The objective of the study reported herein was to evaluate the impact of household septic tank wastes on municipal activated sludge treatment plants. Septage addition was evaluated on a continuous basis over a four-month period in a 7500 l/day (1980 gpd) pilot plant. The septage was combined with municipal wastewater primary effluent in a series of increasing loadings to the activated sludge unit. Results were compared to a control unit receiving primary effluent only. Shock load studies were also conducted in the pilot plant system and with a series of batch aeration tests. Septage addition was found to be feasible on either a continuous or intermittent basis. The response during the continuous feeding studies depended upon the organic loading and the septage characteristics. COD loadings below 3 g COD/g MLVSS/day could be handled without severe upset. Unacclimated systems also responded well when septage was added, and substantial organic removals were obtained within a relatively short time.