methane production in low-cost

Methane production in low-cost, unheated, plug-ow digesters treatingswine manure and used cooking grease

Stephanie Lansing a,*, Jay F. Martin b, Ral Botero Botero c, Tatiana Nogueira da Silva c, Ederson Dias da Silva c

aDepartment of Environmental Science and Technology, University of Maryland, 1445 Animal Sci./Ag. Eng. Bldg., College Park, MD 20742-2315, United StatesbDepartment of Food, Agricultural, and Biological Engineering, The Ohio State University, 590 Woody Hayes Drive, Columbus, OH 43210-1057, United StatescEARTH University, Apartado Postal 4442-1000, San Jose, Costa Rica

a r t i c l e i n f o

Article history:Received 17 November 2008Received in revised form 19 January 2010Accepted 19 January 2010Available online 11 February 2010

Keywords:Anaerobic digestionBiogasRenewable energyCo-digestionWaste

a b s t r a c t

A co-digestion investigation was conducted using small-scale digesters in Costa Rica to optimize theirability to treat animal wastewater and produce renewable energy. Increases in methane production werequantied when swine manure was co-digested with used cooking grease in plug-ow digesters thatoperated at ambient temperate without mixing. The co-digestion experiments were conducted on 12eld-scale digesters (250 L each) using three replications of four treatment groups: the control (T0),which contained only swine manure and no waste oil, and T2.5, T5, and T10, which contained 2.5%,5%, and 10% used cooking grease (by volume) combined with swine manure.The T2.5 treatment had the greatest methane (CH4) production (45 L/day), a 124% increase from the

control, with a total biogas production of 67.3 L/day and 66.9% CH4 in the produced biogas. Increasingthe grease concentration beyond T2.5 produced biogas with a lower percentage of CH4, and thus, didnot result in any additional benets. A batch study showed that methane production could be sustainedfor three months in digesters that co-digested swine manure and used cooking grease without dailyinputs. The investigation proved that adding small amounts of grease to the inuent is a simple wayto double energy production without affecting other digester benets.

2010 Elsevier Ltd. All rights reserved.

1. Introduction

Creating renewable energy from waste products through anaer-obic digestion results in numerous advantages, including capturingand utilizing methane, a greenhouse gas 21 times more powerfulthan carbon dioxide, decreasing organic loading on receivingwaters, and creation of a high-nutrient, low-solid fertilizer (Archerand Kirsop, 1990). Previous digestion research has focused onindustrialized systems, but with an average cost of over $1.0 mil-lion, these systems are inaccessible to medium and small-scalefarmers (USEPA, 2006, 2009a). Digesters are concentrated in thedeveloping world, with over forty million low-cost digesters in In-dia and China alone, but there has been a paucity of research onoptimizing these low-cost systems for methane production.

1.1. Low-cost digestion systems

The low-cost digesters used in this study are Taiwanese-model,which are plug-ow systems constructed with tubular polyethye-

lene or geomembrane and are not heated, mixed, or contain anymechanical control mechanisms (Botero and Preston, 1987; Charet al., 1999). The systems operate in the lower portion of the mes-ophilic range (2030 C) and have a retention time of 2050 days(Botero and Preston, 1987; Lansing et al., 2008). Studies haveshown that low-cost Taiwanese-model digesters produce high-quality biogas, reduce organic loadings, and create a usable fertil-izer (Thy et al., 2003; Lansing et al., 2008). The produced biogasfrom these digesters have previously been used directly as a heatsource, eliminating the need to buy propane or rewood for cook-ing (Char et al., 1999; Lansing et al., 2008), for electric generation(Lansing et al., in press), and could also be used in boilers, heattransfer pipes, or in refrigeration systems.

During digestion, pathogens are greatly reduced (Archer andKirsop, 1990), organic matter is reduced by 5090%, and a moreeffective fertilizer is created as microorganisms transform the or-ganic pollutants into dissolved nutrients (Thy et al., 2003; Lansinget al., 2008). The entire digestion system can be installed for as lit-tle as $300 and has an estimated life of 10 years (Botero and Pres-ton, 1987). The low-cost of these digesters and the value-addedproducts they produce result in an increase in household incomeand a decrease in water pollution and deforestation (Char et al.,1999).

0960-8524/$ - see front matter 2010 Elsevier Ltd. All rights reserved.doi:10.1016/j.biortech.2010.01.100

* Corresponding author. Tel.: +1 301 405 1197; fax: +1 301 314 9023.E-mail address: [email protected] (S. Lansing).

Bioresource Technology 101 (2010) 43624370

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1.2. Co-digestion

Digesting materials with a high-fat content is expected to in-crease methane yields due to the more negative oxidation stateof the carbon in fats compared to proteins, carbohydrates, and urea(Jerger and Tsao, 1987). Conversely, lipids are known to be difcultto degrade, which can result in a reduction in pH in the digestionenvironment, especially if the slower growing methanogens can-not utilize the organic acids at the production rate of acetogenicbacteria (Cirne et al., 2007). Previous studies have shown thatdigesting materials with high lipid content increases methaneyield (Cirne et al., 2007), digester efciency (Jeyaseelan and Mat-suo, 1995), and is more efcient than digesting manure alone(Ghaly, 1996). Despite this apparent advantage, previous studiesthat digested lipid-rich materials without co-digestion found thatchemical inputs were necessary to prevent the system frombecoming overly acidic (Ugoji, 1997).

Co-digestion is a waste treatment method where different typesof wastes are treated together (Angelidaki and Ahring, 1997). Co-digestion of wastewater with carbon-rich food wastes, such asgrease, has been used in industry due to its positive effect on bio-gas production (Zitomer and Adhikari, 2005), but the mixture isusually a function of availability and not based on knowledge ofan optimal mixture (Gavala et al., 1996; Kbler et al., 2000). In gen-eral, anaerobic digestion of wastewater and food wastes has notbeen widely practiced in the United States until recently (USEPA,2009b) due to lack of experience and information regarding imple-mentation, correct operating conditions, and cost (Zitomer andAdhikari, 2005).

It has been established that manure is the best co-digestionmaterial for high-fat content wastes due to the high alkalinity ofmanure, which increases digester resistance to acidication(Angelidaki and Ahring, 1997; Murto et al., 2004; Gelegenis et al.,2007). Additionally, manure has high ammonium levels, whichare important in bacterial growth. Mladenovska et al. (2003) foundthat co-digesting manure with materials containing 2% fat im-proved digestion efciency without an increase in acidity. Co-digesting olive oil mill wastewaters and sh oil (5%) with manuredoubled the production of biogas due to the higher concentrationof lipids and the higher biodegradability of the oil/grease contain-ing wastewaters compared to manure (Angelidaki and Ahring,1997). Additionally, Gelegenis et al. (2007) concluded that co-digestion of olive mill wastewaters with diluted poultry manureincreased methane production by 150% without chemical addition.

All of these previous co-digestion studies were conducted onheated, mixed lab-scale systems of less than 5 L (Jeyaseelan andMatsuo, 1995; Mladenovska et al., 2003; Spajic et al., 2009) orsmall, un-replicated pilot-scale systems that are, thus far, inappro-priate for technology transfers to small-scale farmers (Angelidakiand Ahring, 1997; Gelegenis et al., 2007). Additionally, there havebeen co-digestion studies that have showed that the lab-scaledynamics can be quite different in eld-scale applications (Davids-son et al., 2008). All of these systems are indicative of what Santanaand Pound (1980) and Char et al. (1999) found in their literaturereviews: digestion research has focused on highly specialized sys-tems that are expensive and energy intensive to build and main-tain. These systems are largely inaccessible to small-scale farmers.

1.3. Study objectives

In order to increase the use of low-cost digestion systems, thesystems must be optimized for methane production to increase en-ergy availability and protability. Low-cost digesters are optimalfor co-digestion due to the dispersed nature of small-scale wasteproduction (Gavala et al., 1996). In addition, a digester will createa safe and protable method for disposal of household grease,

whey, and restaurant waste, which will extend the life of septictanks, the only waste system available in most rural areas. Dueto the low volume of wastewater being treated in small-scale sys-tems, knowing correct proportions is extremely important.

This study investigated co-digesting swine manure and usedcooking grease in 12 eld-scale digesters with three replicationsof each mixture in a nine-month study. Additionally, a methaneproduction batch study was conducted after the nine-month studyto determine if manure loading could be interrupted in co-diges-tion systems without an additional stabilization period. By utiliz-ing simple systems that treat ample waste sources, this studyseeks to determine the optimal ratio of grease and manure forincreasing methane production in existing systems and addingeconomic incentives for further dissemination of low-cost diges-tion systems. Additionally, the results of this study will help deter-mine if co-digestion is feasible in non-mixing conditions due to theimmiscible nature of wastewater and oil.

2. Methods

2.1. Site description

The outdoor digester laboratory was built at the dairy andswine farm at the Escuela de Agricultura de la Regin TropicalHmeda (EARTH University) in Costa Rica. EARTH University is lo-cated in the humid tropics (10110 N, 83400 E) at an elevation of50 m and average temperatures ranging from 24 C in January to26 C in May. The annual precipitation is 34 m, distributedthroughout the calendar year. The laboratory is housed in anopen-air, roofed building, with the digesters operating at ambienttemperatures.

Twelve eld-scale Taiwanese-model digesters were constructedin the digester laboratory using 250 L tubular polyethylene bags.Each tubular bag had a diameter of 0.32 m, and length of 3.1 m,and the polyethylene material had a thickness of 0.2 mm (Fig. 1).The digesters had a liquid capacity of 200 L, with up to 50 L avail-able for in-vessel biogas storage. The majority of the produced bio-gas owed by pressure to a 250 L biogas storage bag located aboveeach digester.

In the co-digestion experiment, there were three replications offour treatment groups: the control (T0), which contained onlyswine manure and no waste grease, and T2.5, T5, and T10, whichcontained 2.5%, 5%, and 10% used cooking grease (by volume) com-bined with swine manure. The digesters were fed daily with 5 L ofthe prescribed mixture. The grease and manure treatments were

Fig. 1. Pictured is the outdoor digester laboratory at the Escuela de Agricultura de laRegin Tropical Hmeda (EARTH) University in Costa Rica. The laboratory is anopen-air building with a zinc roof, which houses 12 eld-scale Taiwanese-modeldigesters.

S. Lansing et al. / Bioresource Technology 101 (2010) 43624370 4363

manually measured, mixed in a bucket, and added to the digesterseach morning. Each digester had a 40-day retention time. Beforebeginning the experiment, each digester was fed with only manureto ensure that all digesters had an active microbial community. Themethane production stabilized after 50 days and the nine-monthexperiment (May 2007February 2008) commenced.

The swine manure was collected daily from the EARTH Univer-sity 50-pig farm. The 50-kg pigs were fed a diet of organic wastefrom the campus cafeteria, sugar cane, whey, oating aquatic veg-etation, and protein feed. The manure was diluted 4:1 with washwater in accordance with the wash water use at the swine farm.The used cooking grease was collected monthly from the EARTHUniversity cafeteria and stored in a 250 L drum.

The water quality characteristics of the manure and used cook-ing grease are listed in Table 1. The increase in volatile solids (VS)in the T2.5, T5, and T10 treatments from the control (T0) were113%, 206%, and 453%, respectively, due to the high VS of thegrease compared to that of the manure.

2.2. Biogas analysis

Biogas production was measured three times a week using 12American Meter Company gas ow meters (model AC-250) withIMAC Systems pulse digital counters and a vacuum pump. Biogasquality was analyzed weekly using an IR-30M methane (CH4) me-ter and a Z-900 hydrogen sulde meter (H2S) (Environmental Sen-sors). Methane production data was calculated by multiplying thebiogas production rate by the percentage of CH4 in the producedbiogas.

Biogas production and CH4 data from the beginning of the nine-month study were not used in the analysis due to metering prob-lems associated with the low-pressure of the produced biogas. Theproblemwas rectied, and reliable data were collected for percent-age of CH4 in the produced biogas for the nal seven months (July2007February 2008) and total biogas production for the nal vemonths (September 2007February 2008) of the experiment.

2.3. Methane production batch study

Amodication of the biochemical methane potential (BMP) wasconducted at the end of the nine-month experiment. In a BMP test,a batch sample is placed in an anaerobic environment and CH4 pro-duction is monitored over a period of 56 days or more to estimatebiomass conversion (Owens and Chynoweth, 1993). BMP test arenormally conducted in a laboratory setting using replicated serumbottles.

This methane production batch study was conducted in situusing the existing digesters. The last manure/grease mixtures wereadded on January 30, 2008 (day 0). After this addition, biogas pro-duction rates and biogas quality were measured for 75 days with-out further addition of waste materials. The test was conductedusing the existing digesters with their existing concentration ofmanure and grease that accumulated within the digesters overthe nine-month experiment. Only biogas production and CH4 weremonitored, as the digesters could not be opened to obtain organicmatter samples prior to the experimental period. The digesterswere opened after the 75-day experiment, and the contents were

analyzed for pH, chemical oxygen demand (COD), and volatile sol-ids (VS) using standard methods (APHA, 1998).

2.4. Statistical analysis

Biogas and water data were analyzed using analysis of variance(ANOVA) and TukeyKramer multiple comparisons to determinewhich variables were signicantly different. p-Values

The average ambient temperature was 25.8 C in MayJuly, 26.0 Cin AugustOctober, and 24.7 C in NovemberJanuary.


3.2.1. Biogas quantity and qualityResults from the 75-day batch study showed that T10 had the

highest biogas production rate (66.2 L/day), followed by T5(52.7 L/day), T2.5 (46.5 L/day), and T0 (12.7 L/day) (p < 0.001;F = 53.4) (Table 4). The percentage of CH4 in the produced biogasin T2.5 (69.8%) was signicantly greater than T5 and T10(p = 0.003; F = 5.3), and the H2S concentration of T0 (289 ppm)was signicantly greater than all grease treatments (p < 0.001,F = 7.7) (Table 4).

3.2.2. Biogas differences over timeThe 75-day study was divided into three parts to identify differ-

ences over time (Fig. 3, Table 5). In T0, biogas production in the rst25 days were signicantly greater than days 2550, which were

signicantly greater than days 5075 (p < 0.001, F = 56.2). In T2.5and T10, the rst 25 days were signicantly greater than days2575 (p < 0.001, F = 15.6; and p < 0.001, F = 13.9, respectively).There were no signicant differences in biogas production in T5(p = 0.45, F = 0.84). The average ambient temperature was 24.5 Cin days 025, 25.0 C in days 2550, and 25.5 C in days 5075.

There were no signicant differences in the percentage of CH4 inthe produced biogas over time for T0 and T5 (p = 0.84, F = 0.18; andp = 0.22, F = 1.7, respectively). There were differences over time inCH4 for T2.5 and T10, with days 2550 (70.9% and 70.0%, respec-tively) having signicantly greater percentage of CH4 in the pro-duced biogas than days 025 (69.2% and 65.6%, respectively)(p = 0.04, F = 3.9; and p = 0.001, F = 10.1, respectively).

There were no signicant differences in H2S concentration inthe produced biogas over time for T0 and T5 (p = 0.13, F = 3.4;and p = 0.13, F = 2.4, respectively). There were differences in theH2S concentration in T2.5 and T10, with the rst 25 days (175and 148 ppm, respectively) having a signicantly greater H2S con-centration than days 2550 (38.6 and 48.1 ppm, respectively) and

Table 2Biogas quantity (biogas production), percentage of methane (CH4), hydrogen sulde (H2S) concentration, total CH4 production, and specic CH4 yield are given for the control (T0)and each treatment (T2.5, T5, and T10) from a nine-month experiment on co-digesting swine manure and used cooking grease. All values are averages with standard errors (n).Letters represent signicant differences within each column from the TukeyKramer analysis of difference.

Treatment Biogas production (L/day) CH4 (%) H2S (ppm) CH4 production (L/day) Specic CH4 yield (m3/kg VS/day)

T0 28.8 0.7 (93)A 69.9 0.2 (57)A 356 28 (36)A 20.1 0.29T2.5 67.3 1.5 (117)B 66.9 0.2 (58)B 272 15 (36)B 45.0 0.31T5 57.8 1.4 (117)C 65.9 0.2 (65)C 189 12 (47)C 38.1 0.18T10 69.7 1.8 (114)BD 63.2 0.1 (65)D 178 8.8 (55)C 44.1 0.12

Fig. 2. Methane production rates (L/day) of the four treatment groups in the co-digestion study from September to January. The treatment groups are T0 (0% ), T2.5 (2.5%j),T5 (5% N), and T10 (10% d).

Table 3Biogas production by month (September through January) from the swine manure and used cooking grease co-digestion experiment are given. All values are averages withstandard errors (n). Letters represent signicant differences within each column from the TukeyKramer analysis of difference.

Treatment Biogas production (L/day)

September October November December January

T0 32.1 1.7 (14)A 26.3 1.2 (19)A 28.4 1.0 (21)A 26.8 1.3 (19)A 31.3 2.3 (20)A

T2.5 61.1 1.4 (21)AB 57.6 1.8 (25)B 64.8 2.8 (24)AB 70.3 3.7 (18)AC 80.4 3.9 (29)C

T5 54.7 1.7 (21)A 49.9 1.9 (25)A 50.0 1.7 (24)A 55.9 2.4 (18)A 74.4 3.2 (29)B

T10 63.1 1.6 (20)A 54.5 1.7 (23)B 63.9 2.1 (24)A 68.7 2.3 (18)A 91.7 3.9 (29)C

Table 4Biogas quantity (biogas production), percentage of methane (CH4), hydrogen sulde (H2S) concentration, and total CH4 production are given for the control (T0) and eachtreatment group (T2.5, T5, and T10) from a 75-day CH4 production batch experiment using swine manure and used cooking grease. All values are averages with standard errors(n). Letters represent signicant differences within each column from the TukeyKramer analysis of difference.

Treatment Biogas production (L/day) CH4 (%) CH4 production (L/day) H2S (ppm)

T0 12.7 3.2 (11)A 69.2 1.0 (8)AB 8.8 289 66 (8)A

T2.5 46.5 1.6 (36)B 69.8 0.3 (22)B 32.5 112 22 (20)B

T5 52.7 1.7 (21)BC 67.6 0.6 (22)A 35.6 98 21 (17)B

T10 66.2 1.8 (23)C 67.2 0.6 (20)A 44.5 106 14 (20)B


days 5075 (22.8 and 32.4 ppm, respectively) (p = 0.002, F = 9.0;and p < 0.001, F = 17.6).

3.3. Wastewater characterization in the digestion environment

After the 75-day study was completed, samples were collectedfrom within the digesters and analyzed for pH and organic loading(Table 6). The pH was signicantly different in the treatments,decreasing from T0 (7.2), to T2.5 (7.0), T5 (6.8), and T10 (5.2)(p > 0.005, F = 1340). Additionally, there were signicant higherconcentrations of COD and VS concentration inside the digestionenvironment in T10 than all other treatments (Table 6)(p = 0.014, F = 6.73; and p = 0.005, F = 9.68, respectively).

Accumulation of VS and COD within the digestion environmentafter the nine-month study and the 75-day BMP modied studywas evaluated. The digester VS concentration for T0, T2.5, T5,and T10 was 20.1, 26.6, 80.7, and 563 g/L, respectively, and theCOD was 35.7, 46.1, 29.2, and 187 g/L, respectively. In plug-owsystems, solids have a longer retention time than liquid, and there-fore, accumulation of VS and COD is expected. The concentration ofVS and COD was 46.7% and 78.5% greater, respectively, in T0 afterthe study was completed than the concentration added in the dailyinuent (Table 6). T2.5 had 14.8% less VS concentration and T5 had27.9% less COD in the digester environment compared to the inu-ent concentration. T10 had increases of 88.8% and 572% in the COD

and VS, respectively, in the digestion environment when comparedto the daily inuent concentrations.

The average inuent VS concentration in T0, T2.5, T5, and T10were 13.7, 29.2, 41.9, and 75.8 g/L, respectively, and the averageefuent concentrations was 1.30, 1.33, 1.28, and 1.54 g/L, respec-tively, resulting in percent reductions in VS during digestion of90.5%, 95.4%, 97.0%, and 98.0%, respectively. The average inuentCOD concentration in T0, T2.5, T5, and T10 were 20.0, 37.8, 44.9,and 56.5 g/L, and the average efuent concentrations were 1.82,1.96, 1.96, and 2.62 g/L, resulting in percent reductions in COD dur-ing digestion of 90.9%, 94.8%, 95.6%, and 95.4%, respectively.

4. Discussion

Co-digesting used cooking grease with swine manure increasedtotal CH4 production by 124%, with smaller amounts of grease(T2.5) having higher methane production than larger grease addi-tions (Table 2). These results imply that co-digestion with manureand used cooking grease is benecial in low-cost digestion systemsthat are not heated and do not have mixing components. Methaneproduction more than doubled with a small volume of used cook-ing grease (T2.5), which corresponds to a similarly large increase ininuent VS (113%). No additional benets were seen by increasingthe amount of grease added above 2.5% due to the lower percent-age of CH4 in the produced biogas (Table 2).

Fig. 3. Cumulative methane production during the 75-day methane production batch study illustrating the total amount of methane produced without daily additions ofmanure and grease treatments. The treatment groups are T0 (0% ), T2.5 (2.5% j), T5 (5% N), and T10 (10% d).

Table 5Methane (CH4) production values are given by days (025, 2550, and 5075) for a CH4 production co-digestion batch experiment of used cooking grease and swine manure. Thepercentage of the total methane production (MP) in the 75-day experiment for each timeframe is also given. All values are averages with standard errors (n). Letters representsignicant differences within each treatment group row from the TukeyKramer analysis of difference.

Treatment CH4 (L/day) Percentage of total MP CH4 (L/day) Percentage of total MP CH4 (L/day) Percentage of total MPDays: 025 Days: 2550 Days: 5075

T0 15.3 2.5 (5)A 69.4 4.5 0.2 (3)B 20.3 2.3 0.3 (3)C 10.3T2.5 40.7 2.9 (18)A 45.6 24.1 1.6 (12)B 27.0 24.2 2.6 (6)B 27.2T5 37.3 2.9 (15)A 37.5 30.8 0.6 (4)A 31.0 31.1 1.4 (2)A 31.3T10 51.2 3.2 (14)A 43.2 33.2 2.6 (6)B 28.0 33.3 2.5 (3)B 28.1

Table 6Waste characteristics from inside the digesters of the control (T0) and each treatment (T2.5, T5, and T10). The samples were collected after the nine-month experimental studyand a 75-day batch study. The characteristics given are pH, chemical oxygen demand (COD), and volatile solids (VS). Percent differences between the concentrations inside thedigester bag and the concentrations in the daily inuent input to each treatment group are stated. All values are averages with standard errors (n). Letters represent signicantdifferences within each column from the TukeyKramer analysis of difference.

pH % Diff. COD (g/L) % Diff. VS (g/L) % Diff.

T0 7.2 0.03 (3)A 4.0 ; 35.7 3.1 (3)A 78.5 " 20.1 2.3 (3)A 46.7 "T2.5 7.0 0.03 (3)B 5.8 ; 46.1 16.2 (3)A 15.9 " 26.6 9.6 (3)A 14.8 ;T5 6.8 0.03 (3)C 7.5 ; 42.9 6.8 (3)A 27.9 ; 80.7 65.6 (3)A 65.6 "T10 5.2 0.01 (3)C 27.9 ; 187 79.4 (3)B 88.8 " 563 68.1 (3)B 572 "

4366 S. Lansing et al. / Bioresource Technology 101 (2010) 43624370

The batch study showed that grease treatments sustained sig-nicantly higher CH4 production over time than the control likelydue to higher amount of residual VS in the digestion environment(Fig. 3, Tables 4 and 5). In T10, grease accumulated within the di-gester (Table 6). The study results imply that manure introductioncould be interrupted for two to three months without re-stabiliza-tion when co-digesting grease and manure, but a stabilization per-iod is necessary for manure-only digesters. It was also determinedthat larger grease additions do result in signicant VS accumula-tion in the digestion environment.

4.1. Specic methane yield

Methane production in T2.5 (45.0 L/day) was 124% greater thanthe control, T0 (20.1 L/day). There was a signicantly higher organ-ic loading in the grease treatments than the control. By looking atthe specic CH4 yield, the VS additions can be related to CH4 pro-duction. T2.5 had a specic CH4 yield (0.31 m3 CH4/kg VS/day) thatwas 5% higher than T0 (Table 2). The specic CH4 yields in T5 andT10 were lower than T0 due to their higher organic loadings notproducing corresponding increases in methane production.

This data suggests that co-digestion with substances that have agreater VS content than manure is benecial in a small amount.When using low-cost, plug-ow mesophilic digesters, adding asmall amount of grease addition (2.5% by volume or 113% moreVS) to swine manure is better than no grease additions. Industrialstandards of co-digesting manure with 520% food or greasewastes does not apply in low-cost systems due to decreased spe-cic CH4 yields and accumulation of the greases within the diges-tion environment over time.

The specic CH4 yields in this study were comparable to otherco-digestion studies (Table 7). The other published studies wereconducted indoors on bench-scale systems having volumes lessthan 35 L, with temperature ranging from 35 to 37 C. This currentstudy is the only co-digestion study to have operated replicateddigesters with a large volume (250 L) in an outdoor setting in alower temperature range (2226 C). The lack of other co-digestionstudies done in replicated eld digesters is likely due to the costassociated with the high-tech equipment used in these bench-scalestudies, which include heating and mixing components.

In a study by Murto et al. (2004), the highest specic CH4 yieldswere produced by pig manure co-digested with wastes fromslaughterhouses, grease traps, restaurants, and vegetable and fruitprocessing. This study also had the highest retention time of theother published studies (36 days) (Table 7). The specic CH4 yieldobtained from co-digesting olive mill wastewater and poultrymanure (0.32 m3 CH4/kg VS/day) was similar to the CH4 yield ofT2.5 (0.31 m3 CH4/kg VS/day) (Gelegenis et al., 2007). Co-digestingcattle manure and glycerol trioleate resulted in a slightly higher,but comparable specic CH4 yield (0.38 m3 CH4/kg VS/day) to thecurrent study (Mladenovska et al., 2003). The sewage sludge andgrease trap co-digestion study (Davidsson et al., 2008) and the Kor-ean food waste and sewage sludge co-digestion study (Heo et al.,2003) were also similar to T2.5 in specic CH4 yield, even thoughthese other studies operated at a high temperature range withmixing components.

In the current study, the 113% increase in VS concentration fromT0 to T2.5 was greater than other co-digestion studies but the dailyVS loading was less than these studies. When olive mill wastewa-ter (OMW) was co-digested with poultry manure the highest bio-gas production occurred at 25% OMW, but the increase in VSconcentration from the control was only 9.8% (Gelegenis et al.,2007). The COD concentrations of the poultry manure and OMWwere 92.4 and 89.5 g/L, respectively, less than the used cookinggrease alone (811 g/L), but much more than T0 and T2.5 (20.0and 37.8 g/L, respectively) (Table 1).

Similarly, when cattle manure was co-digested with 2% glyceroltrioleate, the resulting increase in VS was 33%, with an organicloading rate was 3 and 4 kg VS/m3 day for the manure and 2% glyc-erol trioleate, respectively (Mladenovska et al., 2003). In theDavidsson et al. (2008) study, co-digesting sewage sludge andgrease sludge in a 70:30 (v/v) ratio resulted in the highest CH4yield. This mixture actually had a slightly lower organic loading(2.4 kg VS/m3 day) than a control of only sewage sludge (2.5 kgVS/m3 day).

In the current study, the organic loading to the digester was0.34 kg VS/m3 day in T0 and 0.78 kg VS/m3 day in T2.5 Taiwan-ese-model digestion systems typically have lower organic loadingrates compared to the other studies because these systems useushed manure in a plug-ow system, while most plug-ow sys-

Table 7Comparisons between co-digestion studies that combine substances containing high concentrations of lipids with manure or sewage sludge. Average values or ranges of valuesacross various treatment groups are given.

Methane yield(m3/kg VS/day)

Methane(%)

Biogas(L/L/day)

Temperature(C)

Reactor size(L)

Retention(days)

Current study: manure and used cooking greaseSwine manure and 2.5% used cooking grease 0.31 67 0.34 2226 250 40

Manure and substances containing high concentrations of lipidsPoultry manure and olive oil mill wastewatera 0.250.32 71.8 0.520.48 35 25 20Cattle manure and glycerol trioleateb 0.38 37 3 15Pig manure, slaughterhouse, grease trap, restaurant, fruit and vegetable

wastec0.68 68 2.6 35 3 36

Sewage sludge and substances containing high concentrations of lipidsSewage sludge and grease trap sludged 0.290.34 6669 35 31 1013Sewage sludge and cheese wheye 6376 0.81.7 37 32 1219Sewage sludge and olive mill efuente 6364 1.42.5 37 32 813Sewage sludge and food wastec 0.4 67 35 0.5 720Sewage sludge and food wastef 0.34 6386 0.261.8 35 3.5 20Sewage sludge and food wasteg 0.310.50 6166 0.541.15 35 2.5 15

a Gelegenis et al. (2007).b Mladenovska et al. (2003).c Murto et al. (2004).d Davidsson et al. (2008).e Carrieri et al. (1993).f Heo et al. (2003).g Zitomer and Adhikari (2005).


tems favored in the United States use scrapped manure systems.The higher loading rates of the other studies led to the higher over-all biogas production rates, while the specic CH4 yields, whichcalculate CH4 production per unit of VS, were more similar be-tween the current study and other published studies.

4.2. Organic matter accumulation

When comparing organic matter loading to CH4 production,only T2.5 had an increase in CH4 production (124%) that was great-er than the increase in inuent COD (89%) and near the increase ininuent VS (113%) when compared to the control. The increases inCH4 production from T0 to T5 (89.2%) and T10 (119%) were signif-icantly lower than the increase in inuent COD (125% and 183%,respectively) and inuent VS (206% and 453%, respectively).

The lower CH4 production rate with the higher organic matterloading in T5 and T10 is likely due to accumulating organic mate-rial within the digestion environment. T2.5 did not accumulate VSand only had a 15.8% increase in the COD concentration inside thedigester environment compared to the inuent. While any accu-mulation might seem unacceptable, it should be noted that inplug-ow systems solids have a longer retention time than liquids(Hobson, 1990). All the digesters had 90% reductions in COD and VSfrom the inuent to the efuent of the digesters, but part of thisreduction can be attributed to accumulation within the digestionenvironment. Not all of the accumulation in the digestion environ-ment can be attributed to grease, as the control had 46.7% and78.5% accumulation of VS and COD, respectively (Table 6).

T2.5 had a greater reduction of organic matter than the control,but due to the higher organic loading of T2.5 the overall accumula-tion of organic matter within the digesters was similar to that ofthe control (Table 6). This data suggests that T2.5 was able to suc-cessfully digest 99% of the additional COD loading that was addedto the digester without signicant amounts of grease accumulatingwithin the digestion environment. Davidsson et al. (2008) foundsimilar ndings with greater overall reductions in VS with co-digestions due to higher initial loadings. T10, on the other hand,had a methane production rate similar to T2.5, but had 160% moreVS in the inuent had 2000% more VS accumulated within thedigestion environment than T2.5, suggesting less efcient diges-tion of the organic matter.

4.3. Temperature effects

The lower efciency of VS breakdown with T10 can be partiallyattributed to the temperature within the digestion environment.The digesters in this study operated in the lower portion of themesophilic range (2226 C). At this temperature, methanogenscan survive, but their substrate utilization rates, and thus, theirCH4 production rates are decreased (Kettunen and Rintala, 1997;McHugh et al., 2006).

One signicant nding in this study was that when the digest-ers were at their coldest temperatures in January (22 C), the CH4production rates were higher in all treatments with grease addi-tions, but not signicantly different in the control group (Table 3).This data suggest that the increase in CH4 production was due toincreased lipid hydrolysis at lower temperatures due to their abil-ity to out-compete other bacteria. This increased production oflong chain fatty acids from the breakdown of the lipids did nothamper the CH4 production rate, but actual enhanced it.

This data suggests that the methanogens were well adjustedand able to keep up with the increased fatty acid production. Meth-ane production efciency appears to be increasing over time. Onereason for the increase in efciency may be the higher quantityof organic material that accumulated in the digesters over the pre-vious eight months. As suggested by McHugh et al. (2006) in their

study of methanogens at low-temperatures, more experiments inanaerobic microbiology need to be conducted on long-term,in situ systems. Maximum CH4 production rates were observedin the grease treatment groups at the end of a nine-month studyand would likely not be replicated in a short-term laboratoryexperiment.

4.4. Biogas quality methane content of the biogas

The percentage of CH4 in the biogas signicantly decreased asthe amount of grease increased (Table 2). Decreasing percentageof CH4 with increasing organic loading rate was also found in co-digestion studies of food waste and activated sludge (Heo et al.,2003; Zhang et al., 2007), industrial food waste and sewage sludge(Murto et al., 2004), olive oil mill wastewater (Boubaker and Ridha,2007), olive mill wastewater and poultry manure (Gelegenis et al.,2007), and cheese whey and sewage sludge (Carrieri et al., 1993).The lower pH of the co-digesting material compared to manurein the current study and the studies cited above likely led to thelower biogas quality. Additionally, when carbon dioxide in the bio-gas is above 30%, which was seen in all of the grease treatments,carbon dioxide can induce the pH to drop even further in the diges-tion environment, as acid fermentation increases (Geradi, 2003). Itshould be noted that even with the addition of grease at a pH of4.6, the pH range of all treatment groups was kept within the opti-mal range for methanogens (6.57.5) due to the high alkalinity ofthe manure. Nevertheless, on account of the lower percentage ofCH4 produced with high grease treatment levels and the lowerpH in the digestion environment, it is recommended that low-costdigestion systems limit grease addition to 2.5% (113% increase inVS).

In this study, H2S was monitored to determine if grease addi-tions would increase harmful substances in the biogas. H2S isharmful to human health when released at concentrations as lowas 10 ppm, with a LC50 (lethal dose concentration) of 800 ppmwith 5 min of H2S exposure. In addition, when exposed to mois-ture, H2S forms H2SO4, a powerful acid that can corrode electricgenerators, broilers and other metal xtures in the system. Duringdigestion excessive H2S production can be toxic to methanogensleading to a lower CH4 yield, decreased organic matter removalefciency, and foul odors (Hulshoff Pol et al., 2001). H2S concentra-tions of biogas can rage from 50 to 5000 ppm. The H2S levels in thisstudy were relatively low (178356 ppm), with the grease treat-ments having the lowest H2S concentrations. The control had high-er H2S and CH4 levels in the produced biogas and lower biogasproduction than the grease treatments. While no statistically sig-nicant correlations were found between CH4 and H2S and ele-mental sulfur levels were not determined for the grease andmanure treatments, the results suggest that the sulfur levels ofthe manure might have been much greater than that of the usedcooking grease, leading to higher H2S concentrations in the biogaswhen manure was the only source of organic material.

4.5. Biogas quantity

The liters of biogas produced per day for T2.5 were higher thanall the other cited studies, but when dividing the production rateby the liters of the reactor, bench-scale systems overwhelmingoutperformed T2.5. When scaling up laboratory-scale studies, bio-gas production can decrease, as was seen in the Davidsson et al.(2008) study, which produced 7590% of the specic methaneyield of 2 L laboratory reactors with 35 L pilot-scale digesters.The higher volume of sludge being processed in large-scale sys-tems likely results in a more heterogenic environment with zonesof reduced microbial activity due to mixing, heating, or attachmentsites differentials.


T2.5 had signicantly higher biogas production rates than T5,but was not signicantly different from T10. One likely explanationfor the relatively higher CH4 production in T2.5 compared to the T5is the location of the digesters within the outdoor laboratory. Thelaboratory has a roof and a 1-m high concrete wall around theparameter, with the top two meters of the perimeter open to theenvironment and, thus, to sunlight. Direct westerly sunlight couldbe seen striking the most westerly digester (T2.5, replication 1)throughout the afternoon, and statistical analysis showed thatthe biogas production from replication 1 was signicantly greaterthan replications 2 and 3, which were located in the center ofthe outdoor laboratory.

The benets seen here by direct sunlight on the digesters sug-gests that low-cost digestion systems would benet from direct so-lar light, but degradation of the polyethylene bags was observedwith digesters in direct sunlight, which is why roong is recom-mended (Botero and Preston, 1987). Modifying Taiwanese-modeldigesters to reap the benets of solar heating, as was done in anEgyptian solar heated digestion study (El-Mashad et al., 2003),might increase the biogas production, but solar heating increasesthe cost of the system and decreases in efciency as the size ofthe digester increases (El-Mashad et al., 2003). This study suggeststhat co-digestion could play an even bigger role in increasing over-all CH4 production, without the costs associated of solar heating.


The objective of the batch study was to determine if constantmaterial inputs are a requirement for successful digestion or ifthe microbial population could be sustained on the existing mate-rial in the digester when daily inputs are interrupted. This objec-tive is especially pertinent in Costa Rica and surrounding LatinAmerican countries due to the cultural practice of raising severalpigs throughout the year to be slaughtered on Christmas Eve andbuying new piglets in the new year to raise for the followingChristmas festivities. This practice results in a period of one tothree months of no or substantially decreased amounts of manureinputs to anaerobic digesters. The CH4 production in T0 from days0 to 25 was 75.9% of the CH4 production seen with daily wasteadditions, but after 25 days, the CH4 production was less than18% of normal production (Table 5). The low CH4 production sug-gests that interrupting additions to manure-only digesters formore than a month will result in signicantly lower CH4 produc-tion and a stabilization period is necessary to restore the methano-genic population.

Methane production in the grease treatment groups decreasedfrom days 0 to 10, but was sustained from days 10 to 75 (Table 5,Fig. 3). Methane production in T2.5 was 90% of normal CH4 produc-tion from days 0 to 25 and 54% from days 25 to 75. Due to the accu-mulated organic matter in T5 and T10, the average CH4 productionin the rst 25 days of the experiment was equal to normal CH4 pro-duction. Methane production from days 25 to 75 averaged 78% ofnormal production. Maintaining high CH4 production levels with-out any additions with the grease treatments suggests that inter-ruption of co-digesting materials for up to three months does notrequire a re-stabilization period, especially with higher greaseloadings (5% and 10%) and that the accumulated grease in thedigesters will be broken down over time when material inputsare interrupted.

Previous batch studies have shown methane production re-duced to near zero after 10 days for mixed food waste with 24 gVS/L (Cho et al., 1995), 15 days for primary sludge (Jerger and Tsao,1987), 20 days for broiler and cattle manure (Gngr-Demirci andDemirer, 2004), 25 days for mixed food waste of 10 g VS/L (Choet al., 1995), and 40 days, including a lag phase, for waste contain-ing 540% lipids (Cirne et al., 2007). Ergder et al. (2001) found

similar results with cheese whey: 5525 mg/L COD ceased produc-tion after 15 days, while 11,050 mg/L COD had production for25 days, and 22,100 mg/L had production for 45 days.

The current study was able to sustain 5078% of normal meth-ane production levels for 75 days with grease treatments. Oneexplanation for the longer methane production time of the currentstudy compared to other batch studies is the lower temperaturerange at which the current study was conducted, which resultedin slower substrate utilization rates. Gngr-Demirci and Demirer(2004) found that CH4 production in mixtures of cattle manure andbroiler waste ceased after 20 days at 35 C, with 300 mL of gas pro-duced, but when operated at ambient temperature, CH4 productionwas still occurring at 30 days when the experiment ended, with100 mL of gas produced.

The slower rate of substrate utilization in low-temperature,low-cost digesters results in sustained methanogen populationsover an extended period of time when substrate loading is inter-rupted. Without co-digestion, CH4 production rates are quickly re-duced when manure inputs cease, likely due to both the loweramount of accumulated organic matter and a smaller populationof methanogens accumulated within the digestion environment.

5. Conclusions

The investigation proved that co-digesting used cooking greasewith swine manure in low-cost digesters is a simple way to doubleenergy production. A small volume of grease (2.5%), which corre-sponded to a 113% increase in organic matter, increased methaneproduction by 124%. Accumulation of grease, a decrease in specicCH4 yield, and a decrease in the pH of the digestion environmentwere observed with grease treatments greater than 2.5%. The spe-cic methane yield in T2.5 was similar to many laboratory-scale,completely mixed reactors operating at 3537 C. The longerretention time and solids accumulation in low-cost digesters likelyled to digestion efciency similar to higher temperature systems.

This current study emphasized the need for more co-digestionstudies in large scale, replicated systems at low-temperatures.Data obtained after a 50-day stabilization period and nine monthsof experimental study suggested that the digestion environmentwas operating at a signicantly greater efciency over time. The re-sults from this study can be used by small and medium-scale farm-ers in tropical climates to increase their methane production, andthus, the economic value of their digestion systems. Additionally,by doubling methane production, co-digesting manure and usedgrease could result in a greater adaptation of digestion technologyas a method for small and medium-scale farmers to treat agricul-tural wastewaters while obtaining renewable energy and organicfertilizers. Additionally, the batch study suggested that seasonalproducers of manure and grease wastes could successfully utilizelow-cost digesters.

Acknowledgements

This material is based upon work supported by the National Sci-ence Foundation (project #60012470), Department of Energy(award number #DE-FG02-04ER63834), and the Ohio State Univer-sitys Targeted Investments in Excellence Carbon-Water-ClimateProject. We would like to thank the laboratory and research staffat EARTH University for their assistance in the research, includingBert Kohlmann, Jane Yeomens, and Herbert Arrieta. We also wishto thank the student workers at EARTH University and our CentralState University counterparts, Sritharan Subramania and BryanSmith. Additional thanks are also extended to Richard Fortner, Da-vid Hansen, Pat Rigby, and Carol Moody for their administrativeguidance.


References

Angelidaki, I., Ahring, B.K., 1997. Co-digestion of olive mill wastewaters withmanure, household waste or sewage sludge. Biodegradation 8, 221226.

APHA, 1998. In: Clesceri, L.S., Greenberg, A.E., Eaton, A.D. (Eds.), Standard Methodsfor the Examination of Water and Wastewater, 20th ed. American Public HealthAssociation, Washington, DC.

Archer, D.B., Kirsop, B.H., 1990. The microbiology and control of anaerobic digestion.In: Andrew Wheatley (Ed.), Anaerobic Digestion: A Waste TreatmentTechnology. Critical Reports on Applied Chemistry 31, 4391.

Botero, R., Preston, T.R., 1987. Low Cost Biodigesters for the Production of Fuel andFertilizer from Animal Excreta: Manual for Installation, Operation, andUtilization. Centro para la Investigatin en Sistemas Sostenibles deProduccin Agropecuaria, Cali, Colombia.

Boubaker, F., Ridha, B.C., 2007. Anaerobic co-digestion of olive mill wastewater witholive mill solid waste in a tubular digester at mesophilic temperature.Bioresource Technology 98, 769774.

Carrieri, C., Di Pinto, A.C., Rozzi, A., Santori, M., 1993. Anaerobic co-digestion ofsewage sludge and concentrated soluble wastewaters. Water Science andTechnology 28 (2), 187197.

Char, J., Pedraza, G., Conde, N., 1999. The productive water decontaminationsystem: a tool for protecting water resources in the tropics. Livestock Researchfor Rural Development 11 (1). .

Cho, J.K., Park, S.C., Chang, H.N., 1995. Biochemical methane potential and solidstate anaerobic digestion of Korean food wastes. Bioresource Technology 52,245253.

Cirne, D.G., Paloumet, X., Bjrnsson, L., Alves, M.M., Mattiasson, B., 2007. Anaerobicdigestion of lipid-rich waste effects of lipid concentration. Renewable Energy32, 965975.

Davidsson, ., Lvstedt, C., la Cour Jansen, J., Gruvberger, C., Aspegren, H., 2008. Co-digestion of grease trap sludge and sewage sludge. Waste Management 24,986992.

El-Mashad, H.M., van Loon, W.K.P., Zeeman, G., 2003. A model of solar energyutilization in the anaerobic digestion of cattle manure. Biosystems Engineering84 (2), 231238.

Ergder, T.H., Tezel, U., Gven, E., Demirer, G.N., 2001. Anaerobic biotransformationand methane generation potential of cheese whey in batch and UASB reactors.Waste Management 21, 643650.

Gavala, H.N., Skiadas, I.V., Bozinis, N.A., Lyberatos, G., 1996. Anaerobic codigestion ofagricultural industries wastewaters. Water, Science and Technology 34 (11),6775.

Gelegenis, J., Georgakakis, D., Angelidaki, I., Christopoulou, N., Goumenaki, M., 2007.Optimization of biogas production from olive-oil mill wastewater, by co-digesting with diluted poultry-manure. Applied Energy 84, 646663.

Geradi, M.H., 2003. The Microbiology of Anaerobic Digesters. John Wiley & Sons,Inc., Hoboken, New Jersey.

Ghaly, A.E., 1996. A comparative study of anaerobic digestion of acid cheese wheyand dairy manure in a two-stage reactor. Bioresource Technology 58, 6172.

Gngr-Demirci, G., Demirer, G.N., 2004. Effect of initial COD concentration,nutrient addition, temperature and microbial acclimation on anaerobictreatability of broiler and cattle manure. Bioresource Technology 93, 109117.

Heo, N.H., Park, S.C., Lee, J.S., Kang, H., Park, D.H., 2003. Single-stage anaerobiccodigestion for mixture wastes of simulated Korean food waste and wasteactivated sludge. Applied Biochemistry and Biotechnology 105108, 567579.

Hobson, P.N., 1990. The treatment of agricultural wastes. In: Wheatley, A. (Ed.),Anaerobic Digestion: A Waste Treatment Technology. Critical Reports onApplied Chemistry 31, 93138.

Hulshoff Pol, L.W., Lens, P.N.L., Weijma, J., Stams, A.J.M., 2001. New developments inreactor and process technology for sulfate reduction. Water Science andTechnology 44 (8), 6776.

Jerger, D.E., Tsao, G.T., 1987. Feed composition. In: Chynoweth, D.P., Isaacson, R.(Eds.), Anaerobic Digestion of Biomass. Elsevier Science Publishing Co., NewYork, New York, pp. 6590.

Jeyaseelan, S., Matsuo, T., 1995. Effects of phase separation anaerobic digestion ondifference substrates. Water Science and Technology 31 (9), 153162.

Kettunen, R.H., Rintala, J.A., 1997. The effect of low temperature and adaptation onthe methanogenic activity of biomass 529 C. Applied Microbiology andBiotechnology 48, 570576.

Kbler, H., Hoppenheidt, K., Hirsch, P., Kottmair, A., Nimmrichter, R., Nordsiech, H.,Mche, W., Swerev, M., 2000. Full scale co-digestion of organic waste. WaterScience and Technology 41 (3), 195202.

Lansing, S., Botero, R.B., Martin, J.F., 2008. Wastewater treatment and biogasproduction in small-scale agricultural digesters. Bioresource Technology 99,58815890.

Lansing, S., Vquez, J., Martnez, H., Botero, R., Martin, J., in press. Optimizingelectricity generation and waste transformations in a low-cost, plug-owanaerobic digestion system. Ecological Engineering.

McHugh, Sharon, Collins, G., OFlaherty, V., 2006. Long-term, high-rate anaerobicbiological treatment of whey wastewaters at psychrophilic temperatures.Bioresource Technology 97, 16691678.

Mladenovska, Z., Dabrowski, S., Ahring, B.K., 2003. Anaerobic digestion of manureand mixture of manure with lipids: biogas reactor performance and microbialcommunity analysis. Water Science and Technology 48 (6), 271278.

Murto, M., Bjrnsson, L., Mattiasson, B., 2004. Impact of food industrial waste onanaerobic co-digestion of sewage sludge and pig manure. Journal ofEnvironmental Management 70, 101107.

Owens, J.M., Chynoweth, D.P., 1993. Biochemical methane potential of municipalsolid waste (MSW) components. Water Science and Technology 27, 114.

Santana, A., Pound, B., 1980. The production of biogas from cattle slurry, the effectsof concentration of total solids and animal diet. Tropical Animal Production 5(2), 130135.

Spajic, R., Burns, R.T., Mooday, L., Kralik, D., Poznic, V., Bishop, G., 2009. AnaerobicDigestion of Swine Manure with Different Types of Food Industry Wastes.ASABE Meeting Presentation Paper No. 097209 for the 2009 ASABE AnnualInternational Meeting. Reno, NV. June 2124, 2009.

Thy, S., Preston, T.R., Ly, J., 2003. Effect of retention time on gas production andfertilizer value of biodigester efuent. Livestock Research for RuralDevelopment 15 (7). .

Ugoji, E.O., 1997. Anaerobic digestion of palm mill efuent and its utilization asfertilizer for environmental protection. Renewable Energy 10 (2/3), 291294.

USEPA, 2006. Market Opportunities for Biogas Recovery Systems: A Guide toIdentifying Candidates for On-farm and Centralized Systems. AgSTAR Program:USEPA, USDA, and DOE Energy and Pollution Prevention. EPA-430-8-06-004.

USEPA, 2009a. Anaerobic Digestion Capital Costs for Dairy Farms. AgSTAR Program:USEPA, USDA, and DOE Energy and Pollution Prevention. February 2009.

USEPA, 2009b. Anaerobic Digesters Continue Growth in US Livestock Market.AgSTAR Program: USEPA, USDA, and DOE Energy and Pollution Prevention.February 2009.

Zhang, R., El-Mashad, H.M., Hartman, K., Wang, F., Liu, G., Choate, C., Gamble, P.,2007. Characterization of food waste as feedstock for anaerobic digestion.Bioresource Technology 98, 929935.

Zitomer, D., Adhikari, P., 2005. Extra methane production from municipal anaerobicdigesters. Biocycle 46 (9), 6466.


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