solid state anaerobic co-digestion of yard waste and food waste for biogas production

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Solid state anaerobic co-digestion of yard waste and food waste for biogas production Dan Brown, Yebo Li Department of Food, Agricultural, and Biological Engineering, The Ohio State University/Ohio Agricultural Research and Development Center, 1680 Madison Ave., Wooster, OH 44691-4096, United States highlights " Solid state anaerobic digestion (SS-AD) of food waste and yard waste. " The highest methane yields were obtained at feedstock/effluent (F/E) ratio 1. " Increasing F/E ratio from 1 to 2 and 3 caused decreases in methane yield. " The AD was upset at F/E ratio 3 except yard waste only. article info Article history: Received 21 June 2012 Received in revised form 16 September 2012 Accepted 22 September 2012 Available online 29 September 2012 Keywords: Solid-state anaerobic digestion Biogas Municipal solid waste Food waste Co-digestion abstract Food and yard wastes are available year round at low cost and have the potential to complement each other for SS-AD. The goal of this study was to determine optimal feedstock/effluent (F/E) and food waste/yard waste mixing ratios for optimal biogas production. Co-digestion of yard and food waste was carried out at F/E ratios of 1, 2, and 3. For each F/E ratio, food waste percentages of 0%, 10%, and 20%, based on dry volatile solids, were evaluated. Results showed increased methane yields and volumet- ric productivities as the percentage of food waste was increased to 10% and 20% of the substrate at F/E ratios of 2 and 1, respectively. This study showed that co-digestion of food waste with yard waste at spe- cific ratios can improve digester operating characteristics and end performance metrics over SS-AD of yard waste alone. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Solid-state anaerobic digestion (SS-AD) has been successfully used to convert various lignocellulosic biomass feedstocks to bio- gas (Li et al., 2011a). SS-AD has been the dominant AD system in- stalled in Europe since the early 1990s for the treatment of municipal solid waste (MSW), and typically operates at 15–50% to- tal solids (TS) content (Li et al., 2011b; Baere and Mattheeuws, 2008; Guendouz et al., 2010). SS-AD provides many benefits over liquid AD in digesting lignocellulosic biomass such as treating more organic solids in the same size digester and producing a com- post-like finished organic material that is easier to handle and can be applied to agricultural land for fertilizer (Martin et al., 2003a; Li et al., 2011b). The SS-AD system also features fewer moving parts and lower energy inputs needed for heating and mixing (Li et al., 2011a), and it has a greater acceptance of inputs containing glass, plastics, and grit. Furthermore, SS-AD can overcome other common problems existing in the liquid AD process such as floating and stratification of fats, fibers, and plastics (Chanakya et al., 1999). The start-up period of an SS-AD system is considered the most critical phase in batch digestion. The feedstock/effluent (F/E) ratio, an operating parameter that measures the amount of substrate to the amount of inoculum on a dry volatile solids (VS) basis, has been shown to be a critical factor affecting the performance of SS-AD (Li et al., 2011b). SS-AD may require up to 50% of digested residue for a rapid startup, which decreases reactor utilization efficiency (Martin et al., 2003b; Rapport et al., 2008; Li et al., 2011b). A highly concentrated and active inoculum source is important to reduce digestion time, improve digester efficacy, and increase TS in the finished product (Forster-Carneiro et al., 2008). Co-digestion of mixed substrates offers many advantages, including ecological, technological, and economic benefits, com- pared to digesting a single substrate (Rughoonundun et al., 2012). However, combining two or more different types of feedstocks requires careful selection to improve the efficiency of 0960-8524/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2012.09.081 Corresponding author. Tel.: +1 330 263 3855. E-mail address: [email protected] (Y. Li). Bioresource Technology 127 (2013) 275–280 Contents lists available at SciVerse ScienceDirect Bioresource Technology journal homepage: www.elsevier.com/locate/biortech

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Page 1: Solid state anaerobic co-digestion of yard waste and food waste for biogas production

Bioresource Technology 127 (2013) 275–280

Contents lists available at SciVerse ScienceDirect

Bioresource Technology

journal homepage: www.elsevier .com/locate /bior tech

Solid state anaerobic co-digestion of yard waste and food wastefor biogas production

Dan Brown, Yebo Li ⇑Department of Food, Agricultural, and Biological Engineering, The Ohio State University/Ohio Agricultural Research and Development Center, 1680 Madison Ave.,Wooster, OH 44691-4096, United States

h i g h l i g h t s

" Solid state anaerobic digestion (SS-AD) of food waste and yard waste." The highest methane yields were obtained at feedstock/effluent (F/E) ratio 1." Increasing F/E ratio from 1 to 2 and 3 caused decreases in methane yield." The AD was upset at F/E ratio 3 except yard waste only.

a r t i c l e i n f o

Article history:Received 21 June 2012Received in revised form 16 September 2012Accepted 22 September 2012Available online 29 September 2012

Keywords:Solid-state anaerobic digestionBiogasMunicipal solid wasteFood wasteCo-digestion

0960-8524/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.biortech.2012.09.081

⇑ Corresponding author. Tel.: +1 330 263 3855.E-mail address: [email protected] (Y. Li).

a b s t r a c t

Food and yard wastes are available year round at low cost and have the potential to complement eachother for SS-AD. The goal of this study was to determine optimal feedstock/effluent (F/E) and foodwaste/yard waste mixing ratios for optimal biogas production. Co-digestion of yard and food wastewas carried out at F/E ratios of 1, 2, and 3. For each F/E ratio, food waste percentages of 0%, 10%, and20%, based on dry volatile solids, were evaluated. Results showed increased methane yields and volumet-ric productivities as the percentage of food waste was increased to 10% and 20% of the substrate at F/Eratios of 2 and 1, respectively. This study showed that co-digestion of food waste with yard waste at spe-cific ratios can improve digester operating characteristics and end performance metrics over SS-AD ofyard waste alone.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Solid-state anaerobic digestion (SS-AD) has been successfullyused to convert various lignocellulosic biomass feedstocks to bio-gas (Li et al., 2011a). SS-AD has been the dominant AD system in-stalled in Europe since the early 1990s for the treatment ofmunicipal solid waste (MSW), and typically operates at 15–50% to-tal solids (TS) content (Li et al., 2011b; Baere and Mattheeuws,2008; Guendouz et al., 2010). SS-AD provides many benefits overliquid AD in digesting lignocellulosic biomass such as treatingmore organic solids in the same size digester and producing a com-post-like finished organic material that is easier to handle and canbe applied to agricultural land for fertilizer (Martin et al., 2003a; Liet al., 2011b). The SS-AD system also features fewer moving partsand lower energy inputs needed for heating and mixing (Li et al.,2011a), and it has a greater acceptance of inputs containing glass,

ll rights reserved.

plastics, and grit. Furthermore, SS-AD can overcome other commonproblems existing in the liquid AD process such as floating andstratification of fats, fibers, and plastics (Chanakya et al., 1999).

The start-up period of an SS-AD system is considered the mostcritical phase in batch digestion. The feedstock/effluent (F/E) ratio,an operating parameter that measures the amount of substrate tothe amount of inoculum on a dry volatile solids (VS) basis, has beenshown to be a critical factor affecting the performance of SS-AD (Liet al., 2011b). SS-AD may require up to 50% of digested residue fora rapid startup, which decreases reactor utilization efficiency(Martin et al., 2003b; Rapport et al., 2008; Li et al., 2011b). A highlyconcentrated and active inoculum source is important to reducedigestion time, improve digester efficacy, and increase TS in thefinished product (Forster-Carneiro et al., 2008).

Co-digestion of mixed substrates offers many advantages,including ecological, technological, and economic benefits, com-pared to digesting a single substrate (Rughoonundun et al.,2012). However, combining two or more different types offeedstocks requires careful selection to improve the efficiency of

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276 D. Brown, Y. Li / Bioresource Technology 127 (2013) 275–280

anaerobic digestion (Álvarez et al., 2010). The purpose of co-diges-tion is to balance nutrients (C/N ratio and macro- and micronutri-ents) and dilute inhibitors/toxic compounds to enhance methaneproduction (Hartmann et al., 2004; Sosnowski et al., 2003).

Xu and Li (2012) found that an F/E ratio of 2 achieved higheraccumulative methane yields than at higher F/E ratios of 4 and 6for the same dog food to corn stover ratio. The study also foundthat co-digestion improved methane yield compared with usingcorn stover or dog food as the sole substrate due to improvementsin reactor characteristics. The study also concluded that co-diges-tion of dog food with corn stover reduced start-up time and vola-tile fatty acid (VFA) accumulation in SS-AD. A study by El-Mashad and Zhang (2010) found that inclusion of food waste, atrates of up to 60% of feedstock VS, with dairy manure resulted inhigher biogas yields and production rates as compared to thedigestion of dairy manure alone. Panichnumsin et al. (2010) exam-ined the potential of co-digestion of cassava pulp with pig manureusing a semi-continuously fed, stirred tank reactor. The studyfound a maximum methane yield and VS removal of 306 L/kgVSadded and 61%, respectfully, when the cassava pulp accountedfor 60% of the feedstock VS. However, at higher (>X%) cassava pulpratios, the reactor failed due to rapid VFA accumulation and insuf-ficient buffering capacity.

In the United States, MSW such as yard and food wastes, whichare available year round, are often landfilled, incinerated, or com-posted. Food waste is the largest waste stream in MSW, exceptfor recyclables, and accounted for 14.3% (34.7 million tons) of thetotal MSW in 2009 (USEPA, 2011). Collection of food waste fromrestaurants, grocery stores, and processing plants, which are singlelarge sources, can ease logistical issues and reduce collection costscompared to household pick up. Yard waste, which includes grass,leaves, and various wood chips, had a total annual availability inthe United States of 30.9 million metric tons (28 million tons) (Mil-brandt, 2005). Collection of yard waste from cities and town’s col-lection service and tree trimming businesses can provide a largeamount of yard waste at low cost. Grass, leaves, and maple woodchips were determined to have C/N ratios of 17, 11, and 567,respectively (Michel et al., 1993; Wong et al., 2001; Zagury et al.,2006). Food waste collected from restaurants, which was foundto have a C/N ratio of 15 (Zhang et al., 2007), could be added to bal-ance the C/N ratio of yard waste. The final mixture of liquid ADeffluent, yard waste, and food waste should have a C/N ratio inthe range of 20–30 for optimum microbial performance. In orderto maximize biogas production, the volumetric loading of foodwaste should be maximized . Increasing the volumetric loadingof food waste can be accomplished by: (1) increasing the F/E ratiowith a constant substrate composition that includes a certain per-centage of food waste, (2) increasing the percentage of food wastein the feedstock while keeping the F/E ratio constant, or (3) com-bining these two approaches.

Currently, there are no reported studies on solid-state co-diges-tion of food waste with yard waste. This study could provide base-line data for the adoption of SS-AD in the United States usinginexpensive and available feedstocks that complement each other.Therefore, the major objective of this study was to determine meth-ane yields and volumetric productivities for solid state co-digestionof different food waste to yard waste ratios at different F/E ratios.

2. Methods

2.1. Feedstock and inoculum

Yard waste was obtained in June 2011 from the OARDC Woostercampus and contained leaves and tree branches. The feedstock wasoven dried at 40 �C for 48 h in a convection oven (Precision Thelco

Model 18, Waltham, MA) to obtain a moisture content of less than10%, and then ground with a hammer mill (Mackisik, Parker Ford,PA) to pass through a 5 mm screen, and stored in air tight contain-ers until used. Food waste was collected in August 2011 from thefeeding hopper of quasar energy group’s liquid anaerobic digesterin Wooster, Ohio, USA. The food waste originated from severalWal-Mart grocery stores nearby. The food waste collected wascut and ground up using a standard kitchen blender. Food wastewas stored in air-tight buckets at 4 �C in a walk-in cooler untilused.

Effluent from a mesophilic liquid AD system fed with foodwastes, fats, oils and greases (FOG), and sewage sludge (operatedby quasar energy group in Columbus, OH, USA) was used as inoc-ulum. Due to the low TS content (7.7%), the effluent was centri-fuged (Thermo Scientific Sorvall Legend T+) at 3500 rpm (2634g)for 30 min to obtain the required TS content of 15%. The decantedliquids were removed from the solids by turning the plastic con-tainers (600 ml for each) upside down and letting the liquid por-tion run out. The solids attached at the bottom of the containerwere collected to be used as inoculum for SS-AD. Effluent was keptin air-tight buckets at 4 �C in a walk-in cooler. Prior to use, theinoculum was starved for 1 week and incubated at 37 �C to reacti-vate microbiological activity and remove the easily degradable VS.

2.2. Solid-state anaerobic digestion

The effect of F/E ratios (1, 2 and 3) and percentage of food waste(0%, 10%, 20%, based on dry VS) in the feedstocks on the perfor-mance of SS-AD was studied. A wide range of volumetric loadingrates of food waste (0.0–166 g/L) was studied in the digesters.The inoculum, food waste, and yard waste were mixed by ahand-mixer (Black & Decker, 250 watt mixer, Towson, MD, USA)for 10 min. Well-mixed materials were loaded into a 1 L glass reac-tor and incubated in a walk-in incubation room for up to 30 days at36 ± 1 �C. Duplicate reactors were run for each condition. Inoculumwithout any feedstock addition was used as a control. Biogas gen-erated was collected in a 5 L Tedlar gas bag (CEL Scientific, Santa FeSprings, CA, USA). The composition and volume of biogas weremeasured every 1–3 days during the 30 day SS-AD.

2.3. Analytical methods

The Standard Methods for the Examination of Water andWastewater were used to analyze the TS and VS contents of feed-stocks, inoculum, and material taken at the beginning and end ofthe AD process (Eaton et al., 2005). Samples were taken and pre-pared to determine total carbon and nitrogen contents by an ele-mental analyzer (Elementar Vario Max CNS, Elementar Americas,Mt. Laurel, NJ, USA). Total volatile fatty acids (TVFA) and alkalinity(total inorganic carbon) were measured using a two-step titrationmethod (McGhee, 1968). Samples for pH, TVFA, and alkalinity mea-surements were prepared by diluting a 5 g sample with 50 ml ofdeionized water. The dilution was then analyzed using an auto-titrator (Mettler Toledo, DL22 Food & Beverage Analyzer, Colum-bus, OH, USA). The TVFA/alkalinity ratio was calculated using theempirical formula to determine the risk of acidification, a measureof the process stability (Anderson and Yang, 1992). The extractivecontent of feedstocks was measured according to the NREL Labora-tory Analytical Procedure (Sluiter et al., 2008b). Extractive-free so-lid and solid fractions before and after digestion were furtherfractionated using a two-step hydrolysis method based on theNREL Laboratory Analytical Procedure (Sluiter et al., 2008a). Mono-meric sugars (glucose, xylose, galactose, arabinose, and mannose)and cellobiose in the acid hydrolysate were measured by HPLC(Shimadzu LC-20AB, MD, USA) equipped with a Biorad Aminex(Biorad, CA, USA) HPX-87P column and a refractive index detector

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D. Brown, Y. Li / Bioresource Technology 127 (2013) 275–280 277

(RID). A deionized water flow rate of 0.6 mL/min was used as themobile phase. The temperatures of the column and detector weremaintained at 80 and 55 �C, respectively.

The volume of biogas collected in a Tedlar bag was measuredwith an acidic water displacement column and the compositionof biogas (CO2, CH4, N2, and O2) was analyzed using a GC (AgilentTechnologies, HP 6890, DE, USA) equipped with a Thermal Conduc-tivity Detector at 200 �C and a 10 ft stainless steel column 45/60Molecular Sieve 13X. Helium was used as a carrier gas at a flowrate of 5.2 ml/min. The temperature of the column oven was ini-tially programmed at 40 �C for 4 min, then elevated to 60 �C at20 �C/min and held for 5 min.

Methane yield expressed in L/kg VSfeedstock was calculated as thevolume of methane gas produced per kg of VS loaded into the reac-tor at start-up, corrected by subtracting the methane yield ob-tained from the control reactor and adjusted linearly based onthe amount of seed VS (Angelidaki et al., 2009). Methane produc-tivity of lignocellulosic biomass is expressed in Vmethane/Vwork,which is the volume of methane gas produced (Vmethane) per unitworking volume of reactor (Vwork). The working volume of thereactor was taken as the maximum working volume after expan-sion had taken place.

2.4. Statistical analysis

Statistical significance was determined by analysis of variance(ANOVA) using SAS software (Version 8.1, SAS Institute Inc., Cary,NC, USA) with a threshold p-value of 0.05.

3. Results and discussion

3.1. Composition of inoculum and feedstocks

The effluent from the liquid mesophillic AD was centrifuged toincrease the TS from 7.7% to 14.9% as shown in Table 1. Centrifuga-tion also increased the VS from 4.1% to 7.3%, C/N ratio from 3.0 to5.0, and pH from 7.8 to 8.2, and it decreased the VFA/alkalinity ra-tio from 1.00 to 0.66. Centrifugation was required to ensure thatthe TS of all materials in the reactors were around 20% or above.The food waste had TS of 15.2% a C/N ratio of 11.4, and a pH of4.1. The yard waste was very dry with TS of 94.3%, a C/N ratio of55.3, non-detectable pH. The extractive, lignin, cellulose, and hemi-cellulose contents of yard waste were 14.7%, 23.0%, 24.3%, and9.7%, respectively.

It can be seen from Table 2 that as the percentage of food wasteincreased for each F/E ratio, the TS and C/N ratio decreased due tothe low TS and C/N ratio of food waste. The TS (19.3–30.2%) and C/N ratios (16.9–32.2) of feedstooks fell in the favorable operatingrange for SS-AD (Fernandez et al., 2008; Hills, 1979).

Table 1Material characterization.

Material Effluent Centrifuged e

TS (%) 7.7 ± 0.0 14.9 ± 0.0VS (%)b 4.1 ± 0.0 7.3 ± 0.0C:N ratioa 3.0 ± 0.0 5.0 ± 0.0Extractives (%)b ND NDLignin (%)b ND NDCellulose (%)b ND NDHemicellulose (%)b ND NDpH 7.8 ± 0.0 8.2 ± 0.0TVFA/alkalinity ratioc 1.00 ± 0.21 0.66 ± 0.04

a As total weight of sample.b As TS of sample.c mg HAceq/mg CaCO3 Data shown are the average and standard deviation based on

3.2. Biogas production

At an F/E ratio of 1, increasing the amount of food waste by 20%and 10% in the substrate led to a 2.8- and 1.5-fold increase, respec-tively, in the peak daily methane production (Fig. 1a). Food wasteaddition also delayed the peak methane production from 6.3 days(0% food waste) to 9.2 days (10% and 20% food waste). This delaywas probably caused by the faster hydrolysis of easily digestiblematerial, which initially led to over production of fatty acids thatinhibited the methanogenesis process. The addition of food wastealso resulted in higher daily methane yields that occurred laterin the digestion process. Fig. 1b shows the daily methane yieldsat an F/E ratio of 2. The digester with 20% food waste had a signif-icantly lower (p < 0.05) maximum daily peak methane yield thanthat of the digester with 0% food waste. Due to an apparent severeVFA inhibition, this digester also had almost zero biogas produc-tion from day 10 to 20 at which time biogas production began toincrease. The digester with 10% food waste had a 1.4-fold(p < 0.05) higher peak production than that of the digester with0% food waste. The time to reach peak daily production also in-creased from 6.7 to 9.4 days when 10% food waste was included.Fig. 1c shows the daily methane yield at an F/E ratio of 3. The sub-strate with 100% yard waste was the only digester that producedmethane for the entire 30 day period. The other two digestersstopped producing methane around day 8, suggesting that a highpercentage of food waste at higher F/E ratios might introduceexcessive quantities of easily digestible material and cause thereactor to fail. A study by Xu and Li (2012) also showed decreaseddaily methane yields that approached 0 near day 8 at an F/E ratio of4 when 100% and 50% dog food was co-digested with corn stover.

The daily methane content of the biogas produced from thedigesters with F/E ratios of 1, 2, and 3 are shown in Fig. 2a–c,respectively. Most of the digesters had methane contents between50% and 70% between day 8 and day 30, which is considered steadystate, with the exception of reactors with 20% food waste at an F/Eratio of 2 and reactors with 10% and 20% food waste at an F/E ratioof 3. The highest average methane contents at steady state (67%and 65%) were observed in the reactors with 20% food waste atan F/E ratio of 1 and with 10% food waste at an F/E ratio of 2,respectively. Reactors with 100% yard waste at F/E ratios of 1, 2,and 3 had similar methane contents (56–58%). Fig. 2 clearly showsthat the addition of food waste of up to 20% at an F/E ratio of 1 andof up to 10% at an F/E ratio of 2 increased the methane content ofthe biogas. This phenomenon was also observed in the anaerobicbatch digestion of solid potato waste where the methane contentimproved with an increasing concentration of potato waste andan increasing inoculum to substrate ratio (Parawira et al., 2004).

The total methane yield and volumetric productivity of thedigesters can be found in Fig. 3. The methane yield increased 2.3-fold at an F/E ratio of 1 as the percentage of food waste in the sub-

ffluent Yard waste Wal-Mart food waste

94.3 ± 0.1 15.2 ± 0.291.7 ± 0.2 13.8 ± 0.255.3 ± 3.2 11.4 ± 0.014.7 ± 0.5 ND23.0 ± 0.7 ND24.3 ± 1.0 ND

9.7 ± 1.0 NDND 4.1 ± 0.0ND ND

duplicate runs ND = not determined.

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(a)

(b)

(c)

Fig. 1. Daily methane yield during 30 day SS-AD of tested substrate compositions at(a) F/E ratio of 1, (b) F/E ratio of 2, and (c) F/E ratio of 3.

Table 2Characteristics of tested effluent–feedstock compositions.

F/Eratioa

Food waste% insubstratea

Volumetric loadingWal-Mart foodwaste (g/L)

Totalsolids (%)

Carbon:nitrogenratio

1 0 0.0 ± 0.0 20.8 ± 0.0 19.4 ± 0.010 36.8 ± 0.1 20.0 ± 0.0 18.2 ± 0.020 68.8 ± 0.3 19.3 ± 0.0 16.9 ± 0.0

2 0 0.0 ± 0.0 25.8 ± 0.0 27.3 ± 0.010 67.2 ± 0.0 24.1 ± 0.0 25.4 ± 0.020 123.7 ± 3.2 22.5 ± 0.0 23.4 ± 0.0

3 0 0.0 ± 0.0 30.2 ± 0.0 32.2 ± 0.010 90.7 ± 0.7 27.4 ± 0.0 30.0 ± 0.020 166.0 ± 0.1 25.0 ± 0.0 27.5 ± 0.0

a Based on dry volatile solids (VS).

(a)

(b)

(c)

Fig. 2. Daily methane composition of biogas during 30 day SS-AD of testedeffluent–substrate compositions at (a) F/E ratio of 1, (b) F/E ratio of 2, and (c) F/Eratio of 3.

278 D. Brown, Y. Li / Bioresource Technology 127 (2013) 275–280

strate increased from 0% to 20%. This effect was also seen in thedigesters with an F/E ratio of 2. The methane yield increased 2-foldas the percentage of food waste increased from 0% to 10% but thendecreased 9.7-fold when the food waste was increased to 20% dueto an excessive amount of easily digestible material which causedVFA inhibition. This was also seen in a study of the solid-state co-digestion study of dog food and corn stover where the methaneyield decreased significantly with 50% or more dog food and anF/E ratio of 4 (Xu and Li, 2012). At an F/E ratio of 3, the digesterwith 100% yard waste was the only reactor which had significantmethane production. It can also be seen that digesters with the

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Fig. 3. (a) Methane yield and (b) volumetric productivity during 30 day SS-AD oftested effluent–substrate compositions.

Table 3Changes of pH and VFA/TIC ratios during 30 day co-digestion SS-AD.

F/Eratioa

Food waste% insubstratea

pH VFA/alkalinity ratiob

Initial Final Initial Final

1 0 7.6 ± 0.0 8.3 ± 0.0 0.92 ± 0.14 0.74 ± 0.1610 7.4 ± 0.0 8.3 ± 0.0 0.98 ± 0.04 0.56 ± 0.1120 7.2 ± 0.2 8.0 ± 0.0 1.22 ± 0.07 0.70 ± 0.12

2 0 8.1 ± 0.1 8.2 ± 0.0 1.19 ± 0.10 0.73 ± 0.0510 7.5 ± 0.3 8.2 ± 0.0 0.95 ± 0.25 0.62 ± 0.1820 7.3 ± 0.4 5.7 ± 0.0 0.92 ± 0.00 3.17 ± 0.23

3 0 8.2 ± 0.1 8.2 ± 0.0 0.92 ± 0.21 0.92 ± 0.0910 7.6 ± 0.3 6.4 ± 0.2 0.89 ± 0.08 4.20 ± 0.0220 6.8 ± 0.1 5.5 ± 0.0 1.79 ± 0.09 4.16 ± 0.42

a Based on dry volatile solids (VS).b mg HAceq/mg CaCO3.

D. Brown, Y. Li / Bioresource Technology 127 (2013) 275–280 279

same substrate composition had decreasing methane yields as theF/E ratio increased. Zero methane yields are seen in Fig. 3 for reac-tors with 10% and 20% food waste at an F/E ratio of 3 which impliesthat addition of feedstock had an adverse impact on methane yield.

Volumetric productivity comparisons closely resemble those ofthe methane yield comparisons at the same F/E ratios. Increasingthe food waste from 0% to 20% at an F/E ratio of 1 increased the vol-umetric productivity 1.7-fold (p < 0.05). The volumetric productiv-ity at an F/E ratio of 2 increased by 1.4-fold (p < 0.05) as the foodwaste was increased from 0% to 10%. Similar to the methane yield,the volumetric productivities were also much lower for the reac-tors with 20% food waste at F/E ratios of 2 and 3 and with 10% foodwaste at an F/E ratio of 3. The highest volumetric productivity(8.6 Lmethane/Lwork) was achieved in the digester with 10% foodwaste at an F/E ratio of 2. It is also noted that the highest volumet-ric productivity for the other F/E ratios occurred with 0% foodwaste at an F/E ratio of 3 (7.7 Lmethane/Lwork) and with 20% foodwaste at an F/E ratio of 1 (6.7 Lmethane/Lwork).

Fig. 4. Reduction of volatile solids (%) during 30 day SS-AD of tested effluent–substrate compositions.

3.3. Reactor characteristics

Reactor failure and low methane yield may be caused by imbal-ances of hydrolytic, fermentative, and acetogenic bacteria, andmethanogenic archea. Often, these imbalances are caused by accu-mulation of VFAs which may lead to a dramatic drop in pH if thereis not enough buffer capacity. The drop in pH inhibits methanogen-ic archea and disrupts the performance of the anaerobic digester.Therefore, pH and VFA/alkalinity ratios are common stress indica-tors used for monitoring the AD process (Anderson and Yang,1992). The pH and VFA/alkalinity ratios of the digesters at 0 and30 days of digestion are shown in Table 3.

The stability criterion for anaerobic digestion is often expressedby the ratio of total VFA to the buffering capacity measured asalkalinity – total VFA/alkalinity ratio (Koch et al., 2010). Althoughthe optimal total VFA/alkalinity ratio of each AD reactor is unique,

a ratio of less than 0.4 is generally regarded as optimal for liquidAD and a ratio exceeding 0.6 is regarded as indicative of overfeed-ing (Lossie and Pütz, 2010). As seen in Table 3, the initial VFA/alka-linity ratios ranged from 0.92 to 1.79 which are considerablyhigher than the ratios provided by Lossie and Pütz (2010). Themajority of final VFA/alkalinity ratios fell in the range of .56–.92,but the digesters with both high F/E ratios and a higher percentageof food waste in the substrate had VFA/alkalinity ratios of 3.17–4.20 which are significantly (p < 0.05) above the other digesters.

The initial pH in each F/E ratio cluster decreased as the percent-age of food waste in the substrate increased and ranged from 6.8 to8.2, which is close to the recommended operating pH of 7.4 pro-posed by Lahav and Morgan (2004). The majority of the digestershad ending pH values in the range of 8.2–8.3, but the digesterswith both high F/E ratios and a higher percentage of food wastein the substrate had ending pH values significantly below(p < 0.05) the rest of the digesters.

It was observed that reactors with significantly low final pH val-ues also had significantly higher final VFA/alkalinity ratios whichindicate reactor failure. The failure was likely due to the accumula-tion of organic acids due to overfeeding. In order to overcome thislimitation, pH adjustment and/or buffer addition may improve SS-AD performance.

3.4. VS reduction

VS reduction of the feedstocks is shown in Fig. 4. It can be seenthat by increasing the amount of food waste in the substrate from0% to 20% at an F/E ratio of 1, a 1.6-fold increase (p < 0.05) in VSreduction was achieved. Also, a 1.3-fold increase (p < 0.05) in VSreduction was achieved by increasing the amount of food waste

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in the substrate from 0% to 10% at an F/E ratio of 2. The VS reduc-tion was highly correlated with the methane yield (Fig. 3). HigherVS reduction was observed in reactors with higher methane yields.More than 20% of VS reduction was observed in the failed reactorseven though there was almost no methane production. This wasdue to the conversion of VS to intermediate products such as VFAs.

4. Conclusions

Co-digestion of food waste with yard waste increased bothmethane yield and volumetric productivity considerably over SS-AD of only yard waste. Increased methane yields and volumetricproductivities were observed with increases in the percentage offood waste to 10% and 20% of the substrate at F/E ratios of 2 and1, respectively. The highest volumetric productivity of 8.6 Lmethane/Lwork obtained at a loading of 10% food waste at an F/E ratio of 2which is also where the maximum reduction of VS (43%) occurred.

Acknowledgements

This project was supported by Ohio Agricultural Research andDevelopment Center (OARDC) SEEDS Program (2008-043) andthe Ohio Third Frontier Program (10-059). The authors would liketo thank Mrs. Mary Wicks (Department of Food, Agricultural andBiological Engineering, OSU) for reading through.

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