Kinetics of thermophilic batch anaerobic digestion of thermal hydrolysed waste activated sludge
Enhancing waste activated sludge (WAS) anaerobic digestion by thermal pretreatment has become of high interest. However, thermal treatment has been mainly combined to mesophilic anaerobic digestion. This paper presents the combination of sludge thermal pretreatment (110, 165 and 220 °C) and batch thermophilic anaerobic digestion (55 °C). Optimal conditions of thermal pretreatment were shown to be 165 °C, involving a chemical oxygen demand (COD) and volatile solids (VS) solubilisation of 18 and 15% and a biodegradability increase from 47 to 61%. Treatments at 165 °C were carried out in electric and steam modes and no significant difference on the impact of heating mode on sludge anaerobic biodegradability was observed. Moreover, it may be recommended not to carry out successive batch experiments to assess thermophilic BMP of sludge as accumulation of volatile fatty acids (VFA), particularly propionate, and a decrease of VFA uptake rates may occur. However, thermal pretreatment at 165 °C allowed the decrease of propionate accumulation and an higher methane production.
Biochemical Engineering Journal 46 (2009) 169–175
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Biochemical Engineering Journal
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Kinetics of thermophilic batch anaerobic digestion of thermal hydrolysed waste
A. Motteta,b, J.P. Steyera, S. Déléris b, F. Vedrenneb, J. Chauzy c, H. Carrère a,∗
aINRA, UR50, Laboratoire de Biotechnologie de l’Environnement, Avenue des Etangs, Narbonne, F-11100, France
bVeolia Environnement R&D, Centre de Recherche sur l’Eau, F-78603 Maisons-Lafﬁtte, France
cVeolia Water Direction Technique, 1 rue Battista Pirelli, F-94410 Saint Maurice, France
Received 20 October 2008
Received in revised form 30 April 2009
Accepted 5 May 2009
Volatile fatty acids
Enhancing waste activated sludge (WAS) anaerobic digestion by thermal pretreatment has become of
high interest. However, thermal treatment has been mainly combined to mesophilic anaerobic digestion.
This paper presents the combination of sludge thermal pretreatment (110, 165 and 220 ◦C) and batch
thermophilic anaerobic digestion (55 ◦C). Optimal conditions of thermal pretreatment were shown to be
165 ◦C, involving a chemical oxygen demand (COD) and volatile solids (VS) solubilisation of 18 and 15%
and a biodegradability increase from 47to 61%. Treatments at 165◦C were carried out in electric and steam
modes and no signiﬁcant difference on the impact of heating mode on sludge anaerobic biodegradability
was observed. Moreover, it may be recommended not to carry out successive batch experiments to assess
thermophilic BMP of sludge as accumulation of volatile fatty acids (VFA), particularly propionate, and a
decrease of VFA uptake rates may occur. However, thermal pretreatment at 165◦C allowed the decrease
of propionate accumulation and an higher methane production.
© 2009 Elsevier B.V. All rights reserved.
Biological processes are developed as the main process to
improve efﬁciently the quality of the efﬂuent in municipal
wastewater treatment plants. Waste activated sludge (WAS), as a
by-product, are generated in large and increasing quantities. Since
the development of the biogas sector in France, following the July
2006 publication of new sufﬁciently attractive prices, WAS is more
and more considered as renewable energy source and becomes an
interesting substrate to anaerobic digestion. However, WAS anaer-
obic digestion is more difﬁcult than for primary sludge and, with
usual technologies nowadays available, only approximately 20–30%
of the sludge organic matter are mineralized .
Anaerobic digestion process follows four major steps: hydroly-
sis, acidogenesis, acetogenesis and methanogenesis. Hydrolysis is
a slow step and is considered as the rate-limiting step of the over-
all process in the case of sludge degradation . During hydrolysis
step, organic compounds, such as polysaccharides, proteins and
fats, are hydrolysed by extracellular enzymes to monomer or dimer
∗Corresponding author. Tel.: +33 468 425 168; fax: +33 468 425 160.
E-mail addresses: firstname.lastname@example.org (A. Mottet), email@example.com
(J.P. Steyer), Stephane.DELERIS@veolia.com (S. Déléris),
Fabien.VEDRENNE@veolia.com (F. Vedrenne), Julien.CHAUZY@veoliaeau.fr
(J. Chauzy), firstname.lastname@example.org (H. Carrère).
Over the last years, pretreatment steps, such as physical treat-
ment with bead mill, sonication or high-pressure homogenizer,
biological treatment with enzymatic hydrolysis, chemical treat-
ment with alkaline addition were applied to improve hydrolysis
of particulate organic matter and substantially biodegradability of
sludge. Thermal treatment has been widely combined to anaero-
bic digestion performed in the mesophilic range and this resulted
in an increase of biogas production and of the kinetic rates  and
energy costs can be covered by biogas production [4,5]. Climent
et al., Bougrier et al. and Jeong et al. [6–8] underlined the impact
of solubilisation of particulate organic matter on the biogas pro-
duction enhancement during anaerobic digestion. Another way to
increase anaerobic digestion performances consists in operating
digestors in thermophilic mode. Indeed, thermophilic anaerobic
digestion allows to improve degradation rates, hence shorter reten-
tion times and provides an enhancement of the biogas production
. However, very few studies deal with the combination of ther-
mal pretreatment and thermophilic anaerobic digestion of sludge
The objective of this work was to carefully analyse the impact
of thermal pretreatment on thermophilic anaerobic digestion of
WAS. Efﬁciency of thermal pretreatment was evaluated by solu-
bilisation of COD and volatile solids (VS). Methane production and
kinetics of volatile fatty acids (VFAs) and soluble chemical oxygen
demand (COD) were measured during batch anaerobic biodegra-
dation in order to investigate in details the hydrolysis of pretreated
and untreated organic matter. Moreover, electric and steam modes
1369-703X/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
170 A. Mottet et al. / Biochemical Engineering Journal 46 (2009) 169–175
for thermal pretreatment at 165 ◦C were compared. Indeed, electric
mode is generally used at laboratory scale whereas steam mode is
used in industrial plants .
2. Materials and methods
2.1. Waste activated sludge samples
Sludge samples were taken from a wastewater treatment
plant (France), treating urban wastewaters and working with
high load aerobic process at a sludge retention time of 0.4 days.
Characteristics of WAS or raw sludge were: Total solids (TS):
−1, volatile solids (VS): 36.6 ±1.8gL
−1, total suspended
solids (TSS): 40.5 ±2.0gL
−1(87%TS), volatile suspended solids
(VSS): 31.5 ±1.6gL
−1(86%VS), COD: 64.0 ±9.6 gCOD L−1and soluble
COD (<0.45 m): 5.6 ±0.3 gCOD L−1.
2.2. Thermal hydrolysis
Thermal pretreatments were performed in a 10 L agitated auto-
clave (Autoclave, class IV), allowing a temperature increase by
electric mode or by steam mode. Sludge sample volume was around
6 L. Temperatures of treatment were 110 ◦C, 165 ◦C and 220 ◦Cin
electric mode and 165◦C in steam mode. Once temperature was
reached, treatments lasted for 30min. In general, at laboratory
scale, autoclave has only a temperature increase by electric mode.
Thus the process, used for thermal pretreatment, allowed us to
compare performances obtained in electric mode against perfor-
mances obtained in steam mode that is generally used at industrial
The solubilisation of COD and VS were chosen to evaluate
the degree of disintegration of pretreated sludge. This parameter
deﬁnes the transfer of particulate organic matter to soluble organic
matter. The solubilisation was expressed as a percentage, following
where SSand SS0are the concentrations in soluble fraction, mea-
sured in treated and untreated sludge, respectively; Xp0is the
concentration in particulate fraction, measured in untreated sludge.
2.3. Determination of anaerobic degradation tests
Biochemical methane potential (BMP) tests were used to estab-
lish anaerobic biodegradability and determination of the methane
potential of WAS samples. The method was based from Bufﬁere et
al. .Anaerobic batch reactors were operated under thermophilic
conditions. The temperature was kept at 55 ◦C by water circulation
in a water jacket. Six reactors, with a volume of 3.5 L each, were
used in parallel. The inoculum was taken from a full scale sludge
anaerobic digester. One reactor was used with no feed to quan-
tify the endogenous activity of the inoculum. Others reactors were
fed with untreated sludge and with sludge treated at 110 ◦C, 165 ◦C
(electric and steam modes) and 220 ◦C. Organic loading was 0.5 gCOD
of sludge per gVS of inoculum. Large inoculation ratio ensured high
microbial activity,low risk for overloading and low risk of inhibition
. For each condition, four successive 22 days batch experiments
were carried out in a same reactor to minimise the effect of the
inoculum and two successive batch experiments were separated
by 2 days. At the beginning of each BMP test, the reactors were
purged with N2/CO2(75/25) gas mixture . Biogas production
and pH were measured continuously. An electronic volumetric gas
counter was used to monitor biogas production. BMP is expressed
as the produced methane relatively to the amount of introduced
COD (mLCH4gCODin−1), in STP conditions.
It has to noted that endogenous activity decreased from batch
1 to 2. It represented about 12% of cumulated methane production
from raw sludge in batch 1, 6% in batch 2, 3% in batch 3 and 5% in
The biodegradability of the sludge was estimated from the BMP
value and the theoretical maximal methane yield of 350mL gCOD−1
in STP conditions, following the equation:
Biodegradability (%) =100 ×BMP(mLCH4gCOD introduced
During each batch anaerobic digestion, total and soluble COD, VFA,
biogas composition were regularly monitored in order to follow
the formation of by-products, involved in the biological reactions
Newman–Keuls tests, with a conﬁdence interval at 90%, were
realised to compare biodegradability values of each treated sludge
and raw sludge and to estimate the signiﬁcant differences between
each biodegradability values.
The maximum accumulation rates of propionate and acetate
and the maximum production rate of methane can be used to
evaluate the impact of thermal pretreatment on the steps of aceto-
genesis and methanogenesis. Propionate and acetate accumulation
rates (Kpropionate and Kacetate) were calculated from the variation
of propionate and acetate concentrations versus time during the
accumulation phases. Kpropionate and Kacetate were determined by
the slope of the linear regression line between day 0 and the
day corresponding to the maximal accumulation. These param-
eters were expressed in gCOD L−1day−1. Accumulation of each
compound is the result of production minus consumption. For
example, acetate accumulation is the result of acetate production
minus acetate conversion into methane and carbon dioxide. How-
ever, acetate production rate could not be calculated by taking
into account methane production because we could not determine
the parts of methane originating from acetoclastic methanogene-
sis and from hydrogenotrophic methanogenesis. For the methane
production rate, the methane volume was converted to gCOD and
KCH4, expressed in gCOD gCODintro−1day−1, was determined from
the maximum value of the methane speciﬁc production rate,
which was obtained by the derivative of the methane speciﬁc
The particulate fraction was separated by centrifugation at
50 000 ×g, 15 min and 5◦C. Supernatant was then ﬁltered through
a cellulose acetate membrane with 0.45 m pore size to obtain the
COD measurements were realised on total and soluble fractions
and measurements of TS, VS, TSS and VSS were realised on sludge
and on solids of centrifugation, according to standard methods .
The oven-dried step, at 105 ◦C for 24 h, involved a volatilisation of
VFA. TS and VS concentrations were thus corrected by taking into
account VFA concentrations.
Proteins were measured according to the Lowry method .
The technique quantiﬁed the peptidic bounds. By using different
known solutions of bovine serum albumin (BSA), a calibration curve
was obtained and protein concentrations were determined in BSA
equivalent gram per litre. Carbohydrates were measured with the
anthrone reduction method . It dosed carbohydrate concentra-
tions by quantifying the carbonyl functions (C O). A calibration
curve was obtained from glucose (Gluc) and carbohydrate concen-
trations were determined in glucose equivalent gram per litre.
VFA concentrations were measured by using gas chromatograph
(GC-8000 Fisons instrument), equipped with a ﬂame ionisation
detector with an automatic sampler AS 800. The internal stan-
A. Mottet et al. / Biochemical Engineering Journal 46 (2009) 169–175 171
COD and VS solubilisation, methane potential and biodegradability of untreated and pretreated WAS at different conditions.
Conditions Solubilisation of COD (%) Solubilisation of VS (%) BMP (mLCH4gCODin−1) Biodegradability (%)
Raw sludge 165 ±17 47
110 ◦C 3.8 ±0.2 1.9 ±0,2 186 ±10 53
165 ◦C, electric 18.0 ±1.0 16.0 ±2.0 195 ±956
165 ◦C, steam 17.8 ±0.4 14.0 ±1.0 215 ±761
220 ◦C27.0±1.0 24.0 ±2.0 142 ±22 41
dard method allowed to measure acetate, propionate, butyrate and
iso-butyrate, valerate and iso-valerate concentrations.
The composition of biogas was determined with a gas chromato-
graph (Shimadzu GC-8A), with a CTRI Alltech column, with argon
as the carrier gas, equipped with a thermal conductivity detector
and connected to an integrator (Shimadzu C-R8A).
3. Results and discussion
3.1. Impact of thermal pretreatment on sludge solubilisation and
Solubilisation of COD and VS are often used to evaluate the
impact of pretreatment on the sludge maximal biodegradability.
Results are summarised in Table 1. As already shown [6,7], the val-
ues of COD and VS solubilisation increased with temperature until
220 ◦C, from 3.8 to 27% and from 1.9 to 24%, respectively. The ther-
mal pretreatment led to a transfer of particulate organicmatter into
the soluble phase, lower than 0.45 m and could be assimilated
to a thermal hydrolysis. Thus, the application of thermal pretreat-
ment on a largely particulate rawsludge (86%VS) should make more
available organic components tothe anaerobic microorganisms and
could imply an increase of degradation ratesand of produced biogas
Nevertheless, the values of biodegradability (Table 1) showed a
threshold value in the increase of methane production. Indeed, it
increased with thermal pretreatment until a temperature of 165 ◦C,
from 165mLCH4gCODin −1for untreated sludge to 215 mLCH4gCODin−1
for sludge pretreated at 165 ◦C in steam mode. Thus, among the
tested temperatures, the optimum was 165 ◦C. It is worth noting
that an identical optimal temperature pretreatment was found for
anaerobic digestion under mesophilic condition .
Moreover at 220 ◦C, although a large solubilisation of particulate
organic matter occurred, sludge biodegradability was lowered to
raw sludge biodegradability with 142 mLCH4gCODin−1. This may be
explained by the composition of solubilised organic matter (Fig. 1).
At 220 ◦C, protein solubilisation was similar to the one obtained
at 165 ◦C in steam mode, i.e. 40.1% and carbohydrates solubilisa-
tion strongly decreased from 15% at 165◦C to 1.2%. However COD
solubilisation increased from 18 to 27%. Thus, at 220 ◦C, carbo-
Fig. 1. Impact of treatment temperature on solubilisation of WAS.
hydrates in the soluble phase reacted with other components to
form products slowly or hardly biodegradable. These results are
in agreement with those of Bougrier et al., Müller and Stuckey
and McCarty [5,20,21]. They suggested that the presence of “burnt
sugar” reactions and Maillard reactions for high pretreatment tem-
peratures. The brown colour of the soluble phase of sludge treated
at 220 ◦C conﬁrmed the presence of new compounds, like Amadori
compounds and melanoidins which are recalcitrant to anaerobic
An objective of this study was to assess the impact of heating
modes on solubilisation and biodegradation results. Solubilisation
of COD, VS and carbohydrates, obtained at 165 ◦C in both modes,
did not show signiﬁcant differences: it reached around 18, 15 and
15%, respectively with both modes. On the other hand, protein sol-
ubilisation was slightly higher for sludge treated with steam (40.2%
against 34.5% in electric mode). Moreover, the Newman–Keuls test
showed that the difference of BMP between treatments at 165 ◦C
(in electric and steam modes) were not signiﬁcant with a 90% con-
ﬁdence interval (Table 2). Thus, it can be concluded that laboratory
thermal hydrolysis carried out in electric mode properly represents
industrial thermal hydrolysis carried out with steam injection.
Newman–Keuls test also showed that the improvement of
sludge thermophilic anaerobic biodegradability was not signiﬁcant
after the 110 ◦C pretreatment and conﬁrmed the reduction of sludge
biodegradability after 220 ◦C treatment.
Solubilisation values and methane production values, repre-
senting initial and ﬁnal conditions of the anaerobic digestion, are
not sufﬁcient to obtain a full understanding of anaerobic digestion
mechanisms. Kinetics of intermediate products (VFAs and soluble
COD) and ﬁnal products of anaerobic digestion (CH4and CO2)are
indeed interesting to explain biodegradability differences, involved
by thermal pretreatment and to observe the potential degradation
3.2. Kinetics of batch anaerobic digestion
The batch anaerobic digestion experiments of the ﬁve sludge
samples were realised in ﬁve reactors, which were used in parallel.
As in Bufﬁere et al. , four successive batch experiments were
Comparison of each methane production obtained with different thermal pretreat-
ment conditions (mean values of 4 successive batch experiments).
172 A. Mottet et al. / Biochemical Engineering Journal 46 (2009) 169–175
Fig. 2. Soluble compound concentrations during batch anaerobic digestion of each
sludge (batch 4).
carried out in a same reactor for each substrate in order to evaluate
the inoculum adaptation. The experiments were monitored during
Fig. 2 represents the measured CODsoluble minus VFAs in batch
4. It can represent the transfer of compounds from the particu-
late phase to the soluble phase, i.e., the behaviour of hydrolysis,
thus the production of soluble compounds (<0.45 m), before their
degradation by acetogenesis and acidogenesis.
Fig. 3 represents methane production and VFA concentrations
for batch 2, 3 and 4 for raw sludge and sludge treated at 110, 165◦C
and 220 ◦C. VFA concentrations were not monitored for batch 1.
It is important to deﬁne the VFA concentration variations moni-
tored during experiments; they result from the VFA production by
acidogenesis and from the VFA uptake by acetogenesis and aceti-
clastic methanogenesis. For example, an accumulation of acetate
can be due to the step of acetate production faster than the step
Fig. 3. Methane production and VFA concentrations during batch anaerobic digestions of (A) untreated sludge, (B) pretreated at 110 ◦C, (C) pretreated at 165◦C in electric
mode, (D) pretreated at 165◦C in steam mode and (E) pretreated at 220 ◦C.
A. Mottet et al. / Biochemical Engineering Journal 46 (2009) 169–175 173
of acetate uptake. These values can bring interesting information
into the understanding of mechanisms involved in the methane
The soluble COD concentration variations (Fig. 2) of sludge pre-
treated in optimal conditions (165 ◦C) showed a direct degradation
of soluble organic matter and no signiﬁcant accumulation of soluble
COD. Thus, pretreatment allowed a better accessibility of organic
matter, which was directly available for the biological steps, and
could minimise the limiting effect of biological hydrolysis.
For the sludge pretreated at 220◦C, despite a large solubilisa-
tion of COD (27%), soluble compound concentration at the end of
experiment was high and decreased very slowly. It seems to indicate
that compounds, formed at 220 ◦C (e.g. memanoidins and Amadori
compounds) were hardly biodegradable.
In the case of raw sludge and pretreated at 110 ◦C, a ﬁrst phase of
degradation of particulate organic matter from day 0 to day 3 and
a second phase of degradation from day 3 to day 6 were observed.
The signiﬁcant accumulation of soluble components seems to lead
to the conclusion that two types of particulate matter constitute the
sewage sludge: one fraction is quickly hydrolysed whereas the sec-
ond fraction is more slowly hydrolysed. These two fractions are less
signiﬁcant on the soluble COD concentration variations of sludge
pretreated at 165 ◦C. But they can be clearly observed for all sludge
samples in Fig. 2.
Indeed methane production curves showed two phases (Fig. 3).
A ﬁrst one, with a fast production corresponding to the degra-
dation of readily accessible organic matter, like monomers and
dimers compounds or exopolymers, lasted from day 2 to day 6. It is
also characterised by the production and the accumulation of VFAs
(from day 0 to day 4). The second phase, with a slower produc-
tion, corresponding to the degradation of hardly accessible organic
matter, like particulate macromolecules strongly linked in sludge
structure or compounds located inside the cells lasted from day 6
to days 12–14. Finally, methane production was null or very low
from day 14 to day 22.
The variations of acetate and propionateconcentrations could be
associated to the ﬁrst and the second phases of methane produc-
tion, respectively. Batch experiment 2 with sludge treated at 165◦C
in electric mode (Fig. 3C) could conﬁrm this hypothesis. It was
observed that the ﬁrst phase of methane production was mainly
associated to the acetate uptake, from the day0 to day 6. During this
phase, the highest methane volume was produced and the acetate
concentration decreased strongly. The second phase of methane
production was associated to the propionate uptake. Indeed the
start-up of the second methane production phase with the uptake
of propionate at day 7 could be observed and a plateau was reached
at day 12 when the VFA uptake was complete. According to Vavilin
et al. , the acetogenesis and methanogenesis can be the rate-
limiting steps in anaerobic digestion of a complex substrate at a
high organic loading. These observations seem to show that the
acetate and propionate degradation steps can also be considered as
the limiting step for the anaerobic digestion of a complex substrate
(waste activated sludge) with an organic loading of 0.5 gCOD per gVS
As discussed above, total methane production increased with
thermal pretreatment temperature.However, a signiﬁcant decrease
of methane production for raw sludge was observed through suc-
cessive batch experiments (Fig. 3A). This seems to be linked to the
propionate accumulation. Thus successive thermophilic digestion
batches of raw sludge may lead to an accumulation of propionate
as already observed by Speece et al. .
In the case of sludge treated at 220 ◦C, the ﬁrst phase of methane
production, which was from day 0 to day 6 for the batch 2, had
a tendency to increase in time through consecutive batch experi-
ments, while a small methane production and a slow degradation
rate of acetate were observed. This seems to indicate that the
Fig. 4. Acetate (A), propionate (B) accumulation maximum rates and methane
(C) production speciﬁc maximum rates with different conditions of pretreatment
through batch experiments.
hardly biodegradable components produced at 220◦C are slowly
biodegradable. Thus the results can conﬁrm that the melanoidins
and Amadori compounds are hardly and slowly biodegradable.
The enhancement of methane production observed during batch 4
could arise from an overestimation of produced volume (resulting
from very slowly biodegradable compounds from previous experi-
ments) and not from an adaptation of inoculum.
The values of propionate and acetate accumulation maximum
rates, Kexpressed in gCOD L−1day−1, can be used to accurately eval-
uate the impact of thermal pretreatment on the mechanisms of
acetogenesis and methanogenesis steps (Fig.4A and B). This param-
eter was determined from the slope of the linear regression line
associated to the accumulation phase of propionate and acetate
from day 0 to the day corresponding to the maximal accumulation
of each batch experiment.
The values of Kpropionate did not show statistically signiﬁcant dif-
ferences (Fig. 4A). However, important differences were observed
between the variations of propionate concentrations of each sludge
with an accumulation of propionate in the case of the raw sludge
174 A. Mottet et al. / Biochemical Engineering Journal 46 (2009) 169–175
and the sludge pretreated at 220◦C(Fig. 3A and E), which could
be correlated to the values of Kacetate. Indeed, these propionate
accumulations were associated to a low acetate accumulation max-
imum rate which may be associated to a low acetate production
rate (Fig. 4B). The values of Kacetate were below 0.10gCOD L−1day−1
when the propionate accumulation took place. Thus, propionate
accumulations involved a partial inhibition of acidogenesis.
Fig. 4C presents the maximum methane speciﬁc production of
each sludge through consecutive batch experiments, KCH4being
expressed in gCOD gCODintro day−1. This was determined from the
maximum methane production speciﬁc rate throughout experi-
If we consider raw sludge, KCH4decreased which was followed
by the decrease of ﬁnal methane volume through consecutive batch
experiments (Fig. 3A). This was associated to a strong propionate
accumulation and a decrease of Kacetate. This comment could con-
ﬁrm that a high propionate concentration can involve a partial
inhibition of acidogenesis and methanogenesis steps. This was
already underlined by Hyun et al.  who observed an inhibition of
acetate degradation by a high propionateconcentration (1 gCOD L−1)
during the anaerobic digestion of propionate-enriched mixed cul-
tures at 35 ◦C.
The values of Kacetate through consecutive batches for the
sludge treated at 220 ◦C were low with a value lower than
0.10 gCOD L−1day−1, and a large production of acetate was observed
with a slow degradation of acetate, which was associated to a low
KCH4. Thus the produced compounds (melanoidins and Amadori
compounds) had a negative effect on acetate degradation and
methane production. These results conﬁrmed that a fraction of sol-
ubilised organic matter were hardly and slowly biodegradable.
For the sludge samples pretreated at 110 ◦C and 165 ◦C (both
modes), Kacetate values were not signiﬁcantly different and were
stable. However, they showed important and statistically signiﬁcant
higher values than for raw sludge. For example in batch 2, Kacetate
was equal to 0.0939 gCOD L−1day−1and 0.1593 gCOD L−1day−1in the
case of digestion of raw sludge and sludge pretreated at 165◦Cin
steam mode, respectively. This enhancement was observed in each
batch experiment. Moreover, the propionate accumulations were
less important than in the case of raw sludge and sludge pretreated
at 220 ◦C. Thus, the solubilisation induced by the thermal pretreat-
ment has an important impact on the step of acetate production
from propionate and soluble components. Fig. 2 showed that the
thermal pretreatment allowed a decrease of the limiting effect of
the anaerobic digestion hydrolysis step by solubilising a large part
of organic matter,which is more available by the anaerobic microor-
ganisms, in particular in the case of sludge pretreated at 165 ◦Cin
both modes. The solubilised organic matter was directly degraded,
allowing higher acetate and methane production rates, no or low
propionate accumulation and a higher quantity of degraded organic
The thermal hydrolysis enhanced the availability of organic mat-
ter. Thus, the solubilisation allowed to bring an available substrate
in high quantity and it could allow maximising acidogenesis by a
better development of acidogens and no propionate accumulations.
A second hypothesis is possible to explainthe non-accumulation
of propionate in the case of sludge pretreated in optimal con-
ditions: anaerobic digestion can be stimulated by element trace
metals. Indeed, Zitomer et al., Kim et al. and Speece et al. [25,26,23]
showed that the supplementation of trace metals, like cobalt, iron
and nickel had a positive effect on the degradation of propionate
and acetate. Zitomer et al.  obtained a statistically signiﬁ-
cant increase of degradation of propionate and acetate for 77% of
tested thermophilic biomass samples, upon supplementation with
either nickel, cobalt or iron. The propionate degradation rate more
frequently increased upon nutrient addition, especially in ther-
mophilic systems, whereas the acetate degradation rate increased
less frequently. In the sludge, the total metal concentrations may
appear adequate but the metals may not be present in bioavailable
form for the microorganisms. Thus, the disintegration of the sludge
structure, induced by the thermal hydrolysis, could allow to make
the trace metals more available for the microorganisms and could
enhance the degradation of propionate. However, this hypothesis
could not be validated since the measurements of trace metals were
Optimal conditions of thermal pretreatment were obtained at
165 ◦C, involving a COD and VS solubilisation of 18 and 15% and
a biodegradability increase from 47 to 61%. At 220 ◦C, although a
large solubilisation of particulate organic matter occurred, sludge
biodegradability was lowered than other sludges (42%). Thus, a too
high temperature treatment leads to the production of slowly and
hardly biodegradable compounds, like Amadori compounds and
The Newman–Keuls test showed that the difference of BMP
between treatments at 165 ◦C (in electric and steam modes) was
not signiﬁcant. Thus, the laboratory thermal hydrolysis carried out
in electric mode represents properly industrial thermal hydrolysis
carried out with steam injection.
For all untreated and pretreated sludge, successive thermophilic
batch digestion tests were not necessary to evaluate maximal
methane production, as an adapted inoculum was used and VFA
accumulations occurred through successive batches.One batch test,
monitored until VFA concentrations reaches zero, seems sufﬁcient
to measure the maximal methane production of WAS.
Methane production kinetics were strongly linked to the acetate
and propionate kinetics. The results showed that the degradation of
propionate and acetate had a limiting effect on the methane produc-
tion. The acetate degradation was associated to the ﬁrst methane
production phase and the propionate degradation was associated
to the second methane production phase.
The thermal hydrolysis could minimise the limiting effect
of hydrolysis due to a large solubilisation of organic matter,
thus improved the availability of particulate organic matter and
improved the acetate production rate, inducing a better conversion
of propionate to acetate that avoided propionate accumulation.
Further work will concern the modeling of these results by
ADM1, a complete model for anaerobic digestion. Special attention
will be paid to the description of the hydrolysiswhich is the limiting
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