Methane Production from Anaerobic Co-digestion

of maize and cow dungMohammad Ali Abdoli,a Leila Amiri,a Akbar Baghvand,a Javad Nasiri,b and Edris Madadianc

aDepartment of Environmental Engineering, University of Tehran, #15 Ghods St, Enghlab Ave, Tehran, Iran;Leila_amiri_3224@yahoo.com (for correspondence)bRenewable Energy Organization of Iran, Tehran, IrancDepartment of Bioresource Engineering, McGill University, Quebec, Canada

Published online 13 April 2013 in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/ep.11783

Anaerobic digestion (AD) treatment of agricultural andanimal waste can be considered as a means of enablingenvironmental remediation. This research investigated twoconditions of anaerobic co-digestion of maize waste andcow dung. This was done by placing a mixture of cow dungand maize waste together in a floating drum digester thatwas run at two different cow dung/maize ratios of 10:1 and10:5. Batch conditions were on a bench-scale of AD, 5 L involume, developed to operate under mesophilic (36 6 1

�C).

Results showed that biogas and methane yields from mes-ophilic digestion at the ratio of 10:1 were lower than yieldsobtained at ratio tested in the second run (10:5). Biogasyields were 250 and 480 L/kg VS for first and second runs,respectively. In the case of methane, these amounts were pre-sented at 130 and 300 L/kg VS. Furthermore, the averagemethane content of biogas was calculated as 51 and 62%,in first and second runs respectively. The total biogas produc-tion of the reactor increased by 92% when substrates werefed in second condition ratio compared to biogas productionduring the first condition ratio. In continue the effect of add-ing maize waste on biogas yield from cow dung was eval-uated in batch digesters under mesophilic conditions. Theaddition of maize waste to cow dung presents a viablemethod to improve biogas yield, as well as a means to usemaize waste. VC 2013 American Institute of Chemical Engineers En-

viron Prog, 33: 597–601, 2014

Keywords: mesophilic; co-digestion; maize waste; cowdung; biogas

INTRODUCTION

Anaerobic digestion is the most commonly used methodto dispose of organic waste because of its high level of per-formance in terms of reducing volume and stabilizing biogasproduction thus making the process commercially profitable[1].

In effect the process is simply that of anaerobic break-down of organic waste in a controlled and enclosed way ina landfill that releases methane. Almost any organic materialcan be processed by AD, including low- grade waste paperand cardboard that is unsuitable for recycling because offood contamination, grass clippings, leftover food, industrialeffluent, sewage and animal waste [2].

Biogas is a carbon–neutral source of renewable energy. Itpresents competitive source energy in terms of energy effi-ciency and its minimal environmental impact [3]. Biogas canbe used directly for heating and generating electricity, or as asubstitute for fossil fuel applications such as transport fuel [4].

A bio digester is a physical structure, commonly knownas a biogas plant [5]. Anaerobic digestion involves a series ofmetabolic reactions such as hydrolysis, acidogensis andmethanogensis [6,7].

Co-digestion is a waste treatment method in which differ-ent waste products are mixed and treated together [8]. Toachieve successful digestion, several physical and chemicalfactors must be considered. The most important physical fac-tor is temperature. Anaerobic fermentation can be main-tained at psychrophilic (12–16

�C, in situations such as

landfills, swamps or sediments), mesophilic (35–37�C, such

as that in rumen and in an anaerobic digester), and thermo-philic conditions (55–60

�C; such as in anaerobic digesters or

geothermally heated ecosystems) [9,10]. Technically, onlymesophilic and thermophilic ranges are significant becauseat ambient temperature the rate of anaerobic degradation isextremely slow. The thermophilic process is more sensitiveto changes in ambient conditions than the mesophilic pro-cess [11]. It is more usual for anaerobic digestion to occur inthe mesophilic range. It is also important that the tempera-ture remains constant during the process of AD [9,10].

AD processes can be classified according to the total sol-ids (TS) content of slurry in a digester reactor. Low solidssystems (LS) contain less than 10% TS, medium solids (MS)contain about 15–20%, and high solids (HS) processes rangefrom 22 to 40% [12].

AD processes can be categorized according to numbers ofreactors used, determined as either single-stage or multistageprocesses. Batch reactors are those that are loaded with feed-stock at the beginning of the reaction and products are dis-charged at the end of a cycle [13]. Batch systems may appearto operate similarly to a vessel landfill but in fact theyachieve much higher reaction rates and 50–100% higher bio-gas yields than landfills. This can be attributed to two factors;first because of continuous recirculation of the leachate andsecondly, because they are operated at higher temperaturesthan landfills. The technical simplicity of batch the processmakes it inexpensive and robust. However, the processrequires a larger land area than single-stage HS reactors asthey are much shorter and their OLR (Organic Loading Rate)VC 2013 American Institute of Chemical Engineers

Environmental Progress & Sustainable Energy (Vol.33, No.2) DOI 10.1002/ep July 2014 597

is half that of other reactors [13]. So batch and continuousanaerobic-digesters have been designed to treat MSW (Mu-nicipal Solid Waste) to yield biogas [14].

Other disadvantages are that material settles to the bottomof the reactor and this tends to inhibit digestion and risksexplosion while the reactor is being unloaded [13]. In termsof feeding method, there are three different forms of plants:(1) batch plants, (2) continuous plants, and (3) semi-batchplants.

Batch plants are filled and then completely emptied aftera fixed retention time. Anaerobic batch reactors are usefulbecause they can digestion is quick and equipment used inthe process is simple and inexpensive, so rates of digestionare easily assessed [7,15,16]. In a continuous system the tankis loaded daily after initial loading and gas production [17].In such systems the substrate must be fluid and homogene-ous. Continuous plants are suitable for rural households asthe necessary work fits well into a daily routine. Gas produc-tion is constant, and higher than that in batch plants. Today,nearly all biogas plants operate on a continuous mode [18].

Anaerobic digestion involves a series of metabolic reac-tions such as hydrolysis, acidogensis, and methanogensis [6].

This metabolic reaction for digestion is summarized belowin its simple form.

Stage 1: Hydrolysis. The insoluble solids are broken downinto soluble monomers [19,20]. This stage is also knownas the polymer breakdown stage [21].Stage 2: Acidification. A special group of microorganismsferments the breakdown products to acetic acid, hydro-gen, carbon dioxide and other lower weight simple vola-tile organic acids like propionic acid and butyric acid thatare in turn converted to acetic acid [22].Stage 3: Methanization: The principle acids produced atStage 2 are processed by methanogenic bacteria to pro-duce methane. The reaction that takes place in the pro-cess of methane production is called methanization [21].The relationship between amounts of carbon and nitrogen

present in organic materials is expressed in terms of a car-bon/nitrogen (C/N) ratio [13]. A C/N ratio ranging from 20 to30 is considered optimum for anaerobic digestion [21]. Cowdung and maize have C/N ratios of 24 and 60, respectively[23].

Requirements for anaerobic digestion are strongly de-pendent on properties of the biomass feedstock. There areno special requirements for the process of biogas productionif it only handles manure or sludge straight from a farm.

External feedstock should be handled properly to guaranteeadequate standards of hygiene [24].

Any biodegradable organic material can be used as inputfor processing inside the bio-digester. However, for eco-nomic and technical reasons, some materials are more pre-ferred as input than others [5].

Increased stability and performance in anaerobic reactorscan be achieved if the microbial consortium is retained inthe reactor [25]. This research investigates the biogas produc-tion using anaerobic digestion in terms of optimizing theprocess. In this regard maize waste is used as an invaluableadditive to increase the efficiency of the results. It not onlyraises the efficacy of the process, but also helps to recoveryof this kind of organic waste.

MATERIALS AND METHODS

Three batch digesters, each with a volume of 5 L, wereused to assess performance of mesophilic anaerobic co-digestion for two different cow dung/ maize waste ratios of10:1 and 10:5. Figure 1 demonstrates a schematic representa-tion of the batch digester used in this study.

Dairy manure was collected from a dairy farm near Saveh,Central Province, Iran, during November 2011. Samples werecollected in 15 L vessels and immediately transported to theSaveh Laboratory at the Center for Development of NewEnergies. A bench-scale anaerobic digester, 5 L in volumewas developed to operate under a mesophilic (36 6 1

�C)

batch condition. The digester was a glass cylinder with a di-ameter of 12 cm and a depth of 30 cm. Experiments in thisstudy were done simultaneously in a series of three 5-1itrepure-volume glass reactors, contents of which were stirredthoroughly with a magnetic stirrer (Figure 2). Sampling wastaken by screening and grinding from the bed of the anaero-bic digester.

The main purpose of this study is to identify discrepan-cies in results from anaerobic co-digestion of maize wasteand cow dung in two conditions. Two ratios of cow dungand maize waste were mixed (10:1 and 10:5). Two runs ofexperiments were conducted for each ratio to assess per-formance of mesophilic anaerobic digestion. A separate runwas carried out with cow dung only in order to compare

Figure 1. Detail of the batch digester used in this study.[Color figure can be viewed in the online issue, which isavailable at wileyonlinelibrary.com.]

Figure 2. Glass reactor containing the waste mixture. [Colorfigure can be viewed in the online issue, which is availableat wileyonlinelibrary.com.]

Environmental Progress & Sustainable Energy (Vol.33, No.2) DOI 10.1002/ep598 July 2014

amounts of produced biogas and considered as the controlsample. Levels of biogas production were determined bycomparing results of these tests with the control sample, thusdetermining the effect of maize waste in enhancing biogasproduction.

The relationship between the amount of carbon andnitrogen presented in organic materials is expressed in termsof the carbon to nitrogen (C/N) ratio. A C/N ratio, rangingfrom 20 to 30, is considered optimum for anaerobic digestion[14]. Cow dung and maize have C/N ratios of 24.49 and26.25, respectively.

Calculations of C/N of compounds used in these experi-ments are as follows.

N½1�5%N½1�3Weight of Material½1�N½2�5%N½2�3Weight of Material½2�C½1�5%C½1�3Weight of Material½1�C½2�5%C½2�3Weight of Material½2�C

N½mix1; 2�5

C½1�1C½2�N½1�1N½2�

(1)

N½cow Dung�5%N½cow Dung�3Weight of Material½cow Dung�50:631:751:2N½Maize�5%N½Maize�3Weight of Material½Maize�50:530:02850:001C½cow Dung�5%C½cow Dung�3Weight of Material½cow Dung�514:431:7524:48C½Maize�5%C½Maize�3Weight of Material½Maize�53030:02850:84C

N½First Contdition�5

C½cow Dung�1C½Maize�N½cow Dung�1N½Maize�

524:49

(2)

N½cow Dung�5%N½cow Dung�3Weight of Material½cow Dung�50:631:2550:75N½Maize�5%N½Maize�3Weight of Material½cow Dung�50:530:150:05C½cow Dung�5%C½cow Dung�3Weight of Material½cow Dung�514:431:25518C½Maize�5%C½Maize�3Weight of Material½Maize�53030:153c

N½Second Condition�5

c½cow Dung�1c½Maize�N½cow Dung�1N½Maize�

526:25

(3)

This research employed a Low Solids System (LS) deter-mined by the type of stirrer and laboratory conditions.

In the first run, three batch digesters were fed with 2.27 L(1.70 Kg) cow dung as the main substrate; then specificamounts of crushed maize waste (0.23 L 5 0.028 Kg) wereadded as the co-substrate to each set of three digesters. Afloating drum digester was applied to produce the biogas(Figure 3).

In the second run of experiments three digesters werenourished with 1.66 L (1.25 Kg) of cow dung as the mainsubstrate and 0.84 L (0.1 Kg) crushed maize waste as the co-substrate.

RESULTS AND DISCUSSION

Results of the primary experiments indicated that 230 L/kg VS biogas was produced through a separate run in the di-gester fed only with cow dung. Figure 4 shows rates of dailybiogas production throughout the duration of the experimentfor the first mixing bed (cow dung/maize ratio of 10:1). Asshown, biogas productivity increased by passing the firststage of the process. In this mixing ratio, the hydrolysis stagelasted about 11 days and after that the second stage of bio-gasification began. Series 1–3 show the results from each di-gester in this experiment. A maximum value of 28.1 L/Kg VSbiogas was produced in this condition.

Figure 5 shows rates of daily biogas production through-out the duration of the experiment in the second mixing bedin which the cow dung to maize ratio was 10:5. As canshown, biogas productivity increased after passing the firststage of the process. In this mixing ratio, the hydrolysis stagetook about 4 days and after that second stage, the process ofbio-gasification was activated.

Figure 3. Floating drum digester. [Color figure can beviewed in the online issue, which is available atwileyonlinelibrary.com.]

Figure 4. Daily biogas production rates throughout the experimental period of first mixing bed (cow dung: maize, 10:1) andCow dung only. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

Environmental Progress & Sustainable Energy (Vol.33, No.2) DOI 10.1002/ep July 2014 599

In comparison with the first condition, the second onehad a shorter hydrolysis stage. It was caused by addition ofmaize waste rather than as a result of the initial circum-stance. The maximum value of biogas production in thiscondition was about 41.6 L/Kg VS. It should be noted that itapproximately took 11 days to produce 10 L/Kg VS biogas inthe first case, while for the second one, it decreased to 4days, probably due to an extra addition of maize waste.

Results of the evolution of methane percentage for differ-ent rates of cow dung and maize indicated that in the firstand second ratios, methane contents were in the range ofapproximately 51 and 62%, respectively.

Eze and Ojike [26] stated biogas generated from chaffs,stalks and cobs of maize waste are 67.25% CH4, 66.30% CH4,and 66.20% CH4, respectively. Dubrovskis et al. [27] in theirstudy demonstrated the average methane content of 49.6–59.3% in biogas from different samples of maize biomass.

The rate of maize waste shows that it had a significanteffect on methane content during the whole period of biogasproduction. Thereupon, methane contents increased with ris-ing of maize waste in the bed.

Depending on numerous conditions, a fairly wide rangeof methane yields, between 120 and 658 m3/t VS, wasreported by Braun from anaerobic digestion of differentcrops [28]. Recent German practical experience showedmean methane yields of 348 m3/t VS for maize [29].

In this research, biogas and methane yields from meso-philic digestion of the first condition were lower than yieldsobtained from the second one. Biogas yields were 250 and480 L/kg VS, accordingly and methane yields were 130 and300 L/kg VS, respectively after 28 days of digestion. After 21days, around 93 and 96% of the final biogas yield could beobtained, correspondingly.

As a consequence, in comparison with the biogas pro-duced during the first condition, the total biogas productionof the reactor increased by 92% when substrates were fed inthe second condition.

CONCLUSIONS

This research presented the great potential of using a mix-ture of maize waste and cow dung in anaerobic co-digestionfor methane production. The effect of adding maize on bio-gas yield of cow dung was evaluated in batch digestersunder mesophilic conditions.

Adding maize waste to cow dung in an anaerobic digesterhad three main benefits which are highlighted as follows.

First, maize waste affected the amount of biogas pro-duced from cow dung by anaerobic digestion without add-ing any chemical substances. It can be used to enhancebiogas production from 250 to 480 L/kg VS. In contrast with

the control sample, it indicated growth rates of 8 and 108%in biogas production.

Second, adding more maize waste to cow dung reducedthe hydrolysis stage of biogas production in the anaerobic di-gester from 11 to 4 days.

Third, it led to an increase in the amount of methane con-tent from 51 to 62%.

Furthermore, considering the aforementioned benefits,mesophilic anaerobic co-digestion of cow dung with maizewaste in batch digesters is an economically and environmen-tally friendly method to dispose of cow dung. This may bedue to the availability of both cow dung and maize waste inlocal farms. Nevertheless, an addition of maize waste in theco-digestion process of cow dung presents a feasible methodof improving biogas yield as well as an alternative means ofrecycling maize waste.

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