The uncertainties associated with plant design and operation should be recognized in the design of trickling filters. The best way to do this is to incorporate stochastic concepts and procedures in the design process. To produce an effluent of high quality and to meet the effluent discharge requirements at minimum cost, design engineers must be able to estimate the expected effluent quality and its variations for a given treatment process. Uncertainties and their significance for process performance may be analyzed systematically using methods of probability. A probabilistic approach for design may be used to provide a consistent basis for the analysis of uncertainty and a theoretical basis for the analysis of performance and reliability. In previous studies by Niku et al.6,7 the reliability and stability of activated sludge processes have been examined. This study determines applicability to trickling filter processes of the reliability and stability models developed for activated sludge processes.