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Biochemical Engineering Journal 34 (2007) 20–27

Impacts of thermal pre-treatments on the semi-continuousanaerobic digestion of waste activated sludge

C. Bougrier, J.P. Delgenes, H. Carrere ∗INRA, UR050, Laboratoire de Biotechnologie de l’Environnement, Avenue des Etangs, Narbonne F-11100, France

Received 24 October 2005; received in revised form 7 November 2006; accepted 13 November 2006

bstract

Thermal pre-treatments can be used in order to enhance the efficiency of anaerobic digestion of waste activated sludge. The objective of this workas to study the effects of thermal treatment on the semi-continuous anaerobic digestion of the main sludge compounds (proteins, carbohydrates

nd lipids). Thermal treatment at 190 ◦C was more efficient than treatment at 135 ◦C in terms of total COD, lipids, carbohydrates and proteinemovals and methane production. However, treatment at 190 ◦C produced refractory soluble COD. In all cases, with or without pre-treatments,ipids degradation yield (67% without pre-treatment and 84% with 190 ◦C treatment) was higher than carbohydrates (56% without pre-treatmentnd 82% with 190 ◦C treatment) and proteins (35% without pre-treatment and 46% with 190 ◦C treatment) degradation yields. Methane production

2006 Elsevier B.V. All rights reserved.

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eywords: Thermal treatment; Anaerobic processes; Biogas; Waste treatment;

. Introduction

At present, the enforcement of the European legisla-ion regarding the Urban Wastewater Treatments Directive91/271/EEC) leads to a non-negligible increase in sludge pro-uction. At the same time, disposal routes are subject to moreocial and legal constraints. Urban sludge land disposal haseen restricted in France since July 2002 (French law 92–3 ofJanuary 1992). Incineration is quite expensive and needs the

reatment of flue gas in order to remove toxic compounds; it ishus highly debated. The main disposal route is land applicationor agricultural use), but it is subject to reservations from farm-rs and consumers. Therefore, this increasing sludge productionauses a large problem to communities and wastewater treat-ent plant operators. So, it is necessary to find more efficient

reatment in order to reduce sludge production in the wastewater

The use of anaerobic digestion of sludge is of high interest.ndeed, this treatment is well known and can allow a reduction

∗ Corresponding author. Tel.: +33 4 68 42 51 68; fax: +33 4 68 42 51 60.E-mail address: carrere@supagro.inra.fr (H. Carrere).

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369-703X/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.bej.2006.11.013

ins; Carbohydrates; Lipids

f sludge quantity of about 30% with a production of biogashich can be considered as a renewable source of energy [1].oreover, according to literature [2] and life cycle assessment,

naerobic digestion combined with agricultural land applica-ion is the most environmentally friendly process thanks toew emissions and a low energy consumption. In wastewa-er treatment plants, anaerobic digestion is generally appliedo mixture of primary and secondary (waste activated) sludge.ut waste activated sludge (WAS) are known to be more diffi-ult to digest than primary sludge [3]. For example, Kepp andolheim [4] presented a production of methane expected dur-

ng anaerobic digestion of 306 L CH4 kg−1 VSfeed for a primaryludge against 146–217 L CH4 kg−1 VSfeed for WAS. Anaerobicigestion process is achieved through several stages: hydroly-is, acidogenesis andmethanogenesis. For WAS degradation, theate-limiting stage is hydrolysis [5]. In order to improve hydrol-sis and anaerobic digestion performance, one possibility is tose lysis pre-treatments. Several pre-treatments can be consid-red: mechanical, thermal, chemical or biological treatments [6].

er from the particles to the liquid fraction) organic compoundsnd especially refractory compounds, in order to make themore biodegradable. Indeed, a linear relation between solubili-

C. Bougrier et al. / Biochemical Engin

Nomenclature

BSA bovine serum albuminCOD chemical oxygen demand (g O2/L)CODp COD in particles (g O2/L)CODs soluble COD (g O2/L)CODt total COD (g O2/L)CST capillary suction time (s)Gluc glucoseR135 reactor fed with sludge treated at 135 ◦CR190 reactor fed with sludge treated at 190 ◦CRc control reactorTS total solids (g/L)TSS total suspended solids (g/L)VFA volatile fatty acid (g/L)VS volatile or organic solids (g/L)VSS volatile or organic suspended solids (g/L)WAS waste activated sludge

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ation and biodegradation has been shown [7]. Final quantity ofesidual sludge and time of digestion can thus be reduced andiogas production can be increased [8–10]. For example, a ther-al pre-treatment led to 60% enhancement of performance of

ludge anaerobic digestion with an increase of CH4 productionrom 115 mL g−1 CODfeed to 186 mL g−1 CODfeed [9]. Besides,n ultrasound pre-treatment (64 s at 31 kHz) allowed to reducehe sludge retention in the digestor from 22 days to 8 days whileolatile solids removal yields did not change (44%) [8].

For this study, thermal treatment has been chosen in ordero improve sludge anaerobic digestion performance. It may alsoe applied to enhance food wastes biodegradation [11]. Ther-al treatment results in the breakdown of the gel structure

f the sludge and the release of intracellular bound water [6].herefore, this treatment allows a high level of solubilisation, an

mprovement in biogas production, modification in sludge char-cteristics (increase in filterability and viscosity reduction) andeduction of pathogen micro-organisms [9,12–14]. The mainarameter for thermal treatment is temperature; time of treat-ent has less influence [5,9,15]. According to several authors,

ptimal temperature is around 170–200 ◦C [5,16,17]. Indeed,or higher temperature, biodegradability of sludge is no moremproved and can decrease [18]. This can be due to the for-ation of refractory compounds linked to Maillard reactions

9,19]. But, a thermal treatment around 175 ◦C, combined withnaerobic digestion, can highly reduce sludge production; thiseduction can reach 50–70% according to the process [20,21].

The objective of this work was to study the effects of ther-al treatment on the semi-continuous anaerobic digestion of the

ure 135 ◦C was chosen according to the European law onanitation of animals by-products and slaughterhouses waste

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eering Journal 34 (2007) 20–27 21

1774/2002/CE) and 190 ◦C was chosen according previoustudies realised at the laboratory [12,22]. Chemical oxygenemand (COD), solids, lipids, proteins and carbohydrates repar-ition were measured on sludge before and after anaerobicigestion. Finally, we calculated if biogas production could per-it to pre-heat the sludge.

. Experimental

.1. Waste activated sludge characteristics

The experiments were carried out using flotation-thickenedAS (secondary sludge) collected from the municipal WWTP

f Toulouse (South of France). This plant had a capacity of00,000 people equivalent. This plant treated domestic andndustrial wastewater and was operated with a high loaded aera-ion tank. Sludge were stored at 4 ◦C. For the experiments, sludgeas diluted in order to obtain total solids concentration (TS) of4.5 g/L (standard deviation 0.7 g/L). The organic solids (or totalolatile solids VS) content was equal to 81% (S.D. 1%) of TS.

.2. Pre-treatment conditions

The reactor used for thermal treatment was a ZipperclaveAutoclave France) controlled in temperature via a regulationsing a proportional integral derivative (PID). It was heated inceramic oven. The 1 L stainless reactor was equipped with aagnetically coupled mixer and with an 0.55 L Hastelloy C tank

n order to avoid corrosion. Sample volume was 0.5 L. The risen temperature lasted around 35 min and 50 min for, respectively,35 ◦C and 190 ◦C. Once temperature was reached, treatmentsasted 30 min (135 ◦C) or 15 min (190 ◦C). The decrease in tem-erature lasted about 30–40 min. Treated samples were storedt 4 ◦C in order to limit sludge variation.

.3. Anaerobic digestion

Reactors used for the anaerobic digestion were semi-ontinuous reactors, they were stirred with a bar magnet. Fig. 1hows a schematic representation of the semi-continuous reac-ors. For experiments, temperature was regulated at 35 ◦C (S.D..5 ◦C) by water circulation in double jackets. Hydraulic reten-ion time was fixed at 20 days and organic loading rate wasround 1 g COD/day/L of reactor. Feeding and withdrawingere realised once a day using peristaltic pumps. Feed sludgeas maintained at 5 ◦C by water circulation and outlet samplesere analysed as soon as possible and stored at 4 ◦C. Biogasroduction was measured by acidified water movement (pH 2)nd biogas composition was determined by gas chromatographyGC-8A, Shimadzu).

Three reactors were run in parallel. They were inoculatedith the same anaerobic sludge. First, they were fed with raw

ludge in order to acclimate micro-organisms and to verify the

22 C. Bougrier et al. / Biochemical Engineering Journal 34 (2007) 20–27

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.4. Samples analyses

.4.1. Sludge compositionIn order to determine sludge composition, several measure-

ents were made on samples, according to Standard Methods23]. First, soluble and particulate fractions were obtainedfter centrifugation (Beckman J2 MC—25,000 × g, 15 min,◦C).

COD was measured according to Standard Methods [23].t was measured on the total sludge and on the supernatantfter centrifugation. For this paper, COD measured on super-atant will be called “soluble COD” (CODs) and the differenceetween total COD and soluble COD will be called “particulateOD” (CODp). The error due to this measure was around 10%.

Measures of total and organic solids (TS and VS) wereealised on sludge and on particulate fraction of centrifugationTSS and VSS) according to Standard Methods [23]. Solids con-entration of the supernatant, that is to say the soluble phase, waseduced from the difference between total solids and suspendedolids concentrations. All these concentrations led to the com-osition in the different parts of the sludge (the whole sludge,he particulate fraction and the soluble fraction). The error dueo this measure was around 3–5%.

In order to better know solids and soluble fractions, pro-eins, carbohydrates and lipids concentrations were measured.roteins concentration was determined on total sludge and onupernatant using the Lowry method [24]. The technique quan-ified the peptidic bounds. After reactions with salts and Folineagent, absorbance of samples was determined at 750 nm, using

spectrophotometer (DV-640, Beckman). Protein concentra-ions were determined in BSA equivalent gram per litre. Therror due to this measure was around 15%.

As for proteins, carbohydrates concentration was determinedn total sludge and soluble fraction. The technique used washe anthrone method [25]. It dosed carbohydrates concentra-ion by quantifying the carbonyl function (C O). After reaction

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ith anthrone and sulphuric acid, absorbance of samples wasetermined at 625 nm using a spectrophotometer (DV-640,eckman). Carbohydrates concentrations were determined inlucose equivalent gram per litre. The error due to this measureas around 10%.Lipids concentration was determined using two techniques.

irst, fatty solids concentration was measured by extractiony hexane according to literature [26]. Samples were acidifiedn order to maintain fatty acids in the soluble fraction (non-issociated form). Hexane was added to samples which weregitated: fatty acids were transferred from sludge to hexaneraction. Then the hexane phase was collected and evaporatedRotavapor R, Buchi). By weighting the extracted compounds,nd by knowing the initial volume of sludge, it was possibleo determine fatty solids concentration. The error due to this

easure was around 15%.Beside, volatile fatty acids (VFA) concentrations were deter-

ined in the soluble fraction, by gas chromatography (GC800,isons Instruments). The internal standard method allowed toeasure total VFA concentration (acetic, propionic, butyric and

so-butyric, valeric and iso-valeric acids) in the range 0.25–1 g/L.he error due to this measure was around 3%.

.4.2. Sludge filterabilityThe filterability was measured using the capillary suction

ime (CST). The apparatus used was a Triton type 319 Multi-ST (Triton Electronics Ltd.). The CST permits to estimate the

ludge ability to dewater: water is absorbed by CST paper byapillary. The CST measure corresponds to the time needed forater to cross a fixed distance in the filter paper.

.5. Statistical analyses

In order to know if results obtained with the three reactorsere significantly different, Student tests were realised. Confi-ence level was chosen equal to 95%.

C. Bougrier et al. / Biochemical Engin

Table 1Sludge characteristics after thermal treatment (inlet) and after anaerobic diges-tion (outlet) (mean of nine samples taken between day 67 and day 99)

CODt (g O2/L) CODs (g O2/L) TS (g/L) TSS (g/L)

Control RcInlet 17.7 5.6 14.9 11.7Outlet 8.6 0.8 10.3 8.4

Reactor R135Inlet 17.3 5.9 14.0 10.2Outlet 7.4 0.9 9.1 6.8

Reactor R190Inlet 16.4 7.6 14.5 8.9Outlet 6.0 1.4 7.3 5.2

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. Results and discussion

Thermal treatment led to COD and solids solubilisation.able 1 presents sludge composition and characteristics of inlettreated sludge) and outlet (digested sludge) of reactors. Theseesults have been obtained after stabilisation of reactors (60 daysfter feeding with treated sludge).

Considering sludge samples used to feed the reactors, solu-ilisation levels obtained were very low compared to previousesults [12,22] or found in the literature [5,9,16,17]. Indeed, inhis study, COD solubilisation was 34% and 46% after treatmentt 135 ◦C and 190 ◦C, respectively, whereas, for example, a sol-bilisation of 60% after a 170 ◦C pre-treatment was reportedy Li and Noike [5] and Valo et al. [12]. This could be due tostorage problem (sludge underwent variations even at 4 ◦C)

r to a high level of soluble compounds in raw sludge. Indeedntreated sludge underwent solubilisation: soluble COD wasnitially 7% of total COD and reached 32% after 75 days oftorage. In the same time, treated sludge underwent “refloc-lation” as soluble COD was around 44% (135 ◦C) and 60%

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able 2emoval yields obtained with anaerobic digestion reactors and results of Student test

CODt CODs

emoval (%) (variance)Control Rc 0.52 (0.0023) 0.84 (0.00Reactor R135 0.58 (0.0012) 0.84 (0.00Reactor R190 0.63 (0.0050) 0.81 (0.00

tudent’s t-testRc–R135 2.93 0.002Rc–R190 3.92 1.27R135–R190 2.09 1.36

or a confidence interval of 95% and 16 degrees of freedom, the critical t value is equa

190: reactor fed with sludge treated at 190 ◦C). t =√

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alue of removal yield and S2i is the variance of the removal yield

eering Journal 34 (2007) 20–27 23

.1. Effects of thermal treatment on digested sludgeomposition

.1.1. COD variationTotal and soluble COD removal yields obtained in three

eactors and statistical analyses to compare them are shownn Table 2. The total COD removal yield increased from 52%or control reactor (Rc) to 58% for reactor R135 (treatment at35 ◦C) and to 63% for reactor R190 (treatment at 190 ◦C). Thus,reatment allowed an increase in total COD removal efficiency.

oreover, statistical analyses could assure that results on totalOD obtained with treated sludge were significantly different

han those obtained from control, even with treatment at 135 ◦C.evertheless, soluble COD removal yields were not significantlyifferent for the three reactors. For reactor R135 (treatment at35 ◦C), CODs at the outlet of the reactor was almost equalo control CODs: around 0.9 g O2/L (Table 1). On the otherand, for reactor R190, outlet CODs increased strongly to 1.42/L (that is to say a 70% increase). This was due to the forma-

ion of refractory compounds, which were not degraded duringnaerobic digestion. Indeed, according to literature [9,18,19],or temperature higher than 170 ◦C it is possible to create refrac-ory compounds like melanoidines, which are very difficult toegrade [27]. This phenomenon was not observed at 135 ◦C,ut supernatant of digested sludge became more brown that thene of control reactor. Nevertheless, it has not been possible toetermine if these compounds could be degraded with higherydraulic retention time or with an aerobic process. However,he introduction of this soluble refractory COD in the wastewa-er treatment plant should be considered in terms of additionaleration costs and sludge production.

.1.2. Solids repartitionFig. 2 shows solids repartition and volatile fatty acids con-

entration for each reactors in the inlets and in the outletsf digesters. In the inlets, solids repartition was different forhe three reactors. Mineral solids concentrations (soluble and

s

TS VS

30) 0.31 (0.0011) 0.39 (0.0006)23) 0.35 (0.0003) 0.41 (0.0003)21) 0.49 (0.0067) 0.57 (0.0052)

3.51 1.776.02 7.074.79 6.55

l to 1.746 (Rc: control reactor; R135: reactor fed with sludge treated at 135 ◦C;

, where ni is the number of samples (ni = 9 for the three reactors), Xi the mean

24 C. Bougrier et al. / Biochemical Engineering Journal 34 (2007) 20–27

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ig. 2. Solids repartition and VFA concentration in the inlets and the outlets ofhe biological reactors (mean of nine samples taken between days 67 and 99).

hermal treatment led to an increase of their concentration, espe-ially at 190 ◦C. In the same time, VFA concentration increaseduring thermal treatment at 135 ◦C and decreased during the90 ◦C treatment. During anaerobic digestion, TS concentrationecreased strongly for the three reactors, but the removal yieldf TS was higher for reactor R190: 49% against 35% for reactor135 and 31% for control (Table 2). At the same time, mineral

olids were obviously not degraded: the total mineral concentra-ion remained almost constant around 3 g/L. That implied thathe outlets of reactors were less organic; the organic fraction was6% in reactor R190 and 71–72% for reactor R135 and controlRc). A statistic analysis showed that there were some differ-nces in terms of solids removal performance during anaerobicigestion (Table 2). Thus, thermal treatment at 190 ◦C improvedemoval of organic solids: 57% for R190 reactor against 39%or control. But it could be interesting to understand whicholecules were preferentially degraded during anaerobic diges-

ion and if thermal treatment had an effect on the degradation ofhese organic compounds.

Concerning fatty solids, removal yields were very high forhe three reactors: around 82% for the treated reactors and 67%or the control. At the same time, VFA degradation was alsoptimal for the three reactors with a removal yield around 100%.herefore, it is possible to conclude that thermal treatment, event 135 ◦C, improved lipids degradation.

Fig. 3 presents the concentrations in proteins and in carbohy-rates for each reactor, before and after anaerobic digestion,n the particulate fraction and in the supernatant. Concern-ng carbohydrates, thermal treatment led to an improvementn degradation. The outlet concentrations were lower for bothreated reactors than for control. Removal yields were alsoigh: 82% for reactor R190, 68% for reactor R135 against 56%or control. In the outlets, carbohydrates were principally inhe particulate fraction. Therefore, it was possible to supposehat soluble carbohydrates were highly degraded, for the threeeactors. Moreover, the impact of thermal treatment was also

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ig. 3. Carbohydrates and proteins repartition in the inlets and outlets of bio-ogical reactors (mean of four values measured on a mixing of sludge samplesetween days 67 and 99).

emperature, had a strong effect on the anaerobic degradation ofarbohydrates. However, we have to remain careful concern-ng this conclusion. Indeed, thermal treatment, by action onhemical bound, could modify carbohydrates structure and makehem impossible to dose. Nevertheless, for this study, this phe-omenon did not seem important: carbohydrates concentrationsn inlets of digesters were equal (with or without thermal treat-ent). Therefore, it seemed that the removal of carbohydratesas mainly due to anaerobic degradation. The fact that par-

iculate carbohydrates concentrations were low, could suggesthat carbohydrates were preferentially located in the exopoly-

ers (which assumed sludge structure) rather than inside theicro-organisms.Total protein concentrations were almost identical, in the

hree inlet samples. Thus, proteins were not degraded duringhermal treatments, they were only solubilised. Different resultsere obtained by Inoue et al. [28] who observed a 17% reduc-

ion of proteins concentration after a treatment at 175 ◦C duringh, the autoclave being previously pressurised to 2 MPa usingitrogen gas.

Moreover, proteins were not well degraded during anaero-ic digestion. Total removal yield of proteins were quite low:6% for reactor R190, 43% for R135 and 35% for control.hermal treatment only slightly increased proteins degrada-

C. Bougrier et al. / Biochemical Engineering Journal 34 (2007) 20–27 25

Table 3Biogas composition, methane production and yield obtained with anaerobic digestion reactors and results of Student tests

Biogas composition CH4 % Methane yield (mL CH4/g COD cons) Methane production (mL CH4/g COD added)

Mean (variance)Control Rc 0.73 (0.0007) 339 (5745) 173 (654)Reactor R135 0.74 (0.0007) 323 (1399) 194 (322)Reactor R190 0.73 (0.0014) 325 (1407) 217 (527)

Student’s t-testRc–R135 0.508 0.580 1.942Rc–R190 0.161 0.492 3.844

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Filterability of sludge was assessed by measuring CST.Table 5 presents results obtained for the three reactors, for theinlet and the outlet.

Table 4Biogas production improvement for three different sludge samples: sludge 1 and2 issued from ref. [22]

Sludge Methane production (mL CH4/g VS added)

Control Treatment at 170 ◦C Treatment at 190 ◦C

R135–R190 0.252 0.139

or a confidence interval of 95% and 16 degrees of freedom, the critical t value i190: reactor fed with sludge treated at 190 ◦C) Cons: consumed

ion. But, soluble-particulate repartition was modified. Withreatment, inlets soluble concentration increased: for instanceoluble proteins represented 52% of total proteins for reactor190 against 32% for control. Therefore, for reactor R190,egradation should be easier: hydrolysis was already done.ut, soluble proteins removal yield decreased: 75% for con-

rol and only 68% for reactor R190. This was probably due tohe formation of soluble refractory compounds which were notegraded (like melanoidines). This phenomenon has not beenhown for reactor R135 (soluble removal yield: 78%). Thus,hermal treatment could create refractory compounds for highemperature treatment (higher than 135 ◦C). Moreover, particu-ate proteins concentrations remained quite high in the outlets.his could suggest that proteins were essentially located in theicro-organisms rather than outside (exopolymers of structure).nfortunately, nitrogen concentrations, and especially N-NH4

+

oncentration, had not been measured. Thus, it was impossiblenow if proteins were totally degraded into ammonium or if theyere only simplified in amino acids.Neyens and Baeyens [17] did the same observations while

omparing the anaerobic degradation of different sludge com-ounds: while the carbohydrates and the lipids of the sludge areasily degradable, the proteins are protected from the enzymaticydrolysis by the cell wall.

.2. Biogas production

Thermal treatment allowed to improve removal yields, thusiogas production should be improved too. Table 3 presentsethane content of biogas for the three reactors, methane pro-

uction and yield as well as the comparison of the three reactorsy Student test. The methane content of biogas was not differentor the three reactors: 73–74%. This value has to be related to theomposition of sludge. Methane content in biogas produced byure compounds is 50% for carbohydrates, 68% for lipids and1% proteins [4]. As proteins are the major component of sludge,he methane content of biogas produced by sludge is of theame order of the methane content for proteins. Combining ther-al treatment allowed an improvement in methane production:

rom 173 mL/g of added COD for control to 194 mL/g of addedOD for R135 and 217 mL/g of added COD for R190. This

mprovement has to be linked with COD removal increase inrder to verify results coherency. So, thermal treatment allowed

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n increase in anaerobic digestion performance around 12% (foroth biogas and COD removal) for a treatment at 135 ◦C (R135)nd around 25% for a treatment at 190 ◦C. Methane yield was notignificantly different for the three reactors, and it was approxi-ately equal to the theoretical one (375 mL CH4/g COD). This

ould suggest that reactors worked correctly.These results have been compared with previous results

12,22]. Table 4 gathers published results and results obtainedor this study.

For every case, performances have been more increasedith the higher temperature (170 ◦C previously and 190 ◦C for

his study). But biogas improvement obtained for this studyas lower than previously: only 24% against 75–78% previ-usly. This could be explained by the low solubilisation ratesstorage problems) and by the initial biodegradability level ofludge. Sludge 1 and 2 were initially less biodegradable thanhe sludge used in this study. Therefore, it seemed that if sludgere initially easily biodegradable, the gain obtained with thermalreatment will be low and, on the contrary, if sludge are only lit-le biodegradable, the gain obtained with treatment will be high.his is in accordance with literature results [19]. Indeed, ini-

ial biodegradability could be linked to organic matter content.herefore, before combining thermal treatment and anaerobicigestion, it seems important to evaluate raw sludge anaerobiciodegradability in order to estimate if thermal treatment wille efficient [29].

.3. Effects of thermal treatment on digested sludgelterability

128 228145 256

his study 254 314

ean of 7–10 samples taken on stabilised reactors.

26 C. Bougrier et al. / Biochemical Engineering Journal 34 (2007) 20–27

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The CST for inlet sludge is different and lower for R135 and190. Thus, it can be concluded that for the control there isgreat improvement of CST, especially with a temperature of

90 ◦C. Thermal treatment led to modification in sludge struc-ure: sludge became more fluid and could release more water.n the outlet, CST value was 481 s for control, 233 s for reac-or R135 and 105 s for reactor R190. This showed that thermalreatment could be used before anaerobic digestion without dam-ging filterability. On the contrary, this suggested that sludgeould be easier to dehydrate. However, the amount of polymer

equired for sludge dewatering has not been investigated in thisork and should be considered.

.4. Cost aspects

To determine if thermal treatment could be economicallyiable, it was necessary to estimate the costs due to pre-reatment. However, the results obtained were not sufficient toharacterise the whole plant: it has been necessary to put for-ard several hypotheses. Therefore, following results are only

ndications. We have considered a wastewater treatment planthich produced 350 t TS/year. Thermal treatment temperatureas supposed equal to 190 ◦C and TS concentration to 50 g/L

that is to say that flow was equal to 40 kg/h). Fig. 4 presents achematic view of thermal treatment integration. Sludge wouldrst be pre-heated in the first heat-exchanger (E1). The hot fluidould be sludge heated at 190 ◦C (outlet of the second heat-

able 5ST values measured on the inlet and the outlet of the three biological reactors

mean of three values measured on a mixing of sludge samples between day 67nd day 99)

Control Rc Reactor R135 Reactor R190

Inlet Outlet Inlet Outlet Inlet Outlet

ST (s) 1284 481 700 233 164 105

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xchanger E2). The aim of this first heat-exchanger would beo pre-heat activated sludge and to cool thermally treated sludgerom 190 ◦C to almost 35 ◦C. Thus, theoretically, by consideringhat there were no thermal loss, it would be possible to pre-heatctivated sludge to almost 175 ◦C. The second heat-exchangerE2) would be used in order to heat activated sludge from 175 ◦Co 190 ◦C. The hot fluid would be water vapour. The power nec-ssary was estimated to almost 15 kW. Vapour, after condensingn the heat-exchanger E2, would be heated again in the boiler.his vapour would be produced in a boiler by burning methane.y considering results obtained in this study, we have supposed

hat organic flow was equal to 32 kg/h and that methane produc-ion was equal to 314 mL CH4/g VS added. The system wouldhus produce around 10 m3CH4/h. Burning this methane wouldermit to obtain a power of 100 kW, which would be largelyufficient to heat sludge. This confirms that energy requiredo perform the process can be positively balanced by biogasroduction [20].

Nevertheless, it would be interesting to know if the increasen methane production was enough to pre-treat sludge. In ourase, the increase in methane production was 60 mL CH4/g VSntroduced in the digestor. Burning this methane would allow

power of 19 kW; that is to say almost the quantity neces-ary to heat sludge. So, considering methane production andeating energy led to a positive balance. Excess biogas due tohermal treatment would allow to pre-heat sludge. Therefore,t seems that thermal treatment could be interesting even for anot favourable sludge” with a quite good biodegradability andnot favourable storing conditions”. However, improvement andenefits should be higher with a more adapted sludge (with a badnitial biodegradability).

Nevertheless, this is only a first estimation. For a complete

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C. Bougrier et al. / Biochemical

rocess requires more energy: for the aeration (in the aerationank), due to excess COD which returns to the head of the plant.ut this could be counterbalanced by lower elimination costs.

ndeed, TS removal yield was improved by thermal treatment.n our case, outlet flow after anaerobic digestion would decreaserom 27.6 kg TS/h without treatment to 19.6 kg TS/h with ther-al treatment; that is to say a reduction around 30%. Moreover,

his reduction would be emphasised by a better filterability.

. Conclusion

Thermal treatment was used in order to improve anaerobicigestion of sludge. Results obtained with a treatment at 190 ◦Cere better than those obtained with a treatment at 135 ◦C. With a

hermal pre-treatment at 190 ◦C, COD removal yield increasedrom 52% to 64%. TSS concentration strongly decreased dueo thermal treatment. Considering also the better filterability,hermal treatment could permit to significantly decrease sludgeroduction: more than 30% in our case. Lipids and carbohy-rates degradation yields (up to 82–83%) were higher thanroteins ones (up to 46%). For each compounds, combined pro-ess with a thermal pre-treatment at 190 ◦C, allowed to obtainetter removal yields. But, treatment led to an increase in solubleOD after digestion. This was due to the formation of refractoryompounds during thermal treatment. These compounds werelso observed by the dosage of proteins.

A thermal pre-treatment at 190 ◦C permitted to increaseethane production of 25%. This moderate enhancement could

e explained by a relatively good initial biodegradability ofested sludge and bad storage conditions. However, this increasen biogas production could be sufficient to pre-heat sludge. Inrder to assess if thermal treatment could economically be inter-sting, investment costs, maintenance costs and integration in thehole wastewater treatment process should be considered.

cknowledgement

Authors want to thank ADEME for financial contribution.

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