Since 1970 explicit performance standards have been placed on most activated sludge processes in the U.S. The Federal Water Pollution Control Act Amendments of 1972 (PL 92-500) establish the basis for effluent quality regulation to restore the quality of receiving waters. Discharge requirements vary from plant to plant, but in general are at least as strict as secondary treatment guidelines defined by the U.S. Environmental Protection Agency. Current regulations often require very high average treatment efficiencies and also restrict variations from the allowable average concentration. In most cases penalties have been set for violation of these requirements. Currently, activated sludge wastewater treatment is the most commonly used process for treating wastewaters. Conventional criteria and design procedures used in the design of municipal and industrial wastewater treatment processes assume steady-state conditions. These processes rarely operate under steady-state conditions, however. Fluctuations in input loads, environmental conditions, and in-plant biological and operational variations are given little consideration in the system design. The lack of precise design methods, uncertainties, and the dynamic nature of biological waste treatment processes lead the designer to overbuild units and to overdesign processes. Economical pressure or lack of understanding of the variables that affect effluent quality have caused inadequate processes with an incapability to perform efficiently.