The Chapel Hill high rate trickling filter plant which consists of two parallel and equal lines of treatment units was operated in parallel as two separate plants over a period of 26 months. Each side was operated with various fractions of influent flow and recirculation flow rates. Statistical analysis of operating results indicated that the common mathematical models are not reliable in predicting daily performance at the Chapel Hill plant. They are, however, useful in predicting long term average performance. Recirculation ratios as high as 3.0 proved beneficial at total hydraulic loadings of less than 20 mgad. The hydraulic surface loading of the final settling tanks was found to have a significant effect on overall plant performance. Pilot plant studies using four-foot diameter rock filters indicate a significant advantage for two-stage filtration even through the hydraulic loading on each stage may be double that for single-stage operation. Pilot plant studies of activated sludge treatment of trickling filter effluents were conducted. The process proved effective in improving removal of BOD, if effective final solids removal facilities are provided. The process also proved effective in reducing nitrogenous oxygen demand. (Modified author abstract)