RENPWABLE ENERGY

PERGAMON Renewable Energy 16 (1999) 1094-1097

ANAEROBIC TREATMENT OF LIQUIDIZED ORGANIC WASTES

Shigeki Sawayama. Seiichi Inoue. Kenichiro Tsukahara. Tatsuo Yagishita, Tomoaki Minowa & Tomoko Ogi

Biomass Division, National Institute for Resources and Environment. 16-3 Onogawa. Tsukuba. Ibaraki, 3058569. Japan

ABSTRACT

Kitchen garbage and dewatered sewage sludge were thermochemically liquidized at I75 C and 4 MPa with I h of holding time. The liquidized garbage and sludge were separated into liquid and solid fractions, respectively. The diluted liquid fraction of the liquidized garbage and sewage sludge were continuously anaerobically digested using uptlow anaerobic sludge blanket method. The digestion ratio of the liquid fraction separated from the liquidized garbage was 74-75s at the added TOC concentration of 17.0 gel -’ and the TOC loading rate of 1.8-2.8 mg*cm.‘-granule*day~‘. The digestion ratio in the liquid fraction separated from the liquidized sewage sludge was 57-75’5’0 at the added TOC concentration of 7.9 gal -’ and the TOC loading rate of 2.2 mg*cm-‘-granule*day~’ The solid fraction could be processed to refuse derived fuel according to its decreased moisture content. The energy balances of this treatment process for kitchen garbage and sewage sludge were initially analyzed to be better than those of direct incineration. 0 1998 Elsevier Science Ltd. All rights reserved.

KEYWORDS

Anaerobic digestion. kitchen garbage, sewage sludge, thermochemical liquidization, UASB method, waste treatment

INTRODUCTION

Treatments of wet organic wastes such as kitchen garbage and sewage sludge have been important environmental issues to resolve. Anaerobic digestion is commonly used for treatment of organic wastes and energy is recovered in the form of methane (Fannin et d., 1983); however, anaerobic digestion of organic waste takes long retention time and produces large amount of digested sludge. We reported on thermochemical liquidization and anaerobic treatment of kitchen garbage and organic sludges (Dote et crl., 1993. Minowa et d., 1995. Sawayama cf crl., 1995, 1996. 1997). The liquidized organic wastes can be easily separated to solid and liquid fractions. The liquid fraction could be digested rapidly compared with original organic wastes without producing large amount of sludge. The solid fraction could be processed to refuse derived fuel (RDF).

This paper deals with the thermochemical liquidization of kitchen garbage and dewatered sewage sludge and anaerobic digestion of the liquid fractions separated from the liquidized organic wastes using the upflow anaerobic sludge blanket (UASB) method.

0960-1481/99/$--see front matter 0 1998 Elsevier Science Ltd. All rights reserved PII: .80960-1481(98)00425-X

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MATERIALS AND METHODS

Liauidization of wastes

Kitchen garbage or dewatered sewage sludge was added to a 1000 ml autoclave made of stainless steel. After purging with nitrogen gas, an autoclave was heated with an electric furnace to I75 C and held at 175 C and 4 MPa for I h (Dote ertrl.. 199.1). Separation of the liquidized garbage was conducted by filtration using 20 pm pore size of nylon mesh and that of the liquidized sewage sludge was by centrifugation (2500 g. IOmin).

Anaerobic digestion using UASB method

Seed granular sludge for continuous anaerobic digestion using an upflow anaerobic sludge blanket (UASB) method W;LS offered from Ebara Co.. Kanagawa. Japan. A biowor for the UASB method (40x40~150 mm. made of plastic) was incubated at 3ScC. A liquid fraction separated from a liquidized waste was diluted and wa< continuously added to the bioreactor using a tube pump at flow rates of 5.3-66.0 r&day.‘. A total organic carbon (TO0 concentration was determined by TOC meter (TOG8L, Yanaco, Japan).

Energv analvsis

A preliminary energy balance (EB I. kWh*t-’ ) for the total treatment process of kitchen garbage or sewage sludge was calculated using the following equation:

EB I= -FQ+(LQV,+MQV?) r,-S/r?

FQ is the heating energy for the liquidization of the garbage. LQ is the low heating value of the solid fraction from the liquidized garbage. V, is the ratio of the solid fraction. MQ is the energy of the obtained biogas (kWh*t-‘-liquid fraction). V? is the ratio of the liquid fraction. and r, is the efficiency of available combustion energy (= 0.6). S is the necessary electric power for the belt press dewatering process (= 5.0 kWhq ’ ) and rI is the ekctric efficiency (= 0.3). The necessary heat for the anaerobic treatment could be supplied by the hot liquid fraction separated from liquidized garbage.

RESULTS

A flow diagram of the proposed liquid&ion and anaerobic digestion process is shown in Fig. I. The kitchen garbage and dewatered sewage sludge were successfully liquidized at l7SC. The liquidized kitchen garbage was separated by filtration to liquid (64%. w/w) and solid fractions (36%. w/w), and the TOC concentration of the liquid fraction was 3 I. 8 g*f’ The liquidized sewage sludge was separated by centrifugation to liquid (3 I%. w/w) and solid fractions (69%. w/w), and the TOC concentration of the liquid fraction was 40. I g-f ‘.

The diluted liquid fractions of the liquidized kitchen garbage and sewage sludge were successfully anaerobically digested at 3593 using a UASB method. The digestion ratio of the liquid fraction separated from the liquidized garbage was 74-75% at the added TOC concentration of 17.0 g-1 -’ and the TOC loading rate of I .8-2.8 mg*cm”-granule&$. The TOC removal rate was 2. I mg*cm’-granuloday~’ at this TOC concentration and the TOC loading rate of 2.8 mg.r.m-‘-granuleday-‘. The methane content of the biogas obtained from the anaerobic treatment of the liquidized garbage was 62 46. The digestion ratio of the liquid fraction separated from the liquidized sewage sludge was 57-754 at tbe added TOC concentration of 7.9 g-f .’ and the TOC loading rate of 2.2 mpcm ‘-granule*day~’ The TOC removal rate was I. 3-l. 7 mgvrn-‘- granule*day-’ under the same conditions (Fig. 2).

lhe calculatd low heating valur: of the solid fraction separated from the liquidized kitchen garbage was 1025 kWh+solid fraction-‘. While the energy balance for the process of direct incineration of kitchen

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garbage wa\ calculated to be -c13 kWh*t ‘. EBI in the liquidization and anaerobic treatment process for kitchen garbage was 2-l kWh=r ’ While the energy balance for the process of direct incineration of sewage sludge wx calculated to be -2’32 kWh*1’. EBI in the liquidizotion and anaerobic treatment process for sewage sludge was 1-l kWh.1 ‘.

l__uidization -__-- 175°C. -1 MPa

+

Liquidized garbage or sludge 1 t

Separation t

(Liquid) (SolidfiaL.tion)

!JAS!K treatmenr t i

e m

Fig. I. Flow diagram of anaerobic treatment of liquidized organic wastes.

2 7121619232630334144476165687178

Days

Fig. 2. TOC‘ loading rate of the diluted liquid fraction of the liquidized scwagc sludge (full column) to the UASB reactor and TOC removal rate (dotted column)

DISCUSSION

These results suggest that anaerobic digestion of the only liquid fraction separated from thermochemically liquidized wastes using the UASB method could be useful for fst md high efficient digestion compared

WREC 1998 1097

with digestion of untreated wastes. The degradation of particulates into soluble substrates is the rate- limiting step during anaerobic digestion (Eastman and Ferguson. 1981). Anaerobic digestion of the only liquid fraction separated from liquidized wastes could contribute to fast digestion. Kitchen garbage and sewage sludge can be treated using the same process. therefore, one plant of this method could treat both kitchen garbage and organic sludge. The high removal efficiency of organic compounds using thermal sludge conditioning liquor as influent was reported using an upflow anaerobic hybrid system with granular sludge (Kin-rata et <I/. 1993): therefore. a faster digestion rate of the liquid fraction from the liquidized sewage sludge or the kitchen garbage compared with the present results could be feasible under idea) digesting conditions.

The calculation of energy balance suggests that this treatment process indicates better energy balance for kitchen garbage and sewage sludge compared with that of direct incineration. The solid fraction separated from the liquidized wastes would be a good material for RDF and compost because of its relatively low moisture content compared with the original wastes. Direct incineration at the waste treatment facility would be avoided using this treatment process.

ACKNOWLEDGMENTS

We are grateful to Mr. Yoshiaki Ohnishi, a student in Science University of Tokyo and Ms. Yukiko Fukuda, an assistant staff. for their experimental work and to Ebara Corporation for preparation on seed granular sludge.

REFERENCES

Dote. Y.. Yokoyama. S.. Minowa, T.. Masuta. T.. Sato, K.. Itoh, S. and Suzuki, A. (1993). Thermochemical liquidization of dewatered sewage sludge. Biomass Bioenergy, 4, 243-248.

Eastman. J A. and Ferguson, J. F ( I98 I ). Solubilization of particulate organic carbon during the acid phase of anaerobic digestion. J. Wurer Poll. Conrro/ Fed. 5 3, 352-366.

Fannin, K. F.. Conrad, J. R.. Srivastava, V. J.. Jerger, D. E. and Chynoweth, D. P. (1983). Anaerobic processes. J. WNter Poll. Control Fed. 55, 623-632.

Kimata T.. Kawai. T.. Tada, M.. Tanaka, K.. Shinabe. K. and Shim@ K. (1993). Anaerobic treatment of thermrd sludge conditioning liquor with granular sludge. Wafer Environ. Rex. 65, 6- 14.

Minowa. T., Dote. Y.. Sawayama. S.. Yokoyama, S. and Murakami, M. (1995). Phase changing of garbage from solid to liquid slurry by thermal liquidization. J. Chem. Eng. Jpn., 2 8, 727-73 I.

Sawayama. S.. moue. S.. Yagishita. T.. Ogi. T. and Yokoyama S. ( 1995). Thermochemical liquidization and anaerobic treatment of dewatered sewage sludge. J. Ferment. Bioeng. 79, 300-302.

Sawayama. S.. moue, S.. Tsukahara. K. and Ogi, T. ( 1996). Thermochemical liquid&ion of anaerobically digested and dewateted sludge and anaerobic retreatment. Biores. Technol. 5 5, 141-144.

Sawayama. S.. moue. S.. Minowa T, Tsukahara, K. and Ogi, T. (1997). Thermochemical liquidization and anaerobic treatment of kitchen garbage. J. Ferment. Bineng. 83. 451-455.

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