Bioresource Technology 125 (2012) 233–238

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Bioresource Technology

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Effects of different SRT on anaerobic digestion of MSW dosed with variousMSWI ashes

H.M. Lo a,e,⇑, H.Y. Chiu b,c, S.W. Lo a,d, F.C. Lo e

a Department of Environmental Engineering and Management, Chaoyang University of Technology, 168, Gifeng E. Rd., Wufeng District, Taichung 41349, Taiwan, ROCb Graduate Institute of Biochemical Science and Technology, Chaoyang University of Technology, 168, Gifeng E. Rd., Wufeng District, Taichung 41349, Taiwan, ROCc Central Vocational Training Center, Bureau of Employment and Vocational Training, Councils of Labor Affairs, No.100, Gongyecyu 1st Rd., Situn District, Taichung 407, Taiwan, ROCd Mingdao High School, 497, Sec. 1, Zhongshan Rd., Wuri Dist., Taichung City 41401, Taiwan, ROCe Department of Occupational Safety and Health, China Medical University, 91 Hsueh-Shih Rd., Taichung 40402, Taiwan, ROC

h i g h l i g h t s

" Suitable MSWI ashes addition improved the MSW anaerobic digestion." Biogas enhancement was due to the suitable released metals levels." Co, Mo and W were mostly found to stimulate MSW biogas production." MSW anaerobic digestion favored SRT20 and SRT40 operation.

a r t i c l e i n f o

Article history:Received 16 June 2012Received in revised form 8 August 2012Accepted 10 August 2012Available online 31 August 2012

Keywords:MSWFABASRTAnaerobic digestion

0960-8524/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.biortech.2012.08.084

⇑ Corresponding author. Tel.: +886 4 23323000x44E-mail address: hmlo@cyut.edu.tw (H.M. Lo).

a b s t r a c t

This study investigated different solid retention time (SRT) on municipal solid waste (MSW) anaerobicdigestion with various MSW incinerator fly ash (FA) and bottom ash (BA) addition. Results showed thatbiogas production rates (BPRs, �200 to �400 mL/gVS) with organic loading rate of �0.053 gVS/gVSreactor

(Day 1–435, SRT 20 days, SRT20) at FA 1 g/d (FA1), BA 12 g/d (BA12) and BA 24 g/d (BA24) dosed bio-reactors increased after adaptation. BPRs with SRT10 and SRT5 decreased while BPRs with SRT40showed to increase compared to initial BPRs (�200 mL/gVS) with SRT20. SRT5 operation reduced theBPRs (�10 – �90 mL/gVS) significantly and only BA12 and BA24 dosed bioreactors could recover theBPRs (�100 – �200 mL/gVS) after SRT20 operation (Day 613–617) compared to FA1 and FA3 and con-trol. Released levels of Co, Mo and W at BA12 and BA24 dosed bioreactors showed most potential toimprove MSW anaerobic digestion.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction Renewable energy has currently been the inevitable trend for

MSW has been treated mostly by incineration while partly bylandfilling, anaerobic digestion, composting, gasification and re-source recovery. The MSWI could reduce the MSW volume andhave the potential to produce the steam and electricity while italso produces the residues such as BA and FA. MSWI BA and FAwere reported to contain various metals oxides and recalcitrant or-ganic compounds such as PAHs and PCDD/Fs (Huang and Huang,2008; Lin et al., 2008). Both BA and FA could be used as aggregate,backfill, soil amendment and cement additives (Cioffi et al., 2011)after careful pretreatment, toxicity and TCLP test (Lin and Chen,2006).

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the replacement of fossil fuel and nuclear energy due to the reduc-tion need of carbon dioxide emission and treatment difficulty ofhigh level nuclear radiological waste. Solar, hydro, tidal, geother-mal, wind, biorefinery and biomass energy are thought to be thepotential renewable energy resource in the future. However, it ap-pears currently that energy transfer efficiency of solar, tidal, geo-thermal and wind is limited. Thus, biomass energy becomes anattractive option for the renewable energy utilization. Among bio-mass, MSW (containing lignocellulic materials (Goh et al., 2010)) isthought to have the potential for renewable energy production.MSW is generally treated with thermal and biological methodsthat could recover the electricity or energy utilization. When trea-ted with incinerators, FA and BA will be produced needing thecareful treatment to prevent the secondary pollution. As MSWtreated with biological process such as landfilling or composting,leachate or odor production become environmental problemsand might increase the treatment cost. Thus, anaerobic digestion

Nomenclature

BA: Bottom ash MSWI: MSW incineratorBA12: BA 12 g/d OLR: Organic loading rateBA24: BA 24 g/d PAHs: Polyaromatic hydrocarbonsBPRs: Biogas production rates PCDD/Fs: Polychlorinated

dibenzodioxins/furansCOD: Chemical oxygen demand SRT: Solid retention dayCSTR: Completely stirred tank reactors SRT10: SRT 10 days

EC: Electrical conductivity SRT20: SRT 20 daysFA: Fly ash SRT40: SRT 40 daysFA1: FA 1 g/d SRT5: SRT 5 daysFA3: FA 3 g/d VA: Volatile acidsICP-OES: Inductively coupled plasma-optical emission spectros-

copy VS: Volatile solidMSW: Municipal solid waste

234 H.M. Lo et al. / Bioresource Technology 125 (2012) 233–238

of MSW is thought to be a promising biological treatment methodthat could benefit both methane production and digestate utiliza-tion after nutrition examination and biotoxicity test.

Anaerobic digestion performance of MSW, sludge etc. might beaffected by several factors such as pH, temperature, organic mattercontent, carbon/nitrogen ratio, nutrients and particle size (Lo et al.,2012a,b), solid retention time (SRT) and organic loading rate(OLR)(Zhang et al., 2011; Luste and Luostarinen, 2010; Ferreret al., 2010; Pakarinen et al., 2011; Lee and Rittmann, 2011), pre-treatment of acids, alkali, ultrasonication, radiation, microwaveand ozonation (Rafique et al., 2010; Nizami et al., 2009; Coelhoet al., 2011; Devlin et al., 2011), inoculums source (Elbeshbishyet al., 2012), digester configuration (Nizami and Murphy, 2010; Ka-fle and Kim, 2011; Rubio-Loza and Noyola, 2010), microbial com-munity (Martín-González et al., 2011; Shin et al., 2010; Trzcinskiet al., 2010; Sasaki et al., 2011), co-digested materials (Asamet al., 2011; Li et al., 2011), methane solubility (Serra et al., 2006)and recalcitrant matters (Barret et al., 2010; Bertin et al., 2011; Pat-erakis et al., 2012; Yuzir et al., 2012). Literatures showed that stud-ies on the co-digestion or co-disposal of MSW and MSWI asheswere few (Lo et al., 2010; Lo et al., 2009; Lo et al., 2012a,b; Boniet al., 2007). Results of those investigations indicated that suitableMSWI ashes addition could enhance the biogas production due tothe suitable metals levels released as nutrients for anaerobic diges-tion at a SRT of 20 days. Recently, Lo et al. (2012a,b) have incorpo-rated lots of metals ions levels on the integrated understanding ofstimulation or inhibition of anaerobic digestion or fermentativeprocess. In addition, Lo et al. (2012a,b) also pointed out that recal-citrant compounds might be adsorped onto sludge and biodegradedby microorganisms. Nevertheless, studies on different SRT or OLRfor MSW anaerobic co-digestion of MSW and MSWI ashes werefew and that it might show potential useful information for theco-digestion practice of MSW and MSWI ashes with various SRT.

This study aimed to investigate the various MSWI ashes addi-tion on the MSW anaerobic digestion at various SRT with com-pletely stirred tank reactors (CSTR). Results could provide theuseful information for the anaerobic co-digestion of MSWI asheswith MSW for practical operation.

2. Methods

2.1. MSWI ashes, MSW and anaerobic sludge seeding

Materials used in this study were MSWI FA and BA, MSW andanaerobic sludge seeding. MSWI ashes were obtained from an incin-erator located in central Taiwan. Characteristics of MSWI ashes suchas particle size, pH, loss of ignition, metal content, metals com-pounds, PAHs, PCDD/Fs can be found in Table S-1 and S-2 and re-ferred to Lo et al. (2012a,b). Basic characteristics of MSW andanaerobic sludge seeding were presented in Table 1. MSW (6000 gdry weight) comprised of office paper (30%), newspaper (30%), yardwaste (35%) and food waste (5%) was cut into less than 0.5 mm and

blended with distilled water (94 L, 94000 g) to make the MSW TS�6% (VS �4%). This composition proportionally represents typicalorganic fractions of MSW and could prevent the interference ofunavoidable substances on the effects of anaerobic digestion ofMSW with MSWI ashes. Elemental analysis of synthetic MSW in thisstudy by elementary analyzer (Heraeus varioIII-NCH) such as C, H, Oand N etc., were measured to be about 46, 6, 41 and 1.4%(C38.3H60O25.63N), respectively. Compared to Taichung city and Tai-wan MSW, Taichung city and Taiwan MSW had the close C(%), N(%),S(%), Cl(%) values of about 20.55 and 17.38, 0.50 and 0.47, 0.47 and0.5, 0.0625 and 0.07, respectively. C (46%) and N (1.4%) of syntheticMSW in this study showed higher compared to those of Taichungcity and Taiwan MSW. However, C/N ratios of Taichung city, Taiwanand synthetic MSW were found to have close values in the order of41.1, 39.68 and 32.86 suitable for anaerobic digestion. A C/N ratio of32.86 between 25 and 50 was reported to be suitable for the MSWcomposting and anaerobic digestion. Basic characteristics of MSWsuch as pH (pH 207, Lutron), ORP (pH meter SP-2300, SUNTEX), elec-trical conductivity (EC, Con 400 series, SUNTEX), TS (DS45, DengYing), VS (CMF, 304, Cheng Jang) and metal content such as Ca,Mg, K, Na, Cd, Cr, Cu, Ni, Pb and Zn were also analyzed (ICP-OES, IRIS,Intrepid II, Thermal Electron Corporation) by Standard Methods(AWWA, 1995) as presented by Lo et al. (2012a,b). Basic character-istics and metal content measured in MSW were also analyzed inanaerobic sludge seeding as can be seen in Table 1. Microorganismsof Clostridiales sp., Methanonosarcina barkeri and Methanomicrobi-ales archaeon were found in sludge seeding shown in Table 1.MSW and sludge seeding were measured to have TS �5.6% (VS�4%) and TS �3% (VS �1%), respectively. VS content was thoughtto be the source of biogas production potential (Lo et al., 2012a,b).In general, substrates (carbohydrate, fat, protein and lipid) such asMSW can be biodegraded to be biogas and other matters viahydrolysis, acidogenesis, acetogenesis and methanogenesis asshown in Appendix I. The formula of MSW is calculated to beC38.3H60O25.63N by elemental analysis. Thus, the theoretical biogasproduction (1.03124 L/gVS) such as methane, carbon dioxide andammonia can be obtained via equation S-4 in Appendix I. MSW bio-degradation in batch mode follows the first order reaction (Lo et al.,2010). Detailed mechanisms and kinetics can be seen in Appendix I.

3. Experimental

Anaerobic reactors were 5 L plastic bottle with a working vol-ume of 4 L. Acclimatization of anaerobic sludge seeding to theMSW was the first step to start the experiment. Two liter of anaer-obic sludge (VS �1%) was obtained from Fu–Tien municipal waste-water treatment plant in Taichung, central Taiwan. It was placedonto the anaerobic reactor, then 100 mL synthetic MSW was putonto the anaerobic reactor daily. Biogas production was monitoredand collected with the gas collector by water replacement method.After 20 day later, the total working volume 4 L was reached and aSRT of 20 days was operated. About 40 day later, as the BPRs were

Table 1Basic characteristics of MSW and anaerobic sludge seeding.

Taichung city MSW Taiwan MSWb Synthetic MSW Anaerobic sludge seeding

pH 5.49 ± 0.05 7.90 ± 0.22 6.86 ± 0.09ORP(mV) 14.35 ± 0.9 72.70 ± 4.52 17.25 ± 1.34EC(mS/cm) 6.83 ± 0.25 0.34 ± 0.01 1.96 ± 0.05TS(%) 60 ± 2 5.59 ± 0.19 2.78 ± 0.39VS(%) 0.16 ± 0.02 3.96 ± 0.34 0.74 ± 0.40C(%) 20.55 17.38 46H(%) 3.06 6O(%) 18.53 41N(%) 0.50 0.47 1.4S(%) 0.47 0.5Cl(%) 0.0625 0.07P(%) 0.1402K(%) 0.125C/N ratio 41.1 39.68 32.86lignin(%) 17.7a

a-cellulose(%) 47.4a

hemicellulous(%) 6.9a

ash content(%) 12.3a

low heating value(kcal/kg) 1844Ca(mg g�1) 162.97 ± 8.33 110.03 ± 0.30 12.99 ± 0.55Mg(mg g�1) 25.27 ± 1.00 7.84 ± 0.13 2.25 ± 0.15K(mg g�1) 121.10 ± 2.00 65.31 ± 0.01 2.20 ± 0.07Na(mg g�1) 31.80 ± 1.67 14.98 ± 0.80 0.69 ± 0.03Cd(mg g�1) 0.01 ± 0.00 0.0003 0.10 ± 0.05 0.01 ± 0.00Cr(mg g�1) 0.02 ± 0.00 0.001 0.28 ± 0.04 0.11 ± 0.02Cu(mg g�1) 0.02 ± 0.00 0.001 0.12 ± 0.03 0.20 ± 0.04Ni(mg g�1) 0.25 ± 0.01 0.07 ± 0.00 0.06 ± 0.00Pb(mg g�1) 0.08 ± 0.00 0.02 2.53 ± 0.10 0.07 ± 0.00Zn(mg g�1) 3.18 ± 0.07 0.03 5.16 ± 0.35 1.34 ± 0.12Clostridiales sp. (AB198476.1) Sulfite reducingc

Methanosarcina barkeri (HQ591417.1) Methane producingc

Uncultured Methanomicrobiales archaeon (CU917177.1) Methane producingc

a Goh et al. (2010).b Taiwan EPA (2011).c Nielsen et al. (1999), Akarsubasi et al. (2005), Baker et al. (2003) and Lo et al. (2012).

H.M. Lo et al. / Bioresource Technology 125 (2012) 233–238 235

observed steadily, then experiment started to proceed. Accordingto the experimental design, FA 1 g/d (FA1), FA 3 g/d (FA3), BA12 g/d (BA12) and BA 24 g/d (BA24) (referring to the test amountLo et al. (2012a,b)) were carry out to examine the effects of MSWIashes on MSW anaerobic digestion. Anaerobic reactors withoutashes addition were used as the control. All bioreactors were con-ducted with duplicate totally accounting for 10 bioreactors andthey were placed on an oven maintained at a temperature of35 �C suitable for anaerobic digestion.

Experiment was progressed for about 667 days with variousSRT operation in six stages. The period of six stages were day 1–435, 435–505, 505–538, 538–551, 551–613 and 613–667 operatedat a SRT of 20, 10, 40, 20, 5 and 20 days (SRT20, SRT10, SRT40,SRT20, SRT5 and SRT20) with a corresponding OLR about 2(0.053), 4 (0.111), 1 (0.0256), 2 (0.053), 8 (0.25) and 2 gVS/Lbioreac-

tor (0.053 gVS/gVSbioreactor), respectively.

3.1. Anaerobic parameters

Parameters analysis in the control and dosed bioreactors in-cluded biogas production, pH, EC, VS, alkalinity, volatile acids(VA) and chemical oxygen demand (COD) and metals ions levels.Biogas was measured by biogas collectors with water replacementmethod. pH and EC were analyzed by pH 207 (Lutron) and Con 400series (SUNTEX). VS was measured by a 550 �C furnace (CMF-307,Cheng Jang). COD was measured by a COD titration equipment(765 Dosimat, Metrohm). Metals levels were measured by ICP-OES (ISIS Intrepid II, Thermo). VA and alkalinity were measuredby titration methods. All analytical methods followed the standardmethods for the examination of water and wastewater (AWWA,1995) and Lo et al. (2012a,b).

4. Results and discussion

4.1. Biogas production with various SRT

BPRs were shown in Fig. 1. Results showed that BPRs with aSRT20 operation (OLR, �2 gVS/L; �0.053 gVS/gVS) at all bioreac-tors began to decrease compared to initial BPRs about 140 days ex-cept the FA1 dosed bioreactors which lasting for more 40 days until180 days. BPRs at most BA12 and BA24 dosed bioreactors appearedto increase from day 205 compared to those of initial control untilday 435 with the same SRT20 (Fig. 1c and d). The BPRs were foundbetween �50 and �400 mL/gVS. Some BPRs between day 435 and505 with a SRT10 were found highest (�800 – �1000 mL/L) com-pared to those of initial control (�400 mL/L) (400 mL/day, OLR,�4 gVS/L; �0.111 gVS/gVS). However, most BPRs showed lower(�50 – �175 mL/gVS) compared to initial control (�200 mL/gVS)with unit of mL biogas per gram VS (fed MSW) (Fig. 1c and d).

BPRs between �670 and �220 mL/L appeared to decrease grad-ually with a SRT40 (100 mL/d, days from 505 to 538). Similarly,BPRs with unit of mL/gVS (�675 – �200 mL/gVS) appeared to de-crease gradually, however, their BPRs increased about 1–3.5� thatof initial control of �200 mL/gVS (Fig. 1c and d). From day 538–551, most BPRs showed to decline and below initial average of�400 mL/L and �200 mL/gVS (Fig. 1c and d). As SRT was operatedat SRT5 (from day 551–613), BPRs decreased to �90 mL/L and�10 mL/gVS, respectively (Fig. 1c and d) and anaerobic digestionfailure was nearly found. When SRT was operated back to beSRT20 (day 613–667), BPRs of FA1, FA3, BA12 and BA24 were foundnearly below �50, �10, �62 and 84 mL/gVS, respectively andanaerobic systems were found to be nearly failed. However, BA12and BA24 dosed reactors appeared to recover the anaerobic diges-

0060040020

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S(I)Day 1-435200 mL/d, SRT=20dOrganic Loading Rate=~2 gVS/Lworking volume=~0.053 gVS/gVS

(II)Day 435-505400 mL/d, SRT=10dOrganic Loading Rate=~4 gVS/Lworking volume=~0.111gVS/gVS

(III)Day 505-538100 mL/d, SRT=40dOrganic Loading Rate=~1 gVS/Lworking volume=~0.0256 gVS/gVS

(IV)Day 538-551200 mL/d, SRT=20dOrganic Loading Rate=~2 gVS/Lworking volume=~0.053 gVS/gVS

(V)Day 551-613800 mL/d, SRT=5dOrganic Loading Rate=~8 gVS/Lworking volume=~0.25 gVS/gVS

(VI)Day 613-667200 mL/d, SRT=20dOrganic Loading Rate=~2 gVS/Lworking volume=~0.053 gVS/gVS

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(d)

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Fig. 1. SRT (a), organic loading rate (b), biogas production rate by reactor volume (c) and biogas production rate by fed MSW weight (d), of various ashes added and controlreactor.

236 H.M. Lo et al. / Bioresource Technology 125 (2012) 233–238

tion performance to reach biogas production of �200 – �400 mL/L(�100 – �200 mL/gVS). It was thought that suitable OLR and suit-able released metals levels from added MSWI ashes might be thekey factors which affected the microbial activity thus recoveredthe anaerobic digestion performance. It was concluded that BA12and BA24 dosed reactors could adapt the SRT20 and SRT40 opera-tion and could improve the biogas production rates compared tocontrol and FA1 and FA3 dosed reactors. Reports of anaerobic co-digestion by Zhang et al., (2011) and Luste and Luostarinen,(2010) were also found to favor SRT20 similar to this study. Oper-ation of SRT20 and SRT40 appeared to favor the methanogenic per-formance resulting in the higher biogas production compared tothat of SRT10 and SRT5. SRT10 and SRT5 with higher OLR showedinhibitory effect while SRT40 with lower OLR exerted stimulatoryeffect compared to the control. BA12 and BA24 bioreactors showedhigher BPRs than those at control, F1 and F3 bioreactors at all bio-reactors at six stages. It was particularly found at later stage I andstage VI. This phenomenon indicated that OLR �0.053 gVS/gVS(SRT20) with BA 12 and BA24 could recover and stimulate theMSW anaerobic performance. Daily added ashes could accumulateat bioreactors and could release in various metals levels at differ-ent environment conditions such pH, temperature, ashes particlesize, metal oxide dissolution and MSW adsorption etc., resultingin various scenarios of the potential inhibitory or stimulatory met-als levels on anaerobic digestion. The ashes dosed amount and itsreleased metals levels and OLRs that might affect microbial activitymight explain the MSW anaerobic digestion performance in thisstudy. The metals levels were measured as Table S-3 and Fig. S-3while the dosed amount of ashes was presented in Fig. S-4. Com-paratively higher BA at BA12 and BA24 dosed bioreactors in the

operation of SRT20 and SRT40 was thought to release metals levelssuitable for anaerobic process.

In general, biogas production can be enhanced by pretreatmentsuch as acids, alkali, ultrasonication, radiation, microwave andozonation (Rafique et al., 2010; Nizami et al., 2009; Coelho et al.,2011; Devlin et al., 2011) and by inoculums source (Elbeshbishyet al., 2012), digester configuration (Nizami and Murphy, 2010; Ka-fle and Kim, 2011; Rubio-Loza and Noyola, 2010) and co-digestedmaterials (Asam et al., 2011; Li et al., 2011). The use of chemicaltreatment by Ca(OH)2 and thermal treatment between 50–110 �Ccould increase the biogas production higher to �450 – �700 mL/gVS (Rafique et al., 2010) similar to this study of �375 –�675 mL/gVS with MSWI ashes addition at SRT40. Higher wasteto inoculums (S/X) ratios also could increase the methane produc-tion rate in the order of S/X (4) >S/X (2) >S/X (1) >S/X (0.5) (Elbesh-bishy et al., 2012). Digester configuration showed that CSTR mightbe a safety technology while leach bed followed by a high rate di-gester (such as an upflow anaerobic sludge blanket) might providea hydrolysis and mthanogensis optimization suitable for grass si-lage feedstock (Nizami and Murphy, 2010; Nizami et al., 2009) orhigh VS content of MSW. The ultimate methane productivity interms of volatile solids (VS) was enhanced as 161, 230, 236,361 L/kg VS for filter pressed manure fiber, chemically precipitatedmanure fiber, maize silage and grass silage co-digested with rawpig slurry, respectively (Asam et al., 2011) lower than this studyby the co-digestion of MSW with MSWI ashes which could reachhigher of �375 and �675 mL/gVSadded. Compared to theoreticalbiogas production of �1031.24 mL/gVS in Appendix I, MSW co-di-gested with MSWI ashes with SRT40 was found to be biodegradedwith �20 – �68% (�200 – �675 mL/gVSadded) while with SRT20

H.M. Lo et al. / Bioresource Technology 125 (2012) 233–238 237

MSW co-digested with MSWI BA could be biodegraded with �20 –�35% (�200 – �350 mL/gVSadded) compared to the control of �20%(�200 mL/gVSadded).

Chemical addition such as potential hazardous MSWI ashes forbiogas production improvement were few and the results werepotentially positive Lo et al. (2012a,b). Recalcitrant matters mightbe adsorped and biodegraded by anaerobic microorganisms (Barretet al., 2010; Bertin et al., 2011; Paterakis et al., 2012; Yuzir et al.,2012). Biogas production reported by Zhang et al. (2011) and Lusteand Luostarinen (2010) could reach �396 and �430 mL/gVSadded,respectively. In this study, SRT20 and SRT40 operation in firstand third stage could reach higher to �375 and �675 mL/gVSadded

respectively. Biogas production was found to be SRT20 > SRT14 >SRT25 by the report of Luste and Luostarinen (2010). In this study,the order of biogas production rate was found to be SRT40 >SRT20 > SRT10 > SRT5. Methane of biogas is an effective renewableenergy from anaerobic digestion that might be soluble in liquid/leachate (Serra et al., 2006) causing GHG emissions. However, com-pared to landfill, compost and incineration, anaerobic digestion isstill a promising option which might contribute to less leachate,toxic air pollutants and hazardous ashes even less GHG emissions.Carbon dioxide in biogas might be converted to useful chemicalssuch as methanol or solidified and deposited in deeper layer ofearth shell physico–chemically or might be co-digested withMSW by two phase anaerobic digestion for methane recycle andreutilization. On the other hand, algae can be used to fix the carbondioxide and can be extracted for further biodiesel and healthy foodutilization.

4.2. Anaerobic parameters

Anaerobic parameters were shown in Fig. S-1. pHs in all biore-actors were found to between �5 – �7 (Fig. S-1 (a) and (b)). MostFA and BA dosed bioreactors showed higher pHs than control dueto the released OH�1 from metals oxides leading to the potentialhigher pHs suitable for anaerobic digestion (pH 6.5–7.5). EC rangedbetween �1.2 – �13.3 mS/cm (Fig. S-1 (c) and (d)). Ashes dosedbioreactors had higher EC than control. It was mainly due to the re-lease of metals ions and anions causing the higher EC particularfound at the FA1 dosed bioreactors. Most VS showed to have sim-ilar values for all bioreactors about �2.7 – �5.8% (Fig. S-1 (e) and(f)). Most alkalinity had the values from �340 – �2040 mg/L(Fig. S-1 (g) and (h)). It was noted that alkalinity of BA dosed bio-reactors were slightly higher than those of FA dosed bioreactors.Control bioreactors showed to have the lower alkalinity comparedto the ashes dosed bioreactors. VA of all bioreactors showed tohave the close range between �50 – �2180 mg/L (Fig. S-1 (i) and(j)). COD had close values ranging between �1000 – �10940 mg/L (Fig. S-1 (k) and (l)). Most VA/alkalinity ratios were less than 1(Kafle and Kim, 2011) at BA12 and BA24 dosed bioreactors indicat-ing that they were rather stable than control, FA1 and FA3 dosedbioreactors.

4.3. Metals ions levels on anaerobic digestion

Metals levels were measured by ICP-OES as shown in Fig. S-2.Alkali metals Ca, K, Na and Mg were found to have higher levelsthan any other metals. Ca (�315 – �1330 mg/L), K (�313 –�1145 mg/L) and Na (�205 – �758 mg/L) showed to have thecomparative higher levels at FA3 dosed bioreactors while Mg(�53 – �230 mg/L) had the comparative higher levels at BA24dosed bioreactors. Released levels of Ca, K, Mg and Na appearedto show potential stimulation rather than inhibition by checkingexperimental results in Fig. S-2 with inhibitory or stimulatory datain Table S-3 and Fig. S-3. Table S-3 showed the metals levels of

stimulation and inhibition in the references and measured levelsin this study. Fig. S-3 showed the detailed metals levels at variousSRT operation in six periods. Other 8 major elements such as Si, S,Mn, B, Al, P, Ba and Fe in all bioreactors showed the levels of �4.5 –�55, �1.5 – �44, �0.1 – �10, �0.1 – �5.5, �0.01 – �5, �0.01 –�4.7, �0.04 – �1.65 and �0.01 – �1.8 mg/L respectively. The restof 19 trace metals ions Ag (ND – �0.012), Cr (ND – �0.022), Cu (ND– �0.024), Ti (ND – �0.025), Co (ND – �0.027), Zr (ND – �0.027),Zn (ND – �0.032), In (ND – �0.035), Cd (ND – �0.046), Hf (ND –�0.064), Mo (ND – �0.111), Ta (ND – �0.150), Sb (ND – �0.195),Sn (ND – �0.202), Tl (ND – �0.203), W (ND – �0.281), Pb (ND –�0.304), Ni (ND – �0.327) and V (ND – �0.538 mg/L) in all biore-actors were measured to be less than 1 mg/L. Those released met-als ion levels by checking with Fig. S-3 and Table S-3 showed MSWanaerobic digestion stimulation rather than inhibition potentiallyfound in Co, Mo and W at the BA dosed bioreactors. Similar poten-tial benefit of trace elements such as Ca, Mg, K, Na, Al, Fe, Mn, Mo,Co, Cd, Cr, Cu, Ni, Pb and Zn for the anaerobic digestion was alsoreported by Lo et al. (2012a,b) and Zhang et al. (2011).

Metals released by suitable amount MSWI ashes were found toenhance the biogas production particularly the potential trace me-tal of Co, Mo and W found in this study similar to the reports by Loet al. (2012a,b) and Zhang et al. (2011). Zhang et al. (2011) mea-sured the metals levels of Na, Mg, Al, K, Ca, Cr, Mn, Fe, Co, Ni, Cu,Zn, Mo, Cd and Pb for anaerobic performance assessment. In thisstudy, about thirty metals levels including those mentioned (Zhanget al., 2011) were analyzed to assess the MSW anaerobic co-diges-tion performance with MSWI ashes addition. Biogas productionpresented by Zhang et al. (2011) could reach about 300 –600 mL/gVSadded comparatively slightly lower than the stimulatoryresult in this study highest at about �350 – �675 mL/gVSadded.

The ashes added amount and its released metals levels and var-ious OLRs that might affect microbial activity might explain theMSW anaerobic digestion performance in this study. The metalslevels were measured as Table S-3 and Fig. S-3 while the dosedamount of ashes was presented in Fig. S-4. Comparatively higherBA at BA12 and BA24 dosed bioreactors in the operation ofSRT20 and SRT40 was thought to release metals levels suitablefor anaerobic process.

5. Conclusions

MSW anaerobic co-digestion with MSWI ashes improved BPRs(mL/gVS) at BA dosed bioreactors operated at SRT20 (day 1–435and 613–667) and at BA and FA dosed bioreactors at SRT40.SRT10 operation (�50 – �175 mL/gVS) and SRT5 operation (�10– �90 mL/gVS) showed inhibition compared to initial BPRs of�200 mL/gVS. The BPRs favored SRT40 and SRT20 operation andthe order of BPRs was found to be SRT40 > SRT20 > SRT10 > SRT5.Released metals levels stimulated rather than inhibited anaerobicco-digestion of MSW and MSWI ashes potentially found in Co,Mo and W.

Acknowledgements

The authors are grateful to the National Science of Council ofTaiwan (R.O.C) for providing the financial support with a researchGrant of No. NSC 94-2622-E-324-006-CC3.

Appendix A. Supplementary data

Supplementary data associated with this article can be found,in the online version, at http://dx.doi.org/10.1016/j.biortech.2012.08.084.

238 H.M. Lo et al. / Bioresource Technology 125 (2012) 233–238

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