HIGH RATE BIO METHANATION OF MUNICIPAL SOLID WASTE

Abstract

Growing urbanization and industrialization have led to the generation of large quantities of solidwastes, which can be broadly classified as MSW (Municipal Solid Waste) and ISW (Industrial SolidWaste). Among the options available for solid waste management, biomethanation appears to be themost desirable because it is a technology with triple benefits: yields biogas which can replaceconventional fuels, provides digested sludge which can be used as a soil conditioner and manages theenvironment.

This work involved the development of a three-stage leachate based bioreactor system for solid wastemanagement. The first stage involves extraction of high organic strength (high in chemical oxygendemand - COD) liquid called leachate from the solids. This stage is known as the hydrolysing stage.Acid formation takes place in the second or the acidification stage in a separate reactor known as theacidification reactor. Biogas is generated by treating the acidified leachate in an upflow anaerobicfilter (AF) reactor in the third (or the methanation) stage.

Three stage anaerobic systems arc increasingly finding its place in biogas generation. Two kineticallydissimilar groups of bacteria, acetogenes and methogencs arc physically separated in acetogenicreactor and methanogenic reactor. This separation allows optimization of both acetogenesis andmethanogenesis stages making the process control easier.

In this method, the organic solid waste is cut into small pieces, fed into the hydrolysing reactor withwater and circulated at a fixed rate to wash off organic acids formed until high organic strength isobtained. This is then fed in to the acidification reactor. The acidification phase has retention time ofG days. Anaerobic conditions prevail inside the reactor during the whole process. The residue leftinside from the first reactor is subjected to drying for manure preparation.

The final concentrated leachate obtained after 6 days is fed in to the AF reactor for methanation. Thisphase has retention time of 5 days. The microbial consortia present in the AF sludge destroys highamount of COD, forming biogas comprising of 65% - 75% methane. This system is economicallyviable, the total space and the time required for the process is low compared to conventional singlephaseprocesses that takes 30-40 days. The system has low water consumption because of the reuse ofthe AF reactor overflow to the acidification reactor.