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8 Composting MSW

Drive my dead thoughts over the universe
Like withered leaves to quicken a new birth!
Percy Bysshe Shelley (1792-1822)
Ode to the West Wind

8.1 INTRODUCTION

Organic materials comprise the majority of municipal solid wastes (MSW) generated in the United
States, accounting for about 141 million tons (67%) of the waste stream (U.S. EPA, 1997). Some
organics such as newspaper, office paper, and corrugated cardboard are recovered extensively for
recycling. Other potentially useful materials (e.g., yard waste, food scraps, other grades of paper),
however, continue to be lost from the recycling loop and landfilled.
The organic fraction of MSW can circumvent land disposal and undergo other fates including direct
conversion into fuels, fermentation as a biofuel, pyrolysis, and composting. Composting has been in
practice even before Roman times, and the method has since been streamlined in terms of efficiency,
time of completion, health impacts, and area requirements. Composting of MSW, agricultural wastes
(plant residues, animal manures), food factory waste, and municipal wastewaster treatment solids
(“biosolids”) is increasingly used worldwide as a means of waste management. It is estimated that about
75 million tons of the U.S. waste stream, including 28 million tons of yard waste, 25 million tons of food
waste, and 25 million tons of soiled or unrecyclable paper, is available for composting (see Table 1.3).
A range of programs has been established to divert organic materials from landfills and create
beneficial uses. These include:

● Mixed waste composting at centralized processing facilities that accept MSW and sepa-
rate into composting, recycling, and disposal streams.
● Residential source-separated systems using organic materials separated by the generator,
set out for collection, and processed at a centralized facility.
● Commercial composting operations that utilize materials generated by commercial and
industrial establishments.
● Yard waste composting at centralized facilities.
● Backyard composting of food and yard waste.

8.2 BENEFITS OF COMPOSTING
Composting is defined as a controlled aerobic, biological conversion of organic wastes into a com-
plex, stable material. The final product has a number of beneficial uses, most commonly for agri-
culture and landscaping.
If shredded raw MSW were introduced directly into a soil to be used for agriculture, the organic
component would undergo rapid transformation by soil microorganisms. A number of adverse
effects would result:
● Undesirable reactions. Anaerobic transformations will produce ammonia (NH ), hydro-
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gen sulfide (H S), and methane (CH ) gas. Several gaseous products are toxic to plant
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growth and will additionally cause odor problems.
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