Encyclopedia of Environmental Science and Engineering, Volume I and II

186 COMPOSTING

now removed, the resulting source-separated organic waste

(SSOW) is nutritionally richer, yet, because of increased

consumption of paper products and packaging, it has a

higher cellulosic content.

Most often, a number of different wastes are combined to

achieve the nutrient balance necessary to sustain the micro-

bial population. In general terms, the balance depends on

nutritive waste fractions like food wastes, manures and waste

water sludges (biosolids) combined with bulking agents like

wood chips, leaves, bark, light paper and waxed cardboard.

If necessary, adjustments are made to the substrate to achieve

the proper balance of nutrients (e.g., adding ammonium or

nitrate salts boosts nitrogen levels). Biosolids—because of

their high moisture content (up to 98%), available nitro-

gen and an indigenous microbial population—are ideal for

mixing (cocomposing) with MSW and SSOW. It must be

noted, however, that potential problems characteristic of

biosolids (e.g., the presence of pathogens, toxic metals and

organics, and odor) must be recognized before beginning the

co-composting process and producing an end product.

Preparation

Shredding, chopping or grinding reduces particle size and

increases surface area, exposing more of the substrate to

microbial attack. It also produces a uniform mass that facili-

tates mixing and aeration. Typically, the resulting particle

size is smaller than one inch. Such preparation is not essen-

tial to all compost operations. For example, composting

biosolids and leaves does not call for much preparation—

equipment is needed only for mixing blending and moving

the materials—while MSW and SSOW and yard wastes

are routinely shredded. Optimal preparation of the feed-

stock includes front-end separation and recovery of ferrous

metals, glass, plastic, textile and other recyclable materials

(resource recovery and screening to remove recyclables and

large objects may occur at any point in the process).

Environment and Supplements

In large scale composting operations, efficient decomposition

and odor control are determined by the dependent relation-

ship between moisture, oxygen, temperature and bulk den-

sity. The moisture content of the mixture is a key parameter

in composting because the biochemical reactions that drive

the compost process (providing nutrients that support micro-

bial population growth) take place in the film of water envel-

oping each particle (hence the importance of particle size and

surface area). Oxygen also plays a key role. It must be avail-

able in sufficient amounts to microorganisms throughout the

FIGURE 1 Batch composting using windrows.

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