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  • December, 2010 Vol. 3 No.4 61

    Anaerobic digestion of food wastes for biogas production

    Xiguang Chen, Rowena T. Romano, Ruihong Zhang

    (Department of Biological and Agricultural Engineering, University of California at Davis, Davis, CA 95616, USA)

    Abstract: Five types of food wastes were investigated as feedstock for a potential centralized anaerobic digester system in the

    area of Sacramento, California to produce biogas energy. The wastes were from a soup processing plant, a cafeteria, a

    commercial kitchen, a fish farm, and grease trap collection service. Digestibilities of the food wastes, individually and in

    mixtures, were conducted at mesophilic (35) and thermophilic (50) temperatures and at two food to microorganism ratios

    (F/M) of 0.5 and 1.0, for 28 days. A continuously fed mesophilic single-stage anaerobic digester was evaluated using a

    mixture of the five food wastes at organic loading rates of 0.5 to 1.0 g VS/L/d. In the batch digestion tests, fish and grease trap

    wastes required longer time to complete the digestion and had higher biogas yields than the other wastes. The

    continuously-fed digester required the addition of sodium hydroxide to maintain pH at proper levels in the digester. Alkalinity

    of about 2,500 mg CaCO3/L and pH above 7 was maintained by adding 0.2 g NaOH/g VS. The results of this study indicated

    that it was necessary to use the chemicals, such as NaOH, to control the pH of the single-stage anaerobic digester treating the

    food waste. For commercial applications, the cost of chemicals and proper management of additional salts in the digester

    effluent need to be carefully considered.

    Keywords: anaerobic digestion, bioconversion, biogas, continuous digestion, food waste

    DOI: 10.3965/j.issn.1934-6344.2010.04.061-072

    Citation: Xiguang Chen, Rowena T. Romano, Ruihong Zhang. Anaerobic digestion of food wastes for biogas production.

    Int J Agric & Biol Eng, 2010; 3(4): 6172.

    1 Introduction

    Food waste is the third-largest component of

    municipal solid waste generated from the United States.

    According to a report by U.S. Environmental Protection

    Agency[1], approximately 32 million tons of food waste

    was generated annually. Less than three percent of the

    food waste was separated and treated, primarily through

    composting, and the rest was disposed of in landfills.

    Due to increasing needs for renewable energy generation

    and diversion of organic residuals from landfills to reduce

    the greenhouse gas emissions and other environmental

    impacts, treatment of food waste using anaerobic

    digestion technologies has become a more attractive

    Received date: 2010-03-03 Accepted date: 2010-10-11

    Corresponding author: Ruihong Zhang, Department of

    Biological and Agricultural Engineering, University of California,

    One Shields Avenue, Davis, CA 95616-5294, Tel: 1-530-752-9530.

    Fax: 1-530-752-2640. Email: rhzhang@ucdavis.edu.

    method for food waste management.

    Anaerobic digestion is a controlled biological

    degradation process and allows for efficient capturing and

    utilization of biogas (approximately 60% methane and

    40% carbon dioxide) for energy generation. The

    digestate from anaerobic digesters contains many

    nutrients and can thus be used as plant fertilizer and soil

    amendment. Anaerobic digestion of different types of

    food waste has been studied extensively. Cho et al.

    conducted batch digestion tests of food wastes at 37

    and 28 days retention time[2]. The methane yields were

    0.48, 0.29, 0.28, and 0.47 L/g VS for cooked meat, boiled

    rice, fresh cabbage and mixed food wastes, respectively.

    Heo et al. evaluated the biodegradability of a traditional

    Korean food waste consisting of boiled rice (10%15%),

    vegetables (65%70%), and meat and eggs (15%20%)

    and showed a methane yield of 0.49 L/g VS at 35 after

    40 days retention time[3]. Zhang et al. analyzed the

    nutrient content of food waste from a restaurant, showing

  • Vol. 3 No.4

    that the food waste contained appropriate nutrients for

    anaerobic microorganisms, as well as reported a methane

    yield of 0.44 L/g VS of food waste in batch digestion test

    under thermophilic conditions (50) after 28 days[4].

    Anaerobic digestion of food waste is achievable; however

    different types of food waste result in varying degrees of

    methane yields, and thus the effects of mixing various

    types of food waste and their proportions should be

    determined on a case by case basis.

    The objective of this study was to determine the

    digestibility of five food wastes individually and as a

    mixture under mesophilic and thermophilic conditions.

    The five food wastes were selected based on the results of

    a previous survey that indicated that these were the

    largest food waste streams in Sacramento, CA. The City

    of Sacramento was interested in developing a centralized

    anaerobic digester for these food waste streams. The

    digestibility was evaluated in terms of biogas yield,

    methane yield, and volatile solids reduction. The second

    objective was to evaluate the performance and operating

    requirements of a single stage, mixed digester for treating

    the mixture of five food wastes.

    2 Materials and methods

    2.1 Collection and characterization of food wastes

    and anaerobic inoculum

    The five food wastes were collected from July to

    October 2006. Food waste from a soup processing plant

    was collected after it was dewatered during the pressing

    stage. The waste was sampled on three consecutive

    days by manually collecting and placing the waste into

    one-gallon zipped-locked plastic bags. The soup

    processing plant was reported to be processing beef,

    potatoes, clams, and mushrooms on the first day, chicken,

    corn, pasta, ham, and other vegetables on the second day;

    and mushroom, ham, pasta, and other vegetables on the

    third day. For analysis, the three collections were mixed

    in equal proportions. Food waste from a cafeteria was

    sampled at a composting facility where the food waste

    was delivered on two non-consecutive days. For each

    sampling event, the waste was dumped from trucks onto

    the tipping floor. Samples were collected by shovel and

    placed into a five-gallon bucket. For analysis, the two

    collections were mixed in equal proportions. Food

    waste from the salad preparation line (pre-consumer) of a

    commercial kitchen was collected in a five-gallon bucket

    on one day. The waste consisted of melon rinds, bell

    peppers, cucumbers, onions, and various meats. The

    waste was processed through a meat grinder to obtain a

    homogenous mixture. Fish waste from a fish farm

    consisted of five Sturgeon heads and fish viscera. The

    fish waste was collected in a five-gallon bucket. The

    fish heads were difficult to cut and/or grind, therefore the

    flesh and gills were first stripped, and mixed with the fish

    viscera in a meat grinder. Grease trap waste was

    provided by a grease collecting company. Two grease

    trap samples were collected from two fast-food

    restaurants. The samples were shipped to the laboratory

    in one-gallon plastic jars in coolers. For analysis, the

    two samples were mixed in equal proportions. All the

    food waste samples were kept frozen at 20 until used.

    Mesophilic anaerobic inoculum were collected from a

    working mesophilic anaerobic digester at wastewater

    treatment plant in Davis, CA. Thermophilic culture was

    collected from thermophilic anaerobic digesters at East

    Bay Municipal Utility District (EBMUD) in Oakland, CA.

    All the five food wastes and anaerobic inoculum were

    analyzed for total solids (TS), volatile solids (VS), and

    fixed solids (FS) in duplicate prior to any digestion tests.

    For the batch tests, the entire content of the reactor was

    measured for TS, VS, and FS at the end of the digestion

    period. Initial and final TS, VS, and FS were used to

    determine solid reduction during the digestion period.

    All the analyses were performed according to the

    standard methods[5]. Samples of the five waste streams

    were also sent to A&L Laboratories (Modesto, CA) for

    analysis of nutrients (N, P, K, S, Mg, Ca, Na), metals (Fe,

    Al, Mn, Cu, Zn), chloride, crude protein, crude fat, fiber,

    total carbohydrates, ash, organic matter, and carbon to

    nitrogen ratio (C/N).

    2.2 Batch digestion tests

    2.2.1 Experimental design and set-up

    Mesophilic and thermophilic batch digestion

    experiments were conducted at (352) and (502)

    respectively. Each of the batch reactors had a total and

    effective volume of 1,130 and 500 mL, respectively.

  • December, 2010 Anaerobic digestion of food wastes for biogas production Vol. 3 No. 4 63

    The reactors were loaded with 1.5 g VS of each food

    waste to obtain an organic loading of 3.0 g VS/L. To

    achieve food to microorganism ratios (F/M) of 0.5 and

    1.0, 3.0 g VS and 1.5 g VS of inoculum was added to

    each reactor, respectively. Tap water was used to bring

    the working volume up to 500 mL working volume. All

    the reactors were tightly closed with rubber septa and

    screw caps. The head spaces of the reactors were

    purged with argon gas for five minutes to assure

    anaerobic conditions. In each of the experiments, blank

    reactors with only inoculum and tap water were also

    prepared to correct for the bi

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