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Journal of Environmental Management 147 (2015) 87e94

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Journal of Environmental Management

journal homepage: www.elsevier .com/locate/ jenvman

Co-digestion of the hydromechanically separated organic fraction ofmunicipal solid waste with sewage sludge

Sebastian Borowski*

Technical University of Lodz, Institute of Fermentation Technology and Microbiology, W�olcza�nska 171/173, 90-924, Ł�od�z, Poland

a r t i c l e i n f o

Article history:Received 19 April 2014Received in revised form10 July 2014Accepted 9 September 2014Available online

Keywords:Anaerobic digestionBiogasSewage sludgeMunicipal solid waste

* Tel.: þ48 42 6313484; fax: þ48 42 6365976.E-mail addresses: sebastian.borowski@p.lodz.pl, se

http://dx.doi.org/10.1016/j.jenvman.2014.09.0130301-4797/© 2014 Elsevier Ltd. All rights reserved.

a b s t r a c t

This study investigates the anaerobic digestion of the hydromechanically sorted organic fraction ofmunicipal solid wastes (HS-OFMSW) co-digested with sewage sludge (SS). Eight laboratory-scale ex-periments were conducted under semi-continuous conditions at 15 and 20 days of solids retention time(SRT). The biogas yield from the waste reached 309 to 315 dm3/kgVS and 320 to 361 dm3/kgVS undermesophilic and thermophilic conditions, respectively. The addition of SS to HS-OFMSW (1:1 by weight)improved the C/N balance of the mixture, and the production of biogas through anaerobic mesophilicdigestion increased to 494 dm3/kgVS, which corresponded to 316 dm3CH4/kgVS. However, when SS andHS-OFMSW were treated under thermophilic conditions, methanogenesis was inhibited by volatile fattyacids and free ammonia, which concentrations reached 5744 gCH3COOH/m3 and 1009 gNH3/m3,respectively.

© 2014 Elsevier Ltd. All rights reserved.

1. Introduction

In Poland, municipal and industrial wastes pose great ecologicalhazards. Themost important problems include long-term improperwaste management, a poorly developed system of selection, a lackof modern infrastructure, and troubles with the enforcement oflaws. According to the Central Statistical Office (Bochenek et al.,2013), Poland produced approximately 135 million tonnes ofwastes in 2012, of which 12 million tonnes constituted municipalsolid waste (MSW). The amount of collected municipal solid wastewas 9.58 million tonnes, from which as much as 90% representedunsorted (mixed) waste. Moreover, approximately 75% of the MSWwas deposited in landfill sites, and only 9%was treated by biologicalprocesses (mainly by composting). However, approximately 80% ofthe total municipal waste produced in Poland is organic; and thistype of waste is known as the organic fraction of municipal solidwastes (OFMSW) and is suitable for biogas production. These fig-ures indicate the urgent need to reduce the mass of landfilledwastes and suggest the implementation of sorting installations forMSW prior to biological processing via composting or anaerobicdigestion (AD).

The literature concerning both laboratory investigations andfunctioning AD plants focuses on the treatment of separately

basbor@poczta.onet.pl.

collected or source sorted municipal organic wastes (SS-OFMSW)(Bolzonella et al., 2006b; Cavinato et al., 2013; Davidsson et al.,2007; Forster-Carneiro et al., 2007; Kim and Oh, 2011; Zhanget al., 2008); whereas little has been published about the anaerobictreatment of the mechanically sorted (MS-OFMSW) and watersorted (WS-OFMSW) organic fraction of municipal solid wastes(Bolzonella et al., 2006b; Dong et al., 2010; Provenzano et al., 2013).The increased popularity of installations processing SS-OFMSW isassociated with higher biogas yields and better compost qualityproduced from these wastes, whereas mechanically sortedmunicipal organic wastes produce less biogas and a digestate ofpoorer quality (Bolzonella et al., 2006b). However, implementationof full-scale AD plants treating both types of waste is still limitedbecause of the relatively high costs and technological limitations(Davidsson et al., 2007; Zhang et al., 2008). A solution to thisproblemmay be the co-digestion of OFMSWwith otherwaste typesincluding sewage sludge (SS). Literature data show generally a lowcarbon to nitrogen ratio in sewage sludges typically ranging from 6to 16; however, the ratio for OFMSW can be as high as 25e38. Thusmixing sewage sludge with municipal solid waste provides animproved nutrient balance, and the optimal C/N ratio of 15e30 thatis suggested for anaerobic digestion can be achieved (Castillo et al.,2006; Forster-Carneiro et al., 2007; Nasir et al., 2012; Zhang et al.,2008).

There are numerous examples of successful SS and OFMSW co-digestion operations at the laboratory-, pilot- and full-scale.

Table 1Characteristics of sludge and hydromechanically separated OFMSW used for theexperiments.

Indicator Unit Sewage sludge HS-OFMSW

TS g/kg 155.37 ± 9.11 35.33 ± 10.02VS g/kg 128.37 ± 10.05 18.79 ± 7.71

% TS 82.58 ± 3.01 53.17 ± 2.28COD gO2/kg 168.47 ± 22.50 21.53 ± 6.81

gO2/kg TS 1084.4 ± 144.8 609.4 ± 164.5Potassium gK/kg TS 7.61 ± 0.41 4.52 ± 0.82Sodium gNa/kg TS 3.87 ± 0.28 4.14 ± 0.95Calcium gCa/kg TS 25.01 ± 1.34 9.80 ± 1.10Magnesium gMg/kg TS 5.39 ± 0.72 2.27 ± 0.39Iron gFe/kg TS 8.16 ± 1.25 9.03 ± 0.82Manganese mgMn/kg TS 532 ± 94 202 ± 58Zinc mgZn/kg TS 947 ± 192 157 ± 84Copper mgCu/kg TS 343 ± 133 209 ± 19Lead mgPb/kg TS 58.8 ± 23.1 39.2 ± 5.1Cadmium mgCd/kg TS 31.7 ± 9.8 1.7 ± 0.4Elemental analysisC % TS 64.30 ± 2.10 66.78 ± 1.39N % TS 7.07 ± 0.42 2.08 ± 0.33P % TS 2.51 ± 0.23 0.73 ± 0.17H % TS 5.27 ± 0.15 5.91 ± 0.52S % TS 0.69 ± 0.05 0.05 ± 0.01C/N e 9.09 32.18

± Standard deviation.

S. Borowski / Journal of Environmental Management 147 (2015) 87e9488

However, almost all of these cases treated source-sorted municipalorganic wastes (Agdag and Sponza, 2005; Bolzonella et al., 2006a,2006b; Cavinato et al., 2013; Sosnowski et al., 2003). These au-thors have demonstrated the feasibility of sewage sludge co-digestion with OFMSW in existing digesters at municipal waste-water treatment plants. The digesters treating waste activatedsludge are often oversized andworkwith low organic loading rates.These factors justify the use of additional substrates to achieve co-digestion. Furthermore, previous investigations also showed thatwastewater treatment plants could implement the co-digestion ofsewage sludge with OFMSW without changing (or with minorchanges to) the plant design. These modifications may beeconomically beneficial (Bolzonella et al., 2006a; Cavinato et al.,2013; Krupp et al., 2005).

In this study, the feasibility of the anaerobic co-digestion thehydromechanically separated organic fraction of municipal solidwaste (HS-OFMSW) with municipal sewage sludge was evaluated.The specific objective was to evaluate biogas and methane yieldfrom HS-OFMSW and from the mixture of these wastes withsewage sludge. The emphasis was also put on the stability of thedigestion processes, in particular, the role of ammonia and volatilefatty acids was investigated. The experiments were performed inmesophilic and thermophilic conditions with solids retention time(SRT) values of 20 and 15 days. To the best of the author's knowl-edge, this study is the first to investigate the anaerobic co-digestionof hydromechanically separated municipal organic wastes withsewage sludge.

2. Methods

2.1. Characteristics and origin of materials

Sewage sludge (the mixture of primary and waste activatedsludge) was collected from the Municipal Wastewater TreatmentPlant in Kutno, Poland. The plant treats 20,000 m3/d of wastewaterand serves an equivalent population of 130,000. In this plant, theprimary sludge (PS) and waste activated sludge (WAS) are first pre-thickened together in a gravity thickener to achieve a 2% total solids(TS) content, and then dewatered by centrifugation to approxi-mately 16% TS. In this process, partial stabilization and sterilization(hygienization) is achieved through the addition of lime; howeverlime was not added to the sludge prepared for the purpose of thisstudy. The annual production of dewatered sludge is 30,000 tonnes.

The organic fraction of municipal solid wastes originated from asorting plant at the Municipal Services Office in Puławy. In thisplant, mixed (unsorted) municipal solid wastes are hydro-mechanically treated in the BTA® Process, which is the onlyinstallation of this type in Poland. The BTA® Process was originallydeveloped in Germany in 1984 by the BTA Biotechnische Abfall-verwertung GmbH& Co (now BTA Company GmbH) in cooperationwith the University of Applied Sciences, Munich (BTA, 2014),whereas the installation in Puławy was opened in 2001. This pro-cess comprises a water pulper to remove heavy materials (bones,stones, glass, etc.) and light components (textiles, wood, fibers, foil,plastics, etc.) from wastes followed by a grit removal system. Thecapacity of the plant is 22,000 tonnes of wastes per year.

The characteristics of both the SS and HS-OFMSW are depictedin Table 1. Typically, the sludge was rich in nitrogen (average C/Nratio of 9.09) and phosphorus because this sludge originated from atreatment plant operating with a biological nutrient removal sys-tem. Additionally, the volatile solids (VS) content was high(approximately 82% of TS) contrasting previously published data(Cavinato et al., 2013; Forster-Carneiro et al., 2007; Sosnowski et al.,2003; Zhang et al., 2008). Conversely, the HS-OFMSW was diluteshowing an average total solids content of 3.5% with the volatile

fraction not exceeding 53% of the TS. This classifies HS-OFMSW asnot-easily biodegradable because the VS/TS ratio was lower than0.7 (Pavan et al., 2000). Moreover, the waste displayed low quan-tities of nutrients. The fraction of nitrogen and phosphorus aver-aged 2.1% and 0.73%, respectively, and these figures were over 3-fold lower than those reported for sewage sludge. This is inagreement with the observations of Dong et al. (2010).

2.2. Experiments

Four laboratory scale reactors (each with 5 dm3 of total and3 dm3 of active volume) were used in these experiments. Eachreactor had a cylindrical shapewith an internal diameter of 16 cm, aheight of 25 cm and an active volume of 3 dm3. The reactors wereequipped with helix-type mechanical stirrers operated with80 rpm for 15 min every hour. The reactors were placed in ther-mostats to ensure constant mesophilic (35 ± 1 �C) or thermophilic(55 ± 1 �C) temperatures.

Each reactor was coupled with a 4 dm3 gas collecting tank toprovide anaerobic conditions and to measure the biogas yield by awater displacement method. The digesters were fed once a day (asemi-continuous operation) using a peristaltic pump. In thenomenclature used in this study, R1 and R2 refer to experimentswith HS-OFMSW that were performed in mesophilic conditions,whereas R3 and R4 refer to experiments with HS-OFMSW that wereperformed in thermophilic conditions (Table 2). The other experi-ments were performed with a mixture of SS and HS-OFMSW (1: 1of feed TS) inmesophilic (exp. R5 and R6) and thermophilic (exp. R7and R8) conditions (Table 3). For each substrate and temperature,two SRT values were implemented to provide different operationalconditions.

2.3. Analyses

The analyses of the pH, total and volatile solids, total alkalinity(TAL) and chemical oxygen demand (COD) were performed ac-cording to standard methods (APHA, 2005). The total ammoniumnitrogen (TAN), free ammonia (FAN), orthophosphates (PO4

3�), andvolatile fatty acids (VFA) were analyzed using a DR2800

Table 2Operating parameters and performances of the anaerobic digestion of HS-OFMSW.

Parameter Unit HS-OFMSW mono-digestion

Experiment e R1 R2 R3 R4

Temperature �C 35 35 55 55Duration time d 120 105 120 105SRT d 20 15 20 15Feed TS g/kg 32.03 ± 8.25 38.08 ± 8.33 34.17 ± 7.88 36.73 ± 9.12Feed VS g/kg 16.91 ± 4.17 20.04 ± 4.37 18.21 ± 3.96 19.56 ± 4.86C/N (average) e 32.18 32.18 32.18 32.18OLR kgVS/m3,d 0.85 ± 0.21 1.34 ± 0.29 0.91 ± 0.20 1.30 ± 0.32VSreduction % 45.97 ± 22.91 49.36 ± 20.19 48.05 ± 15.50 50.76 ± 12.72GPR cm3/dm3,d 266 ± 114 413 ± 166 328 ± 119 418 ± 122SGP dm3/kg VSfed 315 ± 135 309 ± 124 361 ± 130 320 ± 94

dm3/kg VSreduced 684 ± 294 627 ± 251 751 ± 272 631 ± 184SMP dm3CH4/kg VSfed 183 ± 78 176 ± 71 190 ± 75 186 ± 55CH4 content in biogas % 58 ± 2 57 ± 2 58 ± 2 58 ± 1

± Standard deviation.

S. Borowski / Journal of Environmental Management 147 (2015) 87e94 89

spectrophotometer with HACH-Lange tests as previously described(Borowski and Weatherley, 2013). Atomic absorption spectroscopy(SOLAAR 969 UNICAM) was used to determine the concentrationsof metals (K, Na, Ca, Mg, Fe, Mn, Zn, Cu, Pb, and Cd). Elementalanalysis (C, N, P, H, and S) was performed with a NA 2500 elementalanalyzer (CE Instruments, UK). The total carbon was divided by thetotal nitrogen to obtain the C/N ratio.

The daily biogas production was measured by a waterdisplacement method as described in numerous sources in theliterature (Cuetos et al., 2011; Sterling et al., 2001). The water usedin gas collecting tanks was saturated to 75% with sodium chlorideand acidified to pH ¼ 2. The content of methane in the biogas wasdetermined using a Varian gas chromatograph coupled with aHaySep T column (13 m � 0.5 m � 1/800OD) and a thermal con-ductivity detector (TCD). The carrier gas was helium.

Analyses of individual samples were performed in triplicates.The calculation of the average values, standard deviations, and theanalysis of variance (single factor ANOVA) were performed inMicrosoft Excel 2007. A confidence level of 0.05 was selected for allstatistical comparisons.

3. Results and discussion

The operating parameters and average biogas yields of mesophilicand thermophilic digesters treating only the organic fraction ofmunicipal solid wastes are shown in Table 2. The operating parame-ters for the co-digestion of HS-OFMSW with sewage sludge aredepicted in Table 3. A comparison of the biogas produced from

Table 3Operating parameters and performances of the co-digestion experiments.

Parameter Unit SS þ HS-OFMSW

Experiment e R5

Temperature �C 35Duration time d 120SRT d 20Feed TS g/kg 61.24 ± 8.61Feed VS g/kg 42.21 ± 5.98C/N (average) e 20.09OLR kgVS/m3,d 2.11 ± 0.30VSreduction % 41.53 ± 7.40GPR cm3/dm3,d 1043 ± 233SGP dm3/kg VSfed 494 ± 110

dm3/kg VSreduced 1190 ± 266SMP dm3CH4/kg VSfed 316 ± 70CH4 content in biogas % 64 ± 1

± Standard deviation.

different substrates andtemperatures is illustrated inFigs.1 and2. Thebiogaswasmonitoreddaily; however, for clarity, the individual pointson the plots represent the mean values of the weekly biogas yields.

The reactors were established by using anaerobically digestedsewage sludge from previous studies. The start-up of mesophilicreactors (beginning with R1 and R5 runs), was with anaerobicallydigested sludge originated from previous experiments described byBorowski and Weatherley (2013). The same sludge but after pre-vious thermophilic treatments (data not published) was used as theinoculum for experiments performed at 55 �C (starting with R3 andR7 runs). Initially the digesters operated with a solids retentiontime of 20 days; this time was then reduced to 15 days. Eachexperimental run provided at least 5 consecutive SRTs under steadystate conditions. As shown in Figs. 1 and 2, all reactors achievedstable operationwithin a few days, indicating a good acclimation ofinocula for both mesophilic and thermophilic conditions, and var-iations in the biogas production are mainly associated with theheterogeneity of the sludge and wastes being treated.

3.1. Anaerobic digestion of OFMSW

The trials with HS-OFMSWalone (R1eR4) were performed withrelatively low organic loading rates (OLR) in the range of0.05e1.34 kgVS/m3,d because of the dilute organic waste derivedfrom hydromechanical separation. Therefore, the biogas produc-tion rates (GPR, calculated as per unit reactor volume) for thesetrials were also low, and the mesophilic treatment process reachedonly 266 and 413 cm3/dm3,d for SRT of 20 and 15 days, respectively

co-digestion

R6 R7 R8

35 55 55105 120 10515 20 1554.37 ± 10.26 47.67 ± 11.27 59.10 ± 12.6337.58 ± 6.07 33.96 ± 7.52 40.12 ± 8.2720.09 20.09 20.092.51 ± 0.41 1.70 ± 0.38 2.67 ± 0.5529.15 ± 13.48 40.95 ± 10.11 31.35 ± 9.131124 ± 263 622 ± 217 739 ± 262449 ± 105 367 ± 128 276 ± 981539 ± 360 895 ± 312 882 ± 313283 ± 66 224 ± 78 166 ± 5963 ± 1 61 ± 2 60 ± 2

Fig. 1. Biogas production (GPR) reported during the anaerobic mono-digestion of hydromechanically separated OFMSW (experiments R1eR4).

S. Borowski / Journal of Environmental Management 147 (2015) 87e9490

(Table 2, experiments R1eR2). These figures for SRT of 20 and15 days correspond to a moderate specific gas production (SGP) of315 and 309 dm3/kgVSfed, respectively, and moderate values ofvolatile solids reduction of 45.97 and 49.36%, respectively. Also, aspecific methane production (SMP) for these experiments wasrather low and did not exceed 200 dm3 CH4/kgVSfed. Thermophilictemperatures did not spectacularly improve either the biogasproduction or VS removal efficiency. The specific gas productionincreased by nearly 15% for an SRT¼ 20 d to reach 361 dm3/kgVSfed(p ¼ 0.0076), however, an SRT ¼ 15 d displayed an insignificantchange in SGP (p¼ 0.4714). Moreover, only a small and insignificantincrease (p ¼ 0.7282) in VS reduction, compared to mesophilicdigestion, was reported. The average values of volatile solidsreduction for thermophilic HS-OFMSW digestion were 48.05 and50.76% for R3 and R4 (Table 2). Numerous papers investigate the AD

Fig. 2. Biogas production (GPR) reported during the anaerobic

of source sorted (or separately collected) OFMSW; however, onlyselected reports can be found investigating mechanically or watersorted wastes. Bolzonella et al. (2006b) reported an average biogasyield of 255 dm3/kgVSfed (140 dm3 CH4/kgVSfed) and a VS removalof 35e40% in a Bassano biogas plant treating mechanically sortedwastes. The corresponding biogas production from source sortedorganic wastes in this plant was 714 dm3/kgVS whereas methaneyield reached 492 dm3 CH4/kgVSfed. Dong et al. (2010) suggestedthat the biodegradability of water sorted OFMSW is higher thanthat of mechanically sorted OFMSW. However, this value is stilllower than that of source sorted OFMSW. In their study, the biogasproduction and methane yield from water sorted organic wastesdid not exceed 300 dm3/kgVS and 200 dm3 CH4/kgVSfed, respec-tively, and the VS removal was in the range 26.1e41.8%. Thesevalues are lower than those presented in our investigation.

co-digestion of SS and HS-OFMSW (experiments R5eR8).

Table 4Characteristics of digestate from experiments with municipal solid wastes.

Indicator Unit HS-OFMSW mono-digestion

Experiment e R1 R2 R3 R4

SRT d 20 15 20 15pH e 7.42 ± 0.14 7.66 ± 0.18 7.60 ± 0.08 7.89 ± 0.10TS g/kg 18.58 ± 7.30 24.05 ± 9.11 25.25 ± 7.01 25.53 ± 4.38VS g/kg 9.14 ± 3.87 10.15 ± 4.05 9.46 ± 2.82 9.63 ± 2.49

% TS 48.92 ± 7.18 41.81 ± 3.15 38.08 ± 8.40 37.44 ± 4.22TAN gN/m3 694 ± 205 746 ± 230 707 ± 79 825 ± 208FAN gNH3/m3 24.5 ± 6.8 49.0 ± 32.0 112.6 ± 19.1 238.3 ± 83.9PO4

3- gP/m3 24.33 ± 17.73 18.27 ± 9.73 33.47 ± 7.91 25.60 ± 9.39VFA

(as acetic)g/m3 903 ± 458 906 ± 415 1999 ± 557 1966 ± 590

TAL g/m3 5817 ± 1710 6427 ± 625 7631 ± 1564 5881 ± 733VFA/TAL e 0.16 ± 0.10 0.14 ± 0.08 0.26 ± 0.08 0.33 ± 0.09

± Standard deviation.

S. Borowski / Journal of Environmental Management 147 (2015) 87e94 91

3.2. Co-digestion of sewage sludge and HS-OFMSW

In runs R5eR8, the mixture of SS and HS-OFMSW used in theexperiment was blended at a ratio of 1: 1 based on dry weight (TS/TS). This mixture had a C/N ratio of 20.1 whichwas within the rangeof 15e30 that is regarded as optimal for anaerobic digestion (Nasiret al., 2012; Zhang et al., 2008). Moreover, many authors have alsosuggested this ratio for the co-digestion of mechanically sorted andsource separated organic wastes with municipal sewage sludge(Agdag and Sponza, 2005; Castillo et al., 2006; Cavinato et al., 2013;Dai et al., 2013; Provenzano et al., 2013). Data for the co-digestiontrials are shown in Table 3. A comparison of the biogas produc-tion fromwastes and their mixture with the sludge is illustrated inFigs. 1 and 2. The addition of sewage sludge to HS-OFMSW signif-icantly increased the digestion performance during mesophilictreatment. The greatest increase in biogas yield was reported for anSRT of 20 days. For this SRT, the gas production rate increasednearly 3-fold compared to the R1 trial to produce 1043 cm3/dm3,d.However, the specific gas and methane production increased byapproximately 60% and reached 494 dm3/kgVSfed, and 316 dm3CH4/kgVSfed, which was the maximum obtained in the study. For an SRTof 15 days, the average GPR and SGP values were 1124 cm3/dm3,dand 449 dm3/kgVSfed, respectively, whereas methane yield aver-aged 283 dm3CH4/kgVSfed (Table 3). These yields are considerablyhigher than the values reported in the literature for the co-digestion of source sorted OFMSW mixed at a similar ratio withsewage sludge (Bolzonella et al., 2006a; Castillo et al., 2006;Cavinato et al., 2013; Dai et al., 2013). Another benefit of co-digestion was the higher methane content of the biogascompared to the HS-OFMSW mono-digestion. The maximal

Table 5Characteristics of digestate from co-digestion experiments.

Indicator Unit SS þ HS-OFMSW co-digesti

Experiment e R5

SRT d 20pH e 7.75 ± 0.09TS g/kg 41.77 ± 6.13VS g/kg 24.68 ± 3.12

% TS 59.27 ± 1.76TAN gN/m3 1891 ± 174FAN gNH3/m3 124.1 ± 20.5PO4

3� gP/m3 42.13 ± 9.94VFA (as acetic) g/m3 1397 ± 389TAL g/m3 9604 ± 1482VFA/TAL e 0.15 ± 0.04

± Standard deviation.

methane content in biogas was 64% which agreed with the highestSGP reported in trial R5. In experiments using only HS-OFMSW, themethane content did not exceed 58%. The low methane content inbiogas produced during HS-OFMSW mono-digestion can be linkedto the specific nature of these wastes, which are mainly composedof carbohydrates. The theoretical methane content in biogas pro-duced from carbohydrates is about 50%, whereas the values forproteins and fats reach 70% and more. However, the real methanecontent in practice is generally higher than the theoretical valuesbecause a part of CO2 is solubilized in the digestate (Esposito et al.,2012; Weiland, 2010). The low methane content in biogas (notexceeding 60%) yielded from municipal solid wastes was also re-ported in several studies (Bolzonella et al., 2006b; Castillo et al.,2006; Davidsson et al., 2007; Zhang et al., 2008).

Simultaneously, volatile solids reduction in mesophilic co-digestion was lower than the corresponding values for HS-OFMSW mono-digestion. The average percentage of the VSremoval in experiments R5 and R6 were 41.53 and 29.15% for SRT of20 and 15 days, respectively (Table 3). This may be linked to thespecific nature of the Kutno WWTP sludge that was used as a co-substrate in this study. This sludge is primarily composed of WASwith small amounts of PS. The VS removal efficiencies reported inthe literature for the AD of sludges with no or minor amounts ofprimary sludge are within a range of 17e34% (Athanasoulia et al.,2012; Cavinato et al., 2013; Dai et al., 2013).

Because of faster reaction rates, the thermophilic digestionshould have a higher degree of organic material degradation and agreater biogas yield compared to mesophilic treatment (Forster-Carneiro et al., 2008; Provenzano et al., 2013). However, in thefollowing study, no such improvements were observed. The VSremoval rates calculated for the mesophilic and thermophilic co-digestion experiments (R5eR8, Table 3) were comparable, and nosignificant differences between the corresponding trials wereobserved (p ¼ 0.8589 and 0.6273 for SRT of 20 and 15 days,respectively). Furthermore, considerably lower biogas yields werealso noted. The average gas production rates in experiments R7 andR8 were 622 and 739 cm3/dm3,d for SRT of 20 and 15 days,respectively. These gas production rates were nearly twice as low asthe values for mesophilic treatments (R5 and R6). The SGP valueswere 367 and 276 dm3/kgVSfed, respectively, and the latter valuewas significantly lower (p ¼ 0.00017) than the corresponding valuemeasured for HS-OFMSW (R4).

3.3. Behavior of ammonia and volatile fatty acids

Tables 4 and 5 depict the characteristics of digestate derivedfrom the individual trials, whereas the changes of VFA and freeammonia in the course of experiments are illustrated in Figs. 3 and

on

R6 R7 R8

15 20 157.68 ± 0.05 8.19 ± 0.27 7.84 ± 0.2645.32 ± 10.03 33.19 ± 6.01 46.31 ± 7.3026.62 ± 5.07 20.05 ± 3.43 27.54 ± 3.6659.20 ± 2.72 60.67 ± 3.67 59.76 ± 2.711610 ± 314 1948 ± 499 1947 ± 16997.6 ± 16.4 1009.0 ± 518.2 562.7 ± 207.539.64 ± 17.32 81.31 ± 29.26 56.79 ± 19.94790 ± 256 4508 ± 1545 5744 ± 14919077 ± 940 8335 ± 992 10,107 ± 15350.09 ± 0.03 0.54 ± 0.17 0.57 ± 0.10

Fig. 3. Changes in VFA and free ammonia concentration during the anaerobic digestion of HS-OFMSW (experiments R1eR4).

S. Borowski / Journal of Environmental Management 147 (2015) 87e9492

4. It is generally known, that the stability of anaerobic digestion ismainly affected by the concentrations of volatile fatty acids and freeammonia. The inhibition of methanogenesis usually starts at a VFAconcentration of 2500e4000 g/m3 (as acetic acid) (Appels et al.,2008; Duan et al., 2012; Kafle and Kim, 2013). A more precise cri-terion for digester stability is the VFA to total alkalinity ratio.Generally, the operation of a digester is stable at VFA/TAL ratiosbelow 0.4, whereas significant instability occurs when the VFA/TALratio exceeds 0.7e0.8 (Callaghan et al., 2002; Kafle and Kim, 2013).The experiments with HS-OFMSW in mesophilic conditions (R1and R2) showed low VFA concentrations of approximately 900 g/m3, whereas the VFA/TAL ratio ranged from 0.14 to 0.16. Thesevalues doubled in experiments R3eR4 (thermophilic) but stillremained below the inhibition threshold (Table 4). Considering theco-digestion experiments (Table 5), the VFA and VFA/TAL values formesophilic treatments (R5eR6) were similar to those for trialsR1eR2. When the mixture of SS and HS-OFMSW was treated atthermophilic temperatures (R7eR8 trials), the concentrations ofvolatile fatty acids considerably increased to 4508e5744 g/m3 and

Fig. 4. Changes in VFA and free ammonia concentrations during the co-digestion of S

the VFA/TAL ratio reached 0.54e0.57. These values corresponded todecreased biogas production in those experiments, which indicateda slight inhibition of methanogenesis.

The deterioration of AD performance in experiments R7eR8might have also resulted from the inhibitory effect of ammonia.Ammonium (NH4

þ) and free ammonia (NH3) are the most pre-dominant forms of nitrogen present in digestate (Appels et al.,2008); the sum of these two forms gives the total ammonium ni-trogen concentration (TAN). The TAN levels that could destabilizeanaerobic digestion are relatively high (1500e7000 g/m3; Calliet al., 2005; Hansen et al., 1998; Rajagopal et al., 2012). However,the inhibition of methanogens is mainly attributed to free ammoniabecause it easily passes through cell membranes, causes a protonimbalance, and alters the intracellular pH (Calli et al., 2005; Chenet al., 2008; Kim and Oh, 2011). The FAN concentration is mainlydependent on TAN level, pH, and temperature, therefore inhibitionis particularly observed during thermophilic digestion of high ni-trogen containing substrates (Hansen et al., 1998; Kim and Oh,2011; Rajagopal et al., 2012). As stated in the literature, the

S and HS-OFMSW (experiments R5eR8; symbols are identical to those in Fig. 3).

Table 6Free ammonia inhibition thresholds of anaerobic digestion reported by differentauthors.

Substrate Reactortemperature

Inhibitionthreshold[gNH3/m3]

References

OFMSW MesophilicThermophilic

215468

El Hadj et al. (2009)

OFMSW Mesophilic 800e1000 Kim and Oh (2011)Swine manure Thermophilic 1100 Hansen et al. (1998)Sewage sludge Mesophilic 400 Duan et al. (2012)Chicken manure Mesophilic 250 Bujoczek et al. (2000)Waste sludge-pig-cattle

slurry mixtureMesophilic 70e250 Buendia et al. (2009)

Sludge from juvenilesalmon hatcheries

Mesophilic 197e230 Gebauer andEikebrokk (2006)

Slaughterhouse waste Mesophilic 1000e1200 Lauterbock et al. (2012)

S. Borowski / Journal of Environmental Management 147 (2015) 87e94 93

inhibition of methanogenesis may start at a concentration of70 gNH3/m3; however stable digestion operations have beenobserved at much higher values of free ammonia concentrations(Table 6).

In the following study, the highest FAN content (1009 gNH3/m3

on average) was observed in experiment R7 inwhich the SS was co-digested with HS-OFMSW at an SRT ¼ 20 days (Table 5, Fig. 4).When the SRT value was reduced to 15 days (R8 trial), the averageFAN concentration nearly halved. Simultaneously, an average VFAconcentration of 4508 g/m3 was measured in experiment R7compared to the 5744 g/m3 reported in experiment R8. This vari-ation in the VFA concentration explains the significant difference(p ¼ 0.027) between free ammonia concentrations in these trials. Ahigher VFA content resulted in a pH drop, which in turn stronglyinfluenced the FAN level. According to Duan et al. (2012), the sta-bility of low-solids anaerobic digestion systems (TS below 15%) isprimarily affected by the VFA concentration, whereas free ammoniainfluences high-solids digestion systems to a much greater extent.This may explain why the inhibition of methanogenesis wasparticularly visible in experiment R8. It should be noted, however,that despite the evident inhibition, the AD process remained stablethroughout experiments R7 and R8 because the content of methanein the biogas was at a fairly high level (60e61%). This value waslower by only 3 percentage points than the corresponding valuesreported in the mesophilic co-digestion experiments (R5 and R6).

4. Conclusions

The anaerobic digestion of hydromechanically separatedmunicipal organic wastes displayed a moderate biogas productionof 309e361 dm3/kgVSfed, and this production was slightly higherunder thermophilic conditions. A 50% addition of sewage sludge(TS/TS) significantly improved the performance of mesophilic AD,displaying biogas yields of up to 494 dm3/kgVS.When thermophilicconditions were applied, the biogas production significantlydecreased because of the inhibition of methanogenesis by volatilefatty acids and, to a lesser extent, by free ammonia. However, astable digestion operation was still possible because the methanecontent in the biogas remained at the relatively high level of60e61%.

Acknowledgments

The author greatly appreciates to Leszek Komarowski fromEKOSPOT Company for financial and logistical support of theresearch. The language help of Prof. Laurence Weatherley from theUniversity of Kansas is also gratefully acknowledged.

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