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A VIGOROUS debate is underway over what materials should be composted, and in particular, whether composting should be limited to source separated organics or applied more broadly to mixed municipal solid waste (MSW). Several important tradeoffs exist between these approaches: quantity, quality and cost. This report focuses on the quality issues, specifically examining the impact of contaminant separation on levels of key metals in finished composts. The data presented were derived from a comprehensive literature search for results of trace metal analysis in solid waste composts in industrialized countries.
The chemical contaminants in MSW include both toxic organic chemicals and heavy metals. Some contaminants are only present at dilute concentrations throughout much of the MSW stream. Others are concentrated in a limited number of products, and are thus more amenable to separation approaches.
Many organic chemicals volatilize or degrade during composting, but they may expose compost workers to potential risks during composting.(4,6) While some organic chemicals such as PCBs and dioxins may be a concern in finished composts, there are not enough data available to assess the impacts of separation on this class of contaminants.
Metals and other inert contaminants do not degrade, but are instead concentrated during the composting process as the organic matter which dilutes them gradually degrades.(9,1) The metals of greatest concern are those which tend to bioaccumulate, causing short or long-term toxic effects to organisms in the environment. Those most commonly regulated include cadmium (Cd), chromium (Cr), copper (Cu), mercury (Hg), lead (Pb), nickel (Ni) and zinc (Zn).(8)
While MSW composts can usually meet regulatory limits for most metals (with the exception of lead), there is an increasing interest in achieving lower levels.(13) Several European countries and Canadian provinces have proposed or enacted much lower standards, which are sometimes based on background levels in the soil.(15, 8)
Whatever standards ultimately are established, solid waste managers and compost facility operators need to understand the effectiveness of different options to reduce contaminant levels. While there are a wide variety of possible approaches, most can be placed in one of five different categories: 1) Reduce or eliminate contaminant levels in products or packages destined to become MSW; 2) Source separate clean organic materials for separate collection and composting; 3) Source separate contaminants for separate...