Bioresource Technology 101 (2010) 5743–5748

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Bioresource Technology

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Influence of low temperature thermal pre-treatment on sludge solubilisation,heavy metal release and anaerobic digestion

Lise Appels a,b,*, Jan Degrève a, Bart Van der Bruggen c, Jan Van Impe a, Raf Dewil a,b

a Chemical and Biochemical Process Technology and Control Section, Department of Chemical Engineering, Katholieke Universiteit Leuven, Willem De Croylaan 46,3001 Heverlee, Belgiumb Laboratory for Environmental and Process Technology, Katholieke Universiteit Leuven, Campus De Nayer, Jan De Nayerlaan 5, 2860 Sint-Katelijne-Waver, Belgiumc Applied Physical Chemistry and Environmental Technology Section, Department of Chemical Engineering, Katholieke Universiteit Leuven, Willem De Croylaan 46,3001 Heverlee, Belgium

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

Article history:Received 17 December 2009Received in revised form 15 February 2010Accepted 17 February 2010Available online 23 March 2010

Keywords:Anaerobic digestionThermal pre-treatmentDisintegrationSludgeBiogas

0960-8524/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.biortech.2010.02.068

* Corresponding author. Address: Chemical and Bioand Control Section, Department of Chemical EngineLeuven, Willem de Croylaan 46, 3001 Heverlee, Belgiu+32 15 31 74 53.

E-mail address: lise.appels@cit.kuleuven.be (L. App

In this work, the influence of a low temperature (70–90 �C) thermal treatment on anaerobic digestion isstudied. Not only the increase in biogas production is investigated, but attention is also paid to the sol-ubilisation of the main organic (proteins, carbohydrates and volatile fatty acids) and inorganic (heavymetals, S and P) sludge constituents during thermal treatment and the breakdown of the organic compo-nents during the subsequent anaerobic digestion. Taking into account the effects of the treatment on thesludge composition is of prime importance to evaluate its influence on the subsequent anaerobic diges-tion and biogas production using predictive models. It was seen that organic and inorganic compoundsare efficiently solubilised during thermal treatment. In general, a higher temperature and a longer treat-ment time are beneficial for the release. The efficiency of the subsequent anaerobic digestion slightlydecreased for sludge pre-treated at 70 �C. At higher pre-treatment temperatures, the biogas productionincreased significantly, up to a factor 11 for the 60 min treatment at 90 �C.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

The production of a huge amount of waste sludge is an inevita-ble drawback of waste activated sludge processes, and sludge han-dling and disposal already accounts for up to 50% of total treatmentcosts of wastewater purification (Neyens et al., 2004).

Anaerobic digestion is of particular interest in sludge treatmentsince it has the ability to reduce the overall amount of biosolids tobe disposed (by ca. 40%), while producing an energy rich biogas(55–70% CH4) that can be valorised energetically (Appels et al.,2008). Other beneficial features include the stabilisation of thesludge, the inactivation and reduction of pathogens, and theimprovement of sludge dewaterability (Climent et al., 2007).

Various pre-treatment methods have been suggested in litera-ture for improving the solids reduction and biogas production rateby enhancing the digestion’s rate limiting step, i.e. the hydrolysis oforganic matter. They all induce the solubilisation of complex par-ticulate matter, so it is more rapidly and completely consumedduring the anaerobic digestion process (Wilson and Novak,

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chemical Process Technologyering, Katholieke Universiteitm. Tel.: +32 15 31 69 44; fax:

els).

2009). Methods that have been shown to have a positive effecton anaerobic digestion include chemical, mechanical, biologicaland thermal processes. An overview is presented in Appels et al.(2008).

Thermal pre-treatment disintegrates sludge cells by applyinghigh temperature (optionally associated with high pressure). Vari-ous temperatures, ranging from 60 to 270 �C have been studied inliterature. The results of some recently published studies areshown in Table 1.

Most literature sources report on high temperature pre-treat-ment (>100 �C). It was shown that, in this range, treatment temper-ature is a more important factor than treatment duration (Valoet al., 2004). The higher the temperature, the more efficient thetreatment. However, temperatures higher than 180 �C lead to theproduction of recalcitrant soluble organics or toxic/inhibitoryintermediates, hence reducing the biodegradability (Wilson andNovak, 2009). The optimum treatment conditions and digestionimprovement are largely depending on the nature of the sludge(Gavala et al., 2003). The most significant drawback of high-tem-perature treatment is its high energy requirement. The surplus ofenergy that can be recovered because of the increased biogas pro-duction is largely compensated by the high energy requirementsfor bringing the sludge to the disintegration temperature (andassociated high pressure). This largely reduces the overall profit-ability of the process.

Table 1Overview of some thermal pre-treatment studies.

Reference Treatment Comments

Kim et al.(2003)

121 �C30 min

Increase of VS reduction by 30%

Valo et al.(2004)

170 �C15 min

59% increase of TS reduction92% higher gas production

Ferrer et al.(2006)

70 �C9–72 h

Studied thermophilic digestionPositive effect on gas productionHigher temperature (110–134 �C) didnot have any effect

Climent et al.(2007)

70–134 �C90 min–9 h

Studied thermophilic digestion 50%increase of biogas production at 70 �C(9 h)No effect for high-temperaturetreatment

Bougrier et al.(2007)

135–190 �C Increased methane production by 25%at the 190 �C treatment

Jeong et al.(2007)

120 �C30 min

Increased methane production by 25%

Nges and Liu(2009)

25, 50, 70 �C48 h

Increased methane production by 11%at 50 �C

Phothilangkaet al.(2008)

Thermo-pressure-hydrolysis process

Increased biogas production by 80%

Perez-Elviraet al.(2008)

170 �C, 3 bar30 min

Increased methane production by 50%

Table 2Basic characteristics of used sludge types.

Secondary thickened sludge Digested sludge

DS ODS DS ODS

g DS/kg sludge % of DS g DS/kg sludge % of DS

Average value 65.08 69.90 46.56 50.65Standard deviation 2.70 0.36 0.16 0.08

5744 L. Appels et al. / Bioresource Technology 101 (2010) 5743–5748

Application of low temperature thermal treatment (<100 �C)could form an alternative to overcome this drawback. It has beenpointed out as an effective treatment for increasing biogas produc-tion for both primary and secondary sludge (Climent et al., 2007).At low temperatures, treatment time plays a more dominant rolethan treatment temperature. Some authors have concluded thatthe solids solubilisation at temperatures around 70 �C is enhancedbecause of biological activity of some thermophilic bacteria popu-lations with optimum activity temperatures in the high values ofthe thermophilic range.

The types of molecules that are solubilised during thermaltreatment play a significant role in the enhancement of the subse-quent anaerobic digestion. A successful anaerobic digestion pro-cess requires an efficient degradation of complex particulatesubstrate to methane gas, and various compounds may promoteor inhibit some steps in this process. It is reasonable to surmisethat such components could be set free during thermal treatment(e.g. heavy metals) or could be produced through the thermalhydrolysis of insoluble structures (e.g. VFAs from degradation oflipids) (Chowdhury et al., 2007).

The objective of the current work is to study the solubilisationof organic and inorganic compounds during low temperaturethermal treatment and the effects on anaerobic digestion. Apartfrom the total and soluble COD, the release of carbohydratesand proteins is analysed, being a primary carbon and nitrogensource for the anaerobic micro-organisms, respectively. The pro-duction of various VFAs through thermal degradation of fats isalso monitored by measuring the VFA concentrations before andafter treatment. Since heavy metals which are largely adsorbedto the organics will also be set free during treatment and hencemay influence the digestion process, their concentration in thesludge water is determined. For the sake of completeness, the mi-cro-nutrients phosphorus and sulphur are also analysed. Theinfluence of treatment time and treatment temperature is deter-mined through variation of both parameters. Through laboratoryscale digestion, the breakdown of organics and the productionof biogas is determined for all treatments. Since all importantparameters are included in the current work, a complete over-view of the effects of low temperature thermal treatment canbe presented.

2. Methods

2.1. Sludge characteristics

For the experiments, sludge samples were taken from the fullscale WWTP of Antwerp-South (Belgium). The samples were takenfrom the sludge buffer, positioned before the digester. The sludgewas stored at 4 �C in the laboratory prior to the experiments(max 4 h). During digestion the sludge was seeded with digestedsludge obtained from the digester of the same WWTP.

Table 2 shows the basic characteristics of both types of sludgeused in the experiments.

2.2. Pre-treatment conditions

Thermal treatment was performed in a thermal reactor (con-trolled in temperature) with a sample volume of approximately1.5 L. The treatment temperature varied between 70 and 90 �Cand the duration of the treatment varied between 15 and 60 min.The sludge was introduced in the thermal reactor at room temper-ature and it took less than 3 min to heat it to the set temperature(70, 80 and 90 �C). After treatment, the sample was immersed intoan ice bath to guarantee a fast cooling. The treatment duration wasdefined as the elapsed time between the moment at which theheating element of the reactor was turned on and the moment atwhich the sample was removed from the reactor and immersedin the ice bath. The sludge is continuously gently stirred duringthe thermal treatment to avoid temperature gradients. An un-treated sample, i.e. the blank sample, was also analysed as thepoint of reference for the treated samples.

2.3. Digester set-up

Twelve anaerobic digesters were built as parallel operatingmesophilic single stage batch reactors, each of 1 L content. Specificattention was paid towards avoiding a scum layer by adapting themixing rate. The digestion experiments were run for 20 days. Amixture of 500 g waste activated sludge (WAS) and 100 g seedsludge was used. By choosing this ratio, the effects of the thermaltreatment were more pronounced and became more noticeable:they would be masked if using a higher degree of (obviously un-treated) seed sludge. For each experimental run, three controldigesters were operated with thermally treated sludge.

The 500 g of WAS (for each reactor) were thermally treated atdifferent temperature and duration levels (as described above)prior to mixing with the seed sludge. The produced gas was col-lected in calibrated glass cylinders. The cylinders were filled withacidified deionised water to avoid losses of CO2 due to the forma-tion of carbonates.

2.4. Component analysis

The following components were analysed before and after ther-mal treatment, and after subsequent digestion: dry solids (DS),organic dry solids (ODS), chemical oxidation demand (COD), carbo-hydrate concentration, protein concentration and concentration of

L. Appels et al. / Bioresource Technology 101 (2010) 5743–5748 5745

volatile fatty acids. The measured inorganic components includeheavy metals (Cd, Cr, Cu, Hg, Ni, Pb, Zn), total sulphur and totalphosphorus content. The analyses were performed on both thesludge and the supernatant to identify total and soluble fractionsof the specific component. The supernatant samples were obtainedafter centrifugation at 12,000g for 15 min and subsequent filtrationthrough 1.3 lm glass microfibre filter paper (Merck Eurolab).Throughout the paper, the analyses performed on the supernatantwill be called ‘soluble’, and the analyses on the sludge will be called‘total’.

The dry solids content (wt.% DS) of the wastewater, and of thesludge cake and filtrate after dewatering was determined as theresidue after drying a sludge sample at 105 �C to constant weight.Further heating at 605 �C (to constant weight) drives off the organ-ic dry solids (ODS), usually called ignitable solids, leaving the min-eral dry solids content (MDS) as residual ash. These methods aredescribed in Standard Methods (APHA, 1992). The COD was deter-mined by a HACH DR/890 Colorimeter using a HACH COD reactor.Prior to volatile fatty acid (VFA) analysis, the sludge samples wereacidified using H2SO4. A fixed amount of an internal standard wasadded to the acidified sample and mixed with diethyl ether. Thismixture was shaken for approximately 2 min. The supernatantether-phase was transferred to a volumetric flask using a Pasteurpipette. A 2-lL volume of the extract was subsequently injectedinto the GC system. The concentrations of each individual VFAwere analysed on a Chrompack CP-9100 gas chromatographequipped with a CP-WAX 58 FFAP (Varian) column (length 25 m,diameter 0.25 mm) and a flame ionisation detector. The operatingtemperatures for the injection port, the oven and the detector were240, 200 and 240 �C, respectively. Helium was used as the carriergas at a flow rate of 25 mL/min. The following VFAs were analysed:acetic acid, propionic acid, iso-butyric acid, butyric acid, iso-valericacid, valeric acid and caproic acid. Carbohydrates were analysedaccording to the Anthrone method (Gerhardt et al., 1994). Theamount of proteins present in the sludge and the supernatantwas measured using the Bicinchoninic Acid (BCA) method (Lowryet al., 1951; Smith et al., 1985).

The biogas composition was analysed on a Varian CP-4900 Mi-cro-GC equipped with a Molecular Sieve 5A PLOT column (length10 m, diameter 0.32 mm), a PoraPLOT Q column (length 10 m,diameter 0.15 mm) and a thermal conductivity detector. On thefirst column, hydrogen, nitrogen, methane and CO are analysed.The second column allows the analysis of CO2, CH4 and H2S. Theoperational temperatures for the injector of column 1, the injector

Table 3Concentration and degradation of the organic components of the untreated and thermally

Parameter Unit Blank 70 �C

15 min 30 min 60

COD mg O2/L 55,300 64,700 56,500 61Soluble COD mg O2/L 400 650 800 11Solubilisation % 0.00 0.45 0.72 1.3Carbohydrates mg Glu-eq/L 8160 8324 7059 73Soluble carbohydrates mg Glu-eq/L 136 139 130 14Solubilisation % 0.00 0.04 �0.08 0.1Proteins mg BSA-eq/L 29,500 30,524 28,429 27Soluble proteins mg BSA-eq/L 125 864 1293 11Solubilisation % 0,00 2,51 3,96 3,4Total VFA mg/L 75 111 618 10Acetic acid mg/L 46.45 65.35 419.99 59Propionic acid mg/L 18.15 25.12 97.22 25Iso-butyric acid mg/L 2.09 4.65 22.73 42Butyric acid mg/L 2.13 5.23 19.36 50Iso-valeric acid mg/L 4.50 7.11 46.13 74Valeric acid mg/L 1.06 2.69 8.23 41Caproic acid mg/L 0.58 0.43 4.38 10

of column 2, the oven of column 1, the oven of column 2 and thedetector were 60, 50, 60, 50 and 60 �C, respectively. Helium wasused as the carrier gas. Heavy metals, sulphur and phosphoruswere measured by ICP-AES, according to the method described inDewil et al. (2007a).

The degree of solubilisation of the organic components is calcu-lated by the following equation:

Degree of solubilisation ð%Þ

¼ soluble concentrationtreated�soluble concentrationuntreated

total concentrationuntreated�100

3. Results and discussion

3.1. Effects of thermal treatment on the sludge solubilisation

Thermal treatment on the thickened sludge was performed forthree different temperatures and treatment durations. An overviewof the results is shown in Table 3. From this table, it is clear that athermal treatment effectively releases the studied components tothe water phase. A discussion on the solubilisation of the variouscomponents is added below.

3.2. Organic matter solubilisation

Fig. 1 shows the increase in soluble COD for the three treatmenttemperatures. At 70 �C, the increase in soluble COD was limitedcompared to the higher temperatures. Even at a treatment timeof 60 min, the value of the sCOD only tripled. At both higher tem-peratures, the sCOD release was more pronounced, although it onlystarted after a treatment time of 15 min. After 30 min of treatmentat 90 �C, the soluble COD is already 18 times higher than that of theuntreated sludge and increases further to a net increase of 2500%.The release of organics is due to the disruption of chemical bondsin cell walls and membranes by thermal treatment. Therefore,intracellular organic material is released. Moreover, extracellularpolymeric substances (EPS), including polysaccharides, proteins,nucleic acids and humic acids, are degraded and released fromthe cell walls. A temperature of 70 �C seems to be too low for aneffective solubilisation, since even after a treatment time of60 min, only a very limited amount of COD was set free.

treated sludge.

80 �C 90 �C

min 15 min 30 min 60 min 15 min 30 min 60 min

,700 59,900 72,300 70,700 68,500 45,600 74,60050 1350 2050 8200 1600 7200 10,2506 1.72 2.98 14.10 2.17 12.30 17.8161 6219 8370 8420 7913 5489 96484 164 208 721 165 722 10940 0.35 0.88 7.17 0.35 7.18 11.74,929 27,833 30,548 33,762 29,786 29,597 29,90555 388 900 2736 748 2495 31629 0,89 2,63 8,85 2,11 8,03 10,2971 988 1177 1277 1733 1963 27448.10 589.88 807.89 130.94 97.14 65.71 207.292.04 227.18 197.03 543.32 835.56 1001.15 1490.14.54 37.36 39.96 21.42 11.62 24.29 22.83.78 42.70 46.89 8.48 8.15 13.98 9.12.76 69.58 58.20 484.70 592.57 711.08 816.58.84 14.60 19.70 6.49 6.55 12.54 7.05.52 6.61 6.89 81.36 181.71 134.58 190.52

Fig. 1. Concentration of COD (mg O2/l) and carbohydrates (mg Glu-eq/l) in theliquid phase of the sludge for 70, 80 and 90 �C treatment temperature.

5746 L. Appels et al. / Bioresource Technology 101 (2010) 5743–5748

3.3. Carbohydrates and proteins degradation

Carbohydrates and proteins were solubilised due to thermaltreatment. Fig. 1 shows the carbohydrate concentration in the sol-uble phase, whereas Fig. 2 shows protein concentrations. The totalcarbohydrate and protein concentration in the sludge samples(shown in Table 3) are considered to be constant, as the error ofmeasure was about 15%. This means that these compounds arenot degraded during low temperature thermal treatment. The sol-uble protein concentration increased strongly for the 80 �C and90 �C treatment, but only after a treatment time of 30 min. About10% of all proteins were set free by both treatments after 60 min.The same trend was observed for the carbohydrate release: again,about 10% of the total amount of carbohydrates was found in theliquid phase. This finding suggests that both decomposition ofextracellular polymer substances (EPS) and cell lysis take place

Fig. 2. Concentration of soluble proteins (mg BSA-eq/L) and total volatile fatty acids(mg/l) for the different temperatures and treatment durations.

Table 4Concentrations of heavy metals, phosphorus and sulphur in the liquid phase of the sludge

Parameter Unit Blank 70 �C

15 min 30 min 60 min

Cadmium mg/L 0.004 0.004 0.004 0.004Chromium mg/L 0.009 0.009 0.01 0.012Cupper mg/L 0.05 0.163 0.168 0.191Mercury mg/L 0.0005 0.0004 0.0006 0.0004Nickel mg/L 0.031 0.101 0.126 0.221Lead mg/L 0.013 0.013 0.013 0.017Zinc mg/L 0.174 0.286 0.33 0.475Total heavy metals mg/L 0.2815 0.5802 0.652 0.9204Phosphorus mg/L 71.5 78.4 87.9 93.6Sulphur mg/L 11.9 22.5 26.7 40

during the treatment. Bougrier et al. (2008) suggested that carbo-hydrates are mainly located in the EPS, whereas proteins aremainly located inside the cells. A solubilisation of both compoundsin equal ratios means that both structures are decomposed. At theapplied temperatures, a reaction of soluble carbohydrates withthemselves or soluble proteins (forming e.g. Amadori compoundsor melanoidins) is not expected. This hypothesis was confirmedvisually since the sludge supernatant remained transparent anddid not turn brown to a large extend after treatment.

3.4. Lipids solubilisation

The degradation of lipids was characterised by measuring theVFA concentration in the sludge. This was mainly done becauseof the following reasons: (i) an accurate measurement of lipids insludge is difficult because of the low extractability of lipids, (ii) asignificant solubilisation of lipids is not expected due to their highhydrophobicity and (iii) it is mainly the VFAs which play a crucialrole during digestion.

A significant increase in VFA concentration was achieved for alltreatment temperatures, and a positive correlation between treat-ment temperature and VFA concentration was found. All tempera-tures tested showed a similar solubilisation pattern. The release oftotal volatile fatty acids was slower at 70 �C, as clearly can be seenin Fig. 2, but still reached a high concentration after 60 min.

The observed increase is due to the degradation of lipids (Sha-nableh and Jomaa, 2001). Indeed, due to thermal treatment, longchain fatty acids may be reduced to form fatty acids of lowermolecular weights, which themselves may be degraded in lowchain fatty acids. VFA production may also originate from proteinsdegradation (Bougrier et al., 2008).

3.5. Solubilisation of heavy metals and inorganic material

The concentrations of heavy metals, sulphur and phosphorus inthe soluble phase were measured before and after thermal treat-ment. The results are presented in Table 4. The results follow a log-ical trend: the more energy that is supplied to the sludge, the moreheavy metals are released into the water phase. The cadmium con-centrations in the sludge water are very low (only just above thedetection limit of the method of analysis) and remain more or lessconstant during treatment. The same is true for mercury. As forchromium, the soluble concentration increased by about 50% atboth higher treatment temperatures. Copper was released to alarge extend: a maximum increase in the liquid phase by a factor6 was observed. Even at low temperatures and treatment time, asignificant release was observed. Lead is only released at high tem-peratures and treatment time. For zinc and nickel, there is an al-most linear relationship between release and treatment time. Forboth metals, the treatment time only has a minor to moderate

.

80 �C 90 �C

15 min 30 min 60 min 15 min 30 min 60 min

0.004 0.004 0.0046 0.004 0.0052 0.00450.01 0.012 0.014 0.01 0.015 0.0140.162 0.212 0.234 0.126 0.318 0.2920.0007 0.0007 0.0006 0.0005 0.0006 0.00040.117 0.191 0.225 0.131 0.196 0.2090.017 0.049 0.055 0.013 0.104 0.0810.375 0.566 0.61 0.373 0.881 0.7430.6857 1.0347 1.1432 0.6575 1.5198 1.3439

73.8 74.4 71.1 71.4 71.2 74.621.7 39.1 47.4 20.9 37.4 42.2

Fig. 3. The release of phosphorus and sulphur, respectively, in the liquid phase ofthe sludge (in mg/L) for different treatment durations.

Fig. 4. Biogas production (in mL/g ODS) after 20 days for the different treatmenttemperatures.

L. Appels et al. / Bioresource Technology 101 (2010) 5743–5748 5747

influence. The same results have been reported by Laurent et al.(2009) for ultrasound disintegration.

The release of heavy metals by thermal treatment is mainly ex-plained by the following mechanisms. Heavy metals in sludge oc-cur in different forms: exchangeable, bound to carbonates, boundto iron and manganese oxides, bound to organic matter and resid-ual (Dewil et al., 2007b). Heavy metals, incorporated in the sludgeflocs, can only be transported from the flocs to the aqueous phaseby diffusion. An elevated temperature increases the rate of extrac-tion as the diffusivity of ions is promoted at high temperatures(Veeken and Hamelers, 1999). Conformational changes of thesludge flocs further enhance the mobility of the metal ions. More-over, a large part of the heavy metals is adsorbed to the EPS, whichpresent a lot of potential binding sites including carboxylates,amines, thiols and phosphates. The degradation of these structuresleads to the release of the adsorbed metals (Dewil et al., 2007b).

In Fig. 3, the release of both phosphorus and sulphur in the li-quid phase is illustrated. The concentration of phosphorus doesnot show a significant increase when treated at elevated tempera-tures. In the sampled WWTP, physicochemical phosphate removalis applied using FeCl3 as precipitant. Therefore, the major part ofphosphorus is present in the form of an insoluble FePO4 precipitateand is hence not solubilised. The sulphur concentration, however,shows an almost linear relationship with treatment duration forall temperatures. The gradual increase of sulphur in the superna-tant can be due to the release of organically bound sulphur duringthe breakdown of organic matter (Dewil et al., 2008).

3.6. Influence of thermal treatment on anaerobic digestion

Table 5 presents an overview of the degradation of the variousorganic components during digestion and the total biogas produc-

Table 5The degradation of organic components after anaerobic digestion.

Parameter Unit Blank 70 �C

15 min

Carbohydrate concentration before digestion mg Glu-eq/L 8160 8324Carbohydrate concentration after digestion mg Glu-eq/L 2941 3785Carbohydrate degradation % 63.96 54.53Protein concentration before digestion mg BSA-eq/L 29,500 30,524Protein concentration after digestion mg BSA-eq/L 22,028 23,056Protein degradation % 25.33 24.47Total VFA concentration before digestion mg/L 75 111Total VFA concentration after digestion mg/L 37 93Total VFA degradation % 50.37 15.55Biogas production mL/g ODS 34.83 28.33Biogas composition % CH4

% CO2

63.3732.89

63.3535.12

tion. First, it is noticeable that the methane concentration in theproduced biogas was not significantly affected by the treatmentand remained between 63% and 67%. This means that the energycontent of the gas remains much or less stable. As shown inFig. 4, it was seen that low temperature thermal hydrolysis effec-tively influences the biogas production during the subsequentanaerobic digestion. For the treatment temperature of 70 �C, thegas volume slightly decreased for all pre-treatment times. Thisobservation suggests that a temperature of 70 �C is too low to en-hance the biogas production. The noticed decrease lies within themeasurement uncertainty. At 80 and 90 �C on the other hand, a sig-nificant increase was observed. The gas production increases withincreasing pre-treatment times for both temperatures. Of the threetemperatures tested, the 90 �C gave superior results. As expected, alarge fraction of carbohydrates and proteins is degraded duringdigestion. The remaining part is most likely present inside theanaerobic microflora in the digester. The VFA concentrations de-creased during digestion, but remained at a relatively high level.This is due to the continuous production and breakdown of VFAsduring the anaerobic digestion.

4. Conclusions

The present paper studied the application of moderate temper-ature thermal hydrolysis (70, 80, 90 �C) as a pre-treatment prior toanaerobic digestion of waste activated sludge. It was seen that or-ganic and inorganic compounds are efficiently solubilised duringthermal treatment. In general, a higher temperature and a largertreatment time are beneficial for the release. The efficiency of thesubsequent anaerobic digestion slightly decreased for sludge pre-treated at 70 �C. At higher pre-treatment temperatures, the biogasproduction increased significantly, up to a factor 20 for the 60 mintreatment at 90 �C.

80 �C 90 �C

30 min 60 min 15 min 30 min 60 min 15 min 30 min 60 min

7059 7361 6219 8370 8420 7913 5489 96483384 3128 3489 2936 2772 3954 3626 317452.07 57.51 43.91 64.92 67.08 50.03 33.94 67.1128,429 27,929 27,833 30,548 33,762 29,786 21,597 29,90523,819 25,028 23,583 24,889 20,694 29,556 18,190 21,59716.21 10.39 15.27 18.52 38.7 0.77 15.78 27.78618 1071 988 1177 1277 1733 1963 2744201 539 819 956 1184 1367 1440 161167.47 49.61 17.12 18.74 7.29 21.11 26.67 41.3033.58 35.32 21.26 48.02 75.64 76.69 141.94 377.5666.9931.97

63.4934.85

63.9235.30

65.9532.64

65.4633.42

64.2035.59

63.0535.97

63.6635.65

5748 L. Appels et al. / Bioresource Technology 101 (2010) 5743–5748

Acknowledgements

The authors would like to thank ‘‘Institute for the Promotion ofInnovation through Science and Technology in Flanders (IWT-Vlaand-eren)” for their financial support.

The authors also would like to thank Christine Wouters of thedepartment of Chemical Engineering (Katholieke UniversiteitLeuven) for the analysis of all the volatile fatty acids. This researchis supported in part by the knowledge platform KP/09/005(SCORES4CHEM) of the K.U. Leuven Industrial Research Fund. J.Van Impe holds the chair Safety Engineering sponsored by theBelgian chemistry and life sciences federation essenscia.

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