COMPOSTING 189
of wastes chosen to provide the appropriate environment
and ensure active decomposition within the windrow. Waste
water sludges are often added to “seed” the pile with micro-
organisms (abundant in waste water sludges) and/or raise
the moisture content of the mixture; water is often added to
bring the moisture content up to optimum levels; and nutri-
ents may be added, specifically nitrogen amendments, if it’s
necessary to balance the mixture’s C:N ratio.
Periodically, the windrows are turned. In the first weeks
after the feedstock has been windrowed, the piles are turned
frequently (e.g., every fifth day), thereby aerating the inte-
rior sections of the pile. Adequate oxygen supply maintains
aerobic conditions and ensures the microbial population
reaches peak metabolic activity. After 1–2 months, the piles
are turned less frequently. At some facilities, forced aera-
tion helps keep the pile aerobic and speeds decomposition:
Windrows are heaped on a composting pad constructed witha build-in ventilation system or, at more rudimentary facili-
ties, simply piled over a ventilator pipe. Negative aeration
pulls air down through the pile; positive aeration forces air
up through the pile from below.
In batch operations, windrows are monitored for mois-
ture content; long retention times are required to stabilize the
feedstock. Kept in windrows for several months, the waste
material undergoes a substantial change in character, becom-
ing a dark, earthy, soil-like humus.Continuous/Intermittent CompostingContinuous-flow or intermittently-fed composting processes
(Figure 2) have been used as a method of processing MSW
since the 1920s. Many European plants built since the 1950s
are continuous flow operations. In the United States, a
number of continuous-flow, in-vessel processes are currentlyFIGURE 2 Continuous (or intermittently fed) compoting using biological reacter.C003_003_r03.indd 189C003_003_r03.indd 189 11/18/2005 1:00:20 PM11/18/2005 1:00:20 P