Food Waste Disposal Using Anaerobic Digestion

Technical Brochure No.66

WASTE – MUNICIPAL

SummaryThe ever-increasing quantity andhigh moisture and salt content ofKorean food waste causeproblems with its disposal. Highlyurbanised and populated towns donot have enough space for landfillsites and the high moisturecontent of food waste does not

The Anyang City Anaerobic Digestion Plant.

▼ Food wastes converted to a combustible gas

▼ Cheaper alternative to incineration for the disposal of food wastes

Highlights

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allow effective incineration andenergy recovery.

A plant at Anyang City hasdemonstrated a two-phase anaerobicdigestion system which successfullyprocesses municipal solid wastecontaining food waste to producecompost, biogas and recoveredmaterials.

Project BackgroundIn 1991, Korea disposed of 92,200 tonnes/day of municipalsolid waste (MSW). By 1994, thishad decreased to 58,000 tonnes/dayfollowing the enforcement, since1991, of source-sorting ofrecyclable materials such as paper,plastics and bottles. However, theproportion of the food wastecomponent in MSW is graduallyincreasing, from 27% in 1991 to50% in 1995. Food waste is difficultto treat or recycle since it containshigh levels of sodium salt andmoisture and is mixed with otherwaste during collection.

In 1992, a proposal to develop aprocess for the disposal of foodwaste was submitted by the KoreaInstitute of Energy Research (KIER)

to the Ministry of Trade, Industryand Energy (MOTIE). A basic studyfor the process development,investigating the biochemicalmethane potential (BMP) of wastefood components in Korea, beganin 1993, with the help ofgovernment funding.

The concept of two-phasedigestion was developed usingbench-scale and mini-pilot units in1994 and 1995. In 1996, a pilot-scale plant, with a treatmentcapacity of 5 tonnes/day of foodwaste, was funded, constructedand operated by Halla Engineeringand Heavy Industries Ltd, in co-operation with KIER. Details of thepilot-scale plant are described inthis brochure. More recently, afull-scale 15 tonnes/day foodwaste treatment plant was built inEuiwang City, near Seoul, andstarted operation in March 1997.

The ProjectThe pilot-scale plant is located at theAnyang city incineration plant siteand the city co-operates in thecollection and supply of source-sorted food waste. The wholeprocess consists of four differentsteps: pre-treatment, digestion,composting and gas utilisation.

The pre-treatment step is theremoval of non-degradable waste.The waste (in plastic bags) is storedin a bunker before being fed to ashredder. The shredded material ispassed to a 50 mm drum screen toremove light fractions such asplastic bags, clothes and woodenmaterials. A magnetic separatorremoves ferric components such asbottle tops.

The moist and heavy wastestream from the drum screen isfed into an acid reactor where thesedimentation of heavy objectssuch as bones and shells occurstogether with the acidification oforganic materials. The acidreactor is mildly agitated toensure mixing and is equippedwith a skimmer to remove someremaining light fractions such aspieces of plastic and lightvegetables which float to the topsurface of the liquid in thereactor.

In the second step, the uniformlysuspended food wastes arehydrolysed and degraded toorganic acids in the acid reactor.The fluid, which contains a highconcentration of acid, is pumpedinto the methane reactor where theacids are converted to methane.The almost ‘organic-free’

alkaline overflow from themethane reactor is recycled to the acid reactor to preventexcessive acidification of the acid reactor.

Part of the slowly degradingorganics and anaerobic sludgesare withdrawn from the methanereactor, dehydrated in a screwpress, and fed into an aerobiccomposter, where the humus(anaerobically-stabilised organics)is treated aerobically to producecompletely stabilised organicfertiliser.

The biogas produced in thereactors contains about 70%methane and is temporarilystored in a gas holder. The gascan be used for cooling/heatingof adjacent facilities, forelectricity generation or as acombustion aid in an incinerator.

This anaerobic treatment processhas several advantages overconventional anaerobictreatments of organic wastes:

▼ the process is a two-phase system with two reactors (an acid reactor and a methane reactor) in series, where the acidreactor functions as a pre-treatment tank as well as an acidification reaction tank to remove the heavy and light rejects;

▼ the anaerobic digestor type of methane reactor is specially designed for the easy withdrawal of digested sludge, and for the easy recycling of treated methane reactor effluent to the acid reactor;

▼ the whole system and layout of the equipment (including pre-treatment, reactors, humus treatment and gas utilisation) areoptimised for simple automatic operation.

PerformanceThere were several problems duringthe early stage of testing.Mechanical problems such asclogging in the conveyor line werecaused by heavy rejects such asbones and pieces of metal. Almostall the problems were caused by ahigh proportion of foreign materialsin the source-separated food wastescontained in the plastic bags. Forexample there was an unexpectedlylarge number of bottle openers, andof fresh whole fruit which isdifficult to digest because of theskin. However, all the problems

have been overcome and theequipment adjusted to accept anddeal with the waste food input.

The two-phase anaerobic processusing two reactors in series, with 15 m3 and 45 m3 effective capacities,recently started steady-stateoperation. In steady-state operation,the treatment of 3 tonnes of foodwaste produces about 100 kg ofhumus (70% moisture), 230 m3 ofbiogas (70% methane) and 2 tonnesof anaerobically-treated waste water.Around 73% of the degradable wasteis estimated to be converted tobiogas.

EconomicsThe initial investment required forthis type of plant in Korea isestimated to be below $125,000 pertonne/day (where $ is the US dollar)

for a plant with a capacity of over15 tonnes/day. The operating costof the plant is estimated to be$60/tonne. On the other hand, inKorea, the cost of incineration isestimated to be about $90/tonneand for landfill about $25/tonne ofMSW. However, there are no newlandfill sites available in Korea,and the use of landfill for foodwaste disposal is almost impossiblebecause of the environmentalimpacts such as leachate and badodour etc.

The utilisation of the biogas isdependent on the size of plant andthe plant location. Severalpossibilities, heating or cooling ofservice buildings, utilisation as afuel in incinerator or for powergeneration for internal utilisationand/or grid connection areavailable.

Technical Brochure No.66

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M

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Food waste

Storage bunker

Heating and cooling system

Electric power

Incineration and fuel

Compost

Water treatment plant

Shredder

Drum screen

Magnetic separator

Gas holder

Dehydrator

Waste water

Compostor

Stack

CombustiblesIncineration bunker

Recycling tank

Landfill

Acid reactor

Gas (CH4)

Methane reactorY

Metal fragments

Schematic of the solid waste anaerobic digestion process.

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CADDET Centre for Renewable EnergyETSU, Harwell Oxfordshire OX11 0RAUnited KingdomTel: +44 1235 432719Fax: +44 1235 433595E-mail: caddet.renew@aeat.co.uk

KR 97 501 First printed 1998

Please write to the address below if you require more information.

Host OrganisationKorea Institute of Energy Research71-2 Jangdong Yusongku, Taejon KoreaContact: Mr Soon Chul ParkTel: + 82 42 860 3550Fax: + 82 42 860 3132

Information OrganisationCADDET Korean National TeamRaCER3F, 935-34 Bangbae-DongSocho-Ku, Seoul137-060 KoreaContact: Dr ChangSeob KimTel: + 85 2 522 4298Fax: + 82 2 522 8093/8094E-mail: cskim@netra.racer.or.kr

The shredder and drum screen for pre-treatment of the waste.

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