Effects of Mixture Ratio and Hydraulic Retention Time on Single-Stage Anaerobic Co-digestion of Food Waste and Waste Activated Sludge

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  • This article was downloaded by: [University of California, San Francisco]On: 05 September 2014, At: 04:08Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: MortimerHouse, 37-41 Mortimer Street, London W1T 3JH, UK

    Journal of Environmental Science and Health, PartA: Toxic/Hazardous Substances and EnvironmentalEngineeringPublication details, including instructions for authors and subscription information:http://www.tandfonline.com/loi/lesa20

    Effects of Mixture Ratio and Hydraulic RetentionTime on Single-Stage Anaerobic Co-digestion of FoodWaste and Waste Activated SludgeNam Hyo Heo a b , Soon Chul Park b & Professor Ho Kang aa Department of Environmental Engineering , Chungnam National University , Daejon,Koreab Biomass Research Center , Korea Institute of Energy Research , Daejon, KoreaPublished online: 16 Aug 2006.

    To cite this article: Nam Hyo Heo , Soon Chul Park & Professor Ho Kang (2004) Effects of Mixture Ratio and HydraulicRetention Time on Single-Stage Anaerobic Co-digestion of Food Waste and Waste Activated Sludge, Journal ofEnvironmental Science and Health, Part A: Toxic/Hazardous Substances and Environmental Engineering, 39:7, 1739-1756,DOI: 10.1081/ESE-120037874

    To link to this article: http://dx.doi.org/10.1081/ESE-120037874

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  • JOURNAL OF ENVIRONMENTAL SCIENCE AND HEALTH

    Part AToxic/Hazardous Substances & Environmental Engineering

    Vol. A39, No. 7, pp. 17391756, 2004

    Effects of Mixture Ratio and Hydraulic Retention Timeon Single-Stage Anaerobic Co-digestion ofFood Waste and Waste Activated Sludge

    Nam Hyo Heo,1,2 Soon Chul Park,2 and Ho Kang1,*

    1Department of Environmental Engineering,

    Chungnam National University, Daejon, Korea2Biomass Research Center, Korea Institute of Energy Research, Daejon, Korea

    ABSTRACT

    The biochemical methane potential (BMP) test was used to evaluate the anaerobic

    biodegradabilities of food waste (FW), waste activated sludge (WAS), and the

    mixtures having the ratios of 10:90, 30:70, 50:50, 70:30, and 90:10 (FW:WAS) on

    a volatile solid (VS) basis. The carbon/nitrogen (C/N) ratio and the biodegrad-

    ability of the mixtures improved from 6.16 to 14.14 and increased from 36.6 to

    82.6% as the FW proportion of the mixture increased from 10 to 90%,

    respectively. The stability and performance of the single-stage anaerobic digester

    (SSAD) for the co-digestion of FW and WAS were investigated, operated at the

    hydraulic retention times (HRTs) of 10, 13, 16, and 20 days with five mixtures at

    35C, respectively. During all the experiments, there were no indication of failure

    such as low pH, insufficient alkalinity, ammonia inhibition, and the accumulation

    of volatile fatty acids (VFAs) in any of the digesters, and the buffer capacity was

    the highest in the digester fed with a feed mixture of 50:50. The optimum

    operating conditions of the SSAD were found to be an HRT of 13 days and a

    mixture of 50:50 in terms of the buffer capacity of the digester and the effluent VS

    *Correspondence: Ho Kang, Professor, Department of Environmental Engineering,

    Chungnam National University, 220 Gungdong, Yuseongku, Daejon 305-764, Korea;

    Fax: (82) 42-822-5610; E-mail: hokang@cnu.ac.kr.

    1739

    DOI: 10.1081/ESE-120037874 1093-4529 (Print); 1532-4117 (Online)

    Copyright & 2004 by Marcel Dekker, Inc. www.dekker.com

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    concentration, the methane content of the biogas produced and the specific

    methane production (SMP). The VS removal efficiency, biogas production rate

    (GPR), and SMP in this condition achieved 56.8%, 1.24m3m3d1 and 0.321m3

    CH4 kg1VS1fed with an organic loading rate (OLR) of 2.43 kgVSm

    3d1.

    Key Words: Anaerobic co-digestion; Biochemical methane potential (BMP)

    test, Biogas production; Food waste; Mixture waste; Performance; Stability;

    Waste activated sludge.

    INTRODUCTION

    In 2001, food waste (FW) generated in Korea was about 11,091 t d1, of which56.7% is recycled as aerobic composting, feedstuff, and methanization and the rest isdisposed by landfill and incineration.[1] In 2005, landfilling of FW, which causesvarious problems such as a foul odor, emission of greenhouse gas (CH4, CO2) at thelandfill site and surface contamination by leachate, will be prohibited. Food waste isconsiderably difficult to treat or recycle by composting or feedstuff because itcontains high level of sodium salt and moisture as well as a sanitary problem asanimal feed.[2] Whereas, most of particulate materials in FW consist of easilybiodegradable organic components such as grains, vegetables, fruits, fish, and meat.[1]

    Therefore, anaerobic digestion has been considered as a feasible alternative ofwaste volume reduction and a renewable energy recovery as the methane. PresortedFW with total solid (TS) of 1530% has a high volatile solid (VS) content (8892%of TS), and anaerobic biodegradability in batch BMP (Biochemical MethanePotential) test was estimated to be 86% with the methane potential of 0.472m3

    CH4 kg1 VS1fed.

    [3] A characteristic of FW that contains highly soluble organicmaterials, the soluble organics would be rapidly converted to volatile fatty acids(VFAs) in the early stage of digestion. Therefore, the reactor configuration has beenconsidered a two-phase anaerobic digestion system, consisting of acid and methanephases, for the effective treatment of FW with a high organic loading rate (OLR)and methane production.[35]

    One of the interesting alternatives for the treatment of FW is the anaerobicco-digestion together with sewage sludge in existing digester of municipal wastewatertreatment plant (MWTP).[68] A major concept of the co-digestion involves thetreatment of several wastes to get higher biogas production in a single treatmentfacility that makes the operation of biogas plants to be more economically feasible.The feasibility of anaerobic co-digestion of waste activated sludge and simulatedorganic fraction of municipal solid waste (OFMSW) was examined by Poggi-Varaldo and Oleszkiewicz.[9] The co-digestion has been successfully applied in full-scale for the treatment of OFMSW and sewage sludge.[10] In 2002, the number ofMWTP operated in Korea reach to 183 facilities. About 80% of the MWTP are notin full operation. Therefore, adding FW to existing anaerobic digester in the MWTPmay be a good alternative without significant investments.

    The objective of this study was to evaluate the feasibility of the anaerobicco-digestion of FW and WAS. The specific objectives of this study were as follows:(a) to evaluate the anaerobic biodegradability based on cumulative methane

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    productions of FW, WAS, and different mixtures using the batch BMP test; (b) toinvestigate the effects of hydraulic retention time (HRT) and mixture ratio of twobiowastes on the stability and performance of the single-stage anaerobic digestionprocess.

    MATERIALS AND METHODS

    Feedstocks and Mixtures

    To simulate FW generated in Korea, a traditional Korean food called Bibimbab,which has a similar composition to FW, was used in this study. Food waste had anaverage 18% TS and consisted of boiled rice (1015%), vegetables (6570%), andmeat and eggs (1520%). The FW was crushed down to 24mm using a cook mixerand the concentration of TS was controlled by adding tap water. WAS was obtainedfrom the secondary clarifier in MWTP located in Daejon, Korea, and itsconcentration was adjusted to 3% TS using a centrifuge. The mixture ratios ofFW and WAS (FW:WAS) were 10:90, 30:70, 50:50, 70:30, and 90:10 on a VS basis,respectively, and the mixtures were used in experiments.

    Biochemical Methane Potential (BMP) Test

    To evaluate the anaerobic biodegradability of FW, WAS and the mixtures, theBMP test was performed in 500mL Erlenmyer flask at 35C using a modifiedmethod of Owen et al.[11] The inoculum used in the BMP test, in which themicroorganisms were well adapted to the mixture of FW and sewage sludge, wasobtained from a laboratory scale anaerobic co-digestion process operated at 35C inthe Korea Institute of Energy Research. The characteristics of WAS, SFW, andinoculum used in the BMP test are shown in Table 1. The inoculum was filled to250mL in each flask, and the S/I ratio was brought to 0.2 with the addition of FW,

    Table 1. Characteristics of the feedstocks and inoculum used in the BMP test.

    Item

    Feedstocks of the mixture

    WAS FW Inoculum

    TCOD (gL1) 35.5 64.4 23.8

    SCOD (gL1) 0.26 11.5 0.31

    TS (gL1) 30.3 50.8 21.7

    VS (g L1) 22.2 46.7 14.2

    pH 6.80 5.50 7.40

    Alkalinity (gCaCO3 L1) 0.70 0.25 3.65

    NH4 -N (gL1) 0.01 0.03 0.81

    aVFAs (gL1) 0 0.15 0

    aVFAs: C2C6.

    Anaerobic Co-digestion of FW and WAS 1741

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    WAS and the mixtures, and the assays were purged with nitrogen gas to eliminateair. The S/I ratio is given by the ratio of the VS mass of feed substrate to the VS massof inoculum added. All assays were shaken by hand three times per day andincubated for 40 days in an incubator. Biogas produced from each assay wasmeasured with a glass syringe of 10mL volume. The biogas volume was thennormalized on standard temperature and pressure (0C, 1 atm) by compensatingboth the thermal expansion according to Charless law and the volume occupied bywater vapor.

    The anaerobic biodegradability was calculated by Eq. (1),[3,11,12] in which VC iscumulative methane production (CMP) obtained from the BMP test, VT istheoretical methane production (TMP). Generally, the organic wastes arerepresented with a generalized formula of the form CaHbOcNd. Assuming completeconversion of the biodegradable organic constituents to CO2 and CH4, the TMYcan be estimated using Eq. (2).[13,14]

    Biodegradability % VCVT

    100 1

    CaHbOcNd 1=44a b 2c 3d H2O ! 1=84a b 2c 3d CH4 1=84a b 2c 3d CO2 dNH3

    2

    Single-Stage Anaerobic Co-digestion of the Mixtures

    The characteristics of the feedstocks and five mixtures fed to the digester areshown in Table 2, and the mixtures were prepared every week and kept in a

    Table 2. Physicochemical characteristics of the feedstocks and five mixtures.

    Items

    Feed stocksc dMixture ratios (gVSFW:gVSWAS)

    FW WAS 10:90 30:70 50:50 70:30 90:10

    TCOD (gL1) 110 36.8 43.3 48.7 60.0 77.8 97.4

    SCOD (gL1) 42.6 0.21 2.58 3.44 11.4 19.1 33.5

    TS (gL1) 85.0 30.0 3.16 3.34 3.81 4.80 6.28

    VS (gL1) 80.0 22.0 2.40 2.60 3.18 4.04 5.57aVFA (gL1) 0.75 0 0.72 0.41 0.44 0.92 2.21

    pH 4.50 6.80 6.10 5.40 4.10 4.07 4.03bAlkalinity (g L1) 0.15 0.80 0.87 0.53 0 0 0

    NH4 -N (g L1) 0.29 0.03 0.07 0.14 0.16 0.15 0.14

    C/N ratio 16.2 5.60 5.97 6.99 8.90 11.0 14.7

    aVFA: C2C6.bAlkalinity is expressed as CaCO3.cNumbers are the mean measured during all experiments.dNumbers are the mean measured during the start-up period of each feed mixture.

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    refrigerator at 4C. A single-stage anaerobic co-digester (SSAD) made ofPlexiglassa semi-continuously fed and mixed reactor (SCFMR), was used in thisstudy. The volume of the digester was 4.0 L and its working volume was 3.5 L. FourSSADs were arranged in a water bath and inoculated with the same inoculum usedin the BMP test. The digester was operated with a fill and draw method andmechanically stirred at 80 rpm by an electric motor. The water bath was maintainedat 35C by circulating water through a water jacket by a temperature controlledcirculator (Haake, Karlsruhe, Germany). The start-up period of each mixture was 50days to obtain reliable results and the HRT of four digesters were maintained from10 day to 13, 16 and 20 days with various ranging of the OLRs, respectively. Thebiogas produced from each digester was collected in Tedlar bags. The biogas volumewas measured by a wet gas meter (Sinagawa, Tokyo, Japan) and then normalized onstandard temperature and pressure (0C, 1 atm). The influent, effluent, and biogasproductions of the co-digester were measured three times per week during thestart-up of each mixture.

    Analytical Methods

    The pH and temperature were monitored. Chemical oxygen demand (COD), TS,VS, alkalinity, NH4 -N concentrations of the samples were determined accordingto Standard Methods.[15] The sample for the analysis of soluble COD (SCOD),NH4 -N, VFA was prepared by filtration using a 0.45 mm cellulose acetate membraneafter centrifugation at 15,000 rpm for 15min. Elemental composition of the feed-stocks were analyzed by elemental analyzer (CHN-1000, LECO Co., USA) andsulfur analyzer (SC-432DR, LECO Co., USA). The percentages of methane andcarbon dioxide were analyzed using a gas chromatography (HP-5890A GC, USA)equipped with a thermal conductivity detector (TCD) and a 6 ft stainless columnpacked with Hayesep Q (80/100mesh). The injection and detector temperatures were120 and 150C, respectively, and the column oven operated isothermally at 60C.The concentration of VFA was determined using the same GC equipped with a flameionization detector (FID) and a capillary column (25m 0.2mm 0.33 mm; HewlettPackard-FFAP, USA). The injection and detector temperatures were 200 and 220C,respectively. The initial temperature of the column oven was 80C and increasedg...

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