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Journal of Environmental Management 133 (2014) 268e274

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Journal of Environmental Management

journal homepage: www.elsevier .com/locate/ jenvman

Anaerobic co-digestion of food waste and dairy manure: Effectsof food waste particle size and organic loading rate

Fred O. Agyeman, Wendong Tao*

Department of Environmental Resources Engineering, College of Environmental Science and Forestry, State University of New York,1 Forestry Drive, 402 Baker Lab, Syracuse, NY 13210, USA

a r t i c l e i n f o

Article history:Received 9 November 2013Received in revised form11 December 2013Accepted 12 December 2013Available online 7 January 2014

Keywords:Anaerobic digestionDairy manureDewaterabilityFood wasteMechanical pretreatmentOrganic loading rate

* Corresponding author. Tel.: þ1 315 470 4928.E-mail address: wtao@esf.edu (W. Tao).

0301-4797/$ e see front matter � 2013 Elsevier Ltd.http://dx.doi.org/10.1016/j.jenvman.2013.12.016

a b s t r a c t

This study was to comprehensively evaluate the effects of food waste particle size on co-digestion of foodwaste and dairy manure at organic loading rates increased stepwise from 0.67 to 3 g/L/d of volatile solids(VS). Three anaerobic digesters were fed semi-continuously with equal VS amounts of food waste anddairy manure. Food waste was ground to 2.5 mm (fine), 4 mm (medium), and 8 mm (coarse) for the threedigesters, respectively. Methane production rate and specific methane yield were significantly higher inthe digester with fine food waste. Digestate dewaterability was improved significantly by reducing foodwaste particle size. Specific methane yield was highest at the organic loading rate of 2 g VS/L/d, being0.63, 0.56, and 0.47 L CH4/g VS with fine, medium, and coarse food waste, respectively. Methane pro-duction rate was highest (1.40e1.53 L CH4/L/d) at the organic loading rate of 3 g VS/L/d. The energy usedto grind food waste was minor compared with the heating value of the methane produced.

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1. Introduction

In 2011, more than 36 million tons of food waste was generatedin the U.S. (U.S. EPA, 2013). Food waste has higher biochemicalmethane potential. Anaerobic digestion of food waste not onlyproduces methane for energy recovery, but also treats waste forenvironmental and social benefits (Fuchs and Drosg, 2013; Izumiet al., 2010; Zhang et al., 2013). However, mono-digestion of foodwaste often leads to digester instability and even failure at higherorganic loading rates (OLR), especially under thermophilic condi-tions, due to accumulation of volatile fatty acids and ammonia(Banks et al., 2012; Banks et al., 2008; Ghanimeh et al., 2012; Nagaoet al., 2012; Zhang et al., 2012, 2013).

Animal feeding operations generate significant amounts of an-imal manure, which is typically applied to cropland (ASABE, 2010;USDA, 2009). Concentrated animal feeding operations often do nothave adequate land to absorb all of their manure, having to consideron-farm treatment. Anaerobic digestion is increasingly applied toliquid manure to stabilize organic matter, reduce pathogens,eliminate offensive odors, and recover energy from methane(USDA, 2009; U.S. EPA, 2010). However, cattle manure contains high

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contents of non-biodegradable substances and has low C/N ratios(Frear et al., 2010; Zhang et al., 2012, 2013), thus having a lowmethane yield in anaerobic mono-digestion of cattle manure (El-Mashad and Zhang, 2010; Frear et al., 2010; Hartmann andAhring, 2005). Banks et al. (2011a) recommended on-farm co-digestion of dairy cattle slurry and source-separated domestic foodwaste as the most effective means of making dairy cattle slurrydigestion economically viable. Co-digestion of cattle manure andfood waste can increase biogas production and improve processstability (El-Mashad and Zhang, 2010; Zhang et al., 2012, 2013).

Hydrolysis is generally the rate-limiting stage in anaerobicdigestion of organic solid waste (Angelidaki and Sanders, 2004;Izumi et al., 2010; Palmowski and Müller, 2000). Good contactbetween biomass and substrate is a prerequisite for hydrolysisbecause the organisms secreting hydrolytic enzymes are benefitedby adsorption to the surface of particulate substrates (Angelidakiand Sanders, 2004). Methanogens in anaerobic digestion offlushed dairy manure have high affinity to fibrous solids as well(Frear et al., 2010). Reducing substrate particle size through pre-treatment such as grinding could increase surface area available foradsorption of hydrolytic enzymes and subsequently produce morebiogas (Izumi et al., 2010; Kim et al., 2000; Palmowski and Müller,2000). However, excessive particle size reduction could over-stimulate hydrolysis and acidogenesis, resulting in accumulationof ammonia and volatile fatty acids which could become inhibitory

F.O. Agyeman, W. Tao / Journal of Environmental Management 133 (2014) 268e274 269

to methanogens. The effects of particle size on anaerobic digestionof food waste were investigated in two studies only through shortbatch tests (Izumi et al., 2010; Kim et al., 2000). The effects of foodwaste particle size have neither been addressed in continuously fedflow-through anaerobic digesters, nor in co-digestion of food wasteand dairy manure. Moreover, the additional energy consumption toproduce finer particles and dewaterability of digester effluent hasnot been reported along with the effect of particle size on methaneproduction. The major objective of this study was to assess theeffects of food waste particle size on anaerobic co-digestion of foodwaste and dairy manure in continuously fed anaerobic digesters atdifferent OLRs. The effects were assessed comprehensively in termsof energy consumption for grinding food waste, biogas productionrate, specific methane yield, reduction efficiency for volatile solids(VS), and digestate dewaterability over four periods as OLRs wereincreased stepwise from 0.67 g VS/L/d to 3 g VS/L/d.

Successful long-term mono-digestion of food waste has beentypically limited to OLRs below 2.5 g VS/L/d unless enhancementmeasures such as supplementation of trace elements, solids returnand co-digestion are taken (Banks et al., 2011b, 2012; Ghanimehet al., 2012; Nagao et al., 2012; Zhang et al., 2012). A number ofstudies have addressed methane production and ammonia inhibi-tion in co-digestion of food waste and cattle manure at differentsubstrate combination ratios and OLRs (El-Mashad and Zhang,2010; Hartmann and Ahring, 2005; Marañón et al., 2012; Zhanget al., 2012, 2013). Nevertheless, the combined effect of OLR andfood waste particle size in stable co-digestion of food waste anddairy manure is unknown.

Treatment and disposal of digestate account for a great portionof the operational cost of pilot- and full-scale anaerobic digestionprojects (Fuchs and Drosg, 2013). Digestate processing can becomea bottleneck to scaled-up applications (Gebrezgabhera et al., 2010).Typically, digestate is separated into liquid and solids by filtration,screw pressing, or centrifugation. However, dewaterability ofdigestate has rarely been addressed. This paper evaluates digestatedewaterability along with methane production and solid removal.

Table 1Characteristics of inoculum and feedstock made from dairy manure and food waste.

Dairymanure

Foodwaste

Feedstock Inoculum

2. Materials and methods

2.1. Setup and operation of anaerobic digesters

Three 2-L complete-mix anaerobic digesters were set up in alaboratory. Each digester as shown in Fig. 1 was built with amodified Duran GLS80 glass reactor with a magnetic impeller. AThermo scientific hotplate/stirrer was used to heat each digester

Hotplate/stirrer

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Fig. 1. Sketch of a bench-scale, semi-continuously fed anaerobic digester.

and drive its impeller at 140 rpm. The digestate temperature wastargeted at 36 �C. The digesters were initially filled with bacterialinoculum to a working volume of 1.8 L. The inoculum was madefrom anaerobically digested sludge from a municipal wastewatertreatment plant and anaerobically digested dairy manure withcoarse materials (>2.06 mm) sieved out. The inoculum had a VSconcentration of 1.33%, with one half (by mass) from the digestedsludge and the other half from the digested manure. Comparedwith food waste and dairy manure separately, it had a slightly basicpH and generally balanced concentrations of macro- and micro-nutrients (Table 1).

Based on earlier studies (El-Mashad and Zhang, 2010; Zhanget al., 2012, 2013), it appears that a VS ratio of manure to foodwaste around 1 is the optimum combination for co-digestion ofcattle manure and food waste. The feedstock used in this study wasprepared by combining domestic food waste and dairy manure at aVS ratio of 1 and stored frozen at �21 �C in plastic tubes. Table 1summarizes the characteristics of the feedstock and its two com-ponents. The food waste was collected from a Sheraton Hotel’srestaurant over five days and ground through a MG800 Waring ProProfessional meat grinder with three cutting plates having differentaperture diameters (2.5, 4, and 8 mm) for the three digesters, calledfine, medium and coarse food waste, respectively. Energy con-sumption to grind food waste was recorded with a Watts up? PROelectricity watt meter (Electronic Educational Devices, Inc., Denver,CO, USA). Dairy manure was taken from a storage vessel of liquidmanure which was scraped from concrete lots of a large-size dairyfarm at Cayuga County of New York, USA.

The digesters were operated in a semi-continuous mode. Thefeedstock was thawed in a refrigerator at 4 �C and fed to the di-gesters every 2 d. OLR was increased from 0.67 g VS/L/d to 1, 2, and3 g VS/L/d stepwise over 178 d of operation. Digestate (67e90 mL)was discharged every 6 d at the OLRs of 0.67 and 1 g VS/L/d, 4 d at2 g VS/L/d, and 2 d at 3 g VS/L/d. This study aimed at dry digestion.Only 25e45 mL of tap water was used to wash the feedstock stor-age tubes and maintain the working volume after discharge.Hydraulic retention time or solids retention time was 160 d atthe OLRs of 0.67 and 1 g VS/L/d, 80 d at 2 g VS/L/d, and 54 d at3 g VS/L/d.

pH 6.6 4.4 6.6 7.7Total volatile solids, % 9.68 29.3 14.6 1.33Total dissolved solids, g/L 16.9 16.9 16.8 7.52Crude protein, g/kg VS 167 266 273 335Fat, g/kg VS 40 350 231 90Non-fiber carbohydrate,

g/kg VS623 325 380 543

Neutral detergent fiber,g/kg VS

616 196 291 543

Total N, %TS 1.9 3.8 3.6 3.0Total C, %TS 39.9 48.4 46.3 32.4Orthophosphate, g P/L 0.78 No data No data 0.33Total ammonia, g N/L 1.71 No data No data 1.68Sulfur, g/kg TS 6.1 3.4 4.8 11.9Total Ca, g/kg TS 20.6 1.7 13.3 26.0Total Mg, g/kg TS 8.5 0.7 5.2 12.2Total K, g/kg TS 23.8 9.6 18.0 20.3Total Na, g/kg TS 7.25 10.1 8.9 12.1Total Fe, mg/kg TS 705 41 374 18300Total Zn, mg/kg TS 233 32 136 900Total Cu, mg/kg TS 123 5 46 569Mn, mg/kg TS 176 8 98 269Mo, mg/kg TS 1.6 0.3 1.2 11.3

F.O. Agyeman, W. Tao / Journal of Environmental Management 133 (2014) 268e274270

2.2. In-situ measurements and laboratory analyses

Each reactor had a head space of 0.69 L at theworking volume of1.8 L. Biogas production was recorded with in-line gas meters andconverted to daily production rate under standard conditions (stp:0 �C and 760 mm Hg). While sampling biogas, headspace temper-ature was measured with a Hach H160 pH meter connected to anISFET pH stainless steel NMR tube probe. Biogas samples (0.1 mLeach) were collected with a gas-tight syringe through rubber septaand diluted with air in 10-mL Wheaton serum vials. The biogassamples along with air samples were analyzed for CH4 and CO2percentages using a Shimadzu GC-2014 gas chromatograph systemwith a flame ionization detector (Shimadzu Corporation, Tokyo,Japan). Helium was used as carrier gas. The detection limits were0.1 ppm for CH4 and 10 ppm for CO2.

Digestate temperature and pH were measured with the pHmeter while collecting biogas samples. Digester effluent sampleswere collected for determination of total solids and total volatilesolids concentrations, following Standard Methods 2540 B and E,respectively (APHA, 1998). Total dissolved solids concentration was

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Fig. 2. Variations of (a) biogas production rate and (b) specific biogas yield with food waste pdigestion of dairy manure and food waste.

measured with a Hach HQ40d meter. After separating suspendedsolids in the digester effluent via centrifugation at 1600 g for30 min, total ammonia concentration in the centrate was deter-mined colorimetrically with a Hach DR 2800 spectrophotometer(Hach Company, Loveland, Colorado, USA). Free ammonia concen-tration in digestate was calculated with measured total ammoniaconcentration, temperature and pH (Pitk et al., 2013). Time-to-filterwas determined with the small-volume Standard Method 2710H(APHA, 1999) to reveal dewaterability of digester effluent.

The feedstock and its components as well as the inoculumweremeasured for pH, total solids, total volatile solids, total dissolvedsolids, and total ammonia, using the same methods as mentionedabove. Crude proteins, fats, neutral detergent fiber, and non-fibercarbohydrates were determined at Dairy One Forage Laboratory(Ithaca, New York), following AOAC International standards. Totalcarbon and nitrogen contents were determined for oven-driedsamples with an elemental analyzer (Calo Erba NC2500, CostechAnalytical Technologies Inc., Valencia, California, USA). Ortho-phosphate in centrate of the inoculum and dairy manurewas determined colorimetrically with the Hach DR2800

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article size (fine, medium, and coarse) and organic loading rate (OLR) in mesophilic co-

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Fig. 3. Effects of food waste particle size (fine, medium, and coarse) on digestereffluent solid concentrations in co-digestion of dairy manure and food waste atincreasing organic loading rates (OLR).

F.O. Agyeman, W. Tao / Journal of Environmental Management 133 (2014) 268e274 271

spectrophotometer. The other macro- and micro-nutrients wereanalyzed with an inductively coupled plasma radial spectrometerafter microwave digestion.

2.3. Statistical analysis

One-way analysis of variance (ANOVA) was performed todetermine whether there were statistically significant differencesamong the three digesters. The significance level (p value) was setat 5%. If there were significant differences, least significant differ-ence (LSD) was calculated to further identify the pairs of means thathad significant differences (Townend, 2002). Spearman’s rankcorrelation analysis was performed to assess the relationship ofbiogas production with digestate ammonia concentration, givingcorrelation coefficient r. Spearman rank trend test was performed

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Fig. 4. Dynamics of biogas composition in anaerobic co-digestion of dairy man

with a backward elimination approach to identify the period ofstable operation at a given OLR (Townend, 2002).

3. Results and discussion

The first 52 days of digester operation at the OLR of 0.67 g VS/L/d was taken as a startup phase and not monitored regularly. Resultsfrom 42 d of anaerobic digestion at each of the successive OLRswere used to diagnose the effects of food waste particle size,evaluate methane production and treatment performance, andidentify the optimum OLR. As illustrated in Fig. 2 for biogas pro-duction rate and specific biogas yield, trend tests showed that ittook more than 19, 15, and 10 d to reach stable operation at theOLRs of 1, 2, and 3 g VS/L/d, respectively. Whenever OLR wasincreased, there was an increase in microbial biomass as reflectedby the effluent VS concentrations (Fig. 3). Both biogas productionrate and specific yield increased over time initially and becamestable, likely as microorganisms were acclimated, at given OLRs(Fig. 2).

3.1. Performance of co-digestion

The co-digestion in this study attained higher methane contentsin biogas (Fig. 4) compared with earlier studies on co-digestion ofcattle manure and food waste (El-Mashad and Zhang, 2010;Hartmann and Ahring, 2005; Zhang et al., 2012, 2013). Moreover,this study achieved higher specific methane yields during the sta-ble operation periods (0.46e0.63 L CH4/g VS) compared with thosein most earlier studies on co-digestion of food waste and cattlemanure (0.14e0.46 L CH4/g VS). The higher methane yield in thisstudy relative to those in the earlier studies could be attributed tothe long solids retention times associated with dry digestion andthe relatively higher lipid content of the feedstock in this study(Table 1). This study also confirmed the synergistic effect of co-digestion with the higher specific methane yields compared with�0.46 L CH4/g VS in mono-digestion of food wastewithout additionof trace elements (Banks et al., 2011b, 2012; Ghanimeh et al., 2012;Nagao et al., 2012; Zhang et al., 2012, 2013) and�0.25 L CH4/g VS inmono-digestion of cattle manure (El-Mashad and Zhang, 2010;Frear et al., 2010; Hartmann and Ahring, 2005; Zhang et al.,2013). Specific methane yield in anaerobic digestion of foodwaste has been limited mainly because of the operational

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/d 2.0 g VS/L/d 3.0 g VS/L/d

ure and different particle sizes of food waste (fine, medium, and coarse).

F.O. Agyeman, W. Tao / Journal of Environmental Management 133 (2014) 268e274272

instability at higher organic loading rates (Banks et al., 2012; Zhanget al., 2012). High fiber content is the main reason for lowmethanepotential of cow manure (El-Mashad and Zhang, 2010; Frear et al.,2010).

The methane production rates during the stable operation pe-riods at the final OLR of 3 g VS/L/d were 1.53, 1.41, and 1.40 L CH4/L/d in the digesters with fine, medium and coarse food waste,respectively. Like specific methane yield, the methane productionrates of co-digestion in this study were higher compared to those inmono-digestion of food waste, �1.39 L CH4/L/d (Banks et al., 2011b,2012; Ghanimeh et al., 2012; Zhang et al., 2013), and mono-digestion of dairy manure, �0.10 L CH4/L/d (Zhang et al., 2013).

Most enzymes and co-enzymes need a minimal amount ofcertain trace elements for their activation and activity (Appels et al.,2008). As Table 1 shows, dairy manure generally has higherconcentrations of macro- and micro-nutrients than food waste.Combination of dairy manure and food waste improves availabilityof nutrients for anaerobic digestion of food waste. Such combina-tion of digester feed also resulted in pH values (Fig. 5a) non-

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Fig. 5. Dynamics of (a) digestate pH; (b) total ammonia concentration; and (c) freeammonia concentration in co-digestion of dairy manure and different particle sizes offood waste (fine, medium, and coarse) at increasing organic loading rates (OLR).

inhibitory to methanogens (Angelidaki and Sanders, 2004) andbalanced C/N ratios.

It is challenging to separate solids from liquid in anaerobicallydigested dairy manure, which has time-to-filter at 246e348 min(Xia et al., 2012). Anaerobically digested sludge has time-to-filter ata few minutes (Cheumbarn and Pagilla, 2000; Zhang et al., 2010).The digesters in this study started up at their full working volumeswith anaerobically digested dairy manure and digested sludge,which had time-to-filter (initial values in Fig. 6) slightly shorterthan that of anaerobically digested dairy manure. Time-to-filterdecreased considerably across the OLRs of 1, 2 and 3 g VS/L/d inthe co-digestion (Fig. 6), indicating improved dewaterability ofdigester effluent compared with mono-digestion of dairy manure.

3.2. Effects of food waste particle size

Table 2 presents biogas production during the stable operationperiods. The average biogas production rates of the three digesterswere significantly different (p� 0.05) at the OLRs of 1 g VS/L/d (LSD¼ 0.06 L/L/d), 2 g VS/L/d (LSD¼ 0.08 L/L/d), and 3 g VS/L/d (LSD¼ 0.06 L/L/d). The three digesters had significantly differentspecific biogas yields (p� 0.05) at the OLRs of 1 g VS/L/d (LSD¼ 0.06 L/g VS), 2 g VS/L/d (LSD¼ 0.04 L/g VS), and 3 g VS/L/d (LSD¼ 0.02 L/g VS) as well. The digester with fine food waste hadmethane production rate 10e29% higher and specific methaneyield 9e34% higher than those with coarse food waste. Althoughthe biogas production rate and specific yield in the digester withmedium food waste fell between those with fine and coarse foodwaste, the differences were only statistically significant at the OLRof 2 g VS/L/d. Food waste particle size did not make significantdifferences in methane content of biogas (p¼ 0.30e0.63) except fora significantly lower average methane content in the digester withcoarse food waste at the OLR of 2 g VS/L/d (Table 2).

Energy consumed for grinding food waste to fine particleswas 0.130Wh/g VS, 0.069Wh/g VS to medium particles, and0.054 Wh/g VS to coarse particles. The lower heating value ofmethane is 10.67 Wh/L under the standard conditions (Metcalf andEddy Inc, 2003). Considering the specific methane yields at the OLRof 3 g VS/L/d (Table 2), energy consumption for grinding amountedto only 1.1e2.4% of energy carried by methane produced fromanaerobic digestion. It is, therefore, cost-effective to grind food

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Fig. 6. Effect of food waste particle size (fine, medium, and coarse) on digester effluentdewaterability in co-digestion of dairy manure and food waste.

Table 2Biogas production in co-digestion of dairy manure with different particle sizes of food waste during stable operation periods at increasing organic loading ratesa.

Biogas production rate (stp L/L/d) Specific biogas yield (stp L/g VS) CH4 content in biogas (%) Specific CH4 yield (stp L/g VS)

Organic loading rate¼ 1 g VS/L/d from day 52 to day 94Fine food waste 0.79� 0.06 0.79� 0.06 67.5� 6.9 0.53� 0.04Medium food waste 0.74� 0.08 0.74� 0.08 63.7� 11.7 0.47� 0.05Coarse food waste 0.72� 0.05 0.72� 0.05 64.2� 4.2 0.46� 0.03

Organic loading rate¼ 2 g VS/L/d from day 94 to day 136Fine food waste 1.69� 0.05 0.85� 0.02 74.1� 5.3 0.63� 0.02Medium food waste 1.60� 0.06 0.80� 0.03 70.3� 5.8 0.56� 0.02Coarse food waste 1.45� 0.14 0.73� 0.07 64.9� 5.6 0.47� 0.05

Organic loading rate¼ 3 g VS/L/d from day 136 to day 178Fine food waste 2.12� 0.07 0.71� 0.02 72.2� 4.0 0.51� 0.02Medium food waste 2.03� 0.06 0.68� 0.02 69.5� 5.4 0.47� 0.01Coarse food waste 2.00� 0.09 0.67� 0.03 69.8� 4.5 0.47� 0.02

a Mean� standard deviation.

F.O. Agyeman, W. Tao / Journal of Environmental Management 133 (2014) 268e274 273

waste into finer particles for greater methane yield in co-digestionof food waste with dairy manure.

The effects of particle size on biogas production are attributed tothe larger specific surface area provided by smaller particles forenhanced hydrolysis (Izumi et al., 2010; Palmowski and Müller,2000). The optimum particle size for anaerobic mono-digestion offood waste has been examined in different ranges. Kim et al. (2000)reported that the maximum substrate utilization rate coefficientdoubled as the average particle size decreased from 2.14 to 1.02 mmin thermophilic batch digestion tests. Izumi et al. (2010) investi-gated the effect of particle size in a narrow range (0.391e0.888 mm) and found that the optimum particle size (0.718 mm)resulted in 28% more biogas than that with the worst particle size(0.888 mm) in mesophilic batch digestion tests. The effects ofparticle size should be further addressed in a wider range withregard to not only methane production, but also energy con-sumption for particle size reduction and digestate dewaterability inthe future.

The long time-to-filter as presented in Fig. 6 manifests the dif-ficulty to dewater the digester effluent. Nevertheless, there weresignificant differences in time-to-filter among the three digesters atall the three OLRs (p� 0.001; LSD¼ 7e10 min). The shortest wasrecorded in co-digestionwith fine foodwaste and the longest in co-digestion with coarse food waste. This was consistent with thepositive impact of organic waste particle size on dewaterability ofdigestate as Mata-Alvarez et al. (2000) reported.

3.3. Optimum organic loading rate

As Fig. 2 shows, biogas production rate increased withincreasing OLR. As Table 2 shows for the periods of stable operation,however, biogas production rate increased by 101e116% when OLRwas increased from 1 to 2 g VS/L/d and only by 25e38% when OLRwas further increased from 2 to 3 g VS/L/d. Specific methane yieldpeaked at the OLR of 2 g VS/L/d in the digesters with fine andmedium food waste. Similarly, earlier studies (Hartmann andAhring, 2005; Zhang et al., 2013, 2012) on co-digestion of cattlemanure and food waste at higher organic loading rates (3.3e16 g VS/L/d) attained lower specific methane yield (0.14e0.41 CH4/g VS). Therefore, the optimum OLR for deep co-digestion of dairymanure and food waste was close to 3 g VS/L/d, although OLR couldbe increased further for higher methane production rates.

Theoretical methane potential under the standard conditionswas estimated to be 0.53 L CH4/g VS for the feedstock in this study,based on biochemical composition of the feedstock (Table 1) andmethane potentials of proteins, lipids and carbohydrates as sug-gested by Angelidaki and Sanders (2004). The methane yieldsexperimentally determined in this study were 89e119% of the

estimated methane potential, suggesting little limitation of traceelements and inhibition due to ammonia and volatile fatty acids.

Anaerobic digestion converts organic nitrogen to ammonia,which exists in ionized ammonium and free ammonia, dependingon pH and temperature. Free ammonia inhibits more than ionizedammonium to methanogens (Yenigun and Demirel, 2013). Thehydrophobic free ammonia may diffuse into cells, causing protonimbalance and potassium deficiency in microorganisms, particu-larly in methanogens (Pitk et al., 2013). Ammonia could be carriedwith liquid dairy manure and inoculum into mixed liquor andproduced in anaerobic digestion, resulting in inhibition to meth-anogenesis at high OLRs. The inoculum and dairy manure in thisstudy had high total ammonia concentrations, 1.7 g N/L (Table 1). AsFig. 5b shows, total ammonia concentration in the digester effluenttended to increase up to 3090e3420 mgN/L after 178 d of opera-tion. Free ammonia concentration increased rapidly to 202e340 mgN/L at the OLR of 1 g VS/L/d, decreased at the OLR of 2 g VS/L/d, then stabilized between 148 and 237 mgN/L at the OLR of3 g VS/L/d (Fig. 5c). As reviewed by Yenigun and Demirel (2013),ammonia inhibition to mesophilic anaerobic digestion with accli-mated inoculum is triggered at very different concentrations,mostly from 2800 to 6000 mgN/L total ammonia and 337e800 mgN/L free ammonia (Yenigun and Demirel, 2013). Therefore,ammonia concentration in this study might not be high enough tosignificantly affect biogas production. There were insignificantcorrelations between biogas production rate and biogas yieldwith either total ammonium or free ammonia concentrations(r< critical r).

There were insignificant differences in effluent solids concen-trations between the digesters (p¼ 0.08e0.64). Total dissolvedsolids concentrations were reduced by 40.7e42.6% and total vola-tile solids concentrations by 80.9e82.7% on average in the threedigestersat at the OLR of 1 g VS/L/d. The concentrations of totaldissolved solids and total volatile solids in digester effluentincreased with the increasing OLRs (Fig. 3). At the end of the periodwith the OLR of 3 g VS/L/d, the reduction efficiencies decreased to18.2e23.0% for total dissolved solids and 66.6e69.6% for total vol-atile solids through the three digesters. Solids removal efficiency atthe end of this study became moderate compared with that inmono-digestion of food waste and dairy manure at similar OLRs(Hartmann and Ahring, 2005; Nagao et al., 2012). This suggestsagain that the optimum OLR is approximately 3 g VS/L/d for mes-ophilic co-digestion of dairy manure and food waste.

4. Conclusions

Reduction of foodwaste particle size from8 to 2.5 mm increasedmethane production rate by 10e29% and specific methane yield by

F.O. Agyeman, W. Tao / Journal of Environmental Management 133 (2014) 268e274274

9e34% in co-digestion of dairy manure and food waste. Dewater-ability of digester effluent was significantly improved by reducingfood waste particle size. The energy consumed to grind food wastedown to 2.5 mm was minor compared to the heating value of themethane produced.

The co-digestion could be loaded up to 3 g VS/L/d withoutammonia inhibitionwhile reducingmore than 67% of volatile solidsand producing 1.40e1.53 L CH4/L/d. The highest specific methaneyields, however, were achieved at the OLR of 2 g VS/L/d.

Acknowledgments

This study was supported by a fellowship to Fred Agyeman fromFord Foundation-IFP and Association of African Universities. Theresearch was partially supported by a U.S. EPA grant to Dr. Tao(SU835331). We would like to thank Dr. Philippe Vidon, Mr. DavidKiemle, and Pat Rook for their help with biogas analysis. Our thanksalso go to Mr. Steve McGlynn at Twin Birch Diary and Mr. Paul Enoat Sheraton Syracuse University Hotel for providing with thefeedstock materials.

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