Production of biogas from solid organic wastes through anaerobic ...

Appl Microbiol Biotechnol (2012) 95:321–329 DOI 10.1007/s00253-012-4152-7

MINI-REVIEW

Production of biogas from solid organic wastes through anaerobic digestion: a review Ismail Muhammad Nasir & Tinia I. Mohd Ghazi & Rozita Omar

Received: 16 December 2011 / Revised: 3 May 2012 / Accepted: 3 May 2012 / Published online: 24 May 2012 # Springer-Verlag 2012

Abstract Anaerobic digestion treatments have often been used for biological stabilization of solid wastes. These treatment processes generate biogas which can be used as a renewable energy sources. Recently, anaerobic digestion of solid wastes has attracted more interest because of current environmental problems, most especially those concerned with global warming. Thus, laboratory-scale research on this area has increased significantly. In this review paper, the summary of the most recent research activities covering production of biogas from solid wastes according to its origin via various anaerobic technologies was presented. Keywords Anaerobic digestion . Biogas . Methane . Solid waste

Introduction Million tons of solid waste are produced annually from municipal, industrial, and agricultural sources. The indiscriminate decomposition of these organic wastes results in large-scale contamination of land, water, and air. Of all the forms of solid organic waste, the most abundant is animal dung primarily from small farms, and it is from these farms that the pollution problem originating from waste disposal is more intense. Research continues to focus on the treatment of cattle dung for biogas production and possible optimization methods which could be used to enhance the production for practical applicability of the technology. Omar et al. (2008) observed an improvement in biogas yield up to I. Muhammad Nasir : T. I. Mohd Ghazi (*) : R. Omar Department of Chemical and Environmental Engineering, Faculty of Engineering, Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia e-mail: [email protected]

0.207 m3 kg−1 VS added with average methane content of 65 % in the anaerobic treatment of cattle manure by addition of palm oil mill effluent as an inoculum in a laboratory scale bioreactor. In another study, Ounaar et al. (2012) obtained biogas production of 26.9 m3 with an average methane content of 61 % during the anaerobic digestion of 440 kg of cow dung with an energy equivalent of 164.5 kWh. These results are encouraging for the use of animal waste available to produce renewable energy and clean environment. According to Yu et al. (2002), decomposition of 1 MT of grass waste can possibly release 50–110 m3 of carbon dioxide and 90–140 m3 of methane into the atmosphere. Methane is an important greenhouse gas with the ability of global warming 25 times greater than that of carbon dioxide, and its atmospheric concentration has been increasing in the range of 1 to 2 % per year (IPCC 2007). Conventional municipal solid waste (MSW) management has been mainly disposal by land filling (Sosnowski et al. 2003). However, waste from landfills has been identified as the major source of anthropogenic methane emission and an essential contributor to global warming (IPCC 1996). Therefore, the increased production of MSW accompanied with environmental and economic difficulties facing the conventional methods of disposal have resulted in great efforts to find alternative methods of disposal (Zsigraiova et al. 2009). The most promising alternative to incinerating and composting these solid wastes is to digest its organic matter employing the anaerobic digestion (Bouallagui et al. 2005). Anaerobic digestion for biogas production has become a worldwide focus of research, because it produces energy that is renewable and environmentally friendly. Special emphasis was initially focused on anaerobic digestion of MSW for bioenergy production about a decade ago (Braber 1995; Kiely et al. 1997). Anaerobic biological treatment can be an acceptable solution because it reduces and stabilizes solid

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wastes volume, produces biogas comprising mainly methane and carbon dioxide, and traces amount of other gases (Stroot et al. 2001). In addition to biogas, a nutrient-rich digestate is also produced which provide either fertilizer or soil conditioner properties. Biological treatment of MSW to biogas by anaerobic digestion processes including sourcesorted and mechanically sorted MSW has been previously discussed (Gunaseelan 1997). Literature is available about the applications and importance of the anaerobic digestion treatments for solid waste treatment, especially focusing on promoting process efficiency and performance. Therefore, the main objective of this review paper is to summarize the research activities on the effects of both operational and process performance parameters, covering the anaerobic conversion of various solid waste substrates via various anaerobic systems.

Production of biogas by anaerobic digestion processes Many research papers have been published regarding the performance of different anaerobic technological systems digesting organic solid wastes. Most of them concentrate on the concept of anaerobic digestion of the organic fraction of municipal solid waste (OFMSW). Anaerobic treatment of OFMSW has been an attractive feedstock for biogas production. Nevertheless, pretreatment of MSW before the digestion is the initial stage, and these wastes are characterized by a high percentage of moisture and VS above 90 % with high biodegradability. Rao et al. (2000) referred to these wastes as municipal garbage, which is the main constituent of MSW (40–45 wt.%), emanating from different sources as food waste (FW) such as households, fruit and vegetable markets, canteens, hotels, etc., and they are rich in organic matter and can be used for biogas generation by anaerobic digestion. Anaerobic digestion of solid organic wastes has been studied recently, attempting to develop technology that offers waste stabilization accompanying resources recovery. About 90 % of the full-scale plant presently in operation in Europe for anaerobic digestion of OFMSW relies on one-stage system, and this are divided into wet and dry digestion (De Baere 2000). A likely reason for this is that the industrialists prefer a one-stage system over the two-stage or multistage systems because a simpler design system suffers less frequent technical failures and are economical. The dry digestion systems digest waste as received, while the wet digestion systems need to slurry the waste with water to about 12 % TS (Vandevivere et al. 2002). However, from a technical point of view, the dry digestion systems appear more robust as regular technical failures are reported with wet systems due to sand, plastics, wood, and stones. Many researchers have already reported various studies on laboratory scale, pilot-scale, and full-

Appl Microbiol Biotechnol (2012) 95:321–329

scale anaerobic digestion for the treatment of organic solid waste. From a review of literature on the preliminary design procedure for anaerobic digesters for the treatment of MSW for biogas production, Igoni et al. (2008) noted that proper reactor size reduction must be considered for the anaerobic digestion of organic wastes. They further explained that the most important aspect of digestion processes, such as, temperature, hydrogen ion concentration, carbon nitrogen (C/N) ratio, organic loading rate (OLR), moisture content, and heat content, need to be manipulated so as to achieve optimal performance for anaerobic digester. They recommend that the batch digestion system should be increasingly employed because it is cost-effective and economical for treating the ever abundant MSW to useful energy. Therefore, a summary of the anaerobic digestion processes employed for these wastes will be showed in this section, and the overview of the studies are also presented in an orderly manner in Table 1. A laboratory scale batch anaerobic digestion of municipal garbage was studied by Rao et al. (2000) at temperatures of 25 °C and 29 °C, with a concentration range between 45 and 135 g TS/L. They found out that the methane content from the biogas varied between 62 and 72 %, and a conversion efficiency of about 85 % was obtained. In a similar study, Rao and Singh (2004) investigated the batch digestion of municipal garbage under room temperature (26±4 °C) to estimate its bioenergy potential and conversion efficiencies at an HRT of 15 days. They reported a high yield of 0.56 m3 biogas kg−1 VS added with 70 % methane content and a VS reduction of 76.3 %. These results demonstrated that municipal garbage has a high potential to be a bioenergy source. López and Espinosa (2008) evaluated the effect of pretreating OFMSW with lime in the anaerobic digestion process. The laboratory scale experiment was carried out in a completely mixed reactor operated on a batch basis. The maximum yield of methane obtained under the anaerobic digestion of the pretreated waste was 0.15 m3 kg−1 VS added. This result is nearly 172 % increase in the methane yield over the control without pretreatment. In addition, under the same condition, soluble COD and VS removal were 93 and 94 %, respectively. The outcome implied that the chemical pretreatment with lime, followed by anaerobic digestion, gives the best result for OFMSW stabilization. Elango et al. (2007) reported data on the influence of domestic sewage on the biogas production from municipal solid waste using the anaerobic digestion process. They operated a batch reactor at temperatures from 26 to 36 °C with a fixed HRT of 25 days and different OLR in the range of 0.5 to 4.3 kg VS m3 day−1. They obtained a maximum amount of biogas production of 0.36 m3 kg−1 VS added at OLR of 2.9 kg VS m3 day−1. This OLR was referred to as the optimum OLR because the maximal removal of TS (87 %), VS (88.15), and COD (89.3 %) occurred at this stage. They concluded that the disposal problem of MSW

2-stage CSTR and UASB 2-stge

Tubular reactor (18 L)

2-phase system (18 L)

Batch system

Batch

ASBR (2 L)

ASBR (2 L)

Semicont. (2 L)

1–stage and 2-stage (30 L) UASB (0.84 L) and APB (0.7 L) CSTR (4.5 L)

Pilot-scale (35 L)

Batch (5 L)

Sosnowski et al. (2003)

Fongsatitkul et al. (2010)

Bouallagui et al. (2003)


Zhang et al. (2007)

Forster-Carneiro et al. (2007b)



Alvarez and Liden (2008)

Schober et al. (1999)

Angelidaki et al. (2006)

Davidsson et al. (2007)

Forster-Carneiro et al. (2008a)

CSTR (10.4 L)

3-stage semicont.

Batch (1.1 L)

Batch (1.1 L)

PFR (1350 L)

Maroun and EL Fadel (2007)

Kim et al. (2006)

Forster-Carneiro et al. (2008c)

Forster-Carneiro et al. (2007)

Sharma et al. (2000)

Parawira et al. (2006)

CSTR and AF

UAF (222 L)

Macias-Coral et al. (2008)

Glass et al. (2005)

Batch (0.5 L)

Parawira et al. (2004)

CSTR

High solid batch (40 L)

Guendouz et al. (2010)

CSTR (4.5 L)

Batch (1.7 L)

Fernandez et al. (2010)

Hartmann and Ahring (2005)

Batch (1.7 L)


Linke (2006)

Semicont. batch (5 L)

Elango et al. (2007)

Semicont. (14 L)

Batch (1 L)

Lopez and Espinosa (2008)

Batch (375 L)

Batch (3.25 L)

Rao and Singh (2004)

Nguyen et al. (2007)

Batch

Rao et al. (2000)


Reactor type and volume

Researcher

NR

55

55

37

37

NR

37

35

35

35

26–36

25

25

25 and 29

Temp. (°C)

NR

0.8–3.4

4

NR

0.97

NR

NA

NA

NA

NA

05-4.3

NA

NA

NA

OLR (kg VS m−3 days−1)

SSW

FW/SH-OFMSW/ OFMSW SS-OFMSW, food waste

Food waste

SS-OFMSW/MSOFMSW SS-OFMSW

SS-OFMSW

SS-OFMSW

PW leachate (UASB), PW (APB)

KR

FVW + SW + manure

Abattoir waste + FVW

FVW

FW

Food waste

FVW

FVW

37

55

55

50

35

55

55

55

37

35/55

35

55

55

35

50

35/55

35

40

NA

NR

NR

2.03

NA

2.8

11.4

6.1, 4.7 (UASB, APB)

6

1.3

2.56

1.24

NR

NA

7.5 kg COD m−3 day−1

6 % TS

Sewage sludge + OFMSW 56, 36 0.669 g VSS (CSTR, UASB) dm−3 day−1 OFMSW + RAS 35 NR

Steam-treated OFMSW

Potato processing waste

OFMSW + CM

Leachate

Potato waste/potato waste + beet leaves OFMSW + CM / CGW + CM OFMSW

MSW

OFMSW

OFMSW

MSW + domestic sewage

OFMSW

MSW

MSW

Feed

Table 1 Operational and performance data for different bioreactor designs applied for solid wastes

33.7

90

90

12.4

90

60

15

15

13.2, 10 (UASB, APB)

11

30

20

20

20–60

10/28

20

20

17.3, 44.2 (CSTR, UASB) 28

12

NR

18

60

17

141/151

14

15 (20 % TS) ; 35 (30 % TS) 15

NR

25

NR

15

NR

HRT (days)

71

74, 32.4 (SS-OFMSW, FW)

32.4/73.7/79.4

NR

NR

56

81

30

72, 80 (35 and 55 °C) NR

NR

86.2

79

NR

81

96 % COD

75.9

78

20 % COD, 86 % COD (CSTR, AF) NR

NR

74

61

73

NR

NR

40

NR

NR

88.1

94

76.3

85

Efficiency VSRED (%)

0.7

0.50, 0.180 (SS-OFMSW, FW)

0.18/0.05/0.08

NR

NR

NR

0.3–0.4

0.430

NR

NR

0.320

NR

NR

0.348, 0.435 (10, 28 days) NR

NR

NR

NR

0.024

NR

NR

0.460

0.26

0.3

0.1/0.19

0.42/0.68

0.211

0.11 (20 % TS); 0.007 (30 % TS) NR

NR

0.15

NR

NR

CH4 yield (m3 kg−1 VSadded)

1.05

NR

NR

NR

0.2–0.56

NR

NR

0.71

0.800, 0.830 (35 and 55 °C) NR

1.360

0.73

0.480

0.49

NR

0.705, 0.997 (35 and 55 °C)

0.707

0.73

0.02–0.29, 0.04– 0.47 (CSTR, AF) NR

0.65–0.85

0.710

NR

0.8

NR

NR

NR

NR

NR

0.36

NR

0.560

NR

Biogas yield (m3 kg−1 VS)

NR

68.5, 76.7 (SS-OFMSW, FW)

NR

67.4

40–65

53.4

62

64

59, 66 (UASB, APB)

NR

56

62

60

NR

73

64, 61 (35 and 55 °C)

57

NR

60

NR

58

64

55

58

72

62/84

NR

80 (20 % TS)

NR

68–72

NR

70

72

% CH4

Appl Microbiol Biotechnol (2012) 95:321–329 323

Semi cont. semi-continuous, CSTR continuous stirred tank reactor, MSW municipal solid waste, FVW fruit and vegetable waste, SW slaughter house waste, SSW semisolid waste, CGW cotton gin waste, CM cattle manure, KR kitchen refuse, PW potato waste, RAS return activated sludge, SSW semisolid waste, SS-OFMSW source sorted organic fraction of municipal solid waste, OW organic waste, Temp. temperature, OLR organic loading rate, HRT hydraulic retention time, VSRED volatile solids reduction, VSa volatile solids added, NR not reported, NA not applicable

NR NR 0.39–0.6 NR 20 0.8 OW + sludge Full scale (2,000 m3) Zupancic et al. (2008)

35

68 NR

NR 0.4

NR 72

78 40–60

22.5 60

4–6 36–39 SS-OFMSW mixture Bolzonella et al. (2006)

37 SSW PFR (1350 L)

Full scale (2,200 m3)

Lastella et al. (2002)

Biogas yield (m3kg−1 VS) CH4 yield (m3kg−1 VSadded) Efficiency VSRED (%) HRT (days) OLR (kg VS m−3days−1) Temp. (°C) Feed Reactor type and volume Researcher

Table 1 (continued)

56


% CH4

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and domestic sewage can be resolved substantially. An experiment was performed by Fernandez et al. (2008) to investigate the influence of substrate concentration on dry mesophilic anaerobic digestion of the OFMSW. The experiment was conducted in a batch reactor at 35 °C, during a period of 85–95 days at solid concentrations of 20 % and 30 % TS. Experimental results indicated that the reactor with 20 % TS achieved a higher yield of 0.11 m3 CH4 kg−1 VS removed compared to 0.07 m3 CH4 kg−1 VS removed achieved for 30 % TS reactor. Also, the 20 % TS digestion attained the highest performance with high dissolve organic carbon (DOC) removal (80.69 %), compared to the 30 % TS digestion (69.05 %). Therefore, they concluded that the initial substrate concentration during the anaerobic digestion of OFMSW affects the process clearly. In a similar study, Fernandez et al. (2010) investigated the mesophilic anaerobic degradation of OFMSW in discontinuous lab reactors with two different initial concentrations of 20 % TS and 30 % TS. The anaerobic treatment was favored when it was conducted with a 20 % TS content in comparison to a similar process with 30 % TS. Results showed a higher level of organic matter removed, in terms of DOC and VFA, 18.18 % and 8.09 %, respectively, in the 20 % TS system. Also, the kinetics parameters demonstrated higher active biomass and a higher coefficient for the production of methane at the 20 % TS concentration. Guendouz et al. (2010) found similar biogas and methane yields of around 0.211 m3 kg−1 VS added with 40 % VS reduction conducted in a laboratory scale high-solid batch digestion test of MSW under mesophilic conditions for a 15-day HRT. The results obtained compared well to a larger pilot-scale reactor operation with a yield of 0.205 m3 methane kg−1 VS added. Parawira et al. (2004) examined batch anaerobic codigestion in an experiment with different mixtures of potato waste and beet leaves. They reported an enhanced yield of 0.68 m3 methane kg−1 VS added with a mixing ratio of (24:16%TS), and another yield of 0.42 m3 methane kg−1 VS added from potato waste alone. The processes were both operated under mesophilic conditions at an HRT of 14 days. The codigestion result showed an optimum mixing ratio for successful operation with a higher yield and a shorter HRT. Hence, the batch anaerobic digestion can be applied where low cost and low technology are needed. Macias-Coral et al. (2008) investigated the applicability of a two-phase pilot-scale anaerobic codigestion system for the treatment of OFMSW, dairy cow manure (CM), and cotton gin waste (CGW). Results showed that the single waste digestion of OFMSW and CM produced 0.03 and 0.08 m3 CH4 kg−1 VS added. Meanwhile, the codigestion of OFMSW and CM produced a yield of 0.1 m3 kg−1 VS added, in addition to the CGW and CM which obtained the highest yield of 0.19 m3 kg−1 VS added. They concluded


that codigestion of the combined wastes resulted in high methane yield as compared to the single waste digestion. Fernandez et al. (2005) study the potential of anaerobic digestion for the treatment of fats of different origins through codigestion with simulated OFMSW. In the study, a pilot plant operating semicontinuously in the mesophilic (37 °C) temperature and HRT of 17 days was employed. The biogas and methane yields obtained from the simulated OFMSW at steady state were 0.8 and 0.5 m3 kg−1 VS added, respectively. On the other hand, no significant difference was observed in the performance of the anaerobic codigestion when animal fat was changed with vegetable fat. Also, the yields of biogas and methane were similar to those found with OFMSW, methane content being a bit higher in the presence of fat. They concluded that both fat from animal or vegetable origin are highly degradable (94 % for animal fat and 97 % for vegetable fat) during codigestion with OFMSW. Therefore, this indicates that anaerobic digestion appears to be a suitable technology for the treatment of such wastes and obtainment of a renewable energy source. A study was performed by Nguyen et al. (2007) to evaluate the effect of prestage flushing and microaeration in addition to the effect of leachate percolation in methanogenesis stage enhancement in order to develop combined batch anaerobic digestion systems. The study was conducted in a high solid pilot-scale reactor in two runs. In run 1, prestage flushing and microaeration were carried out to find out their effect in terms of enhancing hydrolysis and acidification in ambient condition. Whereas in run 2, following prestage, new condition was provided in order to enhance the start-up of methanogenesis stage by adjusting the pH to 6.5 and followed by inoculum addition at mesophilic condition (37 °C). The result showed that the application of microaeration achieved 75 % biogas conversion and 61 % VS degradation on the hydrolysis and acidification enhancement. Also, the prestage flushing with lesser volume of water (29.3 l/kg TS) was found to be significant in removing the organic matter from waste bed in preparation for methanogenesis stage. The leachate percolation during methanogenesis stage demonstrated an enhanced biomethanization when compared to the reactors without leachate percolation. The methane yield observed during the process was 0.26 m3 kg−1 VS added with 75 % process efficiency obtained during the process. Hartmann and Ahring (2005) investigated the anaerobic digestion of OFMSW in two thermophilic (55 °C) wet lab scale reactor systems. In the first reactor, the OFMSW was codigested with manure with a subsequent higher concentration of OFMSW at HRT of 14–18 days and OLR of 3.3– 4 kg VS m−3 day−1. In the second reactor, codigestion of OFMSW:manure ratio of 50 % (VS/VS) was maintained as control and also the recirculation of 100 % MSW. Results showed that both the codigestion process and the treatment of 100 % OFMSW with recirculation of process liquid

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demonstrated stable operation in spite of fluctuations in the feed volume. They reported a biogas yield in the range of 0.63–0.71 m3 kg−1 VS in both configurations, with VS reduction up to 74 % when treating the 100 % OFMSW. Linke et al. (2006) digested the potato processing waste anaerobically for biogas production in a CSTR at thermophillic condition. They found out that with an increase of the OLR in the range of 0.8–3.4 kg VS m−3 day−1, the biogas yield decreases. In addition, biogas yields with its methane composition were obtained to be 0.85 to 0.65 m3 kg−1 VS and 58 to 50 %, respectively. They concluded that special importance should be positioned on the reactor performance at steady state and at OLR that does not result in reactor failure. Glass et al. (2005) operated a CSTR and an anaerobic filter (AF) for biogas production from steam-treated MSW wastewater. The study reported the CSTR production to be between 0.02 and 0.29 m3 CH4 kg−1 day−1, while the AF production ranged from 0.04 to 0.47 m3 CH4 kg−1 day−1. They concluded that the difference in the performance of both systems is because the CSTR received wastewater containing suspended solids, while the AF received wastewater free from the suspended solids. Sosnowski et al. (2003) studied the anaerobic codigestion of sewage sludge and OFMSW in a laboratory scale two-phase anaerobic system operated in a quasicontinuous mode, with a CSTR as the acidogenic reactor operated at 56 °C and a UASB reactor as the methanogenic reactor operated at 36 °C. They reported a higher specific methane yield as 0.024 m3 kg−1 VS added. Also, Fongsatitkul et al. (2010) operated a laboratory scale, two-phase, mesophilic anaerobic treatment to determine the digestibility of OFMSW through codigestion with varying amounts of return activated sludge (RAS). They found out that increasing the amount of RAS mixed with OFMSW did not enhance its digestibility, as showed by a decrease in the %VS removal and specific biogas production. They concluded that the optimum ratio appeared at 100 % OFMSW (8 % TS) with specific biogas production of 0.73 m3 kg−1 VS and a VS removal of about 87 % at an HRT of 28 days. According to Bouallagui et al. (2003), the maximum OLR for the mesophilic anaerobic digestion of fruit and vegetable waste (FVW) in a tubular reactor is 6 % of TS with a high yield of 0.707 m3 biogas kg−1 VS added for a 20-day HRT. These results compared well with that reported by Hartmann and Ahring (2005). Thus, it can be deduced that the tubular reactor behaves in a similar way to a CSTR with high stability and process economy. Further, in a similar study, Bouallagui et al. (2004) made a comparison between the performances of the tubular anaerobic digesters operated under thermophilic conditions and those under psychrophilic and mesophilic conditions. At an OLR of 6 % TS (4, 6, 8, and 10 % on dry weight), the yield in the psychrophilic and mesophilic digesters was almost the same

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(Table 1), while the performance of the thermophilic digester was higher than that of the mesophilic and psychrophilic digesters. Also, the highest biogas was obtained in the thermophilic digester at a HRT of 10 days. Hence, these results established a phenomenon for OLR selection with the capability of upgrading the existing mesophilic digesters to the thermophilic range, thereby improving the performance of the digesters. Zhang et al. (2007) conducted a batch anaerobic digestion test to investigate the biodegradability of FW at an HRT of 10 and 28 days. In the study, the highest methane yield of 0.435 m3 kg−1 VS was obtained at the end of the 28-days digestion with VS removal of 81 %, which is followed by 0.348 m3 kg−1 VS at the end of 10-day digestion. These results indicated that FW was a good alternative substrate for anaerobic digestion because of its high degradability and biogas yield. In another study, Forster Carneiro et al. (2008b) experimentally study the biomethanization procedure of FW in six reactors with three different total solid percentages (20 %, 25 %, and 30 % TS) and two different inoculum percentages (20–30 % of mesophilic sludge). The study was designed to select the initial performance parameters (total solid and inoculum contents) in a lab-scale reactor and later, to validate the optimal parameters in a lab-scale batch reactor. The best performance for FW treatment and the methane generation was the reactor with 20 % TS and 30 % of inoculum. They observed a methane yield of 0.49 m3 kg−1 VS added between 20 and 60 days during this operation. In addition, the lab-scale batch reactor shows a classical waste removal with high value of methane yield of 0.22 m3 kg−1 VS added. Finally, they proposed a protocol to improve the start-up phase for dry thermophilic anaerobic digestion of FW. The study of Bouallagui et al. (2009) observed a maximum OLR of 1.24 kg VS m−3 day−1 operating a thermophilic anaerobic sequencing batch reactor (ASBR) treating FVW with a 15-day HRT. They achieved a high biogas yield of 0.48 m3 kg−1 VS added with 60 % methane content and a 79 % VS reduction. In a similar study, Bouallagui et al. (2009) examined the effect of HRT and temperature variations under mesophilic and thermophilic conditions on the performance of an ASBR treating abattoir waste water with FVW. They observed a volatile solid removal between 73 and 86 % and a biogas yield of about 0.3–0.73 m3 kg−1 TVS added at OLRs up to 2.56 kg TVS m−3 day−1 in the ASBR codigestion process. The results of the digesters' performances showed that the variation in HRT under mesophilic conditions had no significant influence on the organic matter removal. However, the biogas yield at 20 days of HRT was significantly increased by increasing the temperature from 35 °C to 55 °C. At 55 °C, the HRT of 10 days resulted in overloading and subsequent failure of the digestion of the AW and the codigestion process. This could possibly be due to the outlet pH and alkalinity


values observed to increase with the increases of the OLR and temperature, which fail the biogas production. Alvarez and Liden (2008) experimentally investigated the potential of a semicontinuous mesophilic anaerobic treatment of solid slaughterhouse waste, fruit–vegetable wastes, and manure in a codigestion process. Anaerobic codigestion resulted in methane yields of 0.3 m3 kg−1 VS added, with methane content between 54 and 56 % at OLRs in the range 0.3–1.3 kg VS m3 day−1. However, the biogas production was observed to decrease with an increase in organic loading and subsequent reduction in the methane yield indicating organic overload or insufficient buffering capacity. They concluded that a combined treatment of various waste types like manure (cattle and swine), solid slaughterhouse wastes (rumen, paunch content, and blood from cattle and swine), and FVW in a mesophilic codigestion process establishes the possibility of treating the waste, which cannot be successfully treated separately. Schober et al. (1999) examined the influence of one-stage and a two-stage continuous bioreactor treating kitchen refuse under mesophilic and thermophilic conditions. The result of the treatment of this kitchen refuse, containing a high amount of easily biodegradable organics, especially lipids, led to a similar yield of biogas of 0.83 m3 kg−1 VS added at thermophilic temperatures and 0.80 m3 kg−1 VS added at mesophilic temperatures. On the other hand, a total VS reduction of 91 % was achieved with a 7-day HRT in a two-stage plant using a concentration unit. They concluded from main the result that the digestion of MSW should be carried out in a two-stage system with a concentration unit between the two stages. By achieving these optimized process conditions, a turnover of the organic matter of 90 % with low retention time could be accomplished. Parawira et al. (2006) studied the performance of two types of bioreactors comparatively treating potato waste leachate. They reported that the upflow anaerobic sludge blanket reactor (UASB) demonstrated a stable process with 66 % methane content in biogas and a high OLR. This proved superior to the anaerobic packed-bed reactor (APB) with 59 % methane content in the biogas at a lower OLR. The authors found out that both reactors gave a comparably high performance when treating organic matter. Angelidaki et al. (2006) evaluated how different operational strategies could minimize the process inhibition due to ammonia accumulation during the anaerobic digestion of Source-Sorted Organic Fraction of Municipal Solid Waste (SS-OFMSW) in a thermophilic CSTR. At an OLR of 11.4 kg VS m−3 day and a 15-day HRT, a stable performance was observed when SS-OFMSW was diluted with fresh water and when process water was recirculated after ammonia stripping. Methane (CH4) yields from both treatments were 0.4 and 0.43 m3 CH4 kg−1 VS added, respectively. These results were in accord with the yield reported


by Hartmann and Ahring (2005) during wet digestion of SSOFMSW at 55 °C with effluent recirculation. Davidsson et al. (2007) conducted a study to investigate the methane yields from the thermophilic pilot-scale digestion of 17 types of domestically SS-OFMSW from households. They reported a VS reduction of about 80 % and a methane yield of 0.3–0.4 Nm3 CH4 kg−1 VS added with a 15-day HRT, corresponding to about 70 % of the potential measured during 50-day batch digestion. They concluded that a sorting and collection system does not significantly affect the amount of methane produced. Forter-Carneiro et al. (2008a) evaluated the performance of two laboratory-scale reactors treating OFMSW, SS-OFMSW, and mechanically selected OFMSW (MS-OFMSW). In the study, discontinuous reactors operated at thermophilic temperature and dry digestion (20 % TS) was used. The reactor treating the SS-OFMSW showed a fast start up beginning within 0–5 days and 20– 30 days with a subsequent stabilization phase. A VS reduction of 45 % with a cumulative biogas of 0.120 m3 was reported in 60 days. Moreover, the MS-OFMSW treatment indicated a methanogenic phase during the whole period of experiment (60 days). They obtained a higher VS removal of 56 % and cumulative biogas of 0.082 m3. These results indicated that both digestion treatments were accomplished and a high level of cumulative methane production was achieved in less than 60 days. A study was conducted by Maroun and EL Fadel (2007) in a CSTR to investigate the mesophilic anaerobic digestion of SS-OFMSW. They reported a biogas yield ranging between 0.2 and 0.56 m3 kg−1 VS at an OLR of 2.03 kg VS m−3 day−1. More so, Kim et al. (2006) investigated the digestibility of FW in a three-stage semicontinuous system at thermophillic condition. They reported 67 % methane content in the biogas at an HRT of 12.4 days. ForsterCarneiro et al. (2008c) conducted a batch experiment to determine the digestibility of shredded organic fraction of the municipals solid waste (SH-OFMSW) and FW separately. Their results indicated that SH-OFMSW is a viable substrate for anaerobic digestion with a high VS removal of 74 % and methane yield of 0.05 m3 kg−1 VS added, while the FW demonstrates the smallest VS removal (32 %) and high methane yield of 0.18 m3 kg−1 VS added. On the other hand, the OFMSW indicated the highest VS removal (79.5 %) and achieved a methane yield of 0.08 m3 kg−1 VS added. Therefore, they concluded that the nature of organic substrate has a significant effect on the biodegradation process and methane yield. Also, pretreatment of waste is not necessary for OFMSW. In another study, ForsterCarneiro et al. (2007) evaluated the effect of different inoculum sources on the performance of laboratory scale reactors treating separately collected organic fraction of municipal solid waste (SC-OFMSW). The experimental conditions selected were: 25 % of inoculums and 30 % of

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total solids operated under single phase thermophilic (55 °C) temperature. Results showed that the highest methane yield was obtained for sludge inoculums reactor, followed by swine/sludge inoculums reactor, and swine inoculums reactor, with values of 0.29, 0.27, and 0.18 m3 kg−1 VS added, respectively. However, a lower yield of methane was obtained for the reactor treating restaurant waste digested with rice hulls, followed by corn silage, and then cattle waste with values of 0.11, 0.22, and 0.03 m3 kg−1 VS added, respectively. They concluded that sludge is the best inoculum for the thermophilic anaerobic digestion of SC-OFMSW at dry condition (30%TS) achieving 44 % and 43 % COD and VS removal, respectively. Sharma et al. (2000) investigated the applicability of a plug flow reactor (PFR) to develop an effective treatment of semisolid waste mixed with sewage sludge. Operating under the mesophilic conditions at 33.7 days HRT, 71 % of VS destruction was reported with a methane yield of 0.7 m3 kg−1 VS added which was equivalent to 1.05 m3 biogas kg −1 VS added. However, at a short HRT of 22.5 days, the methane yield did not show any decrease; thus, it can be operated with a short HRT, thereby reducing reactor volume and cost. Consequently, Lastella et al. (2002) investigated the effect of using different bacteria inoculums on the anaerobic treatment of semisolid organic waste available from the ortho fruit market in a PFR under mesophilic conditions. They obtained 68 % methane content in the biogas and a VS reduction of 72 %. These results were similar to that reported by Sharma et al. (2000) during the treatment of semisolid waste at 37 °C. In another study, Bolzonella et al. (2006) investigated the biogas yield and process performance of two full-scale reactors for the treatment of differently sorted municipal organic wastes. They reported methane yield of 0.4 m3 kg−1 VS added for the reactor treating SS-OFMSW operated under a temperature of 36.7 °C at HRT of 40–60 days. While the reactor treating mixed materials consisting of gray wastes, mechanically sorted organic fraction of municipal solid waste (MS-OFMSW), and sludge showed a methane yield of 0.13 m3 kg−1 VS added maintained at a temperature of 38.6 °C at HRT of 50–70 days. They concluded that the strategy of waste collection affects the characteristics of the organic waste, hence, affecting the reactor yield even under similar operational conditions. Zupancic et al. (2008) investigated the applicability a full-scale experiment on the codigestion of organic waste from domestic refuse and municipal sludge. In the experiment, the OLR of the digester treating municipal sludge at HRT of 20 days was raised by 25 % to 1 kg VS m−3 day−1 by the addition of organic waste. It was observed from the experiment that the biogas production increases by 80 % with an increased yield from 0.39 m3 kg−1 VS added before the experiment to 0.6 m3 kg−1 VS added, peaking at 0.89 m3 kg−1 VS added. They concluded that the experiment achieved high degradation efficiency with virtually

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all the organic waste degraded. They recommend the application of such practice as it will assist in handling organic waste in the future. Anaerobic digestions of organic solid wastes studied have shown to be a renewable energy source that can generate biogas with high methane content. Most of the studies on the anaerobic digestion of solid wastes were conducted on different types of anaerobic reactors at various ranges of operating parameters such as temperature, OLR, and HRT. The effect of these parameters on process performance is very important. The information originating from the literature can be used to draw the following conclusions: –

–

Anaerobic digestion of municipal garbage showed a high performance in the CSTR and two-stage bioreactors than batch and ASBR with a methane yield in the range of 0.1–0.7 m3 kg−1 VS added and a VS destruction >80 % with HRT ranging from 7 to 25 days. Conventional batch, single-stage, and two-stage anaerobic digestion processes have been employed to produce biogas from different solid types of substrates such as municipal solid waste, FVW, FW, etc. The two-phase systems have shown good stability and optimum biogas production. Therefore, more attention should be directed towards the utilization of a two-phase system for optimum bioenergy recovery. However, the operation of the single-phase in the treatment of solid wastes to biogas in the rural areas is another alternative for renewable energy production, especially for developing countries as well as for the developed countries.

Acknowledgments The authors would like to thank Universiti Putra Malaysia for the financial assistance and facilities, and the National Feedlot Corporation for support.

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