Persistence and Degradation of Mutagens in Sludge‐Amended Soil

Abstract

AbstractThe presence of toxic organics and mutagenic activity in sewage sludge presents a potential problem when contaminated sludge is applied to soil. Sewage sludges from Washington, D.C. were assayed for mutagenic activity using the Ames Assay. Raw cake, digested cake, biologically thickened sludge, and composted sludges were extracted with water and ethylene dichloride. No mutagenic activity was detected in any of the water extracts. Only the ethylene dichloride extract of the biologically thickened sludge contained significant quantities of mutagens.Biologically thickened sludge was subsequently added to a Chester loam soil at a rate of 112 Mg ha−1. The soils were incubated for 8 weeks. Periodically, the sludge‐amended soils were extracted and the mutagenic activity of the extract determined as before. Unamended control soils were also extracted as a control.The mutagenicity of the sludge‐amended soil was found to increase during the initial 7 d of the study. Seven days after the initiation of the study, the number of revertants (Salmonella typhimurium strain TA‐98) induced by the extract from the sludge‐amended soil was 150 per plate. The control soil extract exhibited 95 revertants per plate. After 7 d, the quantity of mutagens extracted from the sludge‐amended soil rapidly decreased. This was probably due to the rapid degradation of these compounds in soil.