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Waste Management 28 (2008) 1801–1808

Two-phase anaerobic digestion within a solidwaste/wastewater integrated management system

G. De Gioannis a, L.F. Diaz b, A. Muntoni a,*, A. Pisanu a

a DIGITA, Department of Geoengineering and Environmental Technologies, University of Cagliari, Piazza D’Armi 09123 Cagliari, Italyb CalRecovery, Inc., 2454 Stanwell Drive, Concord, California 94520, USA

Accepted 20 November 2007Available online 10 January 2008

Abstract

A two-phase, wet anaerobic digestion process was tested at laboratory scale using mechanically pre-treated municipal solid waste(MSW) as the substrate. The proposed process scheme differs from others due to the integration of the MSW and wastewater treatmentcycles, which makes it possible to avoid the recirculation of process effluent. The results obtained show that the supplying of facultativebiomass, drawn from the wastewater aeration tank, to the solid waste acidogenic reactor allows an improvement of the performance ofthe first phase of the process which is positively reflected on the second one. The proposed process performed successfully, adopting mes-ophilic conditions and a relatively short hydraulic retention time in the methanogenic reactor, as well as high values of organic loadingrate. Significant VS removal efficiency and biogas production were achieved. Moreover, the methanogenic reactor quickly reached opti-mal conditions for a stable methanogenic phase. Studies conducted elsewhere also confirm the feasibility of integrating the treatment ofthe organic fraction of MSW with that of wastewater.� 2007 Elsevier Ltd. All rights reserved.

1. Introduction

The biodegradable organic matter contained in munici-pal solid waste (MSW) probably is one of the most prob-lematic fractions to deal with: it is difficult to sort ifmixed with the other fractions, it affects the performanceof incinerators in terms of energy recovery and it is thecause of the long lasting emissions from sanitary landfills(Cossu et al., 2003; Soyez and Plickert, 2002; Zach et al.,2000). European legislation requires a drastic reductionof the amount of biodegradable organic residues to belandfilled. The European MSW management policy relieson separate collection in order to allow recycling of mostof the packaging waste and the production of high qualitycompost from the biodegradable organic fraction. How-ever, recycling through separate collection and compostingof the entire MSW biodegradable fraction is difficult to

0956-053X/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.wasman.2007.11.005

* Corresponding author. Tel.: +39 70 6755546; fax: +39 70 6755523.E-mail address: amuntoni@unica.it (A. Muntoni).

achieve. Energy recovery through anaerobic digestion rep-resents a complementary option. Mechanical pre-treatmentof residual MSW can be performed in order to produce twostreams: a first one, characterized by a significantly highheating value, to be utilized as refuse derived fuel (RDF)and a second one which would need biological stabilizationbefore it is disposed. As far as this second stream is con-cerned, a process based on anaerobic digestion followedby aerobic treatment combines well the need for stabiliza-tion with the opportunity for energy recovery through bio-gas production and exploitation (Diaz et al., 1981; DeBaere, 2000).

Currently several companies and researchers proposetheir own anaerobic process schemes, which usually areclassified on the basis either of the total solid (TS) contentas wet digestion (TS content: 10–15%) and dry digestion(TS content: 25–40%), or of the adopted temperatureregimes as mesophilic (30–40 �C) and thermophilic diges-tion (50–60 �C). According to a number of research activi-ties, good results are achievable by a two-phase, wetmesophilic digestion process (Bhattacharayya et al., 2007;

1802 G. De Gioannis et al. / Waste Management 28 (2008) 1801–1808

Bolzonella et al., 2003; Bouallagui et al., 2005; Cecchi et al.,2002; Maharaj and Elefsiniotis, 2001; Mata Alvarez,2002b; Pavan et al., 2000; Schober et al., 1999; Zhanget al., 2005). The separation of the hydrolysis-acetogenesisand methanogenesis phases results, in fact, in a higher pro-cess stability; furthermore, the dilution of inhibiting sub-stances contained in the inflow, the optimization of thecontact between biomass and substrate and the possibilityto treat a wider range of waste (MSW, residues from foodindustry, etc.) are achieved. With respect to the tempera-ture regimes, mesophilic digestion has been widely adoptedfor anaerobic treatment due to the good operational per-formance, whilst the use of thermophilic processes has beenlimited, mainly due to poor supernatant quality and pro-cess instability caused, for example, by chronically highpropionate concentration (Kim et al., 2002).

Although it has been reported that anaerobic consortiaare able to adapt to adverse conditions if they are givenadequate time (Speece, 1983, 1996), anaerobic digestionprocesses are often characterized by an intrinsic instabilityderiving from the high sensitivity of the anaerobic biomassto a number of inhibiting or toxic substances (Garcıa-Heras, 2002; Salminen and Rintala, 2002; Vandevivereet al., 2002), either endogenous (ammonia and organicacids) or exogenous (heavy metals and cations like K+,Na+, Mg++, Ca++). It is a general opinion that the recircu-lation of the effluent in a wet anaerobic digestion processes,often in significant quantities to achieve the desired mois-ture content in the solid waste, can lead to the accumula-tion of inhibiting substances (Gallert et al., 2003).

A two-phase, mesophilic wet anaerobic digestion pro-cess was tested at the laboratory scale using mechanicallypre-treated MSW as the substrate. Recirculation of theeffluent was avoided assuming a possible integration ofthe MSW and municipal wastewater treatment cycles. Infact, this integrated management would allow:

� the supply of facultative biomass and liquid phase to thesolid waste acidogenic reactor through the utilization ofactivated sludge from the aeration tank of a wastewatertreatment plant;� the supply of anaerobic sludge to the solid waste meth-

anogenic reactor drawing it from the sewage sludgedigester of a wastewater treatment plant;� the treatment of the effluent of the solid waste treatment

plant at the wastewater treatment plant; and� the use of excess capacity in conventional sewage sludge

digesters.

The supply of the easy to obtain activated sludge to theacidogenic reactor leads to several advantages: (1) the firstphase of the process, which includes hydrolysis that is con-sidered the rate-limiting step of the entire solid wasteanaerobic digestion process, benefits by the considerablepresence and high growth yields of the facultative biomass(Kim and Speece, 2002; Schober et al., 1999); (2) activatedsludge is characterized by a low content of inert material

and sand (Kim and Speece, 2002); (3) nitrogen, as well asother nutrients, which are not present at sufficient levelsin MSW, are provided (Stroot et al., 2001); and finally,(4) the potential risk of accumulation of inhibiting sub-stances is reduced due to the avoidance of effluentrecirculation.

The expected improved performance of the acidogenicphase and the possible avoidance of inhibiting effects couldallow the achievement of significant conversion efficiencyof the substrate and adequate process stability, despitethe problematic nature of MSW. Therefore, shorter valuesof the methanogenic reactor hydraulic retention time(HRT) could be adopted.

On the basis of these considerations, a research programwas developed aimed at testing, at the laboratory scale, theperformance of a two-phase, mesophilic, wet anaerobicdigestion process, where (a) the effluent recirculation isreplaced as a result of the integration of the MSW andmunicipal wastewater treatment cycles, and (b) mechani-cally pre-treated MSW is used as the sole substrate.

2. Materials and methods

2.1. Materials

Mechanically pre-treated MSW, activated sludge fromthe aeration tank and anaerobic sludge from the sewagesludge digester were sampled at an integrated platformwhere municipal wastewater treatment is performed andMSW and digested wastewater treatment sludge are incin-erated. The solid waste was sampled at a mechanical treat-ment section where the plastic bags containing MSW areopened, the solid waste is shredded and size classificationis conducted by means of a rotating sieve (U = 60 mm).These operations result in two streams: the over-size, whichis introduced into the incinerator and the under-size whichhas to be biologically stabilized prior to landfilling. Sam-ples of the mechanically pre-treated waste were collectedduring three different weeks of the summer season to takeinto account the possible short-term variations of wastecomposition, and, in turn, to test the performance of thesystem under these circumstances. Indeed, the characteris-tics of the sampled residues allowed for the adoption of dif-ferent values of the organic loading rate during the testruns.

The solid waste, the activated sludge and the anaerobicsludge were analyzed to determine total solids (TS), volatilesolids (VS), total organic carbon (TOC), and total nitrogen(Ntot); the ratio of TOC to total nitrogen (C/N) was calcu-lated. All of the analyses were performed in triplicate; theaverage data are reported in Table 1.

2.2. Experimental device and operation

The proposed scheme is shown in Fig. 1. The reactorswere operated in semi-continuous mode by feeding andremoving an equivalent volume on a daily basis.

Table 1Characterization of mechanically pre-treated MSW, activated sludge and anaerobic sludge used during the test runs

TS (%) VS (%TS) TOC (%TS) Ntot (%TS) C/N

Waste 1 (Wl) 59.7 ± 1.35 36.2 ± 0.79 18.4 ± 1.25 0.9 ± 0.08 20.5 ± 3.24Waste 2 (W2) 57.7 ± 1.25 22.0 ± 0.70 18.1 ± 0.95 2.0 ± 0.13 9.1 ± 0.94Waste 3 (W3) 64.3 ± 1.40 44.3 ± 1.83 30.6 ± 1.48 1.4 ± 0.10 21.9 ± 2.6Activated sludge 0.5 ± 0.01 0.3 ± 0.04 – – –Anaerobic sludge 4.0 ± 0.1 2.4 ± 0.26 1.2 ± 0.08 – –

Fig. 1. Process scheme of the tested process (mass flow in % w/w).

G. De Gioannis et al. / Waste Management 28 (2008) 1801–1808 1803

The acidogenic phase was carried out using completelymixed reactors having a volume of 500 ml; temperaturecontrol was not performed. Mechanically pre-treatedMSW and activated sludge were mixed in order to obtain10–15% TS content and the resulting mixture fed to the aci-dogenic reactor. Acclimation of the aerobic biomass toanaerobic conditions was allowed by not feeding the acido-genic reactors during the first 48 h after initial filling. After-wards, the reactors were operated in a semi-continuousmode in order to achieve a hydraulic retention time(HRT) of 3 days, similar to those adopted by other authors(e.g. Viturtia et al., 1995; Scherer et al., 2000).

Reactors of 800 ml, minimally mixed thoroughly shakenby hand for 2 min each day, i.e. 1 min before wasting and1 min after feeding as reported by Stroot et al. (2001), wereused as methanogenic reactors. The temperature was set atthe limit of the mesophilic field (39 �C) and controlled bycirculating warm water through a water jacket. Daily feed-ing of the methanogenic reactors was performed using amixture of 80% w/w of the acidogenic reactor effluentand 20% of anaerobic sludge. The HRT of the methano-genic reactor was set at 8 days, significantly shorter than

Table 2Operational parameters adopted during runs 1 and 2

Run Period Waste Acidogenic reactor feeding(% w/w)

OLR (d)

Activated sludge Waste

1 – Wl 84.0 16.0 19.9 (O2 2.1 W2 82.3 17.7 13.2 (O

2.2 W2 78.0 22.0 17.6 (O2.3 W3 78.0 22.0 33.0 (O

the values usually adopted (i.e. 18 days, Kim et al.(2002); 14.2 days, Scherer et al. (2000); 14–18 days, Hart-mann and Ahring (2005); 17 days, Fernandez et al.(2005)) since a good efficiency of the hydrolysis/acidogenicphase was expected.

Start-up of the methanogenic reactors was performed byfirst filling them with a mixture of 75% v/v of demineralisedwater and 25% v/v of anaerobic sludge. Feeding of themethanogenic reactors was not performed during the fol-lowing 48 h; afterwards, the reactors were operated in asemi-continuous mode by daily wasting 12.5% of the vol-ume and feeding with an equivalent volume of the mixtureof acidogenic reactor effluent and anaerobic sludge in orderto gradually reach the prefixed OLRs, which was achievedin about 2 HRTs.

All the tests were carried out in triplicates and accordingto two runs characterized by different operating conditions(Table 2).

The first run (28 days) was performed using waste W1(Table 1). The mixture used for feeding the acidogenic reac-tors was characterized by a TS content of 9.8% and theorganic loading rate (OLR) was set at 19.9 kg VS/m3 d(OLR1, Table 2).

The second run lasted 60 days; two different wastes (W2and W3) were used and three values of OLR were adopted(Table 2: periods 1, 2 and 3 of run 2). Waste W2, charac-terized by low values of the C/N ratio (9.1) and of theVS content, was used during the first two periods of thesecond run. During period 1 of run 2 (20 days), a mixtureof 82.3% w/w of activated sludge and 17.7% of waste W2,corresponding to a TS content of 10% and an organic load-ing rate of 13.2 kg VS/m3 d (OLR2, Table 2), fed the acido-genic reactors. The mixing ratio was changed during period2 of run 2 (20 days), according to 78% w/w of activatedsludge and 22% of waste W2, corresponding to a solid con-tent of 13.2% and in order to have an organic loading rateof 17.6 kg VS/m3 d (OLR3, Table 2). Waste W3, character-

kg VS/m3 TS (%) WasteC/N

HRT (d)

Acidogenicreactor

Methanogenicreactor

LR1) 9.8 20.5 3 8LR2) 10.0 9.1 3 8LR3) 13.2 9.1 3 8LR4) 14.7 21.9 3 8

Fig. 2. VFA concentration in the acidogenic reactor (horizontal solid line:OLR adopted during each period/run).

1804 G. De Gioannis et al. / Waste Management 28 (2008) 1801–1808

ized by higher values of C/N ratio and VS content, wasused during period 3 of run 2 (20 days). The aim of this lastpart of the research was testing the process when high val-ues of OLR are adopted; in fact, the same waste/activatedsludge mixing ratio of period 2 was adopted but, due to thedifferent characteristics of the solid waste, the resultingOLR was 33.0 kg VS/m3 d (OLR4, Table 2).

The performance of the reactors was evaluated monitor-ing, on a daily basis, the volatile fatty acids (VFA) produc-tion (acidogenic reactors), the VS removal efficiency(acidogenic reactors and methanogenic reactors), the spe-cific gas production (methanogenic reactors) and the pHvalues (methanogenic reactors and, periodically, acido-genic reactors). The concentration of VFA and the pHare considered parameters that allow determining the sta-bility of an anaerobic digestion process (Sans et al., 1995;Schober et al., 1999).

2.3. Analytical methods

The total solid content (TS) was measured by weighingthe sample before and after drying at 105 �C for 12 h.The volatile solids content (VS) was measured by weigh-ing the sample before and after combustion at 550 �C for6 h. The total nitrogen content (Ntot) was assessed bymeans of the dry combustion method with the elementalanalyzer CHN600, Leco Instr. The organic carbon con-tent (TOC) was determined by difference between thetotal carbon content measured by the CHN600 analyzerand the fixed carbon content measured by thermal oxida-tion at 950 �C with a MAC400 instrument.

The biogas production was measured by connecting themethanogenic reactors to a volumetric measurement devicebased on the principle of water displacement. Biogas com-position was assessed using a gas chromatograph (GCHP5890) equipped with a thermoconductivity detector(TCD) and a 2-m stainless steel column packed with Pora-pak T. The dissolved oxygen concentration (DOC) in thereactors was periodically measured (data not shown) bymeans of a digital oxymeter (AMEL Instr. Mod. 360);the concentration of volatile fatty acids (VFA) was deter-mined by means of titration according to the Kapp methodreported by Buchauer (1998).

Table 3Performance of the reactors in terms of VFA production, VS removal efficien

VFA (mg CH3COOH/I) VS removal efficiency

Acidogenic reactor Acidogenic reactor

Average Std dev Average Std de

Run l 693.2 21.1 48.8 2.6Run 2,

period 2.1106.1 7.7 9.8 0.8

Run 2,period 2.2

445.2 24.4 31.5 2.2

Run 2,period 2.3

1644.7 26.2 63.0 5.9

2.4. Data analysis

Statistical significance analysis of the values of differentparameters obtained for consecutive adopted OLRs wascarried out by means of F-test (variance analysis) and Stu-dent’s t-test (mean analysis), both at a 5% level of probabil-ity (p < 0.05).

3. Results and discussion

The data in Table 3 present the performance of the sys-tem in terms of VFA production, VS removal efficiencyand specific gas production. The VS removal efficiency isdefined as the VS content in the influent minus the VS ofthe effluent to be divided by the initial VS content. Theaverage values for each run/period were calculated consid-ering the triplicate series of the obtained data; standarddeviation also is reported.

The VFA concentration in the acidogenic reactor(expressed as mg CH3COOH/l) is shown in Fig. 2. TheVFA concentration profile shows a stable performanceafter an initial transitory decrease during the start-up,assessed also by other authors (Hartmann and Ahring,2005). After the end of the start-up period (about 2 HRTs),VFA concentration evolved in accordance with the differ-ent values of the adopted OLRs, as also reported in otherexperiences (Mata Alvarez, 2002a,b; Vandevivere et al.,

cy and assessed specific gas production (SGPA)

(%) SGPA (assessed) Nl/kg MSW

Entire system Methanogenic reactor

v Average Std dev Average Std dev

71.3 1.0 84.6 9.549.8 7.2 56.6 6.7

58.2 6.9 63.2 13.5

78.0 3.3 99.6 2.3

Fig. 4. VS removal efficiency for the entire system (acidogenic + metha-nogenic reactor) (horizontal solid line: OLR adopted during each period/run).

G. De Gioannis et al. / Waste Management 28 (2008) 1801–1808 1805

2002). During the first run, characterized by an OLR of19.9 kg VS/m3 d, an average concentration of693.2 ± 21.1 mg CH3COOH/l was reached. When theOLR decreased (13.2 kg VS/m3 d), due to W2 waste feed-ing instead of W1, the overall VFA concentration droppedquickly to as low as 106.0 ± 7.7 mg/l. Adopting a higherOLR (17.6 kg VS/m3 d) resulted in an increase of theVFA concentration in the acidogenic reactor up to445.2 ± 24.4 mg/l, and when the highly biodegradablewaste W3 was used, the VFA concentration reached thepeak value of 1644.7 ± 26.2 mg/l. The difference betweenthe VFA concentration values was found to be statisticallysignificant, with p < 0.05 for all of the periods/runs underinvestigation.

Concerning the high VFA concentration values reachedduring period 2.3, some controversy can be found in the lit-erature about possible inhibitory effects (Vavilin et al.,2007). Veeken et al. (2000) underscore that the influenceof pH values also has to be taken into account, concludingthat acidic pH is the main inhibitor factor and that no inhi-bition by VFA or by non-ionized VFA can be measured atpH values between 5 and 7. However, it is difficult to dis-tinguish and separate the role of VFA from that of pH,as well as to assess the influence of pH itself. In fact, Vavi-lin et al. (2007) state that gradients of pH occurring close tothe hydrolizable particles and able to decrease the hydroly-sis rate could not be measurable into the reactor bulkliquid; this could explain some contradictory results foundin the literature. As far as the present research is concerned,indeed no adverse effect was noticed, in accordance withthe pH values measured in the effluent of the acidogenicreactor whose results were always higher than 5.5 (datanot reported), even when the highest OLR value wasadopted and the highest VFA concentration was reached.

The acidogenic reactor VS removal efficiency and theentire system one (acidogenic + methanogenic reactor)are reported in Figs. 3 and 4. It can be noticed that low val-ues of the adopted OLR resulted in low VS removal effi-ciency in the acidogenic reactor. The highest VS removalefficiency (63 ± 5.8%) was achieved when an OLR of33.0 kg VS/m3 d was adopted. The performance of the aci-dogenic reactor can be considered remarkable when com-pared with data provided by other authors. Perot and

Fig. 3. VS removal efficiency in the acidogenic reactor (horizontal solidline: OLR adopted during each period/run).

Amar (1989) report a VS removal efficiency of 20% relatedto a two-stage anaerobic digestion of sewage sludge; Karn-chanawong and Deesopa (2004) underline VS removal effi-ciency values of 7.2% and 9.4% for a two-stage processapplied to MSW digestion (OLR = 15 and 25 kg VS/m3 d, respectively); Sung and Santha (2001) obtained aVS reduction of 21.8–31.4% after the first phase of a diges-tion process applied to dairy cattle manure (OLR variableand mesophilic conditions); and Scherer et al. (2000)report, for a two-stage anaerobic digestion process appliedto mechanically pre-treated MSW, a VS removal efficiencyfrom 5% to 20% related to a OLR increasing from 10 to26 g VS/l d (55 �C).

As far as the entire process is considered (acidogenic andmethanogenic reactors, Fig. 4), the following VS removalefficiency values were achieved: 71.3 ± 1.0% (run 1,OLR1 = 19.9 kg VS/m3 d), 49.8 ± 7.2% (run 2 – period 1,OLR2 = 13.2 kg VS/m3 d), 58.2 ± 6.9% (run 2 – period 2,OLR3 = 17.6 kg VS/m3 d) and 78.8 ± 3.3% (run 2 – period3, OLR4 = 33.0 kg VS/m3 d). The stability of the process,even during period 1 of run 2, characterized by low valuesof the OLR and of the waste C/N ratio, is underscored bythe trend of the daily values of the total VS removal effi-ciency (acidogenic and methanogenic reactors, Fig. 4).The difference between the VS removal efficiency assessedfor the acidogenic phase and that for the entire processwas found to be statistically significant, with p < 0.05 forall of the periods/runs under investigation.

The trend of the assessed specific gas productionexpressed with respect to the unit mass of fed waste (SGPA,Nl/kg MSW) is shown in Fig. 5. The SGP profile shows aninitial transitory increase which is typical for the start-up ofanaerobic processes (Bolzonella et al., 2003; Kim et al.,2002). After about 2 HRTs, a good correlation can benoticed between the SGP and the organic loading rate, asnoticed also by Viturtia et al. (1995). The positive evolutionof the process can be noticed also considering the trend ofthe pH values in the methanogenic reactor (Fig. 6). The pHvalue of 6.8, indicative of stable methanogenic conditions,was achieved within 16 days from the beginning of bothruns 1 and 2. This occurred also when waste W2, character-ized by a C/N of 9.1, considered unfavourable for the pro-

Fig. 5. Values of the specific gas production assessed from the methano-genic reactor (SGPA) (horizontal solid line: OLR adopted during eachperiod/run).

Fig. 6. Evolution of the pH in the methanogenic reactors (average values)during runs 1 and 2.

1806 G. De Gioannis et al. / Waste Management 28 (2008) 1801–1808

cess (Hartmann et al., 2002), was treated. Statistical analy-ses indicated that the difference between SGP values wassignificant, with p < 0.05 for all of the periods/runs underinvestigation. The CH4 content in the produced gas alwaysremained close to 60% v/v.

Table 4 summarizes some data considered for a furtherevaluation of the process performance. Theoretical specificbiogas production values (SGPT0 and SGPTm) were evalu-ated on the basis of the amount of the TOC fed to theentire system and to the methanogenic reactor, respec-tively. The amount of organic carbon available for biogasproduction under the adopted temperature conditions

Table 4Theoretical, expected and assessed specific gas production for the different run

Run TOCto thesystem(g)

TOC to themethanogenicreactor (g)

TOC removedin themethanogenicreactor (g)

SGPT0a

(Nl/kg MSW)SGPT

(methreacto(Nl/k

Run 1 2.62 1.45 0.73 299.3 165.6Run 2.1 1.74 1.55 0.56 198.8 177.1Run 2.2 2.64 1.75 0.54 301.6 199.9Run 2.3 4.66 1.83 0.33 532.3 209.1

a SGPT0: theoretical specific gas production, calculated on the basis of the Tb SGPTm: theoretical specific gas production, calculated on the basis of the T

reactor.c SGPE: expected specific gas production, calculated on the basis of the amod SGPA: assessed specific gas production from the methanogenic reactor.

was estimated using the following relation, as proposedby Bingemer and Crutzen (1987):

TOCb=TOC ¼ 0:014T þ 0:28

where TOCb: content of organic carbon available forbiogasification; TOC: total organic carbon content (Table4) and T: process temperature in �C.

The theoretical specific gas production was then calcu-lated assuming that the biogasification of 1 g of organiccarbon leads to the formation of 1.867 Nl of biogas(CH4 + CO2) (Cossu et al., 1996; Rao and Singh, 2004).

The expected specific biogas production (SGPE) was cal-culated on the basis of the amount of organic carbonremoved during the methanogenic stage (TOC assessed inthe influent to the methanogenic reactor minus TOCassessed in the effluent) and assuming, as reported before,that 1.867 Nl of biogas are produced by the biogasificationof 1 g of organic carbon.

Comparing the theoretical specific gas production forthe entire system (SGPT0) with the assessed specific gasproduction from the methanogenic reactor (SGPA), theorganic carbon conversion efficiency (SGPA/SGPT0) resultsfor the entire system are lower than 30%. However, thisresult should be evaluated taking into account that the bio-gas production from the acidogenic reactor was not mea-sured. In fact, if the performance of the entire system isassessed in terms of organic carbon removal, it can bederived from data reported in Table 4 that it accountedfor 80% and 73% for run 2.3 and run 1, respectively, Fur-thermore, the lowest SGPA/SGPT0 value was reached inrun 2.3, when the highest VS and TOC removals in the aci-dogenic reactor were measured.

Comparing the theoretical specific gas production forthe methanogenic reactor (SGPTm) with the specific gasproduction (SGPA) assessed from the same reactor, it canbe noticed that organic carbon conversion efficiencies(SGPA/SGPTm) up to 51% and 47.7% were reached inrun 1 and run 2.3, respectively. Similar values (49.3%) arereported by Sosnowski et al. (2003) for an UASB reactortreating a mixture of sewage sludge and MSW biodegrad-able fractions. Chae et al. (2008) reported maximum con-

s

mb

anogenicr)

g MSW)

SGPEc

(methanogenicreactor)(Nl/kg MSW)

SGPAd

(methanogenicreactor)(Nl/kg MSW)

SGPA/SGPT0

%

SGPA/SGPTm

%

SGPA/SGPE

%

100.9 84.6 28.2 51.1 83.377.4 56.6 28.5 32.0 73.074.6 63.2 20.9 31.6 84.6

121.7 99.6 18.7 47.7 81.9

OCb (TOC fraction available for biogasification) fed to the system.OCb (TOC fraction available for biogasification) fed to the methanogenic

unt of organic carbon removed in the methanogenic reactor.

Table 5Comparison between the performance of the proposed process and those of some other systems

System Substrate T (�C) HRT(d)

VS removalefficiency (%)

Methane yield (N m3/kg VS)

Proposed process wet,two-phase

Mechanically pre-treated MSW 39 11 50–79 0.24

DRANCO Dry,one-phase

MSW organic fraction from separate collection 50 20 40–70 0.21–0.30

TBW Wet, two-phase MSW organic fraction from separate collection 35–55 14 60 N.A.VALORGA Dry

one-phaseMSW organic fraction from separate collection 37–55 20 60 0.21–0.30

WABIO Wet, one-phase Codisposal of sludge and MSW organic fraction fromseparate collection

35 15–20 57.5 N.A.

G. De Gioannis et al. / Waste Management 28 (2008) 1801–1808 1807

version efficiency of 60.4% for anaerobic digestion of swinemanure.

The data reported in Table 4 show also that the valuesof biogas production assessed for the methanogenic stage(SGPA) are fairly consistent with the expected gas produc-tion (SGPE) calculated for the same stage.

In order to underscore the interesting perspectives of theproposed process, Table 5 provides a comparison with theperformances of other anaerobic digestion systems, mostcharacterized by higher values of the methanogenic reactoroperative temperature and of the hydraulic retention time,as well as by the use of selected substrate (Mata Alvarez,2002b; Vandevivere et al., 2002).

Similar studies have been conducted at the University ofCalifornia at Berkeley which demonstrated the feasibilityof integrating solid waste treatment with wastewater treat-ment and of utilizing excess methanogenic reactor capacityin conventional wastewater treatment plants. The studiesconcluded that sludge methanogenic reactors could beloaded with a mixture of 80% (w/w) refuse (the highlydigestible fraction of MSW) and 20% sludge. Organic load-ing rates on the order of 4.8 g VS/l day achieved a TSreduction of about 78% with a corresponding gas produc-tion of about 0.4 l/ g VS (Diaz et al., 1981).

4. Conclusions

A two-phase, wet anaerobic digestion process was testedat laboratory scale using mechanically pre-treated MSW asthe substrate. The proposed process differs from others dueto the integration of the MSW and wastewater cycles,which allows the avoidance of effluent recirculation.

Despite the variable characteristics of the substrate,the process was performed in a successful way adoptingmesophilic conditions (39 �C) and a relatively shorthydraulic retention time (8 days) for the methanogenicreactor, as well as high values of the organic loadingrate. A maximum of 78% of volatile solid reductionwas obtained with an OLR of 33.0 kg VS/m3 d. More-over, the methanogenic reactors quickly reached (within15–16 days) the optimal conditions for a stable methano-genic phase. The assessed SGP ranged from 56 to around100 Nl/kg MSW.

The results obtained show how a proper integrationbetween the MSW and wastewater cycles could contributeto improve the perspectives linked to the bio-energetic val-orization of the biodegradable fraction of MSW.

Acknowledgements

The authors wish to thank the IGAG – CNR (Environ-mental Geology and Geoengineering Institute of the Na-tional Research Council, Italy) of Cagliari for theprecious cooperation in the activity of chemical–physicalanalysis.

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