Water Research 104 (2016) 53e71

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

Review

Thermal hydrolysis for sewage treatment: A critical review

W.P.F. Barber
Cambi, Inc., 5 Great Valley Parkway, Suite 210, Malvern, PA, 19355, United States

a r t i c l e i n f o a b s t r a c t

Article history: A review concerning the development and applicability of sewage sludge thermal hydrolysis especially
Received 1 June 2016 prior to anaerobic digestion is presented. Thermal hydrolysis has proven to be a successful approach to
Accepted 28 July 2016 making sewage sludge more amenable to anaerobic digestion. Currently there are 75 facilities either in
Available online 30 July 2016
operation or planning, spanning several continents with the first installation in 1995. The reported
benefits of thermal hydrolysis relate to: increased digestion loading rate due to altered rheological
Keywords:
properties, improved biodegradation of (especially activated) sludge and enhanced dewaterability. In
Anaerobic digestion
spite of its relative maturity, there has been no attempt to perform a critical review of the pertinent
Dewatering
Rheology
literature relating to the technology. Closer look at the literature reveals complications with comparing
Sewage treatment both experimental- and full-scale results due to differences in experimental set-up and capability, and
Thermal hydrolysis also site-specific conditions at full-scale. Furthermore, it appears that understanding of thermodynamic
and rheological properties of sludge is key to optimizing the process, however these parameters are
largely overlooked by the literature. This paper aims to bridge these complexities in order to elucidate
the benefits of thermal hydrolysis for sewage treatment, and makes recommendations for further
development and research.
© 2016 Elsevier Ltd. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
2. Mass and energy balance of thermal hydrolysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
2.1. Energy balance around anaerobic digestion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
2.2. Energy balance following anaerobic digestion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
3. Influence of thermal hydrolysis on sludge rheology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
3.1. Influence on dewatering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
4. Role of ammonia in thermal hydrolysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
5. Other impacts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
5.1. Production of refractory organics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
5.2. Odours and growth of marker organisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
5.3. Particle size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
5.4. Foam and scum control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
5.5. Co-digestion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
5.6. Anaerobic digestion retention time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
6. Future considerations and recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
7. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

1. Introduction

Thermal hydrolysis of sewage sludge, which involves the

E-mail address: bill.barber@cambi.com.
application of heat at above autoclave temperature for a defined

http://dx.doi.org/10.1016/j.watres.2016.07.069
0043-1354/© 2016 Elsevier Ltd. All rights reserved.
54 W.P.F. Barber / Water Research 104 (2016) 53e71

time period prior to anaerobic digestion, is an established McCarty, 1984).

commercially available technology since the first full-scale plant in Although the technology has been commercially available for 20
HIAS in 1995 (Ødeby et al., 1996). The heat is typically provided by years and numerous laboratory-scale experiments have been con-
live steam injection at design temperature and concomitant pres- ducted on the process, there has been no systematic review of the
sure which is then rapidly released (exploded), although some information available. Whilst there is a great deal of useful infor-
configurations use standard heat exchange (Pereboom et al., 2014). mation in the literature, closer observation reveals that it is difficult
At the time of writing, there are 75 facilities of which 39 are to cross-reference findings on both laboratory- and full-scale. At
operating and the remaining are in various stages of design. In total, laboratory-scale there appears to be no standardized apparatus for
1.65 million metric dry tonnes of sludge per year are, or will be, thermal hydrolysis testing, nor a standard protocol for running a
processed with thermal hydrolysis. Although facilities cover most thermal hydrolysis unit at that scale. This leads to inconsistent
major continents, the vast majority are in Europe (57) within which findings, such as steam explosion being highly influential
the UK boasts 19 operational plants (Kleiven, personal communi- (Abelleira-Pereira et al., 2015; Perrault et al., 2015) or insignificant
cation). Drivers for installation are geographically market driven (Ngwenya et al., 2015); the requirement for long reaction times (Li
but typically include: increased loading rates (to minimize size of and Noike, 1992) or not (Neyens and Baeyens, 2003; Abelleira-
new digestion plants, or maximize use of existing facilities); Pereira et al., 2015; Donoso-Bravo et al., 2011), as examples.
improved sludge cake dewaterability which reduces downstream Furthermore, there is scarce information provided on the sludge
transport and processing costs; increased production of renewable being tested, especially on rheological, thermodynamic and
energy, and sterilization of sludge. Lists of reported advantages and bacteriological properties, which is surprising considering their
disadvantages are given in Tables 1 and 2. importance in controlling the efficacy of the thermal hydrolysis and
Although most emphasis has been given to use of the technol- downstream anaerobic digestion and dewatering processes. Many
ogy to improve biodegradability of sludge with process efficacy studies present biogas production from BMP tests based on the
being inversely proportional to the initial biodegradability of the pivotal methodology presented by Owen et al. (1979), however
material (Wilson and Novak, 2009), thermal hydrolysis was origi- data is rarely provided on the inoculum source, such as previous
nally seen as a means to improve sludge dewaterability (Lumb, exposure to thermal hydrolysis processed feed material. This be-
1940, 1951). Interest gathered pace in the early 1970s when sig- comes especially important if kinetic data is being collected for
nificant improvements in dewaterability were correlated with heat predictive modelling purposes as it may not be relevant for mature
application to various sludges (Everett, 1972). A few years later, the full-scale systems which have evolved bacteriological populations
concept of applying the technology to improve the biodegradability dependent on configuration (De Vrieze et al., 2015) and would
of sewage sludge e mainly that produced from activated treatment therefore respond differently. At full-scale, difficulty comparing
well known to be poorly biodegradable (Rudolfs and Heisig, 1929) sites is attributable to site-specific variables involving: sludge type
e was conceived (Haug, 1977; Haug et al., 1978; Stuckey and (configuration of wastewater treatment plant; aeration sludge age;

Table 1
Reported advantages of thermal hydrolysis.

Advantages References

Significantly improves the biodegradability of activated sludge Haug, 1977; Haug et al., 1978; Stuckey and McCarty 1978, 1984; Liao et al.,
2016; Xue et al., 2015
Improves the biodegradability of primary sludge Wilson and Novak, 2009
Allows significantly higher loading rates resulting in smaller digestion plants Xue et al., 2015; Ngwenya et al., 2015
Increases rate of biogas production Various
Reduces sludge viscosity Liu et al., 2012; Higgins et al., 2015; Oosterhuis et al., 2014; Bougrier et al., 2006
Improves sludge dewaterability on all dewatering systems Higgins et al., 2015; Phothilangka et al., 2008; Everett, 1972; Oosterhuis et al.,
2014;
Barber, 2010; Haug et al., 1978 Lumb, 1951; Sheerwood and Philips, 1970
Sterilizes sludge providing pathogen-free biosolids
Reduces odour and pathogen regrowth from dewatering Neyens and Baeyens 2003; Chen et al., 2011
Eliminates scum and foaming and produces conditions which do not encourage Jolis and Marneri 2006; Alfaro et al., 2014
foaming
Minimizes inhibition due to hydrogen sulphide
Significantly reduces downstream requirements for drying and other thermal Rawlinson et al., 2009; Merry and Oliver, 2015; Qiao et al., 2012, 2013;
processes Pickworth et al., 2006
Numerous sites successfully operating at full-scale

Table 2
Reported disadvantages of thermal hydrolysis.

Disadvantages References

Parasitic energy demand with some configurations (depends on process)

Higher ammonia concentration than standard digestion Oosterhuis et al., 2014; Wilson and Novak, 2009; Tampio et al., 2014;
Potential for and production of refractory material especially with food-waste Tampio et al., 2014; Liu et al., 2012; Dwyer et al., 2008
Potential increase in polymer demand for dewatering Oosterhuis et al., 2014
More complex than standard anaerobic digestion
Requires boilers
Sludge needs cooling prior to anaerobic digestion
Requires centrifuge thickening to 16e18% DS
Higher release of nutrients with potential for salt crystalization and subsequent
maintenance issues and deterioration of dewaterability
 W.P.F. Barber / Water Research 104 (2016) 53e71 55

industrial loads); anaerobic digestion (configuration, retention sludge processing, and the aim of this paper is to discuss the salient
time, operating temperature, quantity and type of mixing); co- themes regarding the use of thermal hydrolysis for sewage sludge
generation (configuration, size and efficiency) amongst others. In treatment.
spite of these issues, the literature provides valuable information
on expected trends and potential performance expectations based 2. Mass and energy balance of thermal hydrolysis
on use of thermal hydrolysis and these are summarized in Table 3.
The reported optimal operating conditions involve the applica- A key factor regarding thermal hydrolysis is minimization of the
tion of temperature between 160 and 180
C, for a time period of energy requirement needed to reach reaction temperature. Sub-
between 20 and 40 min (Haug, 1977; Haug et al., 1978; Stuckey and sequently, it is important to optimize the quantity and temperature
McCarty, 1984; Li and Noike, 1992; Neyens and Baeyens, 2003; of the sludge being processed. As the sludge moiety has a lower
Bougrier et al., 2006, 2008), although researchers have studied specific heat capacity than water (Xu and Lancaster, 2009),
temperature ranges of 60
Ce275
C, for 10e180 min. increasing the dry solids of the sludge will intrinsically reduce
From Table 3, it is well understood that thermal hydrolysis is energy requirements. Typically, the sludge dry solids are thickened
best suited to materials which contain high concentrations of car- to approximately 15e18% DS (shown in Fig. 3), but further thick-
bohydrates and/or proteins, and has little influence on lipids, which ening may incur heat transfer limitations as well as practical pro-
explains why it is more suited to activated, rather than primary cessing concerns. Considering the obvious differences between
sludge. Wilson and Novak (2009) suggested that carbohydrates, primary and activated sludge, especially inherent energy content
proteins and lipids responded the same way to thermal hydrolysis (Barber, 2015), and rheological properties which influence heat
irrespective of the sludge, and differences in suitability of different transfer (Dick and Ewing, 1967; Lotito and Lotito, 2014; Ratkovich
sludges for hydrolysis were purely down to their composition of et al., 2013; Markis et al., 2016) it is surprising to note that there
those materials. However, as primary sludge contains more lipids is negligible information in the thermal hydrolysis-specific litera-
than activated sludge, thermal hydrolysis produced much higher ture looking at their influence on energy requirements of the
levels of VFAs in primary sludge as breakdown products of unsat- process.
urated lipids (Wilson and Novak, 2009), which would explain its It is not possible to extract data from full-scale mass balances
suitability as a carbon-generator for nutrient removal (Barlindhaug due to numerous site-specific conditions as mentioned previously.
and Ødegaard, 1996; Pinnekamp, 1989). Therefore, Fig. 3 shows calculated water and sludge balance for a
Based on the literature (Bougrier et al., 2008; Liu et al., 2012; Li plant processing all of a 60:40 blend of primary and activated
and Yang, 2007; Chu et al., 2002; Lu et al., 2015), application of sludge with an annual quantity of 10,000 tonnes, based on theo-
increasing temperature to sewage sludge causes a sequence of re- retical considerations and design criteria. After the initial waste-
actions depicted in Fig. 1 viz. water has been thickened to reduce energy requirements of
Increasing thermal hydrolysis reaction temperature up to opti- hydrolysis, it is then diluted twice. Firstly, as a consequence of
mum temperature range: steam addition to reach the reaction temperature, and secondly,
from the addition of treated water to control ammonia inhibition in
 Improves downstream sludge anaerobic digestibility downstream digestion (see later).
 Decreases apparent viscosity
 Increases solubility of carbohydrates 2.1. Energy balance around anaerobic digestion
 Increases solubility of proteins
 Has negligible influence of solubility of lipids The steam demand is influenced by influent sludge tempera-
 Reduces average particle size ture; temperature difference; sludge thickness; and the thermo-
 Increases potential for refractory compound formation (COD, dynamic and physical properties of fluid. Thermodynamics shows
nitrogen, colour) that the inlet sludge temperature presents a linear response against
steam requirement with a negative slope as shown below:
Increasing thermal hydrolysis reaction temperature beyond
optimal temperature to sub-critical water range: Q ¼ 10:476T þ 1729 (1)

 Decreases downstream sludge anaerobic digestibility where Q ¼ steam demand/ton dry sludge at 16.5% DS (although
 Significantly increases production of refractory material and unknown sludge composition); T ¼ inlet sludge temperature pre-
colour senting to thermal hydrolysis (
C).
 Further reduces viscosity The energy required to provide steam can be provided in
 Further improves dewaterability numerous ways including: direct use of boilers running on either
bio- or natural gas; co-generation using reciprocating internal
The increasing production of refractory material with increasing combustion engine with auxiliary boiler running on bio- or natural
temperatures is clearly evident in the work of Stuckey and McCarty gas; or use of gas turbine on larger facilities, although the latter
(1984) who noted a continual deterioration in gas production from comes with a loss of power generation unless operated in
thermally hydrolysed activated sludge which was 27% higher than a combined-cycle (Ferna
ndez-Polanco and Tatsumi, 2016). The
control at 175
C, similar at 250
C, and lower that the control above quantity of energy required is typically described as a fraction of the
that temperature. At thermophilic conditions the impact of biogas generated, however comparing literature data reveals that
increasing temperature on reducing performance was further the energy required does not follow a predictable pattern
exacerbated. However, as well as increasing the quantity of re- (Lancaster, 2015; Pook et al., 2013; Merry and Oliver, 2015). This is
fractory material, increasing reaction temperature has also been due to several parameters which include: type, efficiency, config-
linked to improved dewaterability (Everett, 1972; Higgins et al., uration and availability of co-generation plant; presence and
2015) and enhanced biogas production (Stuckey and McCarty, quantity of gas storage; gas production profiles; configuration,
1978; Hung-Wei et al., 2014), implying different optimal oper- operating temperature and retention time of anaerobic digestion
ating conditions exist depending on required project outcomes. plant, and by far the most important parameter e albeit habitually
Fig. 2 shows how thermal hydrolysis influences downstream overlooked, the sludge composition itself. In order to elucidate the
56 W.P.F. Barber / Water Research 104 (2016) 53e71

Table 3
Observations attributable to thermal hydrolysis of sludge.

Parameter Observation Reference

Solubility of COD Increases in a linear-type fashion with both temperature (130 and 170
C) Hung-Wei et al., 2014
and reaction time (10e60 min)
Solubility increases between 150
C and 180
C and reaction time (0 Everett, 1972
e90 min). However at 170
C and above no further solubilisation between
60 and 90 min
For temperatures lower than 200
C, COD solubilisation was found to Bougrier and Carrere 2007
increase linearly with treatment temperature for different sludge samples
tested.
Increases with temperature between 125
C and 175
C Liu et al., 2012
Solubility of carbohydrates Increase in solubility between 130
C and 210
C but little influence Bougrier and Carrere 2008
between 95
C and 130
C.
Solubility of carbohydrates occurs below 150
C above which solubility of Hung-Wei et al., 2014
proteins becomes more evident.
Increasing solubility with temperature between 130 and 170
C. Strong Liu et al., 2012
linear increase between 10 and 30 min, after which further increases are
negligible
Increases with temperature between 125
C and 175
C Noike et al., 1985
Solubility of proteins Increases in a linear-type fashion with both temperature (130 and 170
C) Hung-Wei et al., 2014
and reaction time (10e60 min)
Increases with temperature between 125
C and 175
C Liu et al., 2012
Solubility of lipids Little influence from either temperature or reaction time. Bougrier et al., 2008
Negligible increases with temperature and reaction time. Hung-Wei et al., 2014
No influence Li and Noike, 1992;
Wilson and Novak, 2009
Viscosity Falls significantly with increasing temperature between 130 and 170
C Higgins et al., 2015
Significant reduction Oosterhuis et al., 2014
Following thermal hydrolysis (175
C for 60 min), viscosities of WAS, Liu et al., 2012
Kitchen Waste and Vegetable and Fruit Residues reduced from 13,500;
36,000; and 6250 to 1625; 1658 and 663 mPa s respectively.
Viscosity reduces further with thermal pre-treatment than it does with Bougrier et al., 2006
either ozonation or ultrasonic treatment.
Thermal treatment of sludge (170
C and 190
C) changes rheological
properties of WAS from non- to pseudo-Newtonian based on use of Ostwald
de Waele relationship
Although thermal hydrolysis reduces apparent sludge viscosity by orders of Dawson and Ozgencil, 2009
magnitude, the viscosity of digested sludge is independent of the presence
or absence of pre-treatment
Anaerobic biodegradability Measured by gas production, increases with temperature between 130 and Hung-Wei et al., 2014
170
C (with constant reaction time of 30 min), with higher production of
biogas noted after 10 days with temperature. However little difference in
biogas production noted in first 24 h.
Minor improvement in volatile solids reduction during digestion with Higgins et al., 2015
increasing thermal hydrolysis processing temperatures. However, methane
yield (m3 methane/kg VS added) higher at 170
C compared to lower
temperatures which were all similar.
WAS digestion significantly improved by 75e80% by over-pressurising to Phothilangka et al., 2008
21 bar although no discussion as to the additional energy requirements of
over-pressurisation
Increase in gas production yield of over 100 l/kg VS fed at lab-scale with Oosterhuis et al., 2014
20:80 primary:activated sludge mixture.
VSR of WAS increases from 26 to 42% (relative increase of 62%)
Relative increase in WAS digestion of 55% at 140
C van Dijk and de Man, 2010
When pre-treating sludge by thermal hydrolysis before digestion, Haug Haug et al., 1978
calculated a 25% increase in energy production compared to conventional
digestion.
Thermal pre-treatment at temperatures below 100
C revealed an increase Hiraoka et al., 1985
of more than 30% in gas production at lower temperatures such as 60 and
80
C, but the low temperature pre-treatment necessitated a longer contact
time than the high temperature treatment.
VFAs Increase in levels of acetic and propionic acids with reaction time, little Hung-Wei et al., 2014
influence on C4 and above acids.
VFA release from primary hydrolysed sludge between 4 and 7 times higher Wilson and Novak, 2009
than from activated sludge due to hydrolysis of unsaturated lipids
Particle size Increase in median size after thermal pre-treatment Bougrier et al., 2006
Decrease in average size from 70 to 35 mm Barber, 2010
Decrease in average size from 107 to 66 mm Neyens et al., 2004
Pressure drop Release of both carbohydrates and proteins increases in an approximately Perrault et al., 2015
linear fashion with increasing pressure drop between 3 and 6 bar. Release of
lipids was uninfluenced by pressure drop.
Biogas production rate is quicker with increasing pressure drop Perrault et al., 2015
Refractory material Solubility of carbohydrates reduces at higher temperatures hypothesized to Bougrier et al., 2008
be due to conversion to refractory material
Equivalent to 11 kg COD/t DS dewatered sludge Oosterhuis et al., 2014
Liu et al., 2012
 W.P.F. Barber / Water Research 104 (2016) 53e71 57

Table 3 (continued )

Parameter Observation Reference

Thermal hydrolysis of both kitchen waste and Vegetable Fruit waste

increases production of high molecular weight Maillard reaction products in
response to increased solubility of both sugars and proteins.
Colour Measured increase in melanoidin for kitchen waste and vegetable fruit Liu et al., 2012
waste
Reducing thermal hydrolysis reaction temperature reduces colour in Dwyer et al., 2008
digestate
Increasing thermal hydrolysis reaction temperature increases colour
production in an approximately linear fashion from 2700 mg PtCo/l at
140
C to 12,500 mg PtCo/l at 165
C
Increasing adsorbance at 254 nm with increasing thermal hydrolysis Wilson and Novak, 2009
reaction temperature
Food and other co-substrates Biogas yield from thermally hydrolysed food-waste lower (5e10%) than un- Tampio et al., 2014
processed material at loading rates between 2 and 6 kg VS/m3.d
Digestability of both kitchen waste and vegetable fruit residues (measured Liu et al., 2012
by BMP spikes) both reduced after thermal hydrolysis (175
C/60 min) by 14
and 8% respectively
Dewaterability Improves with increasing temperature between 130
C and 170
C in a Higgins et al., 2015
linear fashion from approximately 27 to 32% DS
Increase from 25.2 to 32.7% dry solids at 180
C and 21 bar Phothilangka et al., 2008
Dewaterability improves with temperature between 180
C and 210
C Everett, 1972
Improvement from 26% to 33.9% Oosterhuis et al., 2014
Centrate from thermally hydrolysed digested sludge a third higher in COD Barber, 2010
and three times higher in ammonia compared to digested sludge
When thermally hydrolysed digested sludge is mixed with sludge which has
not been exposed to treatment the dewaterability of the mixture is slightly
lower than predicted by weighed average of the separate sludges
Heat treatment of sludge improves dewaterability whether digested or not Haug et al., 1978
Dewaterability of thermally hydrolysed raw sludge and post digested sludge Lumb, 1951; Sheerwood
far better than sludge which has been thermally hydrolysed and then and Philips, 1970
digested
Significantly improved by thermal treatment Brooks, 1968, 1970
Foam and scum control A pilot-scale plant was successful in treating scum comprised of Gordonia- Jolis and Marneri, 2006
type organisms
Thermal hydrolysis at 170
C successfully used to abate foaming issues in Alfaro et al., 2014
downstream pilot-scale digesters. Significant reduction in abundance of
filamentous organisms M. parvicella noted, although in contrast to other
measurements which showed higher foam potential and stability with
thermal hydrolysis
Heat treatment prior to anaerobic digestion removed foaming by destroying Barjenbruch and Kopplow, 2003
hydrophobic materials
Loading rate for digestion Thermal hydrolysis increases loading rate by a factor of 2.3 compared to Oosterhuis et al., 2014
standard anaerobic digestion. Typical design loading rate in the range of 5
e6 kg VS/m3.d
Inhibition Inhibition in downstream digestion above 175
C Stuckey and McCarty 1984;
Haug et al., 1978
No inhibition observed with digester feed of 10.4% DS Oosterhuis et al., 2014
No inhibition observed with digester feed up to 13% DS Chauzy et al., 2008
Polymer demand Increased by thermal hydrolysis at lab-scale from 10.5 to 18 kg active Oosterhuis et al., 2014
polymer/TDS
However, no difference noted at full-scale, with dose of 8 kg active polymer/ Lancaster, 2015
TDS for centrifuge dewatering.
Polymer demand for thickening <6 kg active polymer/TDS
Odours and bacterial regrowth Odorous compounds normally associated with heat treatment are Neyens and Baeyens, 2003
during dewatering significantly reduced during digestion of thermally pre-treated sludge.
Significantly lowest odour potential during dewatering for both belt presses Chen et al., 2011
and high speed centrifuges
Extracellular polymers Completely destroyed by autoclaving (121
C) Barjenbruch and Kopplow, 2003
Destroyed by thermal hydrolysis Neyens and Baeyens, 2003
Hydrolysis Rate increases with decreasing particle size Aldin, 2010
Settleability Settleablity (measured by SSVI) significantly improves with increasing Bougrier et al., 2008
temperature
Digestion temperature Pre-treatment better suited to meso- rather than thermophilic digestion at Bi et al., 2013
thermal hydrolysis temperature up to 120
C
Dissolved organic nitrogen Increases with thermal hydrolysis reaction temperature for activated sludge Dwyer et al., 2008
from approximately 2000 mg/l at 140
C to nearly 3000 mg/l at 165
C
Carbon source Can provide carbon source for nutrient removal immediately after Barlindhaug and Ødegaard, 1996;
hydrolysis with a C:N ratio of 12.5:1 Pinnekamp, 1989

wide-ranging energy demands observed in the literature, it is on steam required for a loss-free system, and clearly highlights the
necessary to look closer at the energy balance for a theoretical importance of thickening the sludge to exploit the lower specific
situation which is independent of the variability of full-scale in- heat capacity of the sludge fraction. This trend has been presented
stallations. Fig. 4 shows the calculated impact of dry solids content elsewhere (Panter, 2013).
58 W.P.F. Barber / Water Research 104 (2016) 53e71

Fig. 1. Sequence of reactions due to increasing temperature of sludge.

From heat balance calculations, the influence of heat recovery DT ¼ internal temperature difference required. The factor of 10.85 is
within thermal hydrolysis on steam requirement is linear as relevant for a loss-free system processing 60:40 primary:activated
follows: sludge mix. Combining Fig. 4 and Equation (2) and introducing a
term to account for efficiency losses, the steam demand can be
S ¼ 10:85DT (2) approximated viz.

where S ¼ kg steam/metric tonne dry solids processed;

Fig. 2. Reported effects of thermal hydrolysis (black boxes) and their influence.
 W.P.F. Barber / Water Research 104 (2016) 53e71 59

Fig. 3. Typical water and sludge balance for standard thermal hydrolysis followed by anaerobic digestion. Key: Blue line (water); brown line (sludge) for plant processing 10,000
tonnes dry solids of 60:40 primary:activated sludge mix. Based on live steam injection systems which make up over 99% of installed capacity. (For interpretation of the references to
colour in this figure legend, the reader is referred to the web version of this article.)

Conducting a sensitivity analysis around Fig. 5 (not shown)

highlights the sludge composition as being fundamentally influ-
10:85 DT h i
S¼ 134  DS1:05 (3) ential on energy balance and subsequently the outstanding energy
911h requirement after accounting for high grade heat. Varying sludge
composition reveals an optimal feed composition for thermal hy-
where S ¼ steam requirement [kg/metric tonne]; DT ¼ internal
drolysis which is different depending on whether all or only the
temperature difference [
C]; h ¼ system efficiency; DS ¼ Dry solids
activated sludge is hydrolysed as shown in Fig. 6.
of sludge entering thermal hydrolysis expressed as a decimal.
The proportion of parasitic demand met by combined digestion
Equation valid for a 60:40 primary:activated sludge mix with the
and co-generation system for thermally hydrolysing all sludge pre-
parameters changing for different compositions of sludge. Fig. 5
digestion with same conditions described in Fig. 5 above shows a
shows the main energy flows for the hypothetical facility of
linear response viz.
10,000 tonnes of sludge with both primary and activated sludge
being thermally hydrolysed. In the example, biogas yield and
composition is determined independently for primary and acti- Q ¼ 0:58 f þ 0:529 (4)
vated sludge using elemental composition and stoichiometry
(Barber, 2015). The expected volatile solids destruction of both un- where Q ¼ quantity of parasitic load met by combined system
and thermally hydrolyzed sludges are also sludge dependent and (biogas production combined with co-generation) and
based on expectations from literature (Vesilind, 2003; van Haandel 4 ¼ proportion of sludge that is primary expressed as a decimal [%].
and van der Lubbe, 2007; Speece, 2008; Phothilangka et al., 2008; Fig. 6 highlights the importance of influent characteristics and
Winter and Pearce, 2010; van Dijk and de Man, 2010; Oosterhuis suggests that there are optimal sludge compositions with respect to
et al., 2014; Hung-Wei et al., 2014; Lancaster, 2015). energy balance. Ironically, energy benefits improve with increasing
In this instance the high grade heat can provide close to 90% of quantities of primary sludge due to it being more biodegradable
the energy required to raise the necessary steam. The power output irrespective of thermal hydrolysis (Haug et al., 1978; WEF, 1987)
is 950 kWhre/t DS processed, compared with an equivalent scenario and it having an inherently higher energy content, both in combi-
with no thermal hydrolysis of 825 kWhre/t DS processed, typical of nation, resulting in a higher biogas yield than an equivalent
UK facilities (Mills et al., 2014). quantity of activated sludge (Barber, 2015). The greater biode-
gradability and subsequent gas production from primary sludge
elevates the heat produced concomitantly reducing the
outstanding energy demand.
In Europe, developments have been made to reduce the energy
demand by altering the configuration of thermal hydrolysis. One
configuration which is gaining traction is the concept of hydro-
lyzing only the activated sludge (as it is more amenable to treat-
ment e Shana et al., 2013). The influence on energy balance is
demonstrated in Fig. 7:
Although the primary treatment is not exposed to thermal hy-
drolysis, the biogas production only falls slightly in accordance with
previous studies (Phothilangka et al., 2008; Oosterhuis et al., 2014;
van Dijk and de Man, 2010; Wilson and Novak, 2009) with an
equivalent power generation (based on same assumptions) of
approximately 900 kWhre/t DS processed. When comparing ther-
mal hydrolysis of all sludge against only activated sludge, Mill's
group (2013) observed a minor fall in biogas yield when processing
Fig. 4. Influence of sludge dry solids on steam requirement for thermal hydrolysis for a only the activated sludge fraction (421 Nm3/tDS and 454 Nm3/tDS
60:40 primary:activated sludge mix. for activated and all sludge processed respectively). However, as
60 W.P.F. Barber / Water Research 104 (2016) 53e71

Fig. 5. Typical energy balance for plant processing 10,000 tonnes dry solids sludge with primary:activated sludge ratio of 60:40. Both primary and activated sludge are thermally
hydrolyzed. Energy balance based on use of internal combustion engine based on: 85% availability; 38% electrical efficiency; 27% high grade heat; 17% low grade heat.

to series operation (Chapman and Muller, 2010). Future optimiza-

tion of the energy balance may involve the use of pinch technology,
common in the process industries, but until now, not necessary in
the water industry (Ferna
ndez-Polanco and Tatsumi, 2016).

2.2. Energy balance following anaerobic digestion

Although thermal hydrolysis enhances performance and biogas

production, the auxiliary energy demand (for the original config-
uration described in Fig. 5) makes the energy benefit relatively
modest when concentrating only on anaerobic digestion and co-
generation. However, where the energy benefit from deploying
thermal hydrolysis is clearly evident, is downstream of anaerobic
digestion. A combination of improved volatile solids destruction
combined with enhanced dewaterability characteristics results in
Fig. 6. Influence of sludge composition on energy benefit (increased energy generation
significantly less digested sludge with, importantly, less water
based on IC engines minus energy lost through diversion of biogas for steam re-
quirements) of thermal hydrolysis based on processing all sludge feed (solid line) or contained within it. The influence of digestion and thermal hy-
only activated sludge fraction (dotted line). Operating conditions of IC engine are as for drolysis coupled with digestion on energy required for drying is
Fig. 5 4 ¼ percentage of sludge that is primary as opposed to activated. shown in Fig. 8 determined from theory and based on expected
dewaterability (Higgins et al., 2015; Phothilangka et al., 2008;
Everett, 1972) and digestion performance (van Dijk and de Man,
only the activated sludge is processed, the hydrolysis unit is 2010; Stuckey and McCarty, 1984; Haug et al., 1978) and comple-
significantly reduced in size, and the steam demand met with no ments a previous study (Barber et al., 2012). The
auxiliary fuel. Another approach showing promise is that of posi- Figure demonstrates that combining thermal hydrolysis with
tioning the thermal hydrolysis unit downstream of anaerobic digestion can reduce the water evaporation requirements of an
digestion to concentrate on the e inherently less biodegradable e equivalent amount of raw sludge processed by approximately 60%
effluent prior to a second stage of digestion (Mills et al., 2013; from 2050 to 780 kWhr/t raw sludge equivalent processed before
Chauzy et al., 2014). The first recorded application of this losses for raw and thermally hydrolysed digested sludge respec-
approach was in Hillerød (Gurieff et al., 2011). Subsequently, pilot- tively. The numbers appear consistent with literature data for full-
scale studies showed an increase in biogas yield of over 10% to scale operating drying plants when losses are accounted for
503 Nm3/tDS when this configuration was compared with standard (Rawlinson et al., 2009). Rawlinson's group showed data where
pre-digestion positioning (Mills et al., 2013) although it was not 13.14 MW of energy was required to dry 40,000 tonnes sludge. This
determined how much of the additional biogas was due to posi- is equivalent to approximately 2900 kWhr/t raw sludge which
tioning of hydrolysis and how much due to improved digestion due aligns with the theoretical value assuming 70% efficiency.
 W.P.F. Barber / Water Research 104 (2016) 53e71 61

Fig. 7. Typical energy balance for plant processing 10,000 tonnes dry solids sludge with primary:activated sludge ratio of 60:40. Only activated sludge thermally hydrolyzed. Energy
balance based on use of internal combustion engine based on same conditions as Fig. 5.

The reduction in downstream energy requirements afforded by improvement in dewatering.

thermal hydrolysis has been exploited at several full-scale facilities.
In Ringsend, Dublin, a planned expansion in drying capacity of 3. Influence of thermal hydrolysis on sludge rheology
300% was reduced to 50% after thermal hydrolysis was installed
(Pickworth et al., 2006; Panter and Kleiven, 2005). In the UK, a Rheology change due to thermal hydrolysis is arguably the most
trend has been observed whereby the process has been used to important consequence of the technology on sludge treatment as it
down-scale or remove sludge drying facilities altogether allows higher digester loading rates e due to increased ease of
(Rawlinson et al., 2009; Merry and Oliver, 2015; Merry and downstream transport e and aids in dewatering (Stickland, 2015).
Fountain, 2014). However, in spite of this, there is very little in the thermal hydro-
Reduced sludge product with better dewaterability has also lysis related literature focused on its influence. In the informative
been exploited to create capacity and reduce operating costs in review by Eshtiaghi et al. (2013), sludge is described as a non-
existing incineration facilities (Barber, 2010; Lancaster, 2015; Mills Newtonian shear thinning thixotrophic fluid, which behaves as a
et al., 2014) and is gaining traction where there is little option for thixotropic colloidal suspension at high shear rates but exhibits
land recycling of biosolids (Qiao et al., 2012, 2013). Although, at first polymeric behavior at low shear. This behavior is fundamentally
glance, it may appear contradictory to intensify energy recovery influenced by temperature.
prior to burning, the improvement in dewatering produces a sludge Increasing temperature by thermal hydrolysis reduces viscosity
cake with similar calorific value to raw sludge as shown in Table 4 in a number of ways. Firstly, free water within the sludge fraction
calculated based on dewatering expectations from literature com- decreases in viscosity in accordance with Arrhenius’ law in a
bined with a specifically derived Dulong equation for sewage reversible fashion (Al-Shemmeri, 2012; Eshtiaghi et al., 2013). In
sludge (British Standards, 2004): addition, material may be thermally destroyed within the sludge in
Clearly, improving destruction of volatiles through gas produc- a partly reversible way, for example the denaturation of protein or
tion reduces calorific value on a dry basis due to a reduction of destruction of extracellular polymers (Farno et al., 2015). Finally,
volatile fraction. This is evident in Table 4 where digestion without interactions between compounds released due to heat treatment
and with thermal hydrolysis reduces calorific value by approxi- have been found to influence rheology (Forster, 1983). Conversely,
mately 20 and 30% respectively. However, when different dew- the decrease in particle size due to steam explosion (Neyens et al.,
aterability characteristics are accounted for the calorific value of 2004) is expected to increase viscosity by increasing surface area of
thermally hydrolysed cake rises to be within 5% lower than raw particles enhancing interactions (Pevere et al., 2006). However,
sludge. Installation of thermal hydrolysis on a digestion facility looking at rheology data from thermal hydrolysis, this increase is
feeding an incineration plant released up 14,000 dry tonnes of clearly outweighed by the aforementioned factors. Lotito and Lotito
spare capacity in the incinerator by increasing biogas production (2014) elegantly summarize the parameters influencing sludge
(less dry matter to burn) and improving dewaterability (higher rheology to be: sludge type; density; solids content; particle size
energy content e Barber, 2010). Additionally, the study showed that and distribution; settleability; abrasiveness; particle friability;
loss in calorific value due to thermal hydrolysis when compared to surface charge; liquid phase conductivity; pH and surface chemis-
digested sludge could be offset by a two percentage point try (Forster, 1983) amongst others. No literature was found which
62 W.P.F. Barber / Water Research 104 (2016) 53e71

explicitly studies these factors with respect to thermal hydrolysis,

despite thermal hydrolysis directly influencing those parameters.
Nevertheless, a number of studies have shown that increasing
the thermal hydrolysis operating temperature decreases apparent
viscosity between temperatures of approximately 130
Ce180
C
(Urrea et al., 2015; Higgins et al., 2015). The decrease in viscosity
due to temperature difference alone would result in a 30% drop
using Arrhenius' relationship, but this is insufficient to describe the
relative reductions observed from analysis of various workers data
(Bougrier et al., 2006; Dawson and Ozgencil, 2009). Additionally,
when sludge is cooled down, irreversibility is noted as commented
on by Baudez et al. (2013) who found that yield stress of sludge
exposed to heat treatment and then cooled to its initial tempera-
ture was lower than in an un-exposed aliquot. Farno et al., 2014,
also reported similar results on sludge treated at temperatures
between 20 and 80
C which were returned to starting tempera-
tures. Liao's group (2016) saw a drop in viscosity of 48.6% due to
heating to 80
C, and when cooled to 33
C sludge viscosity
remained a third lower than an unheated sample. This phenome-
non has been termed “thermal history”. Higgins' group (2015)
showed data which implies “thermal history” whereby the vis-
cosity of digested effluent was related to the initial thermal hy-
drolysis reaction temperature prior to digestion. In the study of
Farno et al. (2014) the measured soluble COD of sludge which
was heated and cooled down was significantly higher than the
soluble COD of initial sludge such that a correlation was found
between an increase in soluble COD and a decrease in yield stress.
The difference could potentially be attributed to irreversible ther-
mal denaturing of protein (Anson, 1954) which influences viscosity
by unfolding and increasing quantities of water bound by protein
molecules. By increasing the hydrodynamic radius of the mole-
cules, viscosity is increased (Anson and Mirsky, 1932 cited in Farno
et al., 2014). The denaturing of proteins in this way, along with the
rate and order of solublization of material during thermal hydro-
lysis may partially explain the drop in viscosity with increasing
reaction temperatures. Bougrier et al. (2006) demonstrated that
sludge became more Newtonian in behavior as reaction tempera-
ture increased, noted by an increase in the dimensionless flow
behavior index n when using the Power Law. This was also
observed by Urrea et al. (2015) looking at thermal hydrolysis of
activated sludge. Fig. 9 is plot based on work presented by Bougrier
et al. (2006) using the Ostwaldede Waele relationship as follows:

Fig. 8. Energy and material balance for drying dewatered sludge cake to 95% dry solids
from ambient temperature of 25
C based on one dry tonne raw sludge equivalent. NB
e Energy balance based on theoretical requirements and does not include for drying
losses. Figure: A) Raw; B) Digested, and C) Thermally hydrolysed digested sludge. Key:
pale blue (water); brown (sludge dry matter); red (latent heat requirement); light red
(heat required to reach boiling point). (For interpretation of the references to colour in
this figure legend, the reader is referred to the web version of this article.)

Table 4
Calculated impacts of thermal hydrolysis on calorific value of sewage sludge for a
60:40 primary:activated sludge mixture.

Sludge type Calorific value [kJ/ Calorific value [kJ/kg wet

kg DS] cake]

Raw 18,100 4530

Digested 15,000 3445
Fig. 9. Influence of thermal pre-treatment (170
C) on sludge (at 20 g/l TS with 76% VS)
Thermally hydrolysed 13,300 4385
on apparent viscosity. Key: untreated sludge (full line); thermally hydrolysed (dashed
and digested
line). e Plot using data presented in Bougrier et al., 2006.
 W.P.F. Barber / Water Research 104 (2016) 53e71 63

hydrogen and electrostatic forces characteristic of a gel-matrix. It is

considered that 80% of the mass of activated sludge is comprised of
t ¼ 4:7  103 g0:89 (5)
extracellular material which influences viscosity (Neyens et al.,
where t ¼ Yield stress (Pa); g ¼ shear rate (1/s) and apparent vis- 2004). This results in the viscosity of activated sludge being much
cosity h is t/g. The interested reader is referred to the manuscript higher than that of primary (Lotito and Lotito, 2014; Dawson and
by Bougrier et al., 2006. Sotiriadis, 2007). Markis et al. (2016) demonstrated that they
The graph clearly demonstrates the viscosity drops as a conse- could increase viscosity of a sludge mixture by increasing the
quence of thermal hydrolysis and suggests that the Ostwaldede proportion of activated sludge within it. These factors become
Waele relationship is valid in the range shown. However, as high- progressively important for thermal hydrolysis systems processing
lighted in the excellent review by Eshtiaghi et al. (2013) use of this only activated or high levels of activated sludge as is becoming
relationship fails at predicting Non-Newtonian fluid behavior at increasingly vogue.
very high shear rates where the viscosity of the fluid remains In an informative study, Lotito and Lotito (2014) showed that the
higher than water. Indeed, the review mentions that at both high viscosity of digested sludge was higher than raw undigested sludge,
and low shear rates different analytical equipment are necessary to and that both shear rates and shear stresses increase with dry solids
make rheology measurements to overcome limitations in testing content (Jiang et al., 2014). This may be due to the generation of
protocol due to wall-slip or end effects. soluble microbial products during digestion (Barker and Stuckey,
When thermally hydrolysed sludge is prepared for digestion, its 1999) which ultimately influence dewaterability characteristics of
viscosity will increase by a factor of at least 3 in accordance with the digestate (Neyens and Baeyens, 2003; Neyens et al., 2004;
Arrhenius’ Law, although the addition of dilution water will influ- Christensen et al., 2015; Lu et al., 2015).
ence this. Analysis of unpublished data shows an increase in vis-
cosity of between 2 and 6 times depending on initial reaction 3.1. Influence on dewatering
temperature. However, the viscosity still remains lower than that of
sludge unexposed to thermal conditioning as shown in Fig. 9. There is unanimous agreement in the literature relating to the
It is well understood that the rheological properties of primary positive influence thermal hydrolysis has on sludge dewaterability
and activated sludge are profoundly different and require different ever since the earliest work when it was viewed as a dewatering aid
relationships to model their behavior (Markis et al., 2014; Eshtiaghi (Lumb, 1940, 1951; Sheerwood and Phillips, 1970; Everett, 1972,
et al., 2013), again, this has been overlooked by the thermal 1974) and later as a means to improve anaerobic digestion (Haug
hydrolysis-specific literature. In a study looking a optimizing sludge et al., 1978). Table 5 shows some of the pertinent literature con-
pumping and mixing systems, Markis et al. (2014) suggested that cerning the influence of thermal hydrolysis on dewatering. Typi-
primary sludge behaved like a colloidal suspension with flocs cally, thermal hydrolysis improves dewaterability of mesophilically
governed by weak Van der Waals forces, whilst activated sludge digested sludge by approximately 10 percentage points dependent
rheology was influenced by extracellular polymers held together by on influent sludge composition and dewaterer, although

Table 5
Influence of pre-digestion thermal hydrolysis on sludge dewaterability.

Material Scale HRT TH Temp Before After Change Reference

Mixed municipal sludge Laboratory 15 130 27.0% Higgins et al., 2015

Mixed municipal sludge Laboratory 15 140 28.0%
Mixed municipal sludge Laboratory 15 150 30.0%
Mixed municipal sludge Laboratory 15 160 31.0%
Mixed municipal sludge Laboratory 15 170 33.0%
Mixed municipal sludge Full 22 165 31.3% Lancaster, 2015
Mixed municipal sludge Full 165 19.0% 30.0% 11.0% Fjordside, 2005
Unspecified Pilot 120 30.0% 43.0% 13.0% Kepp and Solheim, 2000
Mixed municipal sludge Full 20 165 26.0% Riches et al., 2008
Unspecified Pilot Not Given 25.2% 32.7% 7.5% Phothilangka et al., 2008
WAS Laboratory 20 165 26% 33.9% 7.9% Oosterhuis et al., 2014
Unknown Laboratory 30% 43.0% 13% Neyens et al., 2004
WAS Laboratory 20 170 20% 39.6% 20% Liu et al., 2012
Mixed Full- 20% 30.0% 10% Mills et al., 2013
Mixed Lab - WAS only 20% 28.0% 8%
Mixed Pilot - ITHP 20% 32.0% 12%
WAS Full- 13 160e170 28 - 30% Nawawi et al., 2007
Mixed with imports 12 165 >30% Walley, 2007
WAS Pilot- 25% Geraats, 2014
Mixed (50:50) Pilot- 28 -32%
WAS Full 22% 29 - 30% 8% Pereboom et al., 2014
Mixed Pilot- 80 h 130/30 min 31.5% Qiao et al., 2013
Mixed Pilot- 80 h 130/60 min 32.3%
Mixed Pilot- 80 h 130/90 min 34.0%
Mixed Pilot- 80 h 150/30 min 46.6%
Mixed Pilot- 80 h 150/60 min 38.2%
Mixed Pilot- 80 h 150/90 min 42.2%
Mixed Pilot- 80 h 175/30 min 44.0%
Mixed Pilot- 80 h 175/60 min 47.8%
Mixed Pilot- 80 h 175/90 min 45.1%
Mixed Pilot- 80 h 190/30 min 47.8%
Mixed Pilot- 80 h 190/90 min 48.7%
Mixed Pilot- 80 h 150/90 min 46.4%
64 W.P.F. Barber / Water Research 104 (2016) 53e71

dewatering is significantly improved compared with no thermal sludge which had not been exposed to those conditions which
hydrolysis irrespective of dewatering device. peaked between 20 and 25% dry solids (Sheerwood and Philips,
There are many excellent reviews available on the topic of 1970). More recently, installation of thermal hydrolysis down-
dewatering, relating to: development of rheological tools to predict stream of digestion has been found to improve dewaterability
performance (Stickland, 2015); quantification of (Skinner et al., compared with application upstream (Rus et al., 2016).
2015); limitations of (Vesilind and Hsu, 1997); performance over- Haug et al. (1978) also noted that whilst thermal hydrolysis
view (Christensen et al., 2015); even the influence of thermal hy- improved dewaterability potential with increasing reaction tem-
drolysis itself (Neyens and Baeyens, 2003) and the interested reader perature between 100 and 225
C, this potential was always lower
is referred to those texts. than prior to digestion. Interestingly, the authors maintained that it
According to Neyens et al. (2004) advanced digestion pre- was the dewaterability of primary, rather than activated sludge
treatment technology improves dewaterability in two ways, (i) which improved and responded best to thermal processing irre-
the degradation of proteins and polysaccharides from within spective of digestion. This may be due to activated sludge having
extracellular polymeric substances (EPS) thereby reducing its water inherently higher viscosity (Lotito and Lotito, 2014), higher quan-
retaining capacity (Barjenbruch and Kopplow, 2003; Neyens et al., tity of ECPs prior to digestion (Neyens and Baeyens, 2003), and
2004; Chu et al., 2002); and (ii) promotion of flocculation which fundamentally different rheology (Markis et al., 2016; Eshtiaghi
reduces the amount of fine flocs (Bougrier et al., 2008). et al., 2013). As such, the findings of Everett (1972), suggesting
EPS, which constitute approximately 80% of the composition of higher temperatures of 190
C as optimal for activated sludge
activated sludge (Neyens et al., 2004), influence flocculation, dewatering are typical. The application of additional heat (to
settling properties and dewaterability of sludge (Tian and Zheng, 210
C) did not further improve dewaterability but allowed a
2006). Subsequently, the destruction of this material plays a key shorter reaction time to be used (Everett, 1972). Everett interpreted
role in improving dewatering potential (Tian and Zheng, 2006). The the improvements by a reduction is specific resistance to dew-
authors, along with Neyens and Baeyens (2003) proposed that atering caused by destruction of structural integrity of the micro-
increasing low levels of ECP are initially thought to aid sludge organisms, which is consistent with additional destruction of ECP
dewaterability by improving the level of sludge flocculation and also further reduced viscosity (Higgins et al., 2015).
reducing the number of small particles present in the sludge, a
factor that has been shown previously to make sludge easier to 4. Role of ammonia in thermal hydrolysis
dewater. However, once a certain level of sludge flocculation has
been attained, further increases in ECP become detrimental to It is well understood that free unionized ammonia NH3 (also
sludge dewaterability (Neyens and Beyens, 2003). This is also known as free ammonia e FA) controls the upper pH limits of
proposed by Scales (personal communication). Analysing the graph anaerobic digestion. However, there is evidence to suggest that
presented by Houghton et al. (2001) this concentration is between charged ammonium (NHþ 4 ) may inhibit methane synthesizing en-
30 and 40 mg ECP/g suspended solids although there is no infor- zymes directly (Chen et al., 2014).
mation on the quantity of the solids which is volatile, or even the As thermal hydrolysis allows an increase in loading rate due to
composition of ECP measured. In a review by Tian and Zheng altered rheology (Dawson and Sotiriadis, 2007), increases solubility
(2006) ECP are typically 20e55% carbohydrate, 20e80% protein of proteins (Haug, 1977; Li and Noike, 1992; Lu et al., 2015), and
with the remaining 30% comprising a mixture of humic, uronic and improves the breakdown of those proteins (Bougrier et al., 2008),
nucleic acids along with lipids amongst other minor compounds, an increase in ammonia and also alkalinity is noted, resulting in a
and this is in agreement with other work (Neyens et al., 2004). pH rise. Subsequently, the operating pH of a mesophilic digestion
Nevertheless, as well as destroying ECP, thermal hydrolysis in- plant preceded by thermal hydrolysis is routinely between 7.5 and
fluences a variety of other parameters which also influence dew- 8. However, increasing pH (and also temperature) shifts the equi-
aterability such as: viscosity which is inversely correlated to librium position away from ammonium to its free state. Having no
increasing thermal hydrolysis reaction temperature (Everett and charge, FA diffuses easily into a cell and once there, ionizes to form
Brooks, 1971; Everett, 1972; Haug et al., 1978; Higgins et al., ammonium resulting in an intracellular pH imbalance which in
2015); increased dry solids (loading rate) resulting in higher turn, stimulates a Kþ/Hþ anti-porter mechanism. This has been
compressive yield stress (Stickland, 2015); particle size and distri- evidenced by the work of Sprott et al. (1984) who found a corre-
bution (Neyens et al., 2004; Hendriks and Zeeman, 2009); and lation between increasing ammonia concentration and depletion of
protein solubility which influences polymer consumption (Hung- potassium. Interestingly, this may enhance dewaterability charac-
Wei et al., 2014; Murthy et al., 1997), amongst others. teristics of the digested sludge by altering the mono-to divalent
In addition, downstream anaerobic digestion further influences cation ratio according to the divalent cation bridging theory for
dewaterability of sludge by changing: viscosity which increases biofloc formation (Higgins and Novak, 1997; Murthy and Novak,
(Lotito and Lotito, 2014); concentration of extracellular material 1998).
and other soluble microbial products (Aquino and Stuckey, 2008; Such is the perceived influence of ammonia toxicity on thermal
Chu et al., 2002; Barker and Stuckey, 1999); volatile solids con- hydrolysis, that it is currently the rate-limiting design consider-
centration (Skinner et al., 2015); release of nutrients and cations ation. Subsequently, it is necessary to dilute hydrolysed sludge prior
(Barnard and Shimp, 2013) as examples. Subsequently, some to digestion to reduce its influence (as shown earlier in Fig. 3).
workers suggest that the act of anaerobic digestion makes rheology Typically this limits the dry solids content of the feed to approxi-
of digested sludge similar regardless of pre-treatment (Xue et al., mately 10% DS for mixed sludges at full-scale, however this can
2015; Dawson and Sotiriadis, 2007). There is a body of evidence drop for systems only digesting waste activated sludge due to its
to suggest that the aforementioned influences of anaerobic diges- higher nitrogen content.
tion of thermally hydrolysed sludge are fundamentally detrimental Early work has suggested a FA concentration of 150 mg/l to be
with respect to dewatering potential. In the UK, Lumb (1951) inhibitory to anaerobic digestion (McCarty and McKinney, 1961).
showed that thermally hydrolysed raw sludge could achieve dry Fig. 10 shows the influence of sludge type and thermal hydrolysis
solids of 52%, which is far higher than anything observed following on the expected FA concentration, calculated by stoichiometry us-
anaerobic digestion (Table 5). Years later, raw thermally hydrolysed ing elemental composition data for primary and activated sludge
sludge was dewatered to between 40 and 50% DS compared to presented before (Barber, 2015), and volatile destruction data
 W.P.F. Barber / Water Research 104 (2016) 53e71 65

ammonia concentrations in predictive modelling (Haile et al., 2015;

Wett et al., 2014; Ho et al., 2013) although none have been used to
predict future performance. However, in contrast, a thorough study
by Wiegant and Zeeman (1986) showed toxicity inhibited hydrogen
utilizers more than their acetogenic counterparts up to at least
4500 mg/l TAN. In one study, Wilson et al. (2012) investigated the
influence of FA on Ks for unionized acetic acid to explain why vol-
atile fatty acids were observed in digester effluents. Although they
looked at conditions with and without thermal hydrolysis, closer
analysis of the data shows that increasing ammonia concentration
increased Ks irrespective of presence or absence of thermal hy-
drolysis. The influence of FA on Ks derived from data presented by
Wilson et al., is shown viz.

Ks½HAc ¼ 2:077LnðFAÞ þ 12:637 (6)

Fig. 10. Influence of sludge type and thermal hydrolysis on expected free ammonia
concentration in anaerobic digestion. Key: solid line (no thermal hydrolysis, pH 7.3, where Ks[HAc] ¼ Half saturation constant for unionized acetic acid
digestion temperature 36
C); dashed line (thermal hydrolysis of both primary and (mg/l), FA ¼ free ammonia concentration (mg/l). This compares
waste activated sludge, pH 7.8, digestion temperature 40
C); dotted line (thermal with previous work showing Ks of 0.128 mg/l (Kus and Wiesmann,
hydrolysis of only waste activated sludge, same conditions as for thermal hydrolysis of 1995) for a system without thermal hydrolysis e equivalent to a FA
all sludge).
of about 410 mg/l HAc.
A study by Ngwenya's group (2015) showed that reducing
digestion retention time could alleviate ammonia toxicity, implying
typical of full-scale application. that, following thermal hydrolysis, degradation of carbohydrates
Fig. 10 suggests a FA concentration for standard mesophilic was occurring in preference to the breakdown of proteinaceous
digestion of between 20 and 25 mg/l, and a concentration of be- materials as would be expected from the Gibbs free energy for the
tween 150 to over 200 mg/l depending on sludge type and how relevant reactions (McCarty, 1971). TAN figures increased with
much is exposed to thermal hydrolysis. These figures correspond to retention time from 1500, 2500 and 3000 mg/l at retention times of
between 2400 and 3500 Total Ammonia Nitrogen (TAN) mg/l. 10, 15 and 18 days with adjusted loading rates of 5.5 kg VS/m3.d
These findings are similar to those summarized by Rajagopal et al. each. Alkalinity followed a similar trend and pH increased from 7.4
(2013) who reported on the anaerobic digestion of chicken manure to 7.7 and 7.8 from shortest to longest retention time. Although not
and found no toxicity to methanogenesis up to 250 mg/l FA in di- presented, the corresponding calculated FA concentrations are 56,
gesters fed at 10% dry solids. Oosterhuis et al. (2014) also found no 180 and 266 mg/l. No statistical difference was observed on biogas
influence of TAN of 4000 mg/l with lab-scale thermal hydrolysis production between 10, 15 and 18 days suggesting no adverse in-
experiments. If pH is slightly higher, as witnessed on some thermal fluence of FA on performance between 56 and 266 mg/l. The work
hydrolysis facilities, the free ammonia could increase to 300 mg/l implies an optimal hydraulic retention time by when most carbo-
for a feed consisting of exclusively waste activated sludge. As these hydrate and lipid material have been degraded but little protein.
figures are routinely observed at full-scale on well-performing fa- Extending retention time would improve protein degradation
cilities, they do not coincide with the conclusions of McCarty and resulting in higher pH and FA and eventually lower biogas yield.
McKinney from 1961. In fact, looking through the literature One way to relieve ammonia toxicity is to feed carbon-rich co-
related to ammonia inhibition reveals a wide range of concentra- substrates to lower pH (Chen et al., 2014; Kayhanian, 1999). With
tions reported as being inhibitory. Other than pH, ammonia inhi- thermal hydrolysis, FOG (fats oil and grease) supplemented as 50%
bition appears to be influenced by: acclimation (Zhang et al., 2014); COD caused a drop in pH of 0.1 units (Ngwenya et al., 2015). A
bacterial population (Wiegant and Zeeman, 1986; Banks et al., similar drop was noted with the addition of food-waste, albeit at a
2012; Zhang et al., 2014; Fotidis et al., 2013); temperature rate of only 25% additional COD load (Barber et al., 2015). From
(Angelidaki et al., 1993); nutrient content (Banks et al., 2012); and equilibrium calculations, a reduction of 0.1 pH of will reduce FA by
seed inoculum (Yenigün and Demirel, 2013) amongst others, which approximately 20% at the typical TAN concentrations and digester
makes comparison with specific data difficult. Subsequently, in a operating conditions under thermal hydrolysis. The relatively low
review article investigating the impacts of toxicants on anaerobic drop in pH units following thermal hydrolysis may be due to
digestion, Chen et al. (2014) described a study which concluded improved biodegradability of the intermediate products of anaer-
that a threshold for free ammonia which could be tolerated to be as obic digestion demonstrated by data showing fatty acid in-
high as 620 mg/l, far in excess of the typical figures experienced in termediates contribution of only 2e4% of the effluent soluble COD
digestion following thermal hydrolysis. following co-digestion with thermal hydrolysis (Barber et al., 2015).
With respect to thermal hydrolysis, there is debate on how An easier approach to reduce ammonia toxicity is simply to
ammonia influences performance. A wide body of literature sug- reduce digestion temperature. Most digesters following thermal
gests that acetoclastic archaea are more sensitive to free ammonia hydrolysis are operated at approximately 40
C which is the
inhibition than either acetate-oxidizing or hydrogenotrophic optimal temperature for digestion when hydrolysis is rate-limiting
methanogens (Angelidaki et al., 1993; Fotidis et al., 2013; De Vrieze (Tong et al., 1991). Assuming thermal hydrolysis circumvents this
et al., 2015; Zhang et al., 2014; Koster and Koomen, 1988; Wilson and digester operating temperature is dropped to that typical of
et al., 2008; Wang et al., 2015). Consequently, it is expected that mesophilic digestion then FA would drop by 25e30% with other
digesters fed thermally hydrolyzed sludge will provide conditions operating conditions being the same. Wilson et al. (2008) demon-
to selectively favour those trophic groups (De Vrieze et al., 2015; Ho strated negligible difference in digester performance between 35
C
et al., 2013). Subsequently, various attempts have been made to and 42
C, supporting the evidence that FA concentration is not
account for a shift in bacterial population from acetophiles to overly influential. Interestingly, the optimum methane production
hydrogen consumers and acetoclasts in response to increasing of 0.4 lCH4/g VS fed with VSR in excess of 57% was noted at 53
C in
66 W.P.F. Barber / Water Research 104 (2016) 53e71

that study. Based on typical concentrations, this is equivalent to a with control reactors fed non-thermally hydrolysed equivalent
free ammonia concentration of >250 mg/l. The authors stated that substrate. Unfortunately, no literature was found which identified
no signs of inhibition were observed even when ammonia was the threshold concentrations of sugars and amino acids above
supplemented such that concentration was 2900 mg/l TAN at pH which melanoidins are observable.
7.8. That work was in agreement with earlier studies on high dry As the production of melanoidins are temperature dependent,
solids digestion (Lay et al., 1998) which looked at influence of both lowering the thermal hydrolysis reaction temperature, especially
pH (between 6.5 and 9) and ammonium-nitrogen (between 100 below 160
C to avoid production of caramelans (Villamiel et al.,
and 6000 mg/l). Using the Gompertz relationship with gathered 2006), can be an effective abatement strategy. This was seen at
data, Lay's team proposed that it was ionized ammonium not FA full-scale in Oxley Creek, Australia (Dwyer et al., 2008) an area close
which influenced biogas at concentrations between 1670 and to sugar-cane plantations. Laboratory tests performed by Dwyer's
3720 mg/l (highly relevant to thermal hydrolysis systems) and that team showed a pseudo-linear relationship between increasing
resulted in a 10% decrease in biogas production. FA only became an hydrolysis temperature and colour from approximately 3800 mg
issue at levels in excess of 500 mg/l (in accordance with findings of PtCo/l at 140
C to 12,700 mg PtCo/l at 165
C, although at the
Wilson et al., 2008), after which a dramatic shock was noted, and higher reaction temperatures, anaerobic digestion reduced colour
this was equivalent to approximately 6000 mg/l TAN. Observation by a third. BMP measurements showed relative uniformity across
of the data presented on full-scale plants showing high levels of the temperature range studied regarding biogas production, so it
performance (Lancaster, 2015; Merry and Oliver, 2015), processing and it was concluded that operating the thermal hydrolysis plant at
dry solids well above 10% (Chauzy et al., 2008), and at laboratory a lower temperature could alleviate issues related to colour for-
scale where effluent is almost free of biogas precursors (Barber mation with little influence on digestion performance.
et al., 2015), along with the findings of both Wilson's and Lay's Oosterhuis et al. (2014) reported a figure of 11 kg COD/tDS
groups (2008 and 1998 respectively) and others (Oosterhuis et al., processed through thermal hydrolysis as refractory material.
2014; Lui and Sung, 2002) suggests that inhibition due to free Assuming a typical COD:VS ratio for activated sludge, this figure is
ammonia at the typical concentrations observed in digestion approximately 1% of the unprocessed sludge. Chudoba, cited by
following thermal hydrolysis is far less influential than other pa- Barker and Stuckey (1999) proposed that a quantity of 15.7 mg/l per
rameters such as sludge composition and rheological properties. gram of biomass destroyed is a typical quantity of refractory ma-
Subsequently, this also suggests that pre-digestion dilution to terial produced by anaerobic digestion. Combining this figure with
minimize ammonia toxicity is being conservatively managed and the performance expected from thermal hydrolysis results in a
that higher loading rates are possible assuming ammonia is the rate quantity of between 0.75 and 1% of input COD as being refractory. In
limiting design parameter. a study looking at co-digestion with thermal hydrolysis, non-VFA
soluble COD accounted for between 95% and 98% of effluent solu-
5. Other impacts ble COD for sludge and sludge co-digested with food waste at 25%
addition by COD load respectively (Barber et al., 2015). The
5.1. Production of refractory organics increased production of refractory compounds may impact attain-
ment of strict nutrient regulations. Based on measurements,
The production of refractory material as soluble products is Khunjar et al. (2014) predicted an increase in refractory nitrogen of
expected from biological action in response to a variety of stimu- a factor of 3 compared to conditions prior to thermal hydrolysis,
lants (Barker and Stuckey, 1999). Influents typically contain mi- although no information was provided on concentrations, nor as-
crobial products which follow a skewed non-normal distribution sumptions used.
with predominance in the very low molecular weight (MW) frac-
tion (<0.5 kDa), whilst effluent from anaerobic digestion contains 5.2. Odours and growth of marker organisms
material which follows a bimodal pattern with material at low
(<1 kDa) and high (>10 kDa) range (Barker and Stuckey, 1999). The Several studies were set up to elucidate the reasons behind
conditions experienced during thermal hydrolysis influence the odour generation and sudden increases in fecal coliform densities
production of these materials, and there are several reports which immediately after sludge dewatering (Higgins et al., 2008; Murthy
show an increase in production of refractory compounds as a et al., 2009). The work looked at a variety of sludge treatment and
consequence of the technology (Stuckey and McCarty, 1984; Neyens dewatering configurations, and concluded that sludge processed by
and Baeyens, 2003) especially as reaction temperature increases thermal hydrolysis and digestion had minimum odour potential
(Bougrier et al., 2008; Liu et al., 2012). Well documented are the based on measurements of volatile organic sulfur compounds
production of Malliard and Amadori products (including melanoi- especially when dewatered using belt presses (Chen et al., 2011).
dins) which are brown, high molecular weight heterogeneous This finding is consistent with comments of Neyens and Baeyens
polymers (>10 kDa, Liu et al., 2012; Dwyer et al., 2008; Bougrier (2003) who previously concluded that digestion of thermally pre-
et al., 2008) produced by non-enzymatic chemical reactions be- treated sludge resulted in a significant reduction of odours.
tween amino acids and reducing sugars (Martins et al., 2000). In Regarding the potential for reactivation of marker organisms,
addition to being un-degradable, the colour produced can interfere Murthy et al., 2009 showed E. coli DNA density below a threshold
with UV disinfection of the digestate and increase effluent nitrogen figure of approximately 3e5  104/gDS for sludge which had been
(Abelleira-Pereira et al., 2015). Melanoidin production occurs in the thermally hydrolysed, above which reactivation was highly prob-
typical reaction range of thermal hydrolysis of 140e165
C. As able. This result was much lower than when compared with other
thermal hydrolysis increases the solubility of sugars and proteins technology designed for pathogen destruction including pre-
(Lu et al., 2015), these products should be expected. Sugars start to digestion pasteurization and temperature phased digestion.
caramelize into caramelans at 160
C which will act to darken
colour further (Villamiel et al., 2006). As food-waste has high 5.3. Particle size
concentrations of both sugars and proteins, thermal hydrolysis of
food-waste exacerbates the production of melanoidins resulting in It is expected that rapid decompression due to steam release
a decrease in biogas volume per unit of material digested as noted causes rupture resulting in particle size reduction (Sun, & Cheng,
in the work of Tampio et al. (2014) and Liu's group (2012) compared 2002; Negro et al., 2003; Hendriks and Zeeman, 2009).
 W.P.F. Barber / Water Research 104 (2016) 53e71 67

Decreasing particle size has been linked to: improved hydrolysis carbohydrate concentration at elevated temperature encourages
(Hendriks and Zeeman, 2009); increased viscosity (Pevere et al., production of refractory material. Additionally, Higgins et al. (2005)
2006), and altered dewaterability characteristics (Skinner et al., presented a linear correlation between increasing protein concen-
2015). The use of steam explosion to improve hydrolysis is well tration and increasing optimal polymer demand in dewatering. In a
documented since Mason (1926). Following thermal hydrolysis at detailed study, Tampio et al. (2014) discovered that biogas yield
165
C for 0.5 h, Barber (2010) noted a change in average particle from food-waste co-digested with sludge was routinely 5e10%
size of 70 to 35 mm, although no information was provided on lower on a unit basis when compared with food-waste which was
particle size distribution. Neyens et al. (2004) also reported a drop not thermally hydrolyzed under identical conditions independently
in average size from 107 to 66 mm following thermal hydrolysis. of loading rate between 2 and 6 kg VS/m3.d. This result com-
However, this result was in contrast to the findings of Bougrier's plemented the findings of Liu et al. (2012) who investigated a wide
group (2006) who reported seeing an increase in median size after variety of thermal hydrolysis settings with and without co-
thermal pre-treatment with data being almost the exact opposite of digestion of kitchen waste and vegetable fruit scraps with acti-
Barber's findings from 2010, with a mean particle sizes of 36 mm vated sludge. At 175
C/60 min e their default set-up e biogas yield
and 77 mm for unprocessed and thermally hydrolysed (at 170
C) reduced by approximately 15% and 10% for kitchen wastes and
digested sludge respectively. Bougrier et al. (2006) hypothesized scraps respectively. Data provided on UV absorbance at 254
that an increase in particle size was due to temperature catalyzed confirmed a large increase in melanoidin across a molecular weight
creation of chemical bonds. Enhanced interactions between parti- range between <1 and >300 kDa (Liu et al., 2012). However, other
cles has been observed previously (Pevere et al., 2006) leading to work showed no significant reduction in biogas yield with pre-
viscosity increase. Interestingly, measurements of CST were processed food-waste added at a rate of 25% by COD load (Barber
inversely proportional to particle size, where decreased size et al., 2015) and that it was possible to predict biogas production
(caused by ultrasound) required longer CSTs, compared to larger based on stoichiometry and COD supplemented and destroyed.
size (thermal hydrolysis) reducing CST from 150 s (no treatment) to Importantly, this appears to be typical of full-scale operation. There
40 s. Considering the importance of particle size and its influence are successful full-scale applications of thermal hydrolysis for
on dewaterability potential, it is clear that further work is required source separated organics and co-digestion at Lillehammar, Verdal
in this area, especially regarding the impact of thermal hydrolysis and Oslo (Panter, 2011; Kanders and Sargalski, 2014; Sargalski et al.,
conditions and influence of anaerobic digestion. 2007). The operating temperature of thermal hydrolysis means that
it can accommodate a wide range of materials which require a
5.4. Foam and scum control higher standard of treatment (European Community, 2009). For the
Verdal facility processing 16,000 wet t/y food waste with 9000 wet
Foam formation and stabilization requires pre-requisites of t/y sludge, Panter (2011) reported a biogas yield of 534 Nm3/t VS fed
surface active agents, hydrophobic material and a gas phase based on volatile solids reduction (VSR) of 65%, although the sludge
(Speece, 2008) all of which are influenced by thermal hydrolysis. composition was not provided. A VSR of 70% was reported for Lil-
Known foam producing bacteria found in waste activated sludge lehammer, where 14,000 wet t/y of mainly food-waste (70e82%,
include Gordonia amarae (previously Nocardia amarae) and other along with <7% garden waste, <7% paper waste, <12% nappies) are
GALO, Microthrix parvicella, and Rhodococcus spp. amongst others, processed (Sargalski et al., 2007). With full-scale operation, pre-
are all sterilized by exposure to the thermal hydrolysis reaction processing the material prior to thermal hydrolysis is key to suc-
temperatures, eliminating their foam-generating properties. This cessful operation (Kanders and Sargalski, 2014).
was confirmed in the pilot-scale work of Jolis and Marneri (2006)
who used thermal hydrolysis to successfully treat Gordonia-con- 5.6. Anaerobic digestion retention time
taining scum. Alfaro et al. (2014) also abated foaming due to fila-
mentous M. parvicella using thermal hydrolysis at 170
C. As thermal hydrolysis acts to enhance hydrolysis, it is possible to
Upgrading to thermal hydrolysis has also been found to eliminate accelerate the rate of gas production, such that the same amount of
foaming on a full-scale site with previous Microthrix based issues in biogas can be produced in a shorter time. The work of Xue et al.
Naestved (Fjordside, 2005) In addition to sterilization of foam (2015) shows this influence very clearly in graphical format. In
causing organisms, improved solubility caused by thermal hydro- their thorough study which looked at reaction temperatures be-
lysis, reduces potential for a surface tension gradient and subse- tween 60 and 180
C and times between 15 and 180 min, the value
quent Gibbs-Marangoni effect influential for foam stability. of accelerated biogas production is optimal at approximately 10
Stability of foam is also dependent on viscosity which influences days. Observing data taken at a reaction temperature of 140
C,
fluid drainage from a bubble layer and also foam stability. Using an biogas production was approximately 70% higher with thermal
autoclave (121
C), Barjenbruch et al. (2000) discovered that foam hydrolysis than without at a reduced retention time of 10 days.
could be eliminated, and that this coincided with a destruction of When retention time was increased to 20 days, the difference
hydrophobic material and extracellular polymers. Destruction of dropped to <25% additional biogas for the hydrolysed system, as
these materials will also significantly reduce viscosity (a well- the biogas production from the control sluggishly narrowed the
known influence of thermal hydrolysis), and itself important with gap. Interestingly, in the control, the biogas yield at 10 days was
respect to foam stability by influencing liquid drainage from approximately two thirds of that at 20 days, whist the reactor with
plateau borders (Pugh, 1996). pre-treatment was producing almost 95% of the 20 day yield within
10 days. This finding is mirrored by Ngwenya et al. (2015) who
5.5. Co-digestion showed statistically insignicant difference in biogas production
with thermal hydrolysis between 10 and 18 days at the same
Numerous studies have shown the influence of thermal hydro- loading rates at laboratory scale. Their work, along with Bougrier
lysis on the solubilisation of proteins, carbohydrates and lipids (Li et al. (2008) implies that carbohydrates are degraded prior to
and Noike, 1992; Bougrier et al., 2008; Lu et al., 2015; Xue et al., proteins during digestion (regardless of thermal hydrolysis), such
2015; etc), and solubilisation of these materials plays an impor- that ammonia and alkalinity and consequentially pH increase with
tant role when considering thermal hydrolysis of material other retention time, increasing the potential for free ammonia inhibi-
than sludge. As previously mentioned, increasing protein and tion. As the production of extracellular microbial byproducts also
68 W.P.F. Barber / Water Research 104 (2016) 53e71

et al., 2013; Chauzy et al., 2014). These systems are showing similar
performance, but with improved energy balances (as shown in
Figs. 5 and 7) and require smaller systems. However, configurations
bypassing primary sludge would not meet regulatory requirements
for pathogen destruction if applicable. Developments are ongoing
which are taking thermal hydrolysis back to its roots, i.e. as a means
to enhance dewatering, and early data from Germany where a plant
is installed downstream of digestion immediately prior to dew-
atering is showing promise in that regard (Kjorlaug, 2015). An
Fig. 11. Thermal hydrolysis of raw sludge prior to incineration to produce sludge cake interesting variant of that approach is being tested in China (shown
with high dry solids and energy content. Liquors from dewatering are anaerobically in Fig. 11), where raw sludge is thermally hydrolysed and sent for
digested in small footprint high rate anaerobic digester (Adapted from Qiao et al.,
incineration, with the high strength filtrate digested in an
2013).
Expanded Granular Sludge Bed Anaerobic Bioreactor within which
63% of the COD is being removed within 60 h with a loading rate of
increases with retention time (Barker and Stuckey, 1999) with a 11.0 kg COD/m3.d (Qiao et al., 2013).
subsequent increase in viscosity influencing dewaterability It is possible that thermal hydrolysis is also complimentary with
(Neyens and Baeyens, 2003) it may be hypothesized that running thermophilic digestion in spite of an expected increase in free
digestion plants at approximately 10 rather than 20 days retention ammonia toxicity potential by a factor of between 2 and 3 based on
time may be preferable when coupled with thermal hydrolysis. Li equilibrium calculations. Wilson et al. (2008) found significantly
and Noike (1992) concluded optimum digestion retention times higher biogas production at 53
C compared to that measured
between 5 and 10 days based on various tests and observations of under mesophilic conditions. However, biogas production fell
changes in methanogic populations. Chertsey, in the UK, has been sharply as temperature was further increased to 55 and 57.5
C. This
running at hydraulic retention time of approximately 12 days at coincides with the knowledge that hydrogenotrophs naturally
full-scale with good performance (Pook et al., 2013; Walley, 2007) favour thermophilic conditions (De Vrieze et al., 2015; Rajagopal
whilst Wilson et al. (2008) concluded that digestion performance at et al., 2013). Other interesting developments involve the use of
15 days retention time with thermal hydrolysis was equivalent to chemical gel-breakers and other technologies to compliment
that without at 20 days. thermal hydrolysis to achieve similar or improved performance
under milder conditions (Abelleira-Pereira et al., 2015; Alfaro et al.,
2014). Although thermal hydrolysis improves the rate at which
6. Future considerations and recommendations
biogas is produced, it is still limited by antiquated designs of
anaerobic digestion plants which remain similar to Cameron's
Although thermal hydrolysis has been commercially employed
patent from 1900. It is proposed optimizing configurations of
for over 20 years, there remain numerous opportunities for further
downstream digestion is integral to future work in improving ef-
evolution of the technology. The fact remains that, even with
ficacy of thermal hydrolysis. Some of this work has started in China
thermal hydrolysis, the destruction of volatile solids during
with staged digestion following thermal hydrolysis is consistently
anaerobic digestion of sludge remains relatively modest at 60e65%
outperforming a control where the technology is coupled to par-
destroyed. In Europe, there has been a move away from the tradi-
allel digestion (Wu, 2015) and use of high rate digestion (Wang
tionally used configuration of combined processing of primary and
et al., 2009; Qiao et al., 2012, 2013).
activated sludge prior to anaerobic digestion towards systems
Recommendations based on literature findings are given in
processing only activated sludge (Shana et al., 2013; Nawawi et al.,
Table 6.
2007), or digested sludge prior to a second stage of digestion (Mills

Table 6
Recommendations for thermal hydrolysis based on literature findings.

Aim/concern Recommendations

Minimize construction of anaerobic  Thermally hydrolyse both primary and activated sludge prior to anaerobic digestion. Design digestion plant at 10
digestion plant e12 days retention time to maximize size reduction
 Maximize pressure drop to increase biogas production rate to allow shorter digestion retention times
Maximize overall energy balance  Thermally hydrolyse only activated sludge. Optimal proportion of primary sludge is approximately 30% of
around digestion incoming feed
 Reduce thermal hydrolysis reaction temperature
 Thermally hydrolyse digesed sludge prior to a second stage of anaerobic digestion
 Run downstream digestion plant in thermophilic region but no higher than 53
C
Design for best dewatering  If installed prior to digestion, run thermal hydrolysis plant at highest possible reaction temperature. Preferably,
install thermal hydrolysis downstream of anaerobic digestion plant immediately prior to dewatering which
should be conducted at high temperature
Foaming concerns  Run thermal hydrolysis reactors at highest temperature to mimimise viscosity of sludge and solubility of material,
both of which minimize foam stabilization
Concerns with colour and refractory material  Run thermal hydrolysis reactors at lower temperature, (preferably <150
C)
 Avoid addition of sugary material which encourages melanoidin production by combining with protein in
activated sludge
Issues with ammonia toxicity  Operate downstream anaerobic digestion plant at 35 rather than approximately 40
C to reduce unionized
ammonia levels
 Run digestion plant at lower retention times (approximately 10 days) to minimize ammonia production and pH
increase
 Reduce thermal hydrolysis reaction temperature to 150
C to mimimise solubilisation of proteins
 Add un-thermally hydrolysed high-carbon material to build up intermediate products during digestion to
decrease pH. If thermally hydrolysed, the material is rapidly degraded before a significant pH drop is noticed.
 W.P.F. Barber / Water Research 104 (2016) 53e71 69

7. Conclusions Banks, C.J., Zhang, Y., Jiang, Y., Heaven, S., 2012. Trace element requirements for
stable food waste digestion at elevated ammonia concentrations. Bioresour.
Technol. 104, 127e135.
The following can be concluded from the literature: Barber, W.P.F., 2010. The influence on digestion and advanced digestion on the
environmental impacts of incinerating sewage sludgeea case study from the
 Thermal hydrolysis is a well-established technology for sewage UK. Proc. Water Environ. Fed. 2010 (4), 865e881.
Barber, W.P.F., 2015. Influence of wastewater treatment on sludge production and
sludge pre-treatment prior to anaerobic digestion; processing. Water Pract. Technol. J. 10, 178e186.
 The primary benefit of the technology involves changing sludge Barber, W.P.F., Lancaster, R., Kleiven, H., 2012. Thermal hydrolysis: the missing
rheology which allows higher loading rates, and significantly ingredient for better biosolids? Water Wastewater Int. Mag. Aug/Sept, 36 e 38.
Barber, W.P.F., Peot, C., Murthy, S., Higgins, M., Bodniewicz, B., 2015. The potential
improves dewatering by < 10% points irrespective of dewatering for Co-digestion of organics using thermal hydrolysis at blue plains advanced
device; wastewater treatment works. In: Proceedings of WEF Residuals and Biosolids
 The energy benefit of the technology around anaerobic diges- 2015. Water and Environment Federation, Washington DC.
Barjenbruch, M., Kopplow, O., 2003. Enzymatic, mechanical and thermal pre-
tion is relatively neutral as additional biogas produced during treatment of surplus sludge. Adv. Environ. Res. 7 (3), 715e720.
digestion is partly counteracted by a parasitic load to provide Barjenbruch, M., Hoffmann, H., Kopplow, O., Tra €nckner, J., 2000. Minimizing of
reaction temperature for treatment; foaming in digesters by pre-treatment of the surplus-sludge. Water Sci. Technol.
42 (9), 235e241.
 The primary energy benefit from the technology is due to Barker, D.J., Stuckey, D.C., 1999. A review of soluble microbial products (SMP) in
improved dewaterability which reduces downstream transport wastewater treatment systems. Water Res. 33 (14), 3063e3082.
and processing requirements, especially when coupled with Barlindhaug, J., Ødegaard, H., 1996. Thermal hydrolysis for the production of carbon
source for denitrification. Water Sci. Technol. 34 (1), 371e378.
thermal technology where energy required for drying can be
Barnard, J., Shimp, G., 2013. The impacts of biological phosphorus removal on
reduced by two thirds (as demonstrated in Fig. 8); biosolids dewatering. In: Proceedings of Joint IWA e WEF Conference on
 The influence of free ammonia toxicity appears minimal at Nutrient Management, Vancouver.
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ammonia. Subsequently, full-scale plants are unintentionally Bi, X., Liu, C., Ran, X., Lv, W., Wang, J., Liao, Z., 2013. Experimental Studies on pre-
conservatively designed to counteract potential of toxicity due treatment of sludge by combination of thermal hydrolysis and ultrasound
to ammonia, and it is suggested that loading rates can be further and sand removal by hydrocyclone to optimize wastewater and sludge treat-
ment. In: Proceedings of Aquaenviro's 18th European Biosolids and Organic
increased; Resources Conference and Exhibition, Manchester, UK.
 It is suggested that anaerobic digesters following thermal hy- Bougrier, C., Albasi, C., Delgene s, J.P., Carre
re, H., 2006. Effect of ultrasonic, thermal
drolysis are optimized at a retention time of 10e12 days as and ozone pre-treatments on waste activated sludge solubilisation and anaer-
obic biodegradability. Chem. Eng. Process. Process Intensif. 45 (8), 711e718.
approximately 95% of biogas potential at 20 days can be realised. Bougrier, C., Delgene s, J.P., Carre
re, H., 2008. Effects of thermal treatments on five
Longer retention times encourage protein degradation which different waste activated sludge samples solubilisation, physical properties and
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Bougrier, D.,J., Carrere, H., 2008. Effects of thermal treatments on five different
statistically significant increase in biogas production; waste activated sludge samples solubilisation, physical properties and anaer-
 It is proposed that rather than a single set of design conditions, obic digestion. Chem. Eng. J. 139 (2), 236e244.
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