS 120 220 26,400 830 16,800 15,970 24.00 429 3250 2821 10.00 NA NA NA NA
WAS 99.8 (g/L) 5 170 850 7376 45,000 37,624 41.66 2200 23,000 20,800 54.83 733 7600 6867 57.62 Donoso-Bravo et al.
(2011)
WAS 10 170 1700 7376 53,000 45,624 49.06 2200 27,000 24,800 64.36 733 10,500 9767 79.60
WAS 15 170 2550 7376 55,000 47,624 50.92 2200 33,000 30,800 78.67 733 9300 8567 70.50
WAS 25 170 4250 7376 55,000 47,624 50.92 2200 32,000 29,800 76.28 733 7000 6267 53.07
WAS 30 170 5100 7376 54,950 47,574 50.87 2200 32,000 29,800 76.28 733 7800 7067 59.13
WAS 76.8 (g/L) 5 170 850 4829 30,000 37,624 30.06 1950 13,000 11,050 34.39 629 7500 6871 69.01
WAS 10 170 1700 4829 36,000 25,171 36.07 1950 15,500 13,550 41.00 629 6800 6171 62.57
WAS 15 170 2550 4829 35,000 31,171 35.07 1950 16,000 14,050 42.32 629 8500 7871 78.21
WAS 25 170 4250 4829 37,500 30,171 37.58 1950 18,000 16,050 47.61 629 7200 6571 66.25
WAS 30 170 5100 4829 42,000 32,671 42.08 1950 17,000 15,050 44.97 629 8300 7671 76.37
S∗is solubilization percentage, SCOD: Difference in initial and final SCOD after thermal pretreatment.
678
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TABLE 3. Chemical changes during the low-temperature thermal treatment
Sludge Treatment Time- SCOD (mg/L) Soluble Protein (mg/L) Soluble Carbohydrate (mg/L)
type TS Time (min) Temp (◦C) temp Before After PT SCOD % S∗Before After PT protein % S∗Before After PT carbohydrate % S∗References
TWAS NA 15 70 1050 400 650 250 0.45 125 864 739 2.51 136 139 3 0.04 Soluble proteins as
mg BSA-eq/L
carbohydrates as
mg Glu-eq/L
Appels et al.
(2010) Soluble
TWAS 30 70 2100 400 800 −320 0.72 125 1293 1168 3.96 136 130 −6−0.08
TWAS 60 70 4200 400 1150 750 1.36 125 1155 1030 3.49 136 144 8 0.1
TWAS 15 80 1200 400 1350 950 1.72 125 388 263 0.89 136 164 28 0.35
TWAS 30 80 2400 400 2050 1650 2.98 125 900 775 2.63 136 208 72 0.88
TWAS 60 80 4800 400 8200 7800 14.1 125 2736 2611 8.85 136 721 585 7.17
TWAS 15 90 1350 400 1600 1200 2.17 125 748 623 2.11 136 165 29 0.35
TWAS 30 90 2700 400 7200 6800 12.3 125 2495 2370 8.03 136 722 586 7.18
TWAS 60 90 5400 400 10,250 9850 17.8 125 3162 3037 10.3 136 1094 958 11.74
WAS NA 30 60 1800 302.52 1089.1 786.54 10.8 124.31 791.1 666.79 11.28 47.16 83.31 36.15 15.6 Jeong et al. (2007)
WAS 30 80 2400 302.52 1865.5 1563.02 18.5 124.31 1280.4 1156.08 20.6 47.16 165.05 117.89 21
WAS 30 100 3000 302.52 1774.7 1472.18 17.6 124.31 1149.9 1025.55 18.5 47.16 175.26 128.1 22.3
S∗is solubilization percentage, is the difference between the SCOD before and after pretreatment.
679
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680 S. Pilli et al.
FIGURE 3. Solubilization of WAS at different time-temperature.
by digestion, 3: AD followed by thermal treatment and recycling of the treated
sludge to AD, 4: AD followed by thermal treatment and second-stage AD) by
incorporating TPT (120◦C for 1 hr). They observed that the dissolved solids
concentration increased from 0.4 g/L (control) to 4.4–6.2 g/L, which implies
that a significant portion of the digested sludge was solubilized through
TPT. Furthermore, the VS destruction increased by 4.5, 6.6, and 9.9 times
for pre, post, and interstage-treatment, respectively, compared to the con-
trol, that is, interstage-treatment (AD-thermal treatment-AD) >posttreatment
(thermal treatment of the anaerobic digested sludge and recycled to AD) >
pretreatment (thermal treatment before AD) >control.
The effect of treatment time and temperature on COD solubilization
reported in the literature is summarized in Table 2. However, comparison
of these results is very difficult, because the solubilization of the sludge
varies with treatment temperature, treatment time, suspended solids con-
centration, and the type of sludge. Furthermore, to compare the impact of
treatment time and treatment temperature (100–200◦C) on COD solubiliza-
tion, for a given sludge solids concentration, time-temperature (min-◦C) was
considered as a parameter. Relation between the time-temperature and per-
centage COD solubilization of waste-activated sludge (WAS) is presented
(Figure 3) for the available literature data. There was a linear correlation be-
tween the time-temperature factor and percentage COD solubilization. The
slope of the line decreased with total sludge solids concentration. However,
to obtain a perfect relationship between time–temperature and COD solubi-
lization, furthermore, COD solubilization data are required at different solids
concentration for a given treatment temperature and time.
3.1.2.2. Carbohydrate Solubilization. The effect of treatment time
and temperature on carbohydrate solubilization reported in the literature
is summarized in Table 2. The soluble carbohydrate concentration increased
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Thermal Pretreatment of Sewage Sludge 681
until 130◦C and further increase in temperature decreased the carbohy-
drate solubilization. For example, the carbohydrate concentration increased
from 0.13 g eqGluc/L to 1.04 g eqGluc/L when the sludge was treated at
130◦C, whereas at 170◦C the carbohydrate concentration decreased to 0.78 g
eqGluc/L (Bougrier et al., 2008). The decrease in carbohydrate concentra-
tion could be explained as follows: the carbohydrate concentration was
measured by spectrophotometry by quantifying the carbonyl group, with
increasing temperature the released carbohydrates reacted with other carbo-
hydrates (“burnt sugar” reactions) or with proteins (Maillard reactions) and
thus the carbonyl groups disappeared; therefore, the compounds could not
be quantified (Bougrier et al., 2008). Mottet et al. (2009) evaluated thermal
hydrolysis of WAS at different temperatures 110, 165, and 220◦C (heated in
electric mode) and 165◦C (heated with steam) and observed that the car-
bohydrates solubilization strongly decreased from 15% at 165◦Cto1.2%at
220◦C. The decrease in carbohydrates concentration was mainly due to reac-
tion with components to form products, which were slowly biodegradable
(Muller, 2000; Bougrier et al., 2008).
3.1.2.3. Protein Solubilization. The effect of treatment time and tem-
perature on protein solubilization reported in the literature is summarized in
Table 2. The soluble protein concentration increased with increase in tem-
perature. An increase in soluble protein concentration from 0.31 eqSAB/L
(BSA =bovine serum albumin) (control sludge) to 5.9 eqSAB/L in sludge
treated at 170◦C was observed (Bougrier et al., 2008). An increase in N–NH4
(from 95.2 to 655 mg/L) was observed as a consequence of protein solubi-
lization during thermochemical pretreatment of microbial biomass at 140◦C
for 30 min and pH 12 (or addition of 26.1 g of NaOH/L) (Penaud et al.,
1999).
Mottet et al. (2009) evaluated thermal hydrolysis of WAS at different tem-
peratures 110, 165, and 220◦C (heated in electric mode) and 165◦C (heated in
steam mode) and observed increased protein solubilization with increasing
temperature. WAS treated at 165◦C with steam mode observed high protein
solubilization compared with electric mode (40.2% with steam and 34.5% in
electric mode). Wilson and Novak (2009) observed that protein solubilization
increased with increase in temperature, but beyond 150◦C caused reduction
in the protein size.
3.2. Effect of Low-Temperature TPT on Sludge Characteristics
3.2.1. LOW-TEMPERATURE EFFECT ON DEWATERABILITY
The effect of low-temperature TPT on sludge dewaterability was evaluated
by various researchers (Mustranta and Vikari, 1993; Lin and Shien, 2001;
Barjenbruch and Kopplow, 2003) and concluded that sludge dewaterability
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682 S. Pilli et al.
is reduced after TPT. The decrease in sludge dewaterability after low-
temperature pretreatment is due to increase in liquid temperature, which
will lower liquid viscosity and promote sludge filterability (Lin and Shien,
2001). The change in dewaterability after TPT reported in the literature is
summarized in Table 1.
3.2.2. LOW-TEMPERATURE EFFECT ON CHEMICAL CHANGES
The chemical changes in terms of COD, carbohydrates, and proteins dur-
ing low-temperature treatment reported in the literature are summarized in
Table 3.
3.2.2.1. COD Solubilization at Low-Temperature TPT. Wang et al.
(1997b) evaluated the organic matter solubilization in the range of 60–120◦C
and 5–60 min treatment time and concluded that solubilization reached equi-
librium at 60◦C in approximately 30 min, which was in agreement with other
researchers (Brooks and Grad, 1968; Wang et al., 1988). Vlyssides and Karlis
(2004) evaluated the COD solubilization and VSS reduction during thermal
hydrolysis at temperatures ranging from 50 to 90◦C and pH 8–11. At all con-
ditions, the COD solubilization increased significantly (at pH ≥10 and temp
≥80, 80% solubilization was achieved). Moreover, change in temperature
(50–90◦C) and pH (10–11) affected significantly the VSS reduction (due to
Maillard reactions, as the treatment time was greater than 2 hr). For exam-
ple, at pH 8, the VSS reduction was 17% at 50◦C and 24% at 90◦C; at pH 11
the VSS reduction was 17% at 50◦C and 43% at 90◦C. Vlyssides and Karlis
(2004) also defined a correlation between soluble COD production and VSS
reduction (a=d(COD)/d(VSS)). The correlation coefficient “a” depends on
the material to be hydrolyzed. The value of afor hydrocarbons, proteins,
and lipids was 1.2, 2.0, and 2.5, respectively (Vlyssides, 1987).
Ferrer et al. (2008) evaluated the mixed sludge solubilization by analyz-
ing the total dissolved solids (TDS). An increase of volatile dissolved solids
(VDS)/VS ratio from 0.05 to 0.44–0.48 (an increase of 780%) resulted after
sludge TPT at 70◦C for 48 hr. Similarly, a negligible (or limited) increase
in soluble COD at 70◦C for 1 hr treatment was observed by Appels et al.
(2010). Lower TPT requires longer treatment time for sludge solids solubi-
lization. From the above discussion and from the literature data presented in
Table 3, we can conclude that low temperature and longer treatment time
showed increased SCOD concentration and the solubilization percentage is
very low. But, comparing the results of low temperature and high tempera-
ture using time–temperature factor (Tables 2 and 3), it can be concluded that
high temperature with shorter treatment time is beneficial to achieve high
COD solubilization.
3.2.2.2. Carbohydrate Solubilization at Low-Temperature TPT. At
low-temperature TPT, the carbohydrate solubilization increases with tem-
perature. The carbohydrate solubilization of WAS at 60, 80, and 100◦Cwas
15%, 20%, and 21%, respectively (Jeong et al., 2007). Appels et al. (2010)
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Thermal Pretreatment of Sewage Sludge 683
revealed that after TPT of secondary sludge (with initial soluble carbohy-
drate concentration of 136 mg Glu-eq/L) at 70, 80, and 90◦C for 60 min, the
concentration of soluble carbohydrates was 144, 721, and 1094 mg Glu-eq/L,
respectively. The effect of treatment time and temperature on carbohydrates
solubilization is summarized in Table 3.
3.2.2.3. Protein Solubilization at Low-Temperature TPT. Increase in
protein solubilization of WAS with temperature was observed by various re-
searchers. For untreated WAS sludge, the soluble protein was 2% (of the total
protein), whereas with treatment at 60, 80, and 100◦C for 30 min the soluble
protein was 12%, 20%, and 18% of the total protein, respectively (Jeong et al.,
2007). At low temperature longer treatment time is required to release the
protein and other materials from the microbial cells. Moreover, the soluble
protein concentration increased with increase in treatment temperature. (The
concentrations reported are tabulated in Table 3.)
4. EFFECT OF TPT ON AD
The primary aim of sludge TPT is to increase the sludge biodegradability
and methane production at the lowest possible residence time in an anaero-
bic digester. Over the decades several researchers have evaluated the effect
of low-temperature (treatment time in days) and high-temperature (treat-
ment time in hours) TPT on sludge degradability to increase the biogas
production. The performance of AD based on biogas production, increased
biodegradability, COD degradation, TSS reduction, VS solids reduction, and
methane production was evaluated by various researchers at low and high
temperatures and is summarized in Tables 4 and 5, and is discussed below.
4.1. High-Temperature Pretreatment Effect on AD
The biodegradability (based on methane production) of thermal pretreated
“WAS” was enhanced through hydrolysis or by splitting complex organic
compounds (Stuckey and McCarty, 1978). The anaerobic sludge biodegrad-
ability increased with pretreatment temperature (Carr`
ere et al., 2008).
However, the biodegradability of WAS decreased at higher pretreatment
temperatures (>200◦C) due to the formation of Amadori and melanoidins
compounds. Bougrier et al. (2008) also concluded that the WAS biodegrad-
ability slightly decreased (but stayed higher than the untreated) at temper-
atures ≥190◦C due to toxic refractory compounds formation (Amadori and
melanoidins compounds). Pretreatment temperature greater than 175◦C en-
hanced the WAS solubilization but not the biogas production (Stuckey and
Mc Carty, 1978; Haug et al., 1983). Pretreatment at 170◦C by direct steam in-
jection for 30 min resulted in increased biodegradability (in terms of specific
methane production or mL CH4/g VSadded) of the pretreated WAS than the
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TABLE 4. Effect of low-temperature thermal pretreatment on biogas generation rate
Temperature
(◦C)
Hydrolysis
time (hr) Increase in biogas generation rate Methane produced References
100 0.5 14% increase in biogas is observed Control 510 mL/day
Pretreated 579 mL/day
Haug et al. (1978)
60 1 30% increase in biogas at organic loading of 3 kg
–VS/m3day
Control 217 mL/g VSadded
Pretreated 283 mL/g VSadded
Hiraoka et al.
(1984)
100 1 More than 50% increase is expected at an organic
loading of 3 kg –VS/m3day
Control 217 mL/g VSadded
Pretreated 344 mL/g VSadded
60 NA 52.1% increase in methane generation at 8 days
HRT
Control 585 mL/L day
Pretreated 344 mL/L day
Wang et al.
(1997a)
70 24 and 96 Pretreating the sludge for 4 days the methane
generation rate increased by 16.2% and at 7.7%,
when pretreated for 7 days
Control 21.4 mmol CH4/g VS
Pretreated for 4 days 24.8 mmol CH4/g VS
Pretreated for 7 days 23.1 mmol CH4/g VS
Gavala et al.
(2003)
70 24 and 96 For secondary sludge at 4 days pretreatment the
methane generation rate increased by 144.6%
and at 1 day pretreatment 25.9%
10.7 mmol CH4/g VS
Pretreated for 4 days
8.5 mmol CH4/g VS
Pretreated for day
70 48 The methane generation from pretreated primary
and secondary sludge is enhanced by 11% and
37.5% compared with untreated primary and
secondary sludge, respectively
Primary sludge – Control 144.6 mL/L day,
pretreated 162 mL/L day
Secondary sludge – control 40 mL/L day,
pretreated 55 mL/L day
Skiadas et al.
(2005)
70 9 68.6% increment in biogas production is observed Control 0.35 L/day,
Pretreated 0.59 L/day
Climent et al.
(2007)
70 9 Biogas production increased up to 30% both in
batch tests and in semicontinuous experiments
The rate of biogas production is around 0.63 L/L
day, and the methane content in the biogas is
around 64%
Raw sludge (0.22 L/g VSadded)
Pretreated 0.28–0.30 L/g VSadded)
Ferrer et al.
(2008)
70 0.5 The pretreatment of the primary sludge resulted in
up to 48% increase of methane potential and up
to 115% increase of methane production rate
Control 13.5 mmol CH4/g VS
Pretreated 20.1 mmol CH4/g VS
Lu et al. (2008)
684
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75 7 Seven hours of thermal pretreatment at 75◦Cledto
50% increase in biogas production
Control 62.5 mL/day,
Pretreated 93.6 mL/day
Borges and
Chernicharo
(2009)
70 48 Methane yield increased from 5% at thermophilic
digestion
Control 247 ±9NmLCH
4/g VSadded.
Pretreated 264 ±8NmLCH
4/g VSadded
Nges and Liu
(2009)
At mesophilic digestion the methane yield
increased by 4.6%
Control (mesophilic digested) 254 ±9NmL
CH4/g VSadded
Pretreated 268 ±7NmLCH
4/g VSadded
50 48 Methane yield increased from 6.25% at
thermophilic digestion
Control 247 ±9NmLCH
4/g VSadded
Pretreated 268 ±6NmLCH
4/g VSadded
Nges and Liu
(2009)
At mesophilic digestion the methane yield
increased by 11%
Control (mesophilic digested) 254 ±9NmL
CH4/g VSadded
Pretreated 284 ±8NmLCH
4/g VSadded
50–65 12–48 Pretreatment temperature had a very clear effect on
methane yield. The degradability was increased
at thermophilic temperature, which was
increased from 21% to 49% with temperature
increasing from 50 to 65◦C, respectively
Approximately 160 mL/g VSadded at 50◦Cwas
observed and approximately 300 mL/g
VSadded at 65◦C, respectively.
Ge et al. (2011a)
50–70 Over different
periods of
15 months
At 50◦C pretreatment during period 1 for186 days,
thermophilic pretreatment did not offer any
advantage over mesophilic pretreatment. But
increasing the pretreatment temperature to 60
and 65◦C (period 2, 100 days and period 3,
67 days, respectively) showed improved
performance, that is, VS destruction increased
from 35% to 42% and 50%, respectively. Further
increase in pretreatment temperature to 70◦C
(period 4, 68 days), there was no increase in VS
reduction
Comparing the hydrolysis coefficient in
mesophilic and thermophilic pretreatment
systems, the hydrolysis coefficient is
enhanced at pretreatment of 60–70◦C.
Ge et al. (2011b)
70–90 0.25 to 1 hr Biogas production increased significantly with
treatment at a temperature of 90◦Cwhen
compared with 70◦C treatment temperature
Biogas 377.56 mL/g Organic dry solids
degraded, at 90◦C
And at 70◦C 35.32 mL/g ODS
Appels et al.
(2010)
685
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TABLE 5. Effect of high-temperature thermal pretreatment on biogas generation rate
Temperature
(◦C)
Hydrolysis
time (hr) Increase in biogas/methane generation rate Methane produced References
175 0.5 60–70 increase Control 406 mL/day
Pretreated 641 mL/day
Haug et al. (1978)
110 0.3 Thermal pretreatment of combined sludge (45%
raw sludge, 45% excess sludge) at 110◦C
enhanced 25% higher biogas compared to the
control sludge
Control 0.9 dm3/day biogas
Pretreated 0.95 dm3/day biogas
Bien et al. (2004)
130 at pH 10 1.0 During the batch thermophilic digestion, the
biogas production increased by 74%
Control 88 mg/g CODadded Pretreated
154 mg/g CODadded
Valo et al. (2004)
170 1.0 During the batch thermophilic digestion for 20d,
the methane production increased by 61%
Control 88 mg/g CODadded Pretreated
142 mg/g CODadded
Valo et al. (2004)
170 0.5 During the batch anaerobic digestion (24 days),
the methane production increased by 76%
Control 221 mL/g CODadded
Pretreated 331 mL/g CODadded
Bougrier et al.
(2007)
170 0.5 During the anaerobic digestion in CSTR
(20 days), the methane production increased
by 51%
Control 145 mL/g CODadded
Pretreated 256 mL/g CODadded
Bougrier et al.
(2006)
120 0.5 The methane gas production for thermally
pretreated WAS increased from 22.4% to 42.9%
Control 185 mL/day
Pretreated 230 mL/day
Jeong et al. (2007)
170 0.5 78% increase in methane production Control 128 mL CH4/g VSadded
Pretreated to 228 mL CH4/g VSadded
Carr`
ere et al. (2008)
190 0.5 Biogas production is improved by 23% at 190◦C
pretreatment
Control 128 mL CH4/g VSadded
Pretreated to 228 mL CH4/g VSadded
Carr`
ere et al. (2008)
180 1.0 Prehydrolyzing WAS by
Thermo-Druck-Hydrolyze process (TDH),
biogas production increased from 90.32
L/kgCOD to 162.82 L/kgCOD
Control 206.5 mL CH4/g VSadded Pretreated
to 254.0 mL CH4/g VSadded
Phothilangka et al.
(2008)
686
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175 0.5 Comparing the performance of anaerobic
sequencing batch reactor (ASBR) and
continuous-flow stirred tank reactors (CSTR)
for the digestion of thermally hydrolyzed
sewage sludge Wang et al. (2009) observed
that the daily biogas production of ASBR is
15% and 31% higher than the CSTR at 20 and
10 days hydraulic retention time, respectively.
At 20 days ASBR 273 mL CH4/g CODadded
CSTR 235 mL CH4/g CODadded
At 10 days ASBR 256 mL CH4/g CODadded
CSTR 207 mL CH4/g CODadded
Wang et al. (2009)
165 0.5 The methane production has increased by 30.3%
at 165◦C (Steam mode)
Control 165 mL/CODinlet
pretreatment 215 mL/CODinlet
Mottet et al. (2009)
170 0.5 The biogas production is enhanced by 40% in
half time, compared to a conventional digester
Control 250 mL CH4/g VSadded
pretreatment 365 mL CH4/g VSadded
Perez-Elvira et al.
(2010)
170–180 1.0 Comparing the thermal hydrolysis with
mechanical ball milling disintegration the
biogas production is 75% compared to
approximately 41%, respectively
247 to 443 L/kg VSSadded
load due to TDH pretreatment
265 to 415 L per kg VSSadded ball milling
Wett et al. (2010)
687
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688 S. Pilli et al.
nontreated WAS (Fern´
andez-Polanco et al., 2008; Perez-Elvira et al., 2008;
Donoso-Bravo et al., 2010, 2011).
The increase in biodegradability could be linearly correlated to COD
solubilization for treatment temperature up to 190◦C. The sludge that was
poorly biodegradable (initially) exhibited a higher impact of TPT than the
easily biodegradable sludge (Bougrier et al., 2007; Carr`
ere et al., 2008). The
poorly biodegradable WAS showed 78% increase in methane production
after treatment at 170◦C (i.e., from 128 to 228 mL CH4/g VSadded) whereas
only 23% increase was observed when initially easily biodegradable “WAS”
was treated at 190◦C (254 mL CH4/g VSadded). Mottet et al. (2009) observed
that heating mode (electrical or steam mode) had no significant impact on
the WAS anaerobic biodegradability.
To overcome the formation of toxic compounds, rapid TPT (170◦C) with
a short retention time of 60 s was studied (Dohanyos et al., 2004), which
enhanced the biogas production from 0.26 L/g CODinlet (untreated sludge)
to 0.39 L/g CODinlet (pretreated sludge). Batch AD of sludge pretreated at
120◦C for 1 hr resulted in an increase of cumulative methane gas production
by 2.4–3.0 times compared to nontreated sludge (Takashima, 2008). In their
continuous experiment (for 91 days), an average methane production for the
control was 0.168–0.004 L/g VS fed. For post-treatment (thermal treatment of
the anaerobic digested sludge and recycled to AD) and interstage treatment
(AD-thermal treatment-AD), the average methane production was increased
by 21% and 17% compared with the control, respectively. The effect of sludge
type, pressure, pretreatment temperature, and treatment time was optimized
by Fernandez-Polanco et al. (2008). They also studied the continuous AD
of the pretreated mixed sludge (170◦C, 30 min) under mesophilic and ther-
mophilic conditions; 445 L/kg VSadded of biogas production was observed in
the mesophilic reactor (55% increment in CH4production over raw sludge)
and 405 L/kg VSadded of biogas production was recorded in the thermophilic
reactor (48% increment in CH4production over raw sludge).
Wang et al. (2009) investigated the performance of anaerobic sequenc-
ing batch reactor (ASBR) with thermally hydrolyzed sewage sludge and ob-
served ASBR performed better than the continuous-flow stirred tank reactors
(CSTR). The anaerobic digestibility and the rate of degradation of thermally
hydrolyzed sludge in a full-scale mesophilic digester increased significantly
(40% higher biogas production in nearly half the time at a residence time of
12 days for thermally hydrolyzed sludge and 20 days for the control sludge)
(Perez-Elvira et al., 2010). In the context of characterizing the effluent color
of the liquor produced during the pretreatment process, Dwyer et al. (2008)
studied the effect of temperature (lowering the temperature from 165 to
140◦C) on biodegradability of secondary sludge and also on TPT effluent
color produced during the pretreatment and concluded that decreasing the
temperature from 165 to 140◦C did not reduce biodegradability, but color
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Thermal Pretreatment of Sewage Sludge 689
of the effluent liquor was reduced, that is, the TPT effluent color decreased
from 12,677 mg PtCo (platinum–cobalt units)/L to 3837 mg PtCo/L.
Based on the report of various researchers, the TPT of WAS in the
temperature range of 120–170◦C for 30–60 min resulted in an increase of
various parameters in the following range increase in methane production
22–43%, methane content increased from 40% to 70%, total solids reduction
up to 59%, increase in VS degradation up to 23%, and COD reduction up to
75% (Valo et al., 2004; Skiadas et al., 2005; Jeong et al., 2007; Bougrier et al.,
2008). Thus, based on various research reports presented and discussed here,
the condition for WAS pretreatment to obtain maximum biodegradability
and biogas generation was 165–170◦C for 30 min (Stuckey and McCarthy,
1984; Li and Noike, 1992). The pretreatment under optimum conditions (i.e.,
165–170◦C) also increased the ease of dewatering (Fern´
andez-Polanco et al.,
2008; Mottet et al., 2009).
4.2. Low-Temperature Effect on AD
In general, the sludge anaerobic biodegradability (biogas production) in-
creased with treatment temperature (60–100◦C); however, higher treatment
time was required at low temperature. Paul et al. (2006) studied the ef-
fect of TPT at temperature below 100◦C by performing respirometric test
on the solubilized COD and showed that only 40–50% of the soluble COD
was biodegradable at a hydraulic retention time of 24 hr (Paul et al., 2006).
Borges and Chernicharo (2009) evaluated the effect of TPT (75◦Cfor7hr)
on the sludge generated from a UASB reactor. They observed 50% higher
anaerobic biodegradability (and increase in biogas production) than the un-
treated sludge. Ferrer et al. (2008) pretreated the sludge at 70◦C for 9, 24, 48,
and 72 hr and observed increased biodegradability (the biogas production
increased by 30% both in batch tests and in semicontinuous experiments)
under thermophilic anaerobic conditions.
The methane generation increased with increase in temperature and pH
during sludge pretreatment (Vlyssides and Karlis, 2004). At 50◦C, pH 8 for 10
hr pretreatment methane generation was 0.07 L CH4/g VSS loading. At 90◦C,
pH 11 for 10 hr pretreatment, methane generation increased to 0.28 L CH4/g
VSS loading. The organic matter destruction increased remarkably during AD
of pretreated sludge at a temperature between 80 and 100◦C (Hiraoka et al.,
1984). Wang et al. (1997b) reported that continuous AD of WAS pretreated
at temperatures below 100◦C (30 min, pH 7) increased methane generation
by 30%. Moreover, the organic matter destruction of thermal-treated WAS in
AD follows the following order with respect to temperature: 100◦C>80◦C
>60◦C>control (Wang et al., 1997a).
Gavala et al. (2003) evaluated the effect of mesophilic and thermophilic
digestion of pretreated (70◦C) primary and secondary sludge, and concluded
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690 S. Pilli et al.
that the effect of pretreatment showed very positive effect on methane pro-
duction rate upon subsequent thermophilic digestion of primary sludge (Ta-
ble 4). But in the case of secondary sludge, it was reported that the methane
production rate was mostly influenced in mesophilic digestion followed by
thermophilic digestion. Furthermore, it was concluded that the selection of
the pretreatment duration and temperature of the subsequent AD depends on
the ratio of primary to secondary sludge. For the primary sludge (pretreated
at 70◦C for 1–4 days), methane generation increased during mesophilic and
thermophilic digestion by 9.2–16.2% and 37.9–85.9%, respectively. For the
secondary sludge (pretreated at 70◦C for 1–4 days), methane generation dur-
ing mesophilic and thermophilic digestion increased by 43–145% and 4–58%,
respectively (Gavala et al., 2003). Pretreatment of the sludge for 2 days at
70◦C was found to be more efficient than control, that is, 28% and 61.7%
higher VSS removal for primary and secondary sludge during AD, respec-
tively, than the untreated sludge (Skiadas et al., 2005).
Ferrer et al. (2008) observed initial biogas production for 9, 24, and
48 hr of pretreated sludge (70◦C and pH 7), which was almost 50% higher
compared to the control. Ferrer et al. (2008) also stated that lower solids
retention time for thermophilic AD of thermally pretreated sludge is more
efficient in terms of energy production, but less efficient in terms of effluent
stabilization. Wett et al. (2010) compared the disintegration of WAS using
mechanical ball milling and thermo-pressure hydrolysis (TPH) on a full-scale
plant and concluded that the biogas production increased by 75% in THD
(biogas production increased from 247 to 443 L per kg VSS with and without
treatment, respectively) and 41% (265–415 L per kg VSS with and without
treatment, respectively) in ball milling pretreatment. Low-temperature TPT
of sludge in enhancing the AD efficiency follows the following order 100◦C
>80◦C>60◦C>control, and required longer treatment time (more than 8
hr). The lower the temperature, the higher is the required sludge treatment
time for enhancing the AD efficiency. Thus in general, with only exception
of Wang et al. (1997b), low TPT (50–100◦C) enhances the sludge biodegrad-
ability but requires longer treatment time (more than 8 hr) to enhance the
biogas production.
5. ENERGY BALANCE OF TPT PROCESS
The primary purpose of TPT is to enhance biogas generation and to have
self-sustainable energy for the treatment process. From the values reported in
the literature, discussed above and summarized in Tables 4 and 5, it is clear
that researchers have reported biogas production either per kg of VSadded or
per kg of CODadded. With this information it is not possible to evaluate the
energy balance or energy ratio of the pretreatment and AD process. To eval-
uate the energy balance and energy output/input ratio for the TPT process
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Thermal Pretreatment of Sewage Sludge 691
TABLE 6. Parametric values considered in the energy analysis
Description Value Units References
Sludge to be treated 40,000 kg dry solids per day Assumed
Temperature of
sludge before
pretreatment
12 ◦C Wastewater treatment plant,
Quebec city
Anaerobic digestion
temperature
35 ◦C Metcalf and Eddy (2003)
Specific heat of
sludge
4200 kJ/m3◦C Metcalf and Eddy (2003)
Density of sludge 1000 kg/m3Metcalf and Eddy (2003)
Retention time (HRT)
Control sludge
Pretreated sludge
20
12
Days Fernandez-Polanco et al.
(2008); Perez-Elvira et al.
(2010)
Heat loss of AD 5 (2–8) % of the sludge
heating
requirement
Zupancic and Ros (2003)
Methane production
rate
0.499 m3CH4/kg VSdestroyed Metcalf and Eddy (2003)
Heating value of
methane
31.79 MJ/m3CH4Metcalf and Eddy (2003)
Thermal pretreatment
temperature
170 ◦C Fernandez-Polanco et al.
(2008); Perez-Elvira et al.
(2010)
Recovery of heat
energy from the
preheated sludge
using heat
exchangers
85 % Lu et al. (2008)
Volatile solids
reduction for
nonpretreated
sludge
45 % Metcalf and Eddy (2003)
Volatile solids
reduction for
pretreated sludge
65 % Haug (1977)
MJ: mega joules
in conjunction with AD, parametric values considered in the energy balance
are provided in Table 6. The energy balance evaluated for the process is
presented in Figure 4. The first step considered in the process is heating
the sludge from 12 to 170◦C (The optimum temperature is adopted from
Fernandez-Polanco et al. (2008) and Perez-Elvira et al. (2010)) and main-
taining it at 170◦C for 30 min. The efficiency of the boiler is considered
as 90% (http://hpac.com/heating/maximizing-small-boiler-efficiency) to in-
crease the sludge temperature to 170◦C. In the second step it is considered
that heat energy (85%, Lu et al., 2008) is recovered from the heated sludge
before adding into the anaerobic digester using heat exchangers. In the next
(third) step of AD hydraulic retention time considered for AD of THP sludge
in the anaerobic digester is given in Table 6.
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692 S. Pilli et al.
FIGURE 4. Thermal pretreatment and anaerobic digestion process considered for energy
balance.
While computing the energy balance, the energy input at different steps
was evaluated, that is, in step one energy required for increasing the temper-
ature of sludge to 170◦C, and the energy loss during boiler operation, in the
third step, the energy required to maintain the temperature of the anaerobic
digester (i.e., heat loss) is assumed to be 5% (2–8%) of the sludge heating
requirement (Zupancic and Ros, 2003) for hydraulic retention time (HRT)
(control and TPT) as mentioned in Table 6. It is also assumed that the pro-
duced biogas is used for mixing the sludge (Nayono, 2009; Van Haandel and
Van Der Lubbe, 2012), and the energy required in compressing the biogas
is very negligible and therefore is not considered in the computations of the
energy balance. The energy output or energy recovery during the process is
step two, where the heat energy is recovered using heat exchangers, and in
step three from the generated biogas. The energy required for dewatering
the pretreated digestate, transportation, and land application is missing in the
literature, which are important factors in evaluating the life cycle analysis.
Therefore, in this article an attempt was made to evaluate the net energy and
energy ratio at different sludge solids concentration (TS). Owing to missing
data life cycle analysis was not performed.
5.1. Computation of Energy Balance at Different Solids (TS)
Concentration
To put the question of energy consumption into perspective for TPT followed
by AD of the pretreated sludge (Figure 4), the detailed energy balance at
1%, 2%, 3%, and 4% w/v total solids concentration was evaluated and pre-
sented in Figure 5. The net energy (output–input) and energy ratio (energy
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Thermal Pretreatment of Sewage Sludge 693
FIGURE 5. Net energy (output–input) generated at different sludge solids concentration.
output/energy input) is increased with increase in total solids concentration
for the control (raw sludge) and the pretreated sludge (Figure 5). Moreover,
the energy ratio (output/input) also increased with increase of total solids
concentration. At total solids concentration of 1%, 2%, 3%, and 4% (w/v%),
the net energy and energy ratio were higher for the thermal pretreated sludge
compared with the control sludge. When the total solids concentration was
greater than ∼1.5% (w/v), the net energy was positive and the energy ratio
was greater than 1, for the pretreated sludge (at 170◦C for 30 min). For the
control sludge the net energy was positive and the energy ratio was greater
than one at ∼2.2% total solids concentration (Figure 5). Thus, it is clear that
it is necessary to optimize the total solids concentration for the control as
well as for the pretreated sludge to have a positive net energy and energy
ratio greater than one. Different treatment temperatures and treatment times
will have different effects on the energy ratio; therefore, further research is
required for TPT at higher total solids concentration (TS >4% w/v) followed
by AD. The commercialized technologies CAMBI and BIOTHELYS Ralso
concluded that the energy generated is self-sustainable to operate the pro-
cesses. However, detailed energy balance is not presented in the literature.
Moreover, the energy required for dewatering, transportation, and land ap-
plication of the pretreated digestate was not considered. Thus, consideration
of reduction in energy requirement during dewatering, transportation, and
land application of the thermal pretreated digestate may render a positive
energy balance when compared with the control.
5.2. Computed Energy Balance for the Literature Results
Results reported by different researchers are not in the same units and ba-
sic data are not provided to evaluate and compare the energy balance.
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694 S. Pilli et al.
Missing data were assumed from Table 6. Moreover, the solids concentration
of sludge, TPT conditions (treatment time and temperature), and AD condi-
tions are not similar to compare the results presented in the literature. There-
fore, to compare the results of the various authors (missing data are assumed
for evaluating the mass–energy balance), energy ratio (output/input) was
calculated and compared with a new parameter, “time-temperature (min-◦C)
factor.” The energy ratio was calculated from the given results in the liter-
ature at different time-temperature factors and with the adopted parametric
values (given in Table 6). The obtained results are summarized in Table 7.
The results summarized in Table 7 show that at higher total solids concen-
tration (greater than 3% w/v), the net energy and energy ratio are higher
and increased with increased solids concentration for the pretreated sludge.
But for the control sludge the results are unclear. The results reported by
Kim et al. (2003) and Haug et al. (1978) are contradictory. During the AD
of the control sludge (with total solids concentration of 38 g/L), Kim et al.
(2003) reported 20.1% VS reduction, resulting in an energy ratio less than
1. Furthermore Haug et al. (1978) reported that after pretreatment (with to-
tal solids concentration of 20.4 g/L) the VS reduction in anaerobic digester
increased by 30% (i.e., VS reduction was 31.8% for control and 41.4% for
thermal pretreated sludge, respectively), but the energy ratio was reduced
when compared with control. Thus we can conclude that sludge characteris-
tics are very critical during pretreatment and AD to have an energy-efficient
process.
5.3. Full-Scale Thermal Sludge Pretreatment and AD
A full-scale plant of thermal sludge pretreatment and AD treating 3200 Mg
of DS/year showed that there was a net energy gain of 223 kW, which was
higher than the conventional treatment (175 kW) and the treated sludge
proved to have high fertilizer value (Kepp et al., 2000). Perez-Elvira et al.
(2008) demonstrated a full-scale feasibility study of the thermal hydrolysis
and concluded that there was a 30% increase in biogas production, which
eventually produces surplus energy in the gas engine (i.e., 246 kW of en-
ergy). The economic value associated with the surplus energy generated was
384.64 $/year. Yang et al. (2010) reported contradictory results while com-
paring with other literature data; they stated that the input energy is very
high compared to the energy generated from the biogas. For example, the
energy input for heating the sludge to 150◦C was 2699.7 kJ/kg WAS and the
energy generated from biogas was only 105.2 kJ/kg WAS.
5.4. Energy Balance of Low-Temperature TPT
Low-temperature TPT requires high treatment time, and the percentage of
biogas generation is not high compared with high-temperature TPT. The
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TABLE 7. Computation of energy ratio for the data adopted from different authors
Total Pretreatment Energy Energy Net
solids temperature Pretreatment Time-temperature input (kWh/ton recovered (kWh/ton energy (kWh/ton Energy
(g/L) (◦C) time (min) (min-◦C) dry solids) dry solids) dry solids) ratio References
20.4 0 0 0 (Control) 741.3 946.5 205.21.28 Haug et al. (1978)
20.4 170 30 5250.0 1454.5 1247.9−206.50.86
40.7 0 0 0.0 (Control) 371.6 1009.6 638.12.72 Haug et al. (1983)
30.7 175 30 5250.0 966.5 1683.8 717.31.74
44.8 0 0 0 (Control) 910.7 1279.8 369.11.41 Ferrer et al. (2008)
68.2 70 540 37,800.0 592.1 1024.5 432.41.73
48.1 70 1440 100,800.0 839.1 1025.7 186.51.22
58.9 70 2880 201,600.0 686.1 828.9 142.71.21
38.0 0 0 0 (Control) 1209.8 563.7−646.00.47 Kim et al. (2003)
38.0 120 30 3600.0 502.2 967.6 465.41.93
12.5 0 0 0 (Control) 1209.8 957.0−252.81 0.79 Valo et al. (2004)
7.0 170 30 5100.0 4101.2 1507.3−2593.90.37
10.9 130 30 3900.0 1927.4 1022.6−904.80.53
695
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696 S. Pilli et al.
energy balance at low-temperature treatment has not been reported. There-
fore, extensive research on energy balance is needed. The study by Lu et al.
(2008) is the only one that evaluated and reported the energy balance for
low-temperature TPT. They observed that the extra energy required for the
pretreatment step will be covered by the energy produced from the extra
methane generation and there would be a significant surplus energy of 2.17
kJ/d.
6. FUTURE PERSPECTIVES
The research on the effect of lower temperature on the chemical param-
eters (proteins, carbohydrates, and lipids) and its corresponding effect on
AD are still at the laboratory scale. Further studies are necessary to have
an energy-efficient TPT process. The alkalinity of sludge plays a major role
during AD of sludge; therefore, the release of calcium and magnesium ions
during TPT might have a significant effect on AD. The resultant effects of
GHG emissions, global warming, and climate change have made it obliga-
tory to quantify GHG emissions from every source. Still extensive research on
optimizing the treatment parameters for enhancing the methane yield (i.e.,
to increase the net energy yield) is required for energy-efficient processes.
Research on TPT has been evaluated since 1960, yet there is no general-
ized method to evaluate the efficiency of the pretreatment process. There
is a need to standardize to evaluate the treatment process and measuring
parameters to compare the results of various authors. Full-scale installation
and the pilot plant studies have evaluated the energy balance for the pre-
treatment process, but the net energy evaluation of the process including the
energy reduction for dewatering and the sludge volume reduction are factors
that need to be considered. A standard method for evaluating the efficiency
of any pretreatment process is by calculating the total energy balance and
the net carbon balance of the total process (including AD of the pretreated
sludge, energy generation from the biogas). Evaluating the total energy bal-
ance and the net carbon balance will help decision makers in choosing the
pretreatment technology.
7. CONCLUSION
The low- and high-temperature TPT of sludge has significant effect on sludge
biodegradability during the AD process in enhancing biogas generation. Lab-
scale studies have concluded that 160–170◦C is the optimum temperature
for better dewaterability and biogas production. The optimum pretreatment
temperature and time for higher biogas production are in the range from
160 to 180◦C and 30 to 60 min, respectively. Full-scale studies proved that
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Thermal Pretreatment of Sewage Sludge 697
high-temperature pretreatment will reduce the sludge retention time, reduce
sludge volume, increase dewaterability, and increase biogas production. At
temperatures >190◦C sludge solubilization is higher but biogas production
is lower (biodegradability is reduced) due to the formation of toxic refractory
compounds (Amadori and melanoidins compounds).
Commercial TPT process, CAMBI thermal hydrolysis, and BIOTHELYS R
process enhance dewaterability. Energy balance of the process concluded
that the energy requirements during the pretreatment step will be covered
by the energy produced from the extra methane production. With increased
solids concentration the net energy and energy ratio is increased. At high
solids concentration (4% w/v TS) the net energy was positive and energy
ratio greater than 1 for the control sludge. When solids concentration was
greater than ∼1.5% w/v, the net energy was positive and the energy ratio
was greater than 1 for the thermal pretreated sludge. In WWTPs, the total
solids, thermal treatment time, temperature, net energy, and energy ratio
need to be optimized before the implementation of the TPT technique.
FUNDING
The authors would like to thank the Natural Sciences and Engineering Re-
search Council of Canada (NSERC) (Grants A4984, Canada Research Chair)
for financial support. The views or opinions expressed in this article are
those of the authors.
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