The objective of this research study was to statistically analyze the performance of a large number of activated sludge processes and to develop methods and procedures for introducing reliability and stability concepts into design and operation of treatment plants. The lognormal distribution was found to fit the observed effluent BOD and SS data more consistently. Using the lognormal distribution and a coefficient of reliability (COR), a probabilistic model and design tables and graphs have been developed for predicting achievable effluent BOD and SS concentrations. The proposed model can be used in design of a treatment process expected to perform at a certain reliability and/or to estimate the reliability of an operating treatment plant. The stability of various activated sludge processes was examined using several statistical measures. The standard deviation was found to be the most appropriate indicator of stability. Plants with effluent values below 10 g/cu m are considered stable; plants with values greater than 10 g/cu m are considered unstable. Correlations and regression analyses have been used to investigate the causes of effluent quality variablity and to identify the extent of the factors contributing to this variability. No single or group of variables was found that could be used to characterize the variability of effluent quality for all plants in general. The use of geometric mean, as a measure of central tendency of daily effluent quality data, was recommended for setting discharge standards. An approach is presented that can be used to design a process stochastically when the effluent standards are deterministic in nature.