Here's one reply.
Yes, sewage sludge contains heavy metals and organic chemicals, including
dioxin. So does the soil around our houses, the food (even organic) we
eat, the air we breath, and even the polar ice caps. Not to mention our
own bodies.
The question is not whether these compounds are there (since we now have the
tools to measure extremely small concentrations), but what levels create
significant impacts for people or the environment. And, as you might
guess, there is some debate, within the scientific community and elsewhere,
about what is "significant," as well as how to model and predict these
impacts.
In 1993, the USEPA promulgated new regulatory criteria for sludges to be
applied to land. These criteria are based on risk assessment models that
incorporated extensive research on contaminant fate and transport, plant
uptake, bioaccumulation in the food chain, and environmental and human
toxicological impacts. The National Research Council used much of the same
information in its analysis, and came up with similar results. Are the
assumptions in these models correct? Are the built-in safety factors
adequate? Is the definition of a "significant" impact appropriate? These
are not just technical questions for the experts to wrangle over. They are
also about values, of both human health and the natural environment.
Many sewage sludges today have contaminant concentrations well below the
regulatory criteria, largely due to industrial pretreatment and pollution
prevention programs. In some cases the contaminant levels are comparable
to those of yard waste composts and manures, and by almost anyone's
assessment would qualify as "safe". Other sludges do not meet the
regulatory criteria, and certainly should not be applied.
The question of where to draw this line is important to a sustainable
agricultural system, since so much of the organic matter and nutrients
harvested from farms are cycled through the urban environment and end up as
sewage sludge.
If you want to learn more about technical and policy issues concerning
"sludge," Cornell Cooperative Extension is sponsoring a
Satellite Conference on this topic in April, with over 75 downlink sites
already nationwide.
April 9 Cornell Cooperative Exension Satelite Network
LAND APPLICATION OF BIO SOLIDS, 1:00 test,
1:30 3:30 program. Contact: Lauri Wellin, (607)
255 1187 for program questions, and Nancy Fey, (607)
255 2135 for satellite questions
******************************************************
Tom Richard
e-mail: tlr2@cornell.edu
Department of Agricultural and Biological Engineering
211A Riley-Robb Hall
Cornell University phone: (607) 255-2488
Ithaca, NY 14853-5701 fax: (607) 255-4080
Cornell Composting Website: http://www.cals.cornell.edu/dept/compost/