Conventional criteria and design procedures employed in the design of municipal and industrial wastewater processes are based on the assumption that steady-state conditions exist with respect to the influent wastewater characteristics. In practice, however, activated sludge systems are seldom operated under steady-state conditions. Effluent quality depends on input loads, environmental conditions, and in-plant biological and operational variables, as well as many other unexpected perturbations beyond operational control. Designing a treatment process is thus a multivariate problem. Fluctuations in input loads and environmental conditions, variations in the nature of wastewater, introduction of toxic materials, mechanical failure of the system, and human errors frequently cause process upset and instability, which may have adverse effects on effluent quality.