JOURNAL OF BIOSCIENCE AND BIOENGINEERINC Vol. 87, No. 4, 554-556. 1999

Treatment of Liquid Fraction Separated from Liquidized Food Waste in an Upflow Anaerobic Sludge Blanket Reactor

KENICHIRO TSUKAHARA,* TATSUO YAGISHITA, TOMOKO OGI, AND SHIGEKI SAWAYAMA Biomass Division, National Institute for Resources and Environment, AIST, MITI, 16-3 Onogawa, Tsukuba,

Ibaraki 305-8569, Japan

Received 12 October 1998/Accepted 15 December 1998

Thermochemical liquidization as a pretreatment for anaerobic digestion of food waste was studied using a laboratory-scale upflow anaerobic sludge blanket (UASB) reactor for a period of 82 d. Model food waste (approximately 90 wt% moisture content) was thermochemically liquidized at 17S’C for 1 h. The liquidized food waste was separated into a solid phase (6-10 wt%> and a liquid phase (85-89 wt%). The diluted liquid phase was continuously treated by anaerobic digestion using a UASB reactor at 35°C. The volumetric loading rate was increased stepwise to 6.47.2g total organic carbon (TOC)/Z-reactor/d. Methane production was found to be approximately 0.35-0.61 Z/g-TOC removed. The range of TOC removal efficiencies was 6769% at an influent TOC concentration of 10.1-11.1 g/Z and a volumetric loading rate of 4.8-5.3 g-TOW-reactor/d. This treatment process using an UASB reactor could be suitable for resource recovery from food waste.

[Key words: anaerobic digestion, food waste, thermochemical liquidization, UASB method, waste treatment]

A large amount of food waste is discharged from food industries and domestic sources. Most is treated by direct incineration without resource recovery. Consider- ing environmental pollution concerns, installation of new incineration plants is problematic in urban areas. Furthermore, the valuable energy contained in organic waste is lost in the energy cycle. Although waste combus- tion systems for electricity generation have been devel- oped, the high moisture content of food waste results in low production efficiency. Accordingly, research into new treatment methods for food waste is necessary.

Anaerobic digestion has been widely studied as a method for the treatment of organic waste (1). This method has the advantages of low treatment cost and methane production for energy recovery. However, anaerobic digestion of solid organic materials such as biomass takes a long time (25 d) using currently available methods (2). The degradation of particulates into soluble substrates is the rate-limiting step during anaerobic diges- tion (3); thus, liquidization pretreatment with subsequent anaerobic digestion of the liquid phase could enable faster digestion of solid organics. Previously, we demon- strated that solid food waste with a water content of approximately 85% could be liquidized by heating to a relatively low temperature (150-200°C) (4) and reported on the applicability of this pretreatment method for anaero- bic treatment of organic sludges and food waste (5-S).

Upflow anaerobic sludge blanket (UASB) systems have been widely used for high-rate treatment of various organic wastes (9). UASB reactors have exhibited superior performance compared to other types of anaerobic reac- tors when utilized with high loading rates (10). UASB reactors do not require packing material to support biomass as they retain the biomass by separation within the reactor. Gas and sludge are separated from the liquid effluent by a gas-solid-liquid separator installed at the top of the reactor. We previously reported that the USAB method could be useful for fast and efficient digestion of liquidized food waste (8). However, the UASB reactor

* Corresponding author.

used in that study was small (40 mm x 40 mm x 150 mm) and the biogas yield was not accurately monitored; there- fore, it was necessary to carry out larger scale experi- ments to clarify the feasibility of liquidization in conjunc- tion with an anaerobic treatment system.

The objectives of this study are to evaluate the process performance of a UASB reactor for the liquid phase of liquidized food waste at different loading rates and con- centrations of organic material, and investigate the feasibility of adopting a UASB system for the treatment of liquidized food waste.

The components of the model food waste used were sliced cabbage (92.4 wt%), boiled rice (5.3 wt%), small dried fish (0.6 wt%), shells (1.1 wt%) and butter (0.6 wt%) (4). The liquidization of the model food waste was carried out as previously described (4, 8). The sample (2260g wet weight) was placed into a 10-f autoclave made of stainless steel. After purging with nitrogen gas, the autoclave was charged with nitrogen at 2 MPa. The reaction was started by heating the autoclave with an electric furnace to 175°C. This temperature was main- tained for 1 h and then the autoclave was cooled to room temperature. The liquidized food waste was separated into solid and liquid phases by filtration using a 20-pm-pore-size nylon mesh.

The UASB reactor was started using granular seed sludge. A polyacrylic laboratory-scale UASB reactor (60mm~6Omm~6OOmm) with a 2-Z effective volume was used in this study. The reactor was operated at 35°C and the settled granular sludge bed volume was 720 cm3. Granular seed sludge from a continuous flow UASB reac- tor was donated by Fujikasui Engineering Co. Ltd., Tokyo. The feed was introduced into the bottom of the reactor using a peristaltic pump at a flow rate of 960 (+-30) ml/d. The granular bed was expanded by the influent flow and gas production. The reactor was con- tinuously supplied with diluted liquid phase of the liqui- dized food waste for 82 d. The performance of biologi- cal fermentation was monitored by determining the total organic carbon (TOC) removal efficiency. When TOC removal efficiency was above 60%, the proportion of the

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FIG. 1. Changes in TOC concentration before and after anaero- bic treatment of the liquid fraction from liquidized food waste by the UASB method, and TOC removal efficiency. The reactor was main- tained at 35°C and at a flow rate of 960 (t30) ml/d. Symbols: 0, TOC before anaerobic treatment; a, TOC after anaerobic treatment; 0, TOC removal efficiency.

waste liquid phase in the feed was increased to the test levels of 20, 30 and 50% (v/v). Incremental changes in volumetric TOC loading rate were made when the effluent TOC fluctuation was less than 10% for at least 1 week. The reactor was monitored daily for temperature, pH, flow rate, and gas production. Chemicals added to the diluted liquid phase were as follows: Fe-EDTA 3 mg/l; CaC& . 2Hz0 2.5 mg/l; MgC12. 6Hz0 2.5 mg/l; CoCIZ .4Hz0 0.5 mg/l; MnClz. 4Hz0 0.5 mg/l; KH2P04 1.6 mg/l; urea 20 mg/Z. The pH of the feed was adjusted to between 7.5 and 8.0 with 1 M NaOH. Biogas produc- tion was measured with a wet gas meter (W-NK-O.SA, Shinagawa, Tokyo). Gas samples were obtained through a septum in a gas line near the top of the reactor.

The composition of the biogas was determined using a gas chromatograph (GC-8A, Shimadzu, Kyoto) with a thermal conductivity detector equipped with a steel column packed with WG-100 (GL Sciences, Tokyo) at 50°C. The TOC concentrations were measured using a TOC analyzer (TOC-SOOOA, Shimadzu, Kyoto) by the combustion-infrared method. Biochemical oxygen de- mand (BOD) was determined as the rate of biochemical degradation of organic matter in a series of dilutions which approximate the amount of labile organic matter in solution (11). Chemical oxygen demand (COD) was determined by the permanganate oxidation method (11). The moisture content was determined from the differ- ence between the weight of sample prior to and after heating at 105°C for 24 h.

The model food waste was liquidized by thermochemi- cal reaction at 175°C with a holding time of 1 h. The thermochemically liquidized food waste was separated by filtration into a solid fraction (6-10 wt%) and a liquid fraction (85-89 wt%). The TOC concentration of the liquid fraction was 28-35 g/l. The diluted liquid phase of liquidized food waste was continuously fed into the reactor with stepwise increases in TOC concentration from

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FIG. 2. Changes in TOC volumetric loading rate to the UASB reactor and TOC reduction rate of the liquid fraction of the liquidized food waste by anaerobic digestion. The reactor was maintained at 35°C and at a flow rate of 960 (t30) ml/d. Solid line: added TOC; broken line: decomposed TOC.

5.9-7.9 to 13.4-14.9 g/f. Figure 1 shows the TOC concen- trations before and after anaerobic treatment, and the temporal variation of TOC removal efficiency over the experimental period. The TOC removal efficiency at 35°C was consistently greater than 60% after 6d of operation at an organic loading rate of 2.8-3.8 g-TOC/ f-reactor/d. TOC removal efficiencies increased even though the organic loading rate was increased from 2.8- 3.8 to 4.8-5.3 g-TOG/I-reactor/d (Figs. 1 and 2). This was possibly due to selection and enrichment of the sludge, which could convert organic compounds in the liquid fraction of liquidized food waste into CH4 and COP. The digestion ratio was 67-69X at the added TOC concentration of 10.1-11.1 g/l (Fig. 1). At this TOC con- centration, when the loading rate of TOC was 4.8-5.3 g/l-reactor/d, the removal rate of TOC by the reactor was 3.3-3.6 g/l-reactor/d (Fig. 2). The organic com- pounds in the liquid phase of the liquidized food waste were removed by the UASB reactor with high treatment efficiency at a hydraulic retention time of 2.1 (kO.1) d. These results indicated that thermochemical liquidization of food waste can be beneficial as a pretreatment for anaerobic digestion and the UASB method can be useful for rapid and efficient digestion of the liquid phase of liquidized food waste. However, an increase in the or- ganic loading rate to 6.9g-TOG/l-reactor/d on day 57 resulted in a slight drop in the TOC removal efficiency probably due to higher TOC concentration, and there- after, removal efficiencies were observed to be 41-43x. Although NaHCOs was added to the influent to avoid reactor acidification, TOC removal efficiencies did not increase again.

After achieving steady-state conditions in each loading phase, BOD and COD were measured. The average BOD and COD removal efficiencies were 96.2% and 83.9% at the TOC loading rate of 2.8-3.8 g/l-reactor/ d, and 80.3% and 68.7% at the TOC loading rate of 4.8-5.3 g/l-reactor/d, respectively. High BOD removal efficiency was achieved in spite of a low COD removal at the TOC loading rate of 4.8-5.3 g/l-reactor/d. A similar

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FIG. 3 Changes in biogas and methane yield during anaerobic treatment of the liquid fraction from liquidized food waste by the UASB method. The reactor was maintained at 35°C and at a flow rate of 960 (f30) ml/d. Symbols: n , biogas yield: 0, methane yield.

tendency has been reported in the anaerobic treatment of alcohol wastewater and the thermal treatment of liquor from concentrated sewage sludge (12-14). Biodegradable materials could almost be completely removed by this method up to 4.8-5.3 g-TOW/-reactor/d; however, recal- citrant organic materials remained in the effluent.

Figure 3 shows changes in daily gas production. The gas yield gradually increased with increasing TOC load- ing rate, and the average biogas production was 0.67, 0.75 and 0.92 I/g-TOC removed at loading rates of 2.8- 3.8, 4.8-5.3 and 6.4-7.2 g-TOW/-reactor/d, respectively. The methane content of the biogas obtained through anaerobic treatment of the liquid phase after liquidiza- tion was 53-74X. The methane yield was between 0.35 and 0.61 I/g-TOC removed (Fig. 3). The methane concen- tration decreased with increasing organic loading. The methane yield of the liquid phase from liquidized food waste is close to that of thermal sludge conditioning liquor (0.30-0.40 I/g-COD removed) (14). Utilization of the biogas produced should be helpful to achieve repay- ment of the initial capital investment for waste treat- ment .

We are grateful to Fujikasui Engineering Co., Ltd. for prepara- tion of the seed granular sludge and to Ms. Yukiko Fukuda and Ms. Tae Kimura for technical assistance.

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