Page 260 - Materials Chemistry, Second Edition

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Composting MSW 231
8.8.5 LEACHATE
Leachate is produced in uncovered piles exposed to excessive quantities of precipitation. The
leachate released from the pile can have elevated biochemical oxygen demand (BOD) and phenols,

resulting from the natural decomposition of organic material. Nitrates (NO ) are also generated by
3
composting grass clippings along with leaves. Leachate composition from a compost pile is shown
in Table 8.6.
Leachate can also contain potentially toxic synthetic compounds, including chlorinated organic
compounds from treated wood, pesticides, polycyclic aromatic hydrocarbons (PAHs), and combus-
tion products of gasoline, oil, and coal. Chlorinated organics and PAHs are resistant to biodegrada-
tion and tend to persist after composting (Gillett, 1992). Microorganisms can partly degrade some
PAHs during composting; however, the resultant compounds can be more toxic than the original
PAHs (Menzer, 1991; Chaney and Ryan, 1992).
Leachate generation can be reduced or prevented by monitoring and correcting the moisture
levels in the composting pile. In some facilities, windrows or piles are installed under a roof to limit
excessive moisture levels arising from precipitation. If the compost feedstock (MSW or yard trim-
mings) contains excess moisture, leachate will be released during the first few days of composting
regardless of any rainfall event. Following this initial release, the volume of leachate generated will
decrease as the compost product matures and humifies, thus improving its water-holding capacity.
The age of the pile will also affect leachate composition. In a mature pile, microorganisms have
decomposed complex compounds and released or consumed substantial carbon and nitrogen. If the

TABLE 8.6
Croton Point, New York, Yard Waste Compost Leachate Composition

Compost Leachate (16 samples)
Average (mg/L) Standard Deviation (mg/L)
Cd ND
Cu ND
Ni ND
Cr ND
Zn 0.11 0.13
Al 0.33 0.38
Fe 0.57 0.78
Pb 0.01 0.02
K 2.70 0.99
NH -N 0.44 0.35
4
NO -N 0.96 1.00
3
NO -N 0.02 0.02
2
Phosphorus 0.07 0.08
Phenols (total) 0.18 0.45
COD 56.33 371.22
BOD 41 a 60
PH 7.75 0.36
Color ND
Odor ND
ND not determined; COD chemical oxygen determined.
a Includes three samples above detection limit of 50 mg/L.
Source: Richard, T. and Chadsey, M., Biocycle, April, 31: 42-46, 1990. Reproduced with kind
permission of The J.G. Press, Inc.