Encyclopedia of Environmental Science and Engineering, Volume I and II

185

COMPOSTING

INTRODUCTION

Composting is the biological decomposition of semi-dry

organic waste by microorganisms under controlled condi-

tions. During the process, a succession of microbial attacks

by bacteria, fungi, and actinomycetes physically and chemi-

cally alters the character and structure of the organic mate-

rial. The volume of the original material is reduced by 30

to 40% as the microorganisms convert the organic waste to

nutrient-rich, nuisance-free soil-like humus resistant to fur-

ther decomposition.

In nature, composting is a continually occurring, ubiqui-

tous process. It is a soil-based, ecological system that recycles

mineral plant nutrients in organic form^1 wherever residues

of vegetation accumulate and whenever favorable environ-

mental conditions (appropriate temperature and moisture)

exist. In agriculture, composted green waste and manures

have been applied to tilled soil to increase crop yield since

land was first cultivated. While compost is still most widely

used to improve soil fertility, its application also helps pro-

vide erosion control, restore wetlands and purify toxic sites

through bioremediation.^2

In the last half century, growth of the United States popu-

lation and improvements in the country’s standard of living,

coupled with changing product consumption habits, has led to

a massive solid waste stream. At present, nearly 2.25 tons of

municipal solid waste (MSW) are generated per capita each

year in the United States^3 —630 million tons per year for the

country as a whole (figures cited do not include tonnage for

industrial or agricultural wastes). Traditionally, this mountain

of waste has been landfilled or incinerated; indeed, for the last

forty years, efforts to produce a soil amendment or low grade

fertilizer from composted MSW have been largely unsuccess-

ful. Poor quality and higher than acceptable costs of the finished

product have contributed to this failure.^4 Lack of public accep-

tance of composting as a valuable recycling method, nuisance

problems and perceived but unfounded health concerns have

also played a part in the closing of some composting facilities.

Over the course of the last 10 years, the study of the

science and engineering of composting has intensified. The

reconsideration of composting as an alternative disposal

method is due, in part, to landfill shortages, rising transpor-

tation costs and tipping fees, and tougher federal and state

regulations for siting and operating landfills and incinerators.

This reconsideration has also been generated by significant

improvements made in the design of facilities, equipment

and process controls, and of equal importance, the wider

dissemination and application of the mechanisms and best

practices involved in successful composting. In communities

throughout the country, large scale composting has begun to

play a significant part in county and municipal integrated

solid waste planning and management.

Renewed interest in composting began specifically with

sewage sludge, expanded to yard wastes, and has now made

inroads into the entire organic fraction of the municipal solid

waste stream.^5 Yard waste composting, with a feedstock of

leaves and yard/park debris mixed with animal manures, has

become the most prevalent form of composting in the United

States. Diverting yard waste from MSW reduces the waste

stream—with seasonal fluctuations—on average, up to 30%.

Several states now permit supermarket and/or restaurant organ-

ics to be included in yard waste composting. 6,7 As of 1992,

2,981 yard waste composting facilities and 12 source-sepa-

rated (food, paper and other organics) composting facilities are

in operation; as of 1993, 182 waste water sludge composting

facilities are in operation; and as of 1994, 17 MSW composting

facilities are in operation with 2 additional plants in the pilot

stage and 3 facilities under construction.^8 While there are many

biochemical, functional and operational similarities between

various compost operations, this discussion will be limited to

the fundamentals, the process mechanics and end products of

MSW, source-separated and yard waste composting.

FUNDAMENTALS

Almost any solid waste with an organic component (e.g.,

vegetable matter, fats, leaves, grass-clipping, sawdust, wood

chips, waste water sludges) can be composted by one of a

variety of process modifications as long as an environment

suitable for microbial growth on a particular feedstock is

maintained. All composting regardless of the facility design

or type of feedstock (substrate) share similar process and

microbiological fundamentals. Figure 1 shows an overview

of the composting process.

Substrate/Mixing and Blending

Composting begins with the substrate. Over the past decade,

recycling and recovery requirements for municipal refuse

have altered the composition of the waste being diverted to

compost facilities. With much of the metal, plastic and glass

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