Potential toxic and carcinogenic chemical contaminants in source‐separated municipal solid waste composts: Review of available data and recommendations

Abstract

Composting and soil amendment could be an excellent alternative to disposal for the biodegradable fraction of residential, institutional, restaurant and commercial wastes. If done properly, composting fits naturally with traditional recycling, and could become one of the next major steps in solid waste management. Issues of public acceptance and the need to protect human health and the environment will be crucial in the development of municipal composting as a viable solid waste management strategy. Reducing the levels of contaminants in composts will help increase public acceptance of composting, markets for composts and associated ecological benefits. Contaminant levels are usually lower when waste generators separate organic feedstocks for composting than when post‐collection separation techniques are applied to mixed solid wastes. For this reason, source‐separated composting is generally favored by environmentalists — and it has been the focus of this work. It is crucial to keep this point in mind when interpreting our results — they are relevant only for composts with contaminant levels similar to those we have used in our calculations, and not for composts with significantly higher contaminant levels. As the composting industry develops, rational standards and guidelines for compost quality will help guide the industry in environmentally responsible and economically workable directions. However, there are at least three distinct — and all arguably rational — approaches to standards development in common use: those based on 1) minimal degradation, 2) acceptable risk and 3) achievable performance. Even so, the broadest possible range of constituencies will support composting if contaminant levels in composts are low enough to satisfy standards based on minimal degradation — because such standards are the most protective environmentally. We developed a minimal‐degradation‐based approach in this work. On the basis of available toxicity information and typical levels in source‐separated composts, we picked three metals — lead, mercury and cadmium — and three classes of organic compounds — polychlori‐nated dibenzo‐p‐dioxins and dibenzofurans (PCDD/Fs), polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs) — for detailed analysis. Using simple, mass‐balance, “box models”; to estimate future contamination levels in rural, agricultural soils receiving annual compost amendments, we asked if the levels of these contaminants in source‐separated composts are high enough to increase soil concentrations significantly above background levels — given compost application on a scale of decades to centuries. Although future patterns of compost application are unknowable, we identified the basic points relevant for policy considerations by looking at two extreme scenarios for application rate and receiving area: 1) a low‐rate/large‐area scenario, and 2) a high‐rate/small‐area scenario. Our results indicate that typical source‐separated composts could be applied to rural, agricultural soils for at least 200 yr under either of our scenarios for application rate and receiving area without increasing soil levels of lead, mercury, cadmium, PCDD/Fs, PCBs or PAHs above the benchmarks that we defined based on current soil levels — even if we take the time constants for the disappearance of these contaminants from the soil to be as large as 200 yr for PAHs, 300 yr for PCBs, and 1000 yr for the other species. Conversely, the application under either of our scenarios of composts containing cadmium and/or mercury at levels permitted by EPA's “503 rule”; for unrestricted land application of sewage sludge would increase the mean concentrations of these metals in receiving soils above our benchmarks within 20–30 years. Advocates of minimal‐degradation‐based compost standards can therefore responsibly support the development of a source‐separated composting industry — while working with other stakeholders to further reduce the ultimate sources of these hazardous contaminants.