The recent development of the trickling filter/solids contact (TF/SC) process has caused renewed interest in trickling filters as a competitive biological treatment process.1 Brown and Caldwell has shown the applicability of the TF/SC process at full-scale operations in seven locations and the process is in design or construction in at least 30 other locations.2 The widespread popularity of the TF/SC process is largely due to advantages which build on the traditional strengths of the trickling filter and the stabilizing aspects of the solids contact features of the process. These advantages are: (1) lower capital costs than the activated sludge and the rotating biological contractor process, (2) lower operation and maintenance costs, (3) simplicity of operation, (4) ease of biological sludge thickening, (5) adaptability to upgrading existing trickling filters, and (6) equivalency of performance to the activated sludge process.