Necessary data are not available in most areas to determine or predict distributions of storm water runoff and combined sewage flows and pollutant concentrations. To alleviate this problem, a procedure was developed to permit rapid, economical, and accurate assessments of system performance based on 3 different and important water quality criteria. The first criterion establishes an absolute maximum pollutant concentration which cannot be exceeded. The second criterion establishes an acceptable distribution of pollutant concentration by specifying the greatest frequency of occurrence for a particular concentration value. The third criterion establishes the maximum acceptable excession frequency for a particular pollutant frequency. A computer model was then used with a modified rational runoff method to perform preliminary design tasks and weigh alternatives to the designs. Then the least costly system developed for the Sacramento, California area as an example.