Twenty Years of Experience Says Sewage Sludge Beneficial

By Don Comis
May 18 1998

A 20-year study of growing crops with municipal sewage sludge offers solid evidence that the practice is safe and beneficial to crops and soil.

A common concern about sewage sludge or "biosolids" is that it will put toxic heavy metals in food crops. But soil scientist Robert H. Dowdy says that won't happen if the sludge is used correctly. Dowdy works at the Agricultural Research Service's Soil and Water Management Unit at St. Paul, Minn. He participated in the municipal sewage sludge study, which tracked corn and reed canary grass growth on a 40-acre watershed near Rosemount, Minn., where biosolids were applied annually.

Each fall after the corn harvest, biosolids were injected into the soil in a liquid suspension. The result: No extra metals showed up in the corn or grass, except slightly higher levels of zinc. The zinc levels were well within safety guidelines and can even be beneficial as dietary nutrients.

The biosolids raised the soil's organic matter content more than did commercial fertilizer used for comparison, according ARS biochemist C. Edward Clapp. Crop yields were slightly higher with the biosolids.

Biosolids hold a slow-release form of nitrogen that is less likely to be lost to surface water or groundwater. Like commercial fertilizers, farmers should apply biosolids only at the rate at which crops can take up the nutrients.

Recycling biosolids also helps return carbon to the soil, rather than losing it to the atmosphere as carbon dioxide during incineration or composting.

On cornfields in the study during the last four years, the applied biosolids supplied an average of 314 pounds per acre of nitrogen (N), 216 pounds phosphorus (P) and 20 pounds potassium (K) annually. On the grass, 433 pounds of nitrogen, 134 pounds phosphorus, and 18 pounds potassium were applied annually. These biosolids, respectively, had a 2-2-0 and 7-2-0 nutrient (N-P-K) content, considerably lower than most commercial fertilizers.

Scientific contact: Robert H. Dowdy, Soil and Water Management Research Unit, St. Paul, Minn., phone (612) 625-7058, fax (612) 625-2208, bdowdy@soils.umn.edu.