The trickling filter/solids contract (TF/SC) process was first successfully demonstrated in 1979 as an outgrowth of the trickling filter process.1,2 Its acceptance and use have spread rapidly in the last few years, largely because it produces an effluent of much better quality than effluent produced by trickling filters alone. This is done economically and reliably. Since that initial work at Corvallis, Oreg., at least 11 other TF/SC plants have been put into operation, and approximately 30 more are in the planning or design stage. In 1984, the U.S. Environmental Protection Agency (EPA) sponsored full-scale studies3 of the TF/SC process under its innovative/alternative technology program to document the performance of existing TF/SC facilities and provide process design operation and information. A total of 29 weeks of field investigations were conducted at four operating TF/SC plants (Oconto Falls, Wisc.; Tolleson, Ariz.; Medford, Oreg.; and Chilton, Wisc.).