Bioresource Technology 101 (2010) 9031–9039

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Bioresource Technology

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Start-up of thermophilic–dry anaerobic digestion of OFMSW using adaptedmodified SEBAC inoculum

L.A. Fdéz.-Güelfo a,*, C. Álvarez-Gallego a, D. Sales Márquez b, L.I. Romero García a

a Department of Chemical Engineering and Food Technologies, Faculty of Science, University of Cádiz, 11510 Puerto Real, Cádiz, Spainb Department of Environmental Technologies, Faculty of Sea and Environmental Sciences, University of Cádiz, 11510 Puerto Real, Cádiz, Spain

a r t i c l e i n f o

Article history:Received 6 April 2010Received in revised form 2 July 2010Accepted 5 July 2010Available online 24 July 2010

Keywords:Anaerobic digestionThermophilic–dryOFMSWStart-up and stabilization

0960-8524/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.biortech.2010.07.021

* Corresponding author. Tel.: +34 956016379.E-mail address: alberto.fdezguelfo@uca.es (L.A. Fd

a b s t r a c t

The work presented here concerns the start-up and stabilization stages of a Continuous Stirred TankReactor (CSTR) semicontinuously fed for the treatment of the Organic Fraction of Municipal Solid Waste(OFMSW) through anaerobic digestion at thermophilic temperature range (55 �C) and dry conditions(30% Total Solids). The procedure reported involves two novel aspects with respect to other start-upand stabilization protocols reported in the literature. The novel aspects concern the adaptation of theinoculum to both the operating conditions (thermophilic and dry) and to the type of waste by employinga modified SEBAC (Sequential Batch Anaerobic Composting) system and, secondly, the direct start-up ofthe process in a thermophilic temperature regime and feeding of the system from the first day of oper-ation. In this way a significant reduction in the start-up time and stabilization is achieved i.e. 110 days incomparison to 250 days for the processes reported by other authors for the same type of waste and diges-ter. The system presents suitable operational conditions to stabilize the reactor at SRT of 35 days, with amaximum biogas production of 1.944 LR/L�d with a CH4 and CO2 percentage of 25.27% and 68.15%,respectively.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Anaerobic Digestion (AD) has been widely applied for the treat-ment of the Organic Fraction of Municipal Solid Waste (OFMSW)(Fannin et al., 1983; Akao et al., 2000; Moorhead and Nordstedt,1993). The main advantages of this process are the low level ofsludge generation, lower energy consumption and increased levelof methane production; the main drawback is the slow rate ofthe process. However, AD under thermophilic (55 �C) and dry(30% Total Solids (TS)) conditions has become increasingly impor-tant in recent years given that this process is much faster, the prod-uct obtained is much cleaner, the hydrolysis stage on the complexorganic/biological material is better and, furthermore, the methaneproduction is greater with respect to mesophilic (35 �C) and wet(5–10% Total Solids) conditions.

The start-up and stabilization of semi-continuous digesterstreating OFMSW requires long time periods and in many casesthese are even longer because the inoculum employed is notadapted to the waste and/or the operating conditions, i.e. thermo-philic and dry. In general, according to the bibliography (Bolzonellaet al., 2003; Michaud et al., 2002), the approach used for the start-up of thermophilic anaerobic reactors consists of an initial meso-

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éz.-Güelfo).

philic stage (35 �C), which can last around 185 days, followed bythe application of thermophilic conditions (55 �C), which can takea further 60 days to achieve stability in the system. During the first8–10 days of the mesophilic stage the reactor is not fed – the aimbeing that the inoculum, which can be anaerobic mesophilicsludge, adapts to the solid organic waste. In addition, on changingfrom one temperature range and another, the reactor is also not fed– in this case in order to avoid possible destabilization processesoccurring. As a consequence, the start-up processes described inthe literature take approximately 250 days to complete.

The SEBAC system has been used successfully for the efficientdevelopment of the anaerobic degradation of the organic fractionof municipal solid waste (Forster et al., 2004; SEBAC Homepage,2003). The use of this technology overcomes the problems associ-ated with stirring inoculation and the instability of conventionalsystems through the exchange of the leachate generated duringthe process. This is the only process described to date that enablesthe transitory effects of inhibition and ‘‘no balance” to be elimi-nated during the start-up stage (Chynoweth, 2000), with accept-able conversions achieved in only 30 days. Also, this system is asimple and cheap technology which allows obtaining appropriateinoculums, adapted to the waste and the operational conditions,in a short period of time.

Similarly, Forster et al. (2007) carried out a study on the optimi-zation of inoculum sources for the thermophilic and dry anaerobic

9032 L.A. Fdéz.-Güelfo et al. / Bioresource Technology 101 (2010) 9031–9039

digestion of OFMSW and confirmed that sludge from wastewatertreatment plants shows the best performance as inoculum frommodified SEBAC system.

The modified SEBAC process employed in this study is based onthe interconnection of two reactors linked. On a daily basis the tworeactors are fed through the recirculation of the leachate from theother reactor. In this way the fresh organic waste (reactor A) to bedigested is inoculated through recirculation of the leachate oreffluent from the reactor containing the digested waste (reactorB), while the leachate generated by the reactor with fresh waste(reactor A) is recirculated to the reactor with the digested waste(reactor B). In this way a flow of microorganisms is establishedto the undigested waste and of organic material to the digestedwaste (Angenent et al., 2002).

In order to reduce the long time periods required for the start-up and stabilization with this type of technology, the work de-scribed here has two main aims:

1. The use of an inoculum adapted to both operating conditions(thermophilic and dry) and type of waste. This objective canbe achieved using a modified sequential batch anaerobic com-posting (SEBAC) system.

2. To start-up the process directly under thermophilic conditionsand to feed the digester from the first day of operation.

2. Methods

2.1. Waste sources and conditioning

2.1.1. Synthetic OFMSWWith the aim of avoiding possible variations in the composition

of the selected OFMSW, it was decided to prepare a syntheticfeedstock based on the nutritional requirements of the majormicroorganisms involved in the process according to the specifica-tions of Martin et al. (1999). This feedstock is characterized by thefollowing main parameters: 0.71 g TVS/g; 112.3 mg DOC/g; 1440mg AcH/L; 207.2 g NH3–N/kg and 4.29 g CaCO3/L. When workingwith a semi-continuous feeding regime it is important that thecomposition of the waste fed into the reactor is uniform in orderto establish accurate process yields.

On the other hand, for the right operation of dry anaerobicdigestion process it is necessary to control the TS content duringthe start-up phase of the process. To achieve this it is necessaryto condition the OFMSW samples in order to adjust the optimumvalues for several parameters. Conditioning of the samples involvesdrying the feedstock at 55 �C for 24 h and subsequently at roomtemperature for 72 h until final moisture of 10% is obtained. Thedry OFMSW is then milled, by means of a high performance mill(mark and model Retsch SM 2000-Retsch), to give a particle sizeof around 1 cm. In this way the colonisable surface area for themicroorganisms is increased and the physico-chemical character-ization of the representative waste can be carried out. The finalconditioning step involves adjusting the moisture to the range70–75% (25–30% in TS, which is characteristic of dry anaerobicdigestion) directly with the inoculum.

2.1.2. InoculumThe inoculum used in this study consisted of a mixture of leach-

ate and sludge, in a proportion 1:1 v/v (Wang et al., 1999; Kimet al., 2002; Lin and Lee, 2002), from the modified SEBAC reactors.

2.2. Preparation of the inoculum through modified SEBAC technology

A schematic representation of the two reactors and the estab-lished leachate flow is shown in the Fig. 1.

The reactors used for inoculum development were a modifiedSEBAC system. It consists of interconnection of two batches anaer-obic 35 L-reactors (25-L effective volume), operating under ther-mophilic conditions (55 �C):

� Reactor ‘‘A” contains 4 total alternate layers (as can be seen inFig. 1) of source selected OFMSW (every one of 2 kg) and pigmanure (with a weigh of 1.5 kg every one) with 30% TS in thiscase. The pig manure accelerates the colonization of the OFMSWsince it is a potential source of anaerobic microorganisms.� Reactor ‘‘B” contains 21 kg of anaerobic mesophilic sewage

sludge from waste water treatment plant (WWTP) was used.

Details of the modified SEBAC reactors employed in the experi-mental work are given below.

2.2.1. Modified SEBAC reactorsEach reactor is a plastic vessel with an effective volume of 25 L.

The vessel consists of central cylindrical body (length 50 cm, inter-nal diameter 30 cm). The vessel is sealed in order to maintainanaerobic conditions. Inside reactor A there is a chamber (depth14 cm) filled with glass balls (1 cm in diameter) and with a work-ing volume of 1.5 L what it is used to stock the generated leachateon a daily basis. The cover of each reactor has three ports: biogasexit, port for leachate spreading from the other reactor and a portfor leachate collection with an internal tube attached to the base ofthe reactor and connected to a pump. The later ports are fitted withvalves to control biogas exit. A peristaltic pump was used daily tocollect the leachate generated in reactor A and to carry out the ex-change cycles between reactors.

2.2.2. Thermostatic chamberThe operating temperature selected for all experiments was

55 �C. This temperature was maintained by carrying out the runsin a temperature-controlled chamber properly isolated.

These reactors were only used to the acclimatization of theinoculum to experimental conditions (thermophilic and dry) andtype of waste.

2.3. Continuous stirred tank 5-L reactor

The reactor used for the semi-continuous process had a totalcapacity of 5 L and a working volume of 4.5 L. The reactor was jack-eted and thermostatically controlled using a circulating 7 L-bath. Aball valve was fitted to the lower part of the reactor in order to dis-charge the contents and in the cover there were several ports: cen-tral hole for agitation system (mixing shaft at 13 rpm), a pH probe,a biogas collector, an opening for the addition of feedstock and twofurther hoses for the pH-control. The pH was adjusted using an on/off controller with 5 N NaOH and 1 N H3PO4 solutions. The initialpH of the synthetic OFMSW (OFMSWSYNTH)–noculum mixturewas 7.2. The operating range of the pH controller was 6.5–8, whichis suitable for the methanogenic microorganisms. The physico-chemical characteristics of the different wastes are summarizedin Table 1.

2.4. Anaerobic digestion of OFMSW

The 5-L reactor was filled with 1.5 kg of dry synthetic OFMSW(10% moisture content) and was mixed with inoculum to adjustthe solid content to 30% in TS. Hydration of the dry waste required4 L of inoculum (i.e. 2 L of sludge and 2 L of leachate). This proce-dure was used in an attempt to obtain a reliable and rapid start-up and to give a system with high stability.

Fig. 1. Structure and leachate exchange cycle in the modified SEBAC system.

Table 1Characterization of the wastes used in the continuous stirred tank reactor.

Parameter Leachateinoculum(1)

Sludgeinoculum(1)

SyntheticOFMSW(2)

OFMSW/inoculummixture(2)

pH 8.62 8.35 7.78 8.70Density (kg/m3) 980 985 750 1116Alkalinity

(g CaCO3/L)21.78 16.54 4.29 5.14

Ammonia(gNH3–N/L)

26.88 14.56 1.68 2.8

Total nitrogen(g NH3–N/L(1);g NH3–N/kg(2))

35.66 21.46 207.2 72.8

TSS (g/L) 14.46 20.46 – –VSS(g/L) 10.73 9.16 – –SFS (g/L) 3.73 11.3 – –TS (g/g sample) – – 0.90 0.31VS (g/g sample) – – 0.71 0.25SFT (g/g sample) – – 0.19 0.07TC (mg/g) 80.78 35.27 112.6 65.07TIC (mg/g) 2.07 0.96 0.29 0.30DOC (mg/g) 78.41 34.31 112.3 64.75Acidity (mg AcH/L) 12403 17353 1440 356

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The start-up and stabilization phases of the digester were sub-sequently carried out and the main variables of the process wereclosely monitored.

2.5. Sampling and analysis

For process monitoring and control, the following analyticaldeterminations have been used: Total Solids (TS), Volatile Solids(VS), alkalinity, pH, Dissolved Organic Carbon (DOC) and ammo-nium. All parameters were analyzed once a day and determina-tions were performed according to Standard Methods (APHA,AWWA, WEF, 1995).

The volume of gas produced in the reactor was directly mea-sured using a high precision flow gas meter – WET DRUM TG 0.1(mbar) – Ritter – through the Tedlar bag. The gas composition(hydrogen, methane and carbon dioxide) was determined by gaschromatography (SHIMADZU GC-14 B) with a stainless steel col-

umn packed with Carbosive SII (diameter of 3.2 mm and length of2 m) and a thermal conductivity detector (TCD). The injectedsample volume was 1 mL and the operational conditions were asfollows: 7 min at 55 �C; ramped at 27 �C min�1 until 150 �C; detec-tor temperature: 255 �C; injector temperature: 100 �C. The carrierwas helium and the flow rate used was 30 ml min�1. A standardgas (by Carburos Metálicos, S.A; composition: 4.65% H2; 5.33% N2;69.92% CH4 and 20.10% CO2) was used for the system calibration.

Individual VFA (from C2 to C7, including iC4, iC5 and iC6) levelswere determined by gas chromatography (SHIMADZU GC-17 A)with a flame ionization detector and a capillary column filled withNukol (polyethylene glycol modified by nitro-terephthalic acid).The temperatures of the injection port and detector were 200 �Cand 250 �C, respectively. Helium was the carrier gas at 50 mlmin�1. In addition, nitrogen gas was used as make up at 30 mlmin�1 flow rate.

3. Results and discussion

3.1. Preparation of the inoculum using modified SEBAC technology

In terms of the microbiology and biochemistry of anaerobicdegradation, different groups of microorganisms coexist in the pro-cess and their metabolic activities are strongly linked. Acidogenicmicroorganisms lead to hydrolysis and fermentation of the organicmatter to VFA along with the production of CO2 and H2. Acetogenicmicroorganisms transform the VFA into acetic acid, with CO2 andH2 also generated. Finally, the methanogenic microorganismstransform these products (acetic acid, CO2 and H2) into methane.This type of coordinated microbial activity confers stability to theoverall process because VFA and H2 inhibit the processes. As a re-sult, the anaerobic inoculum must contain a well-defined microbialecosystem in which the relative proportions of the different groupsof microorganisms enable the process to develop in a balanced andsustainable way.

It can be observed in Fig. 2a and b that the maximum produc-tion of biogas and methane in the digesters occurs between days28 and 30. The daily production of methane increases to 4.7 L/dand 7.1 L/d in reactors A and B on day 28. These results are consis-tent with those reported by Chynoweth et al. (1991), Nopharatana

Fig. 2. Volumes of biogas and CH4 generated in the reactors A (a) and B (b) of the modified SEBAC system.

9034 L.A. Fdéz.-Güelfo et al. / Bioresource Technology 101 (2010) 9031–9039

et al. (1998) and Álvarez-Gallego (2005). The volume of methaneproduced in reactor B then begins to decrease and reaches zeroon day 80. In contrast, in reactor A the volume of methane remainsbetween 2 and 5 L/d in the same period and decrease at the end ofthe run. At this point the biogas production in reactor A may beassociated with the solid materials degradation, as the leachategenerated in the reactor A and transferred to reactor B does notproduce methane in the latter digester.

The evolution of the biogas composition with time in both di-gesters is represented in Fig. 3a and b. It can be observed that sinceday 80 both reactors show a methane-rich composition above 70%.As it was expected, both evolution of biogas composition gets moresimilar at the end of the study.

The information obtained from biogas production and methanepercentage seems to indicate that the inoculum reaches adequateconditions for a time higher than 60 days. However in this experi-ment was extended as consequence of the needs of follow-up theprocess in order to study the global behaviour. Inoculum operationwas performed at day 80, when the target composition wasreached and maintained. Nevertheless, and though the period ofstart-up might have diminished, the used inoculum showed a rapidadjustment to the substrate and it can be considered and adaptedinoculum to development the process.

Bearing in mind the information outlined above, and in order todetermine the suitable time at which the inoculum has the appro-priate characteristics for use in the start-up of a new system (inthis case a continuous stirred tank reactor), the biogas compositionwas considered as good indicators to assess the optimum for inoc-ulation time. The found target composition was a methane-richbiogas without any trace of hydrogen. In these conditions, aceto-

clastic microorganisms are supposed to be favoured versus H2-uti-lizing microorganisms.

Therefore, based on the previous results can be emphasized thata continuous stirred tank reactor can be inoculated directly withsludge from digested waste from a modified SEBAC system withoutthe need for an acclimation period prior to feeding the system.Suitable time to inoculation from modified SEBAC is longer 60 daysof operation because this is the time for which high level of meth-ane composition is achieved with hydrogen-free biogas.

3.2. Start-up and stabilization of the continuous stirred tank reactor

The start-up and stabilization of the reactor was carried out un-der thermophilic conditions (55 �C) with a sequence that consistedof four stages in which the organic loading rate was modified threetimes. The organic loading rate for each of the solid retention times(SRT), expressed as mg DOC/L�d and mg VS/L�d, was constant andthe values are given in Table 2.

Initially the organic loading rate was relatively low (0.702 gDOC/L�d) in order to check if the system evolved appropriately.The results obtained in the first 14 days were promising and the or-ganic load was therefore increased till 1.120 g DOC/L�d withoutwaiting for usual stabilization period after each increment. TheOLR0 value used in first stage differs from the values reported inthe literature (Bolzonella et al., 2003) because the start-up is usu-ally carried out in mesophilic systems with an extremely low or-ganic charge, i.e. below 0.16 g DOC/L�d, which is maintained forapproximately 40 days before this parameter is increased and thetemperature raised. This difference is due to the fact that, in thesystem, the start-up is carried out using the modified SEBAC inoc-

Fig. 3. Percentages of H2 and CH4 and CO2 in reactors A (a) and B (b) of the modified SEBAC system.

Table 2Initial and consumed organic loading rates (OLR0, OLRC) in terms of dissolved organic carbon and volatiles solids and both expressed as mg/L�d.

Stage SRT (day) Operation time (day) DOC (mg/L�d) VS (mg/L�d)

OLR0 OLRC OLR0 � OLRC 100 � (OLR0 � OLRC)/OLR0 OLR0 OLRC OLR0 � OLRC 100 � (OLR0 � OLRC)/OLR0

1 40 14 702 235 467 66.5 4431 2980 1451 32.72 35 17 803 419 384 47.8 5069 3826 1243 24.53 30 25 938 548 390 40.6 5920 4840 1080 18.24 25 50 1120 715 405 36.2 7090 6237 853 12.0

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ulum that has previously been adapted to the waste, the thermo-philic range and the dry conditions. This approach overcomes theneed for the latent phase that arises when the microorganism isnot adapted to the conditions of the medium or the substrate.

3.2.1. Monitoring of the organic loading rate consumed (OLRC) andbiogas production3.2.1.1. OLRC expressed as a function of dissolved organic carbon(DOC). It can be observed from Fig. 4a that during stage 1 there is aadaptation period of 7 days in which the OLRC remains practicallyconstant at a value close to 150 mg DOCc/L�d. This observationcould be due to the occurrence of an adaptation period for themicroorganisms of the inoculum to the waste, a situation due tothe differences between the waste used in the experiment and thatused in the modified SEBAC system. From day 7 until the end ofthis stage the OLRC increases gradually until it stabilizes at a valueof around 320 mg DOCc/L�d.

At the beginning of the stage 2 the OLR0 increases from 702 to803 mg DOC/L�d and this leads to an increase in the overall bio-

mass of the system, which translates into a rapid increase in OLRC

to values close to 400 mg DOCc/L�d, i.e. the microorganisms arecapable of adapting rapidly to changes in the SRT. In the period be-tween days 16 and 25 the OLRC stabilizes in the range 420–425 mg DOCc/L�d and, as a result, it was decided to again modifythe organic loading rate.

During stage 3 the OLR0 increased to 938 mg DOC/L�d and, as inthe previous cases, the OLRC increased rapidly to 530 mg DOC/L�d.After 8 days of this stage, once the reactor had stabilized, the OLRC

remained in the range 540 to 550 mg DOC/L�d.Finally, in stage 4 the OLR0 increased to 1120 mg DOC/L�d and

the system once again showed the same type of behaviour. TheOLRC initially increased to 670 mg DOC/L�d and subsequently re-mained constant until the end of the study, giving an average valueof 708 mg DOC/L�d.

The differences between the organic loading rates consumed(OLRC) and fed organic loading rates (OLR0) for the different SRTvalues investigated are shown in Table 2. A lower value of this dif-ference is representative of the higher biodegradative capacity of

Fig. 4. Evolution of the Organic Loading Rate Consumed (OLRc) in terms of mgDOC/L�d (a) and mgVS/L�d (b).

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the system. The difference (OLR0 � OLRC) represents the fraction ofunconsumed organic material and, as can be seen, this remainspractically constant in the region 400 mg DOC/L�d for the SRT val-ues of 35, 30 and 25 days. This finding indicates that the system isstable and that the microbial population has adapted to the differ-ent experimental conditions used. As a consequence, it can be sta-ted that the value of the parameter (OLR0 � OLRC) is representativeof the recalcitrant fraction of the waste.

3.2.1.2. OLRC expressed as a function of volatile solids (VS). The evolu-tion pattern is similar to that described above (Fig. 4b). The systemadapts rapidly to changes in the SRT values investigated and stabil-ization of the process is achieved. For a SRT of 40 days the OLRC isin the range 2800–3000 mg VSc/L�d; for a SRT of 35 days, between3680–3885 mg VSc/L�d; for a SRT of 30 days, between 4838–4896 mg VSc/L�d and, finally, for a SRT of 25 days, the OLRC isaround 6250 mg VSc/L�d.

The differences between the consumed organic loading rate(OLRC) and fed organic loading rate (OLR0) for the different SRTvalues studied are shown in Table 2. In contrast to the previouscase, when the initial and consumed OLR was expressed in termsof DOC, the organic loading rate that is not biodegraded(OLR0 � OLRCM) does not stabilize for the different SRT valuesinvestigated and decreases continuously. This fact could be dueto the experimental procedure followed in the first day of the studywhen the reactor was loaded. The digester was initially filled witha mixture of OFMSW and inoculum that had very different charac-teristics in terms of VS and feeding of the synthetic OFMSW subse-quently used (Table 1) and, as a result, a longer period ofstabilization is necessary. However, as in the previous case it isseen that in all cases increases in OLR0 lead to increases in OLRC.

This result shows that the system does stabilize and is capable toassume the OLR0 values used.

3.2.1.3. Biogas production. Analysis of data about generated biogasshows that stage 1 can be considered as an adaptation period(Table 3). During this stage solubilization of the components takesplace through hydrolysis of the waste and this leads to itscolonization. As a result, the average methane yield, expressed asLCH4/g DOCc and LCH4/g VSc, is practically negligible. In stage 2the methane yield reaches a maximum value of 1.112 LCH4/g DOCc

and 0.121 LCH4/g VSc and finally, in stages 3 and 4 the yield ofmethane remains essentially constant, with values in the range0.81–0.91 LCH4/g DOCc, and 0.1 LCH4/g VSc, a situation representa-tive of the process stability.

In summary, the overall average methane yield in this studywas 81 L CH4/kg VSc (100 L CH4/kg VSc at two last stages). The spe-cific methane production, in terms of VS consumed, is within theranges reported in the literature (Álvarez-Gallego, 2005; Chughet al., 1999; Chynoweth et al., 1991; Fruteau-de-Laclos et al.,1997; Kayhanian and Rich, 1995; Mata-Álvarez, 1998; Wanget al., 1997).

As far as the daily generation of biogas is concerned, in the firstthree days of stage 1 there is a significant level of production due tohydrolysis of the main mass within the reactor (Table 3). The maingases produced in this period are characteristic of the hydrolyticstage and are H2 and CO2 in a ratio of 30:60. This stage involvesthe breakdown of complex molecules that are readily biodegraded(particularly carbohydrates) into simpler molecules. Given that thepresence of H2 inhibits methanogenesis of VFA, the methanogenicactivity observed is very low and corresponds to the development

Table 3Average biogas production and composition. Also are shown the biogas production and methane and biogas yields.

Stage SRT (day) Biogas production and composition Methane and biogas yields

L/LR�d H2 CH4 CO2 LCH4/g DOCc LCH4/g VSc Lbiogas/g DOCc Lbiogas/g VSc

1 40 0.638 28.09 11.26 60.65 0.034 0.002 1.28 0.222 35 1.944 6.57 25.27 68.15 1.112 0.121 2.91 0.533 30 1.160 0.00 47.89 52.11 0.912 0.103 1.47 0.264 25 1.394 0.00 45.66 54.34 0.817 0.096 1.48 0.23Overall average yield (stages 3–4 average yield) 60.65 (0.865) 0.081 (0.100) 1.785 (1.475) 0.31 (0.245)

Table 4Average removal yields.

Stage SRT(day)

% VSE % TSE % DOCE

1 40 67.26 10.86 33.522 35 75.48 33.00 52.193 30 81.76 45.63 58.444 25 87.98 58.19 63.82Overall average yield excluding stage 1 83.98 48.00 58.93

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of the H2-utilizing Archaeas population. For this reason in the pre-vious figures, stage 1 is considered as an adaptation phase.

In the stage 2 methane yield exhibits a great increase due todegradation of organic material accumulated in the previous stage.During this stage the average daily production and specific yield ofbiogas was higher than following stages (1.944 L/LR�d and 0.53 L/gVSc, respectively). In the final days of stage 2, corresponding to aSRT of 35 days, the percentage of CH4 reaches at around 40%, whichis a value typical of systems in which the production of CH4 corre-sponds to the activity of the H2-utilizing microorganisms. Since

Fig. 5. (a) Evolution of the removal percentage in terms of V

day 22 H2 concentration disappears and its behaviour is similarto that reported by Castrillón et al. (2002). No evidence of aceto-clastic methanogenic activity is obtained till hydrogen dissapearsfrom biogas composition. This profile is characteristic of start-upphases in semi-dry thermophilic anaerobic digestion process(Bolzonella et al., 2003).

For the following stages, average daily production and specificyield of biogas (Table 3) were more similar with those obtainedby Cecchi et al. (1986) on using sludge from wastewater treatmentplants as inoculum. Cecchi et al. (1986) obtained a biogas yield of0.35 L/g VSc on using a SRT of 33 days. In other examples reportedin the literature (Bolzonella et al., 2003; Cecchi et al., 1991;Mata-Álvarez et al., 1993; Pavan et al., 2000; Vallini et al., 1993),which concern the use of semi-dry OFMSW (20–25% in TS), thespecific biogas production was in the range 0.3–0.4 L/g VSc. Biogascomposition in stages 3 and 4 is hydrogen free and close to 50:50CO2/CH4 ratio.

As its showed in Table 3, in stage 3 the average daily productionof biogas decreases to 1.160 L/L�d because the majority of the ini-tial waste loaded into the reactor has been degraded. However,

S and TS. (b) Evolution and removal percentage of DOC.

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the composition of the biogas is stable at this point with percent-ages of CO2 and CH4 at around 50% – indicating that initial transi-tory effects of the start-up are overcome and microbial populationis balanced.

In stage 4 the average daily production of biogas rises to anaverage value of 1.394 L/L�d and this has a composition of 55%CO2 and 45% CH4. This increase in the production of biogas in com-parison to stage 3 is due fundamentally to the increase in OLR0. Theyields of biogas obtained in stages 3 and 4 (0.26 and 0.23 L/g VSc,respectively) are consistent with the average values obtained byFruteau-de-Laclos et al. (1997) and Bolzonella et al. (2003), who re-ported biogas production levels in the range 0.21–0.29 L/g VSc onusing semi-dry OFMSW (20–30% en TS) with SRTs between 13and 55 days.

3.2.2. Overall percentage of removal in terms of TS, VS and DOCThe average percentages of VS and TS removal during stage 1

are 67% and 11%, respectively (Table 4). As a result of the decreasein SRT, the removal efficiencies for both parameters increase andthe system shows a favourable evolution that, despite the increasein OLR0, does not lead to a decrease in the efficiency of the system.In stage 2 the average removal percentage increase to 75% for VSand 33% for TS and these increase further to stable values of 88%in VS and 58% in TS through stage 4. The particularly low percent-age of TS removal observed during stage 1 are related to the initialaccumulation of non-biodegradable solids from the inoculum usedto fill the reactor. As a result, feeding the system causes only aslight increase in this variable.

Concerning the evolution of removed organic material theshape of the curve is similar to that for VS and TS removal butshows a characteristic feature (Fig. 5a) in that from the seventhday to the end of stage 1 the system shows rapid biodegradation,with an increase in the percentage of removal of organic carbonin the daily effluent from 28 to 50% (Fig. 5b). This phenomenonis associated with the removal of dissolved organic carbon due tohydrolysis of the waste initially loaded into the reactor.

The experiment gave rise to maximum percentage of removal(in stage 4) of 88% in VS, 58% in TS and 64% in DOC (Table 4). Theseresults are comparable to the biodegradability values for MSW re-ported in the literature (Álvarez-Gallego, 2005; Davidsson et al.,2007; Ghosh et al., 2000).

Therefore, can be summarized that to start-up the reactor theinitial SRT must be high (40 days in this study), corresponding toa low organic loading rate, with the aim of avoiding irreversibledistortions in the process. The SRT is progressively decreased andthis leads to an increase in the OLR supplied until the conditionsrequired for operation are reached. Stabilization of the system foran SRT of 35 days (OLR0 of 0.805 g DOC/L�d) requires only 30 daysof operation. Under these conditions removal percentage above75% for VS and above 52% for DOC are achieved along with a highlevel of methane production.

4. Conclusions

A successful start-up and stabilization of a CSTR for the thermo-philic–dry anaerobic digestion of OFMSW has been developed. Theeffective start-up of the system is achieved in approximately110 days (80 days to obtain the SEBAC inoculum and 30 days toreach suitable conditions to stabilize the main operational param-eters at SRT of 35 days). In this way stable operation can beachieved in a minimum of approximately 90 days if inoculationis performed with a 60-days effluent from modified SEBAC system.These results represent a significant improvement on those re-ported in the literature for the start-up of similar reactors, whichgenerally require at least 250 days.

Acknowledgements

This work was supported by the Spanish Ministry of Scienceand Innovation (Project CTM2007-62164/TECNO), by the Innova-tion, Science and Enterprise Department of the Andalusian Govern-ment (Project P07-TEP-02472) and by the European RegionalDevelopment Fund (ERDF).

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