Trickling filter and rotating biological contactors (RBC's) may be used with different goals in wastewater treatment: Degradation of BOD5, stabilisation of sludge, nitrification, etc. The design of these processes based on information in design manuals and guidelines does not consider these aspects sufficiently. This paper introduces a design procedure which considers the different design goals. Case studies explain the procedure. In small treatment plants, the alkalinity of waste water is frequently insufficient for complete nitrification. Solutions for this problem are indicated. Careful design of trickling-filters and RBC's results in reliable processes for small treatment plants.