In this report, we review the existing literature on compost leachates, with special emphasis on the chemical and physical characteristics of the leachate constituents. We briefly summarize the biochemical processes occurring during composting and the use and applications of compost in the environment. We then review the chemical and physical characteristics of feedstock, compost, and compost leachate. We finally discuss environmental implications of the compost leachate.
The compost source material, i.e., the feedstock, determines to a large degree the types of leachates. Well-cured compost has been found to eliminate phytotoxic effects of raw feedstock and promote nutrient availability when applied as soil amendment. Feedstock selection and waste-stream separation play an important role in keeping non-degradable contaminants, like heavy metals and plastics, out of the compost. Leachate from compost is yellow to dark-brown in color, mainly due to the presence of dissolved and particulate organic matter. Characterization of the amount and type of organic material (both dissolved and particulate) in compost leachate indicates that, in the early stages of composting, the leached organic matter contains oxidized functional groups, and in the later stages of composting, there is an increase of phenolic (a benzene ring with an OH group) functional groups. The increase of phenolic groups indicates degradation of lignin in the maturing compost.
Together with the leaching of organic matter, there is evidence for leaching of various nutrients and contaminants present in the original feedstock. Contaminants can leach by direct dissolution out of the compost, as well as with the help of dissolved and particulate organic matter, both of which have a high sorption affinity for inorganic and organic constituents, and which can enhance contaminant leaching via the mechanism of colloid-facilitated transport. Organic matter, nutrient, and contaminant concentrations are highest in the initial leachate, and the concentrations decrease considerably with increasing amount of rain or runoff water leaching through the compost.