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 of municipal solid waste with sewage sludge Sebastian Borowski* lczan  ska 171/173, 90-924, Ło dz, Poland Technical University of Lodz, Institute of Fermentation Technology and Microbiology, Wo

a r t i c l e i n f o

a b s t r a c t

Article history: Received 19 April 2014 Received in revised form 10 July 2014 Accepted 9 September 2014 Available online

This study investigates the anaerobic digestion of the hydromechanically sorted organic fraction of municipal solid wastes (HS-OFMSW) co-digested with sewage sludge (SS). Eight laboratory-scale experiments 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 under mesophilic 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 mesophilic digestion increased to 494 dm3/kgVS, which corresponded to 316 dm3CH4/kgVS. However, when SS and HS-OFMSW were treated under thermophilic conditions, methanogenesis was inhibited by volatile fatty acids and free ammonia, which concentrations reached 5744 gCH3COOH/m3 and 1009 gNH3/m3, respectively. © 2014 Elsevier Ltd. All rights reserved.

Keywords: Anaerobic digestion Biogas Sewage sludge Municipal solid waste

1. Introduction In Poland, municipal and industrial wastes pose great ecological hazards. The most important problems include long-term improper waste management, a poorly developed system of selection, a lack of modern infrastructure, and troubles with the enforcement of laws. According to the Central Statistical Office (Bochenek et al., 2013), Poland produced approximately 135 million tonnes of wastes in 2012, of which 12 million tonnes constituted municipal solid waste (MSW). The amount of collected municipal solid waste was 9.58 million tonnes, from which as much as 90% represented unsorted (mixed) waste. Moreover, approximately 75% of the MSW was deposited in landfill sites, and only 9% was treated by biological processes (mainly by composting). However, approximately 80% of the total municipal waste produced in Poland is organic; and this type of waste is known as the organic fraction of municipal solid wastes (OFMSW) and is suitable for biogas production. These figures indicate the urgent need to reduce the mass of landfilled wastes and suggest the implementation of sorting installations for MSW prior to biological processing via composting or anaerobic digestion (AD). The literature concerning both laboratory investigations and functioning AD plants focuses on the treatment of separately * Tel.: þ48 42 6313484; fax: þ48 42 6365976. E-mail addresses: [email protected], [email protected]. http://dx.doi.org/10.1016/j.jenvman.2014.09.013 0301-4797/© 2014 Elsevier Ltd. All rights reserved.

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; Zhang et al., 2008); whereas little has been published about the anaerobic treatment of the mechanically sorted (MS-OFMSW) and water sorted (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 is associated with higher biogas yields and better compost quality produced from these wastes, whereas mechanically sorted municipal organic wastes produce less biogas and a digestate of poorer quality (Bolzonella et al., 2006b). However, implementation of full-scale AD plants treating both types of waste is still limited because of the relatively high costs and technological limitations (Davidsson et al., 2007; Zhang et al., 2008). A solution to this problem may be the co-digestion of OFMSW with other waste types including sewage sludge (SS). Literature data show generally a low carbon to nitrogen ratio in sewage sludges typically ranging from 6 to 16; however, the ratio for OFMSW can be as high as 25e38. Thus mixing sewage sludge with municipal solid waste provides an improved nutrient balance, and the optimal C/N ratio of 15e30 that is 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 codigestion operations at the laboratory-, pilot- and full-scale.

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

However, almost all of these cases treated source-sorted municipal organic wastes (Agdag and Sponza, 2005; Bolzonella et al., 2006a, 2006b; Cavinato et al., 2013; Sosnowski et al., 2003). These authors have demonstrated the feasibility of sewage sludge codigestion with OFMSW in existing digesters at municipal wastewater treatment plants. The digesters treating waste activated sludge are often oversized and work with low organic loading rates. These factors justify the use of additional substrates to achieve codigestion. Furthermore, previous investigations also showed that wastewater treatment plants could implement the co-digestion of sewage sludge with OFMSW without changing (or with minor changes to) the plant design. These modifications may be economically beneficial (Bolzonella et al., 2006a; Cavinato et al., 2013; Krupp et al., 2005). In this study, the feasibility of the anaerobic co-digestion the hydromechanically separated organic fraction of municipal solid waste (HS-OFMSW) with municipal sewage sludge was evaluated. The specific objective was to evaluate biogas and methane yield from HS-OFMSW and from the mixture of these wastes with sewage sludge. The emphasis was also put on the stability of the digestion processes, in particular, the role of ammonia and volatile fatty acids was investigated. The experiments were performed in mesophilic and thermophilic conditions with solids retention time (SRT) values of 20 and 15 days. To the best of the author's knowledge, this study is the first to investigate the anaerobic co-digestion of hydromechanically separated municipal organic wastes with sewage sludge. 2. Methods 2.1. Characteristics and origin of materials Sewage sludge (the mixture of primary and waste activated sludge) was collected from the Municipal Wastewater Treatment Plant in Kutno, Poland. The plant treats 20,000 m3/d of wastewater and serves an equivalent population of 130,000. In this plant, the primary sludge (PS) and waste activated sludge (WAS) are first prethickened together in a gravity thickener to achieve a 2% total solids (TS) content, and then dewatered by centrifugation to approximately 16% TS. In this process, partial stabilization and sterilization (hygienization) is achieved through the addition of lime; however lime was not added to the sludge prepared for the purpose of this study. The annual production of dewatered sludge is 30,000 tonnes. The organic fraction of municipal solid wastes originated from a sorting plant at the Municipal Services Office in Puławy. In this plant, mixed (unsorted) municipal solid wastes are hydromechanically treated in the BTA® Process, which is the only installation of this type in Poland. The BTA® Process was originally developed in Germany in 1984 by the BTA Biotechnische Abfallverwertung GmbH & Co (now BTA Company GmbH) in cooperation with the University of Applied Sciences, Munich (BTA, 2014), whereas the installation in Puławy was opened in 2001. This process 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. The capacity of the plant is 22,000 tonnes of wastes per year. The characteristics of both the SS and HS-OFMSW are depicted in Table 1. Typically, the sludge was rich in nitrogen (average C/N ratio of 9.09) and phosphorus because this sludge originated from a treatment plant operating with a biological nutrient removal system. 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 dilute showing an average total solids content of 3.5% with the volatile

Table 1 Characteristics of sludge and hydromechanically separated OFMSW used for the experiments. Indicator

Unit

Sewage sludge

HS-OFMSW

TS VS

g/kg g/kg % TS gO2/kg gO2/kg TS gK/kg TS gNa/kg TS gCa/kg TS gMg/kg TS gFe/kg TS mgMn/kg TS mgZn/kg TS mgCu/kg TS mgPb/kg TS mgCd/kg TS

155.37 ± 9.11 128.37 ± 10.05 82.58 ± 3.01 168.47 ± 22.50 1084.4 ± 144.8 7.61 ± 0.41 3.87 ± 0.28 25.01 ± 1.34 5.39 ± 0.72 8.16 ± 1.25 532 ± 94 947 ± 192 343 ± 133 58.8 ± 23.1 31.7 ± 9.8

35.33 ± 10.02 18.79 ± 7.71 53.17 ± 2.28 21.53 ± 6.81 609.4 ± 164.5 4.52 ± 0.82 4.14 ± 0.95 9.80 ± 1.10 2.27 ± 0.39 9.03 ± 0.82 202 ± 58 157 ± 84 209 ± 19 39.2 ± 5.1 1.7 ± 0.4

% TS % TS % TS % TS % TS e

64.30 ± 2.10 7.07 ± 0.42 2.51 ± 0.23 5.27 ± 0.15 0.69 ± 0.05 9.09

66.78 ± 1.39 2.08 ± 0.33 0.73 ± 0.17 5.91 ± 0.52 0.05 ± 0.01 32.18

COD Potassium Sodium Calcium Magnesium Iron Manganese Zinc Copper Lead Cadmium Elemental analysis C N P H S C/N ± Standard deviation.

fraction not exceeding 53% of the TS. This classifies HS-OFMSW as not-easily biodegradable because the VS/TS ratio was lower than 0.7 (Pavan et al., 2000). Moreover, the waste displayed low quantities of nutrients. The fraction of nitrogen and phosphorus averaged 2.1% and 0.73%, respectively, and these figures were over 3fold lower than those reported for sewage sludge. This is in agreement with the observations of Dong et al. (2010). 2.2. Experiments Four laboratory scale reactors (each with 5 dm3 of total and 3 dm3 of active volume) were used in these experiments. Each reactor had a cylindrical shape with an internal diameter of 16 cm, a height of 25 cm and an active volume of 3 dm3. The reactors were equipped with helix-type mechanical stirrers operated with 80 rpm for 15 min every hour. The reactors were placed in thermostats to ensure constant mesophilic (35 ± 1  C) or thermophilic (55 ± 1  C) temperatures. Each reactor was coupled with a 4 dm3 gas collecting tank to provide anaerobic conditions and to measure the biogas yield by a water displacement method. The digesters were fed once a day (a semi-continuous operation) using a peristaltic pump. In the nomenclature used in this study, R1 and R2 refer to experiments with HS-OFMSW that were performed in mesophilic conditions, whereas R3 and R4 refer to experiments with HS-OFMSW that were performed in thermophilic conditions (Table 2). The other experiments were performed with a mixture of SS and HS-OFMSW (1: 1 of feed TS) in mesophilic (exp. R5 and R6) and thermophilic (exp. R7 and R8) conditions (Table 3). For each substrate and temperature, two SRT values were implemented to provide different operational conditions. 2.3. Analyses The analyses of the pH, total and volatile solids, total alkalinity (TAL) and chemical oxygen demand (COD) were performed according to standard methods (APHA, 2005). The total ammonium nitrogen (TAN), free ammonia (FAN), orthophosphates (PO3 4 ), and volatile fatty acids (VFA) were analyzed using a DR2800

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

89

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

Unit

HS-OFMSW mono-digestion

Experiment

e

R1

R2

R3

R4

Temperature Duration time SRT Feed TS Feed VS C/N (average) OLR VSreduction GPR SGP



35 120 20 32.03 ± 8.25 16.91 ± 4.17 32.18 0.85 ± 0.21 45.97 ± 22.91 266 ± 114 315 ± 135 684 ± 294 183 ± 78 58 ± 2

35 105 15 38.08 ± 8.33 20.04 ± 4.37 32.18 1.34 ± 0.29 49.36 ± 20.19 413 ± 166 309 ± 124 627 ± 251 176 ± 71 57 ± 2

55 120 20 34.17 ± 7.88 18.21 ± 3.96 32.18 0.91 ± 0.20 48.05 ± 15.50 328 ± 119 361 ± 130 751 ± 272 190 ± 75 58 ± 2

55 105 15 36.73 ± 9.12 19.56 ± 4.86 32.18 1.30 ± 0.32 50.76 ± 12.72 418 ± 122 320 ± 94 631 ± 184 186 ± 55 58 ± 1

SMP CH4 content in biogas

C d d g/kg g/kg e kgVS/m3,d % cm3/dm3,d dm3/kg VSfed dm3/kg VSreduced dm3CH4/kg VSfed %

± Standard deviation.

spectrophotometer with HACH-Lange tests as previously described (Borowski and Weatherley, 2013). Atomic absorption spectroscopy (SOLAAR 969 UNICAM) was used to determine the concentrations of metals (K, Na, Ca, Mg, Fe, Mn, Zn, Cu, Pb, and Cd). Elemental analysis (C, N, P, H, and S) was performed with a NA 2500 elemental analyzer (CE Instruments, UK). The total carbon was divided by the total nitrogen to obtain the C/N ratio. The daily biogas production was measured by a water displacement method as described in numerous sources in the literature (Cuetos et al., 2011; Sterling et al., 2001). The water used in gas collecting tanks was saturated to 75% with sodium chloride and acidified to pH ¼ 2. The content of methane in the biogas was determined using a Varian gas chromatograph coupled with a HaySep T column (13 m  0.5 m  1/800 OD) and a thermal conductivity detector (TCD). The carrier gas was helium. Analyses of individual samples were performed in triplicates. The calculation of the average values, standard deviations, and the analysis of variance (single factor ANOVA) were performed in Microsoft Excel 2007. A confidence level of 0.05 was selected for all statistical comparisons.

different substrates and temperatures is illustrated in Figs.1 and 2. The biogas was monitored daily; however, for clarity, the individual points on the plots represent the mean values of the weekly biogas yields. The reactors were established by using anaerobically digested sewage sludge from previous studies. The start-up of mesophilic reactors (beginning with R1 and R5 runs), was with anaerobically digested sludge originated from previous experiments described by Borowski and Weatherley (2013). The same sludge but after previous thermophilic treatments (data not published) was used as the inoculum for experiments performed at 55  C (starting with R3 and R7 runs). Initially the digesters operated with a solids retention time of 20 days; this time was then reduced to 15 days. Each experimental run provided at least 5 consecutive SRTs under steady state conditions. As shown in Figs. 1 and 2, all reactors achieved stable operation within a few days, indicating a good acclimation of inocula for both mesophilic and thermophilic conditions, and variations in the biogas production are mainly associated with the heterogeneity of the sludge and wastes being treated.

3. Results and discussion

The trials with HS-OFMSW alone (R1eR4) were performed with relatively low organic loading rates (OLR) in the range of 0.05e1.34 kgVS/m3,d because of the dilute organic waste derived from hydromechanical separation. Therefore, the biogas production rates (GPR, calculated as per unit reactor volume) for these trials were also low, and the mesophilic treatment process reached only 266 and 413 cm3/dm3,d for SRT of 20 and 15 days, respectively

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

3.1. Anaerobic digestion of OFMSW

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

Unit

SS þ HS-OFMSW co-digestion

Experiment

e

R5

R6

R7

R8

Temperature Duration time SRT Feed TS Feed VS C/N (average) OLR VSreduction GPR SGP



35 120 20 61.24 ± 8.61 42.21 ± 5.98 20.09 2.11 ± 0.30 41.53 ± 7.40 1043 ± 233 494 ± 110 1190 ± 266 316 ± 70 64 ± 1

35 105 15 54.37 ± 10.26 37.58 ± 6.07 20.09 2.51 ± 0.41 29.15 ± 13.48 1124 ± 263 449 ± 105 1539 ± 360 283 ± 66 63 ± 1

55 120 20 47.67 ± 11.27 33.96 ± 7.52 20.09 1.70 ± 0.38 40.95 ± 10.11 622 ± 217 367 ± 128 895 ± 312 224 ± 78 61 ± 2

55 105 15 59.10 ± 12.63 40.12 ± 8.27 20.09 2.67 ± 0.55 31.35 ± 9.13 739 ± 262 276 ± 98 882 ± 313 166 ± 59 60 ± 2

SMP CH4 content in biogas ± Standard deviation.

C d d g/kg g/kg e kgVS/m3,d % cm3/dm3,d dm3/kg VSfed dm3/kg VSreduced dm3CH4/kg VSfed %

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

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

(Table 2, experiments R1eR2). These figures for SRT of 20 and 15 days correspond to a moderate specific gas production (SGP) of 315 and 309 dm3/kgVSfed, respectively, and moderate values of volatile solids reduction of 45.97 and 49.36%, respectively. Also, a specific methane production (SMP) for these experiments was rather low and did not exceed 200 dm3 CH4/kgVSfed. Thermophilic temperatures did not spectacularly improve either the biogas production or VS removal efficiency. The specific gas production increased by nearly 15% for an SRT ¼ 20 d to reach 361 dm3/kgVSfed (p ¼ 0.0076), however, an SRT ¼ 15 d displayed an insignificant change in SGP (p ¼ 0.4714). Moreover, only a small and insignificant increase (p ¼ 0.7282) in VS reduction, compared to mesophilic digestion, was reported. The average values of volatile solids reduction for thermophilic HS-OFMSW digestion were 48.05 and 50.76% for R3 and R4 (Table 2). Numerous papers investigate the AD

of source sorted (or separately collected) OFMSW; however, only selected reports can be found investigating mechanically or water sorted wastes. Bolzonella et al. (2006b) reported an average biogas yield of 255 dm3/kgVSfed (140 dm3 CH4/kgVSfed) and a VS removal of 35e40% in a Bassano biogas plant treating mechanically sorted wastes. The corresponding biogas production from source sorted organic wastes in this plant was 714 dm3/kgVS whereas methane yield reached 492 dm3 CH4/kgVSfed. Dong et al. (2010) suggested that the biodegradability of water sorted OFMSW is higher than that of mechanically sorted OFMSW. However, this value is still lower than that of source sorted OFMSW. In their study, the biogas production and methane yield from water sorted organic wastes did not exceed 300 dm3/kgVS and 200 dm3 CH4/kgVSfed, respectively, and the VS removal was in the range 26.1e41.8%. These values are lower than those presented in our investigation.

Fig. 2. Biogas production (GPR) reported during the anaerobic co-digestion of SS and HS-OFMSW (experiments R5eR8).

S. Borowski / Journal of Environmental Management 147 (2015) 87e94 Table 4 Characteristics of digestate from experiments with municipal solid wastes. Indicator

Unit

HS-OFMSW mono-digestion

Experiment

e

R1

R2

R3

R4

SRT pH TS VS

d e g/kg g/kg % TS gN/m3 gNH3/m3 gP/m3 g/m3

20 7.42 ± 0.14 18.58 ± 7.30 9.14 ± 3.87 48.92 ± 7.18 694 ± 205 24.5 ± 6.8 24.33 ± 17.73 903 ± 458

15 7.66 ± 0.18 24.05 ± 9.11 10.15 ± 4.05 41.81 ± 3.15 746 ± 230 49.0 ± 32.0 18.27 ± 9.73 906 ± 415

20 7.60 ± 0.08 25.25 ± 7.01 9.46 ± 2.82 38.08 ± 8.40 707 ± 79 112.6 ± 19.1 33.47 ± 7.91 1999 ± 557

15 7.89 ± 0.10 25.53 ± 4.38 9.63 ± 2.49 37.44 ± 4.22 825 ± 208 238.3 ± 83.9 25.60 ± 9.39 1966 ± 590

TAN FAN PO34 VFA (as acetic) TAL g/m3 VFA/TAL e

5817 ± 1710 6427 ± 625 7631 ± 1564 5881 ± 733 0.16 ± 0.10 0.14 ± 0.08 0.26 ± 0.08 0.33 ± 0.09

± Standard deviation.

3.2. Co-digestion of sewage sludge and HS-OFMSW In runs R5eR8, the mixture of SS and HS-OFMSW used in the experiment was blended at a ratio of 1: 1 based on dry weight (TS/ TS). This mixture had a C/N ratio of 20.1 which was within the range of 15e30 that is regarded as optimal for anaerobic digestion (Nasir et al., 2012; Zhang et al., 2008). Moreover, many authors have also suggested this ratio for the co-digestion of mechanically sorted and source 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-digestion trials are shown in Table 3. A comparison of the biogas production from wastes and their mixture with the sludge is illustrated in Figs. 1 and 2. The addition of sewage sludge to HS-OFMSW significantly increased the digestion performance during mesophilic treatment. The greatest increase in biogas yield was reported for an SRT of 20 days. For this SRT, the gas production rate increased nearly 3-fold compared to the R1 trial to produce 1043 cm3/dm3,d. However, the specific gas and methane production increased by approximately 60% and reached 494 dm3/kgVSfed, and 316 dm3CH4/ kgVSfed, which was the maximum obtained in the study. For an SRT of 15 days, the average GPR and SGP values were 1124 cm3/dm3,d and 449 dm3/kgVSfed, respectively, whereas methane yield averaged 283 dm3CH4/kgVSfed (Table 3). These yields are considerably higher than the values reported in the literature for the codigestion of source sorted OFMSW mixed at a similar ratio with sewage sludge (Bolzonella et al., 2006a; Castillo et al., 2006; Cavinato et al., 2013; Dai et al., 2013). Another benefit of codigestion was the higher methane content of the biogas compared to the HS-OFMSW mono-digestion. The maximal

91

methane content in biogas was 64% which agreed with the highest SGP reported in trial R5. In experiments using only HS-OFMSW, the methane content did not exceed 58%. The low methane content in biogas produced during HS-OFMSW mono-digestion can be linked to the specific nature of these wastes, which are mainly composed of carbohydrates. The theoretical methane content in biogas produced from carbohydrates is about 50%, whereas the values for proteins and fats reach 70% and more. However, the real methane content in practice is generally higher than the theoretical values because a part of CO2 is solubilized in the digestate (Esposito et al., 2012; Weiland, 2010). The low methane content in biogas (not exceeding 60%) yielded from municipal solid wastes was also reported 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 codigestion was lower than the corresponding values for HSOFMSW mono-digestion. The average percentage of the VS removal in experiments R5 and R6 were 41.53 and 29.15% for SRT of 20 and 15 days, respectively (Table 3). This may be linked to the specific nature of the Kutno WWTP sludge that was used as a cosubstrate in this study. This sludge is primarily composed of WAS with small amounts of PS. The VS removal efficiencies reported in the literature for the AD of sludges with no or minor amounts of primary 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 digestion should have a higher degree of organic material degradation and a greater biogas yield compared to mesophilic treatment (ForsterCarneiro et al., 2008; Provenzano et al., 2013). However, in the following study, no such improvements were observed. The VS removal rates calculated for the mesophilic and thermophilic codigestion experiments (R5eR8, Table 3) were comparable, and no significant differences between the corresponding trials were observed (p ¼ 0.8589 and 0.6273 for SRT of 20 and 15 days, respectively). Furthermore, considerably lower biogas yields were also noted. The average gas production rates in experiments R7 and R8 were 622 and 739 cm3/dm3,d for SRT of 20 and 15 days, respectively. These gas production rates were nearly twice as low as the values for mesophilic treatments (R5 and R6). The SGP values were 367 and 276 dm3/kgVSfed, respectively, and the latter value was significantly lower (p ¼ 0.00017) than the corresponding value measured for HS-OFMSW (R4). 3.3. Behavior of ammonia and volatile fatty acids Tables 4 and 5 depict the characteristics of digestate derived from the individual trials, whereas the changes of VFA and free ammonia in the course of experiments are illustrated in Figs. 3 and

Table 5 Characteristics of digestate from co-digestion experiments. Indicator

Unit

SS þ HS-OFMSW co-digestion

Experiment

e

R5

R6

R7

R8

SRT pH TS VS

d e g/kg g/kg % TS gN/m3 gNH3/m3 gP/m3 g/m3 g/m3 e

20 7.75 ± 0.09 41.77 ± 6.13 24.68 ± 3.12 59.27 ± 1.76 1891 ± 174 124.1 ± 20.5 42.13 ± 9.94 1397 ± 389 9604 ± 1482 0.15 ± 0.04

15 7.68 ± 0.05 45.32 ± 10.03 26.62 ± 5.07 59.20 ± 2.72 1610 ± 314 97.6 ± 16.4 39.64 ± 17.32 790 ± 256 9077 ± 940 0.09 ± 0.03

20 8.19 ± 0.27 33.19 ± 6.01 20.05 ± 3.43 60.67 ± 3.67 1948 ± 499 1009.0 ± 518.2 81.31 ± 29.26 4508 ± 1545 8335 ± 992 0.54 ± 0.17

15 7.84 ± 0.26 46.31 ± 7.30 27.54 ± 3.66 59.76 ± 2.71 1947 ± 169 562.7 ± 207.5 56.79 ± 19.94 5744 ± 1491 10,107 ± 1535 0.57 ± 0.10

TAN FAN PO3 4 VFA (as acetic) TAL VFA/TAL ± Standard deviation.

92

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

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

4. It is generally known, that the stability of anaerobic digestion is mainly affected by the concentrations of volatile fatty acids and free ammonia. The inhibition of methanogenesis usually starts at a VFA concentration of 2500e4000 g/m3 (as acetic acid) (Appels et al., 2008; Duan et al., 2012; Kafle and Kim, 2013). A more precise criterion for digester stability is the VFA to total alkalinity ratio. Generally, the operation of a digester is stable at VFA/TAL ratios below 0.4, whereas significant instability occurs when the VFA/TAL ratio exceeds 0.7e0.8 (Callaghan et al., 2002; Kafle and Kim, 2013). The experiments with HS-OFMSW in mesophilic conditions (R1 and R2) showed low VFA concentrations of approximately 900 g/ m3, whereas the VFA/TAL ratio ranged from 0.14 to 0.16. These values doubled in experiments R3eR4 (thermophilic) but still remained below the inhibition threshold (Table 4). Considering the co-digestion experiments (Table 5), the VFA and VFA/TAL values for mesophilic treatments (R5eR6) were similar to those for trials R1eR2. When the mixture of SS and HS-OFMSW was treated at thermophilic temperatures (R7eR8 trials), the concentrations of volatile fatty acids considerably increased to 4508e5744 g/m3 and

the VFA/TAL ratio reached 0.54e0.57. These values corresponded to decreased biogas production in those experiments, which indicated a slight inhibition of methanogenesis. The deterioration of AD performance in experiments R7eR8 might have also resulted from the inhibitory effect of ammonia. Ammonium (NHþ 4 ) and free ammonia (NH3) are the most predominant forms of nitrogen present in digestate (Appels et al., 2008); the sum of these two forms gives the total ammonium nitrogen concentration (TAN). The TAN levels that could destabilize anaerobic digestion are relatively high (1500e7000 g/m3; Calli et al., 2005; Hansen et al., 1998; Rajagopal et al., 2012). However, the inhibition of methanogens is mainly attributed to free ammonia because it easily passes through cell membranes, causes a proton imbalance, and alters the intracellular pH (Calli et al., 2005; Chen et al., 2008; Kim and Oh, 2011). The FAN concentration is mainly dependent on TAN level, pH, and temperature, therefore inhibition is particularly observed during thermophilic digestion of high nitrogen containing substrates (Hansen et al., 1998; Kim and Oh, 2011; Rajagopal et al., 2012). As stated in the literature, the

Fig. 4. Changes in VFA and free ammonia concentrations during the co-digestion of SS and HS-OFMSW (experiments R5eR8; symbols are identical to those in Fig. 3).

S. Borowski / Journal of Environmental Management 147 (2015) 87e94 Table 6 Free ammonia inhibition thresholds of anaerobic digestion reported by different authors. Substrate

OFMSW

Reactor temperature

Mesophilic Thermophilic OFMSW Mesophilic Swine manure Thermophilic Sewage sludge Mesophilic Chicken manure Mesophilic Waste sludge-pig-cattle Mesophilic slurry mixture Sludge from juvenile Mesophilic salmon hatcheries Slaughterhouse waste Mesophilic

Inhibition threshold [gNH3/m3]

References

215 468 800e1000 1100 400 250 70e250

El Hadj et al. (2009) Kim and Oh (2011) Hansen et al. (1998) Duan et al. (2012) Bujoczek et al. (2000) Buendia et al. (2009)

197e230

Gebauer and Eikebrokk (2006) 1000e1200 Lauterbock et al. (2012)

inhibition of methanogenesis may start at a concentration of 70 gNH3/m3; however stable digestion operations have been observed 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 in which the SS was codigested 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 average FAN concentration nearly halved. Simultaneously, an average VFA concentration of 4508 g/m3 was measured in experiment R7 compared to the 5744 g/m3 reported in experiment R8. This variation in the VFA concentration explains the significant difference (p ¼ 0.027) between free ammonia concentrations in these trials. A higher VFA content resulted in a pH drop, which in turn strongly influenced the FAN level. According to Duan et al. (2012), the stability of low-solids anaerobic digestion systems (TS below 15%) is primarily affected by the VFA concentration, whereas free ammonia influences high-solids digestion systems to a much greater extent. This may explain why the inhibition of methanogenesis was particularly visible in experiment R8. It should be noted, however, that despite the evident inhibition, the AD process remained stable throughout experiments R7 and R8 because the content of methane in the biogas was at a fairly high level (60e61%). This value was lower by only 3 percentage points than the corresponding values reported in the mesophilic co-digestion experiments (R5 and R6). 4. Conclusions The anaerobic digestion of hydromechanically separated municipal organic wastes displayed a moderate biogas production of 309e361 dm3/kgVSfed, and this production was slightly higher under 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 thermophilic conditions were applied, the biogas production significantly decreased because of the inhibition of methanogenesis by volatile fatty acids and, to a lesser extent, by free ammonia. However, a stable digestion operation was still possible because the methane content in the biogas remained at the relatively high level of 60e61%. Acknowledgments The author greatly appreciates to Leszek Komarowski from EKOSPOT Company for financial and logistical support of the research. The language help of Prof. Laurence Weatherley from the University of Kansas is also gratefully acknowledged.

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