Heightened interest in the application of trickling filters has emerged during the 1980s, largely because this process is less expensive to initiate and maintain than competitive processes.1 Process improvements that use bioflocculation components have produced higher quality effluents than in the past. This improvement eliminates the major advantage of the activated sludge process over the trickling filter. It is now possible for trickling filters to produce an effluent, in terms of biochemical oxygen demand (BOD) and suspended solids (SS), as high-quality as that of the activated sludge process.2 Moreover, newly developed, more efficient plastic media allows construction of smaller trickling filters, which reduces process cost.3,4 Despite this resurgence of interest in trickling filters, relatively little process design information is available for simultaneous nitrification and BOD5 removal in a single unit, commonly termed combined carbon oxidation-nitrification. Moreover, existing data are expressed on different bases and need rationalization to allow reasonable performance predictions. In this paper, data from two recent pilot studies are compared to results from past investigations. Where possible, factors that influence nitrification are assessed. These factors include organic loading, competition between heterotrophs and nitrifiers, ammonia assimilation, and denitrification.