Bioresource Technology 102 (2011) 2213–2218

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Bioresource Technology

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Optimizing feed composition for improved methane yield during anaerobicdigestion of cow manure based waste mixtures

S.M. Ashekuzzaman, Tjalfe G. Poulsen ⇑Department of Biotechnology, Chemistry and Environmental Engineering, Aalborg University, Sohngaardsholmsvej 57, DK-9000, Denmark

a r t i c l e i n f o a b s t r a c t

Article history:Received 6 August 2010Received in revised form 29 September2010Accepted 30 September 2010Available online 8 October 2010

Keywords:Agricultural wastesAnaerobic digestionSingle-component substratesMulti-component substratesMethane potential

0960-8524/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.biortech.2010.09.118

⇑ Corresponding author. Address: Section of EnvironUniversity, Sohngaardsholmsvej 57, DK-9000, Denma+45 9635 0558.

E-mail address: tgp@bio.aau.dk (T.G. Poulsen).

This study investigated methane yield via anaerobic digestion of multi-component substrates basedon mixtures of biodegradable single-component substrates with cow dung as main component. Benchand full-scale digestion experiments were carried out for both single and multi-component substratesto identify the relationship between methane yield and substrate composition. Results from bothbench- and full-scale experiments corresponded well and showed that using multi-component sub-strates increases the methane yield much more than what would be expected from digestion of singlesubstrates. Process stability as indicated by gas production, pH and NHþ4 concentration variations werealso improved by using multi-component substrates compared to digestion of single-component sub-strates. The results, thus, suggest that assessment of methane yield for multi-component substratescannot reliably be based on methane yields for corresponding single-component substrates but shouldinstead be measured directly.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Rural areas of many developing countries often have limited orno public energy supply. In such regions energy needs are met withtraditional biomass fuels. An example is Bangladesh, where about65% of the total population, have limited or no connection to thenational power grid and instead use agricultural residues (45%),wood and wood wastes (35%), and animal dung (20%) as energysources (mainly for cooking). Biomass fuel is estimated to coverabout 62% of the country’s energy consumption (Al-muyeed andShadullah, 2010). Cooking stoves based on biomass burning, how-ever, have a very low energy efficiency of 5–15% (Hossain, 2003)which may lead to high pressure on forest resources, and increasedair pollution from smoke, resulting in public health problems (e.g.,eye infections and respiratory diseases).

An alternative is to produce methane via anaerobic digestion ofsuitable biomass wastes such as manure and household wastes.Digestion can extract energy in the form of methane from even verywet materials (>70% water) while direct burning of such materials isoften much less efficient as significant energy is required for evapo-rating the water. As digestion can be carried out in inexpensive andlow technology plants, this method can treat bio wastes at low cost

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mental Engineering, Aalborgrk. Tel.: +45 9940 9938; fax:

and may therefore potentially improve utilization of such wastesthat are not fully utilized at present. For instance the recycling of bio-degradable organic waste in Bangladesh is only about 11% comparedto 40% and 65% in India and China, respectively (Aktaruzzaman,2003; Chowdhuri et al., 2008). As a result small-scale biogas technol-ogy as a simple and inexpensive solution has gained increased inter-est in many developing countries. For example, India and Nepal haveinstalled family size biogas plants corresponding to 31% and 8% oftheir estimated total capacities, respectively (Gautam et al., 2009;Rao et al., 2010) while Bangladesh has installed less than 1% (Islamet al., 2006; Al-muyeed and Shadullah, 2010). There is at present stilla very large unused potential in many developing countries globally.

Studies show that co-digestion of substrates often improves pro-cess stability, compared to digestion of the substrates separately(Misi and Froster, 2001; Callaghan et al., 1999; Lehtomaki et al.,2007; Alvarez and Liden, 2008). This indicates that methane yieldmay be improved by optimizing the substrate composition. Despitethe large body of literature on biogas production from waste materi-als in existence, only a limited number of studies of co-digestion areavailable. Co-digestion using two component substrates such as ani-mal manure with food waste or with crop residues have been pre-sented by El-Mashad and Zhang (2010), Callaghan et al. (1999),Demirbas (2006), and Magbanua Jr., et al. (2001), but very little re-search has been conducted on digestion of multi-component sub-strates (Misi and Froster, 2001; Alvarez and Liden, 2008) forimproved methane production. While pig manure constitutes themain component of animal agro wastes in most industrialized

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countries, cow manure is the main component in many developingcountries especially in South-East Asia (Wint and Robinson, 2007).

The purpose of this study is therefore to investigate the possi-bilities of improving methane yield from anaerobic digestion ofmulti-component, cow manure based, mixtures of biodegradableagro and household wastes. Mixture composition will be basedon the digestible biomass fractions available in the rural areas ofBangladesh as a representative for the South-East Asian region.Experiments will be carried out both in bench scale laboratoryreactors and in full scale reactors supplying gas to one householdunder local conditions in Bangladesh.

2. Methods

Two sets of experiments for assessing biogas and methane yieldfrom different agro and household waste materials as well asmixtures of these were carried out. Bench-scale experiments werecarried out at Aalborg University, Denmark, under controlled labo-ratory conditions to identify the gas yield for a wider range ofmaterials and mixtures of these. Full-scale digestion experimentsusing agro and household waste obtained locally were carriedout under local conditions in Kawnia village, Rangpur district,Bangladesh in three small-scale biogas plants (Grameen Shakti, 2007;Nes et al., 2005) supplying 1.6–2 normal cubic meters (normalizedto 1 atm, Nm3) biogas per day. The plants consist of a digestiontank with the top exposed to the sun for heating of the contents.The purpose of the full-scale experiments was to confirm findingsfrom the bench-scale experiments under practical conditions.

2.1. Waste material collection and preparation

Agro and household wastes were selected based on the averagewaste composition in a number of project areas in the Rangpur re-gion, where Rangpur–Dinajpur Rural Service (RDRS), a nationalhumanitarian and development non-government organization(NGO), is working on the implementation of biogas plants in col-laboration with DanChurchAid, a Danish humanitarian NGO.According to a survey by RDRS involving 721 families, waste fromthe area consists of 71% cow dung, 7% chicken litter, 3% sheep/goatmanure, 6% food waste, 13% leaves/straw (wet weight) (RDRS,2009). Both sets of digestion experiments were therefore carriedout using these materials.

Materials for the bench-scale experiments were collected nearAalborg, Denmark. Cow dung, chicken litter, sheep manure andbarley straw were collected at local farms, food waste was col-lected over 1 week at a local student dormitory, and fresh leaveswere collected from beech and chestnut trees (equal mix.) on cam-pus, all in the amount of about 2 kg. Cow dung and sheep manurewere fresh and thick slurry, while chicken litter was more dry andconsisted of solid pieces. The food waste consisted of 35% chickenbones, 5% tea bags, 12% bread, 15% banana skins, 6% potato skins,10% potatoes, 13% carrots, and 4% cabbage (wet weight). Leavesand barley straw were mixed 1:1 by weight, cut into about0.5 cm pieces and thoroughly mixed. In addition to the above sin-gle substrates, a multi-component substrate (M0) consisting of 32%chicken litter, 12% sheep manure, 24% food waste and 32% leaves/straw (wet weight) was prepared according to the waste composi-tion in the Rangpur region. Five additional multi-component sub-strates (M1–M5) were then prepared from M0 and cow dung atcow dung: M0 ratios of 80:20, 70:30, 65:35, 60:40, and 55:45(wet weight). This range corresponds to the range of waste compo-sitions available in most of the rural areas in Bangladesh. All singleand multi-component substrates were macerated for about 2–4 min in a 5 l domestic blender to reduce size and to ensureproper mixing. Batches of about 2 kg were used. This resulted in

a homogeneous material with particles smaller than 0.5 cm. Allthe blended materials were subsequently stored at 4 �C for a max-imum of 24 h prior to use.

Substrates for the full-scale experiments, cow dung, chicken lit-ter, goat manure, food waste (mainly potato stems), leaves (mangoand jackfruit) and rice straw were collected locally in the amountsof 100 kg for cow dung and 5–10 kg for the remaining substrates(wet weight). Multi-component substrates M1 and M3 were pre-pared in the amount of 40 kg each. Prior to mixing, leaves, ricestraw and potato stems were cut into small pieces, approximately0.5–2.5 cm long. Substrates were then homogenized by manualmixing and crushing of larger particles using a wooden hammerand a thick earthen dome-shaped plate.

2.2. Fermentation experiments

Prior to digestion, dry matter (DM) and volatile solids (VS) con-tent of all substrates were measured by drying at 105 �C for 24 hand subsequent ignition at 550 �C for about 2 h. Analyses of thematerials collected in Bangladesh were done at Bangladesh Univer-sity of Engineering and Technology (BUET), Dhaka which is locatedabout 400 km from the Rangpur region. Due to difficulties in trans-porting the materials from Rangpur to Dhaka samples of similarmaterials were collected in and around Dhaka and used for DMand VS determination. Care was taken to ensure that the materialscollected were as similar to the materials used in the digesters aspossible although the food waste collected was somewhat differentand consisted of orange skins (45%), carrots (15%), cooked beans(30%), apple pieces (5%), and green beans (5%).

Bench scale digestion tests were carried out in batch reactorsusing digested sewage sludge from a continuous flow reactor atAalborg Wastewater Treatment Plant West as inoculum. The inoc-ulum was starved at room temperature for 4 days before use to re-duce the amount of gas produced from it during the experiments.Duplicate samples were prepared in 250 ml serum bottles by mix-ing 10 g of each substrate with 100 g inoculum yielding a total of22 samples. Two blanks, containing only 100 g inoculum, wererun in parallel with the samples. The headspaces of all bottles wereflushed with nitrogen for 2 min before closing tightly with rubbersepta and screw caps. The bottles were incubated for 48 days at35 �C and the volume of biogas produced was measured everyday during the first 2 weeks and then every other day or less forthe remaining period using a 50 or 100 ml glass syringe (FortunaOptima, Poulten & Graf GmbH, Germany). Gas methane contentwas measured with a CHROMPACK CP 9000 gas chromatograph(GC) equipped with a flame ionization detector (FID) and WCOTFused Silica column (30 m � 0.53 mm). The injector, detector andcolumn temperature were 270, 300, and 85 �C, respectively. Hydro-gen was used as the carrier gas and the sample volume injectedinto the GC was 100 ll.

Upon completion of the digestion experiments (labeled bench-scale experiment I), a second digestion experiment (labeledbench-scale experiment II) was conducted for all single and mul-ti-component substrates using digestate from bench-scale experi-ment I as inoculum. In this case, 40 g inoculum was mixed with4 g of the corresponding substrate, and another 40 g digestate fromthe same culture was used as blank. All samples were prepared induplicate using 120 ml serum bottles and digested at 35 �C for37 days. Table 1 gives an overview of the samples used in bothbench-scale experiments.

After both experiment I and II, duplicate 4 g digestate sampleswere taken from all cultures, mixed with 1 M KCl solution (in theratio of 1:10) and shaken. Approximately 20 ml of each samplewas centrifuged in a Sigma laboratory centrifuge at 4500 rpm(3645 G) for 30 min and filtered using GF-75 glass fiber filter(0.7 lm nominal pore size, Toyo Roshi Kaisha Ltd., Japan) prior to

Table 1Fermentation batch test conditions.

Substrates Sample ID Bench-scale experiment I Bench-scale experiment II

Substrate to inoculum ratio (1:10) No. ofsamples

Substrate to inoculum ratio (1:10) No. ofsamples

Cow dung Cow 10 g substrate: 100 g inoculum in allcultures

2 4 g substrate: 40 g inoculum in allcultures

2Chicken litter Chicken 2 2Sheep manure Sheep 2 2Food wastes Food 2 2Leaves/straw Leaves/straw 2 2Mixtures M0a, M1b, M2c, M3d, M4e,

M5f6 � 2 6 � 2

Blank (onlyinoculum)

Inoculum 100 g inoculum 2 40 g inoculumg 22

a M0 = 32% chicken litter + 12% sheep manure + 24% food wastes + 32% leaves/straw (by weight).b M1 = 80% cow dung + 20% M0.c M2 = 70% cow dung + 30% M0.d M3 = 65% cow dung + 35% M0.e M4 = 60% cow dung + 40% M0.f M5 = 55% cow dung + 45% M0.g Digestate from each culture after experiment I was used as inoculum for respective culture in the experiment II.

Table 2Characteristics of single materials and mixtures used in the digestion experiments (mean ± SD). Note that due to difficulties with transportation data for the full-scaleexperiments were not measured directly on the materials used but instead on similar materials collected in an area near Bangladesh University of Engineering and Technology,Dhaka where measurements were taken.

Material Dry matter (DM) content (% wt.) Volatile solids (VS) content (% of DM) C/N ratioa

Bench scale (n = 3) Full scale (n = 2) Bench scale (n = 3) Full scale (n = 2)

Cow dung 13.75 ± 0.30 25.19 ± 0.21 83.71 ± 1.28 80.09 ± 0.13 24.0Sheep manure 26.73 ± 1.21 – 74.89 ± 1.02 – 19.0Goat manure – 47.86 ± 1.36 – 62.70 ± 0.62 12.0Chicken litter 71.25 ± 0.24 42.82 ± 1.07 35.00 ± 0.28 71.56 ± 1.48 10.0Leaves/straw 76.81 ± 1.57 70.67 ± 0.95 92.03 ± 2.18 84.90 ± 0.20 55.5b

Food wastes 32.84 ± 2.17 20.76 ± 0.34 82.35 ± 2.35 94.21 ± 0.07 14.0M0c 59.14 ± 0.18 45.91 ± 0.34 59.53 ± 3.21 79.37 ± 0.60 26.6b

M1d 24.18 ± 1.52 30.75 ± 0.02 75.53 ± 4.80 79.49 ± 0.12 24.5b

M2e 28.86 ± 0.73 – 72.32 ± 0.27 – 24.8b

M3f 30.00 ± 0.75 33.79 ± 0.01 69.87 ± 4.89 79.52 ± 0.23 24.9b

M4g 31.95 ± 0.32 – 69.96 ± 3.20 – 25.0b

M5h 34.53 ± 0.66 – 65.61 ± 2.55 – 25.2b

Inoculum 3.33 ± 0.02 – 57.02 ± 2.51 – –

(–) Data not measured.a Values taken from literature (Karki et al., 2005; Tsai et al., 2006).b Values are estimated according to the proportion of materials used.c M0 = 32% chicken litter + 12% sheep manure + 24% food wastes + 32% leaves/straw (by weight).d M1 = 80% cow dung + 20% M0.e M2 = 70% cow dung + 30% M0.f M3 = 65% cow dung + 35% M0.g M4 = 60% cow dung + 40% M0.h M5 = 55% cow dung + 45% M0.

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analyses. Sample pH was measured using a Metrohm 744 pH me-ter. Samples were then diluted 100 times and NHþ4 =NH3-N wasdetermined using a Technicon TRAACS 800 autoanalyzer.

The relative VS degraded during the experiments (VSdegr.) wasestimated using a formula (Eq. (1)) from German Standard Proce-dure (VDI 4630, 2006) assuming that VS is converted into eithermethane or carbon dioxide (biogas consists typically of more than98% CH4 and CO2). It was also assumed that 7% of degraded VS wasconverted to bacterial biomass based on Boltes et al. (2008). Thismethod was used as it was not possible to determine digestateVS content after digestion experiment I due to insufficient samplequantity:

VSdegr: ð%Þ ¼ ½ðVgas � CCH4þCO2 Þ � 100�=½m � ðVSÞ � 0:93� ð1Þ

where Vgas is produced biogas volume (ml), CCH4þCO2 is mass concen-tration of CH4 + CO2 in the biogas (g/ml) calculated using a density

of CH4 = 0.000668 g/ml, and CO2 = 0.00184 g/ml, m is the amount ofsubstrate added to the reactor (g), and VS is the total VS concentra-tion in the substrate (g/g).

The three biogas plants used for the full-scale experiments hadbeen digesting cow dung for 4 months prior to the experiment andproduced about 1.6 m3 gas/day. The plants which have digestervolumes of 4 m3 are placed below the soil surface and operatedas batch reactors at ambient temperature. For the experimentone plant was fed with raw cow dung to serve as reference whilethe remaining two were fed substrates M1 and M3, respectively.Forty kilograms of each substrate (wet weight) was mixed with100 l of water in mixing tanks before being introduced to the di-gester. The materials were subsequently stirred using a bamboostick to ensure adequate mixing prior to digestion. The local day-time temperature during the experiment was 20–25 and 15–20 �C during the night. It was not possible to measure the inside

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temperature of the digesters, but based on experience of localoperators, the inside temperature of an underground digester withthe top dome exposed to the sun, is about 22–26 �C. The full-scaledigestion experiments were run for 31 days and the volume of bio-gas produced was measured by recording the number of dailyindependent (i.e. only gas burning through burner) cooking hoursfor the first 5 days, every other day for the following week, andtwice a week for the rest of the period. Cooking hours were con-verted into gas volume using a conversion factor of 0.4 m3 bio-gas/cooking h (IDCOL, 2009). This factor represents average gasconsumption per cooking hour for the normal variety of cookingactivities in a typical household. In addition, a diaphragm gasmeter was used at selected days to measure the volume of biogasproduced. Based on these measurements the volume of gas usedper cooking hour was calculated as 0.398 ± 0.04 m3 (n = 9) whichcorresponds well with the conversion factor used.

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Fig. 1. (a) and (b) Cumulative biogas production from digestion single substrates

3. Results and discussion

3.1. Substrate characteristics

Table 2 shows mean DM, VS contents (measured) and C/N ratio(based on literature) for all substrates used. For the bench-scaleexperiments, the highest DM content was noted for leaves/straw(76.8%) followed by chicken litter (71.3%) and other single andmulti-component substrates being in the range of 13.5–59%. Forthe full-scale experiments DM contents for chicken litter whichwas lower, while for cow dung it was higher. The reason couldbe climatic conditions as well as the conditions under which theanimals were kept. In general, VS/DM ratios were above 60%, ex-cept for the chicken litter used in the bench-scale experiments.As a rule of thumb, materials with VS/DM ratios less than 60%are not considered optimal for anaerobic digestion (Steffen et al.,1998). Estimated C/N ratios for all single substrates used exceptcow dung were far from the optimal range of 25–30 (Sievers andBrune, 1978), particularly chicken litter, goat manure, food waste,and leaves/straw. However, for multi-component substrates allestimated C/N ratios were close to optimal.

(average of two replicates) as a function of time during bench-scale experiments Iand II, respectively. (c) Average methane content of biogas produced from singlesubstrates as a function of time during bench-scale experiments I and II. Y error barsindicate the standard deviation at each data point (n = 4).

3.2. Digestion of single-component substrates

Fig. 1a and b shows the biogas production (corrected for gasproduction by inoculum) for the different materials observed dur-ing the bench-scale experiments. Food waste showed the highestbiogas potential (406 l/kg VS) followed by cow dung (302 l/kgVS), chicken litter (232 l/kg VS), sheep manure (199 l/kg VS) andleaves/straw (125 l/kg VS), respectively after 31 days of digestion.During bench-scale experiment II samples containing food wasteand leaves/straw only produced little biogas. Measurements ofNHþ4 concentration in the food waste samples during experimentII (Table 3) suggest inhibition by ammonia as concentrations wereabove the limit value of 1500 mg/l responsible to initiate inhibition(Deublein and Steinhause, 2008). In all cases pH values remainnear or a bit above neutral, indicating that inhibition by VFA is un-likely. For the leaves/straw the low gas production could be due tolack of nitrogen as indicated by both the high C/N ratio of thismaterial and the low NHþ4 concentration observed duringdigestion.

Diauxia (two phase decomposition) was observed for both foodwaste and leaves/straw during bench-scale experiment I, indicat-ing adaptation by the microorganisms. This is evident from theintermediate period of low or no gas production from 2.5 to12 days for food waste and 4 to 21 days for leaves/straw (Fig. 1a).Diauxic growth was also reported in a few earlier studies (Misiand Froster, 2001; Hobson, 1985).

The methane gas content for the different materials is shown inFig. 1c. Approximately equal amounts of CO2 and CH4 were pro-duced during digestion with average CH4 percentage for the mate-rials after adaptation by the archaea (i.e. methanogens) varyingbetween 45% and 55%. CH4 yields observed for the animal manureswell match observations from previous studies on digestion of ani-mal manures (Moller et al., 2004; Hill, 1984; Misi and Froster,2001; Fantozzi and Buratti, 2009; Alvarez and Liden, 2009). Esti-mates of VS reduction for single substrates were 19–55%, whereleaves/straw showed the lowest and food waste the highest value(Table 4). These values are also comparable with earlier studies(e.g., El-Mashad and Zhang, 2010; Misi and Froster, 2001).

3.3. Digestion of multi-component substrates

Biogas production (corrected for gas production by inoculum)from multi-component substrates (M0–M5) during both bench-scale experiments are shown in Fig. 2a and b. Cumulative gas pro-duction was on average 20% higher during digestion experiment IIcompared to experiment I over the initial 31 days likely because ofmicrobial adaptation during experiment I. Biogas methane contentsfor M0–M5 are shown in Fig. 2c indicating an average methane

Table 3pH and total ammonia of the digestate samples (mean ± SD; n = 4).

Sample ID pH Total ammonia (mg/l)

Experiment I Experiment II Experiment I Experiment II

Cow 8.3 ± 0.07 7.4 ± 0.06 1126 ± 23 1379 ± 52Chicken 8.5 ± 0.13 7.6 ± 0.02 1545 ± 27 2630 ± 658Sheep 8.3 ± 0.05 7.5 ± 0.03 1221 ± 47 1639 ± 233Food 7.3 ± 0.06 6.9 ± 0.21 1860 ± 208 2429 ± 282Leaves/

straw– 7.0 ± 0.20 – 573 ± 111

M0a – 7.5 ± 0.04 – 1701 ± 89M1b 8.3 ± 0.06 7.4 ± 0.04 1384 ± 223 1419 ± 93M2c 8.3 ± 0.08 7.3 ± 0.06 1267 ± 50 1373 ± 76M3d 8.1 ± 0.01 7.5 ± 0.01 1206 ± 7 1364 ± 21M4e 8.2 ± 0.00 7.3 ± 0.03 1248 ± 58 1417 ± 50M5f 8.1 ± 0.01 7.4 ± 0.03 1183 ± 44 1457 ± 74

(–) Not measured.a M0 = 32% chicken litter + 12% sheep manure + 24% food wastes + 32% leaves/

straw (by weight).b M1 = 80% cow dung + 20% M0.c M2 = 70% cow dung + 30% M0.d M3 = 65% cow dung + 35% M0.e M4 = 60% cow dung + 40% M0.f M5 = 55% cow dung + 45% M0.

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Fig. 2. (a) and (b) Cumulative biogas production of multi-component substrates(average of two replicates) as a function of time during bench-scale experiments Iand II, respectively. (c) Average methane content of biogas produced duringdigestion of multi-component substrates as a function of time in bench-scaleexperiments I and II. Y error bars indicate the standard deviation at each data point(n = 4).

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percentage of 45%. In general methane yield increases withdecreasing percentage of cow manure as would be expected basedon the high methane yield of M0 (Table 4). Both total biogas andmethane potentials observed for substrates M0–M5 are 28–177%higher than the gas and methane yields by any of the single mate-rials from which they were prepared substrates. In fact predictionsof methane yield for M0–M5 based on composition in combinationwith single substrate methane yields, are much lower than mea-sured (Fig. 3). Also predicted yields decrease with decreasing frac-tion of cow manure, opposite to measured observations, due to thelow methane yield from leaves/straw. This clearly shows thatdigestion of multi-component substrates can yield methanepotentials much higher than if substrates are digested separately.Thus, it is not possible to estimate methane yield for multi-compo-nent substrates based data for single substrates but direct

Table 4Quantity and quality of biogas produced during anaerobic digestion of single materials and mixtures over a period of 31 days (mean ± SD, where applicable). Values for benchscale are based on data from experiment II, except for food waste, leaves/straw and chicken litter, which are based on experiment I data. All gas production data are given at apressure of 1 atm and 25 �C.

Substrates Biogas potential (l/kg VS) Methane potential (l/kg VS) Methane contenta (%) VS reduction (%)

Bench-scale Full-scale Estimateda Bench-scale Full-scale Estimatedb

Cow dung 302 ± 2 392 – 133 ± 15 172 – 44 ± 6 43Chicken litter 232 ± 16 – – 105 ± 9 – – 45 ± 3 33Sheep manure 199 ± 2 – – 105 ± 7 – – 53 ± 4 26Food waste 406 ± 11 – – 199 ± 26 – – 49 ± 7 55Leaves/straw 125 ± 37 – – 45 ± 23 – – 36 ± 18 19M0c 551 ± 19 – 197 268 ± 38 – 86 49 ± 8 75M1d 428 ± 5 415 254 196 ± 10 191 112 46 ± 3 60M2e 386 ± 2 – 240 189 ± 11 – 105 49 ± 3 53M3f 468 ± 31 447 234 211 ± 15 201 103 45 ± 2 66M4g 436 ± 11 – 229 201 ± 16 – 100 46 ± 4 61M5h 452 ± 9 – 225 220 ± 21 – 98 49 ± 5 62

a Values are estimated based on single substrate data.b Average values have been estimated for the retention time between 18 and 38 days (n = 16).c M0 = 32% chicken litter + 12% sheep manure + 24% food wastes + 32% leaves/straw (by weight).d M1 = 80% cow dung + 20% M0.e M2 = 70% cow dung + 30% M0.f M3 = 65% cow dung + 35% M0.g M4 = 60% cow dung + 40% M0.h M5 = 55% cow dung + 45% M0.

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Bench scale methane yieldFull scale methane yieldBest fit bench scale methane yieldEstimated methane yield

Fig. 3. Measured (symbols) and predicted (based on bench scale single substratedata) methane yield as a function of cow manure addition for digestion of multi-component substrates observed during both bench and full-scale digestionexperiments.

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measurements of methane yield are needed. Use of multi-compo-nent substrates also give a more stable digestion process comparedto single substrates, as indicated by the moderate pH and NHþ4concentration values of the digested samples (Table 3).

Biogas production and methane yield for cow dung, M1, and M3

observed after 31 days of digestion during the full scale experi-ment were 172, 191, and 201 l/kg VS, respectively. These yieldsare fairly similar to the observations from the bench-scale experi-ments (Fig. 3). Again decreasing methane yield with decreasing M0

fraction is observed supporting the results from the bench-scaleexperiments.

The results clearly show that methane yield can be increasedmuch more by mixing substrates than what would be possible ifthey were digested separately. The reason could be that mixingavoids inhibition for instance by excess ammonia or volatile fattyacids but it could also be (macro and micro) nutrient limitationduring digestion of single substrates. In addition to higher gasand methane yield, estimates of VS reduction also indicate thatco-digestion of mixed materials seems to yield higher VS reductioncompared to single materials (Table 4). This means that less quan-tity of digested materials are to be managed after co-digestioncompared to digestion of single-component substrates.

4. Conclusions

Anaerobic digestion of single and multi-component substratesincluding animal manure and food waste, carried out under bothbench and full-scale conditions showed that methane yield frommulti-component substrates were significantly higher than whatwould be expected based on the methane yields from the single-component substrates from which they were prepared. This meansthat it is not possible to achieve reliable estimates of methane yieldfrom multi-component substrates based on methane yields fromdigestion of corresponding single-component substrates but directmeasurements of methane yield from multi-component substratesare necessary.

Acknowledgements

The authors would like to thank Professor Emeritus Jens AageHansen, Aalborg University for providing manure substrates fromhis farm. The authors are grateful to Helle Blendstrup and Soren

M. Karst for their help during the laboratory work. Special thanksto Dr. S.M. Sharifuzzaman, Institute of Marine Sciences and Fisheries,University of Chittagong for his inspiration and cooperation.Danish State Scholarship for M.Sc study, Travel Grant from DanidaFellowship Centre, and cooperation during full-scale experimentsfrom Dr. Syed Samsuzzaman, Director of Resources & Environment,RDRS Bangladesh are highly acknowledged.

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