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Probabilistic Based Discharge Standards
Author: Salar Niku, Edward D. Schroeder
Date: 7/182
Presented at 1982 National Conference on Environmental Engineering, Environmental Engineering Division, American Society of Civil Engineers, July 14-16, 1982

This paper addresses the appropriateness of present standards for secondary discharges from wastewater treatment plants. Present U.S. Environmental Protection Agency (EPA) standards for both 5-day biochemical oxygen demand (BOD) and suspended solids (SS) are based on arithmetic average values over a 7-day or 30-day period. Some states have similar standards based on 1-day composite figures. These arithmetic averages are maximum values, that is, they are not to be exceeded. As an example, a BOD standard for a plant might state that the 30-day arithmetic average of all samples will not exceed 30 milligrams of BOD per liter of effluent (30 mg/l). Engineers designing a process to meet this standard must take into account the high degree of variability in effluent concentrations and must over-design to insure that the standard will at no time be exceeded. For example, they must design a plant for, let us say, 15 mg/l under anticipated conditions, so that when the unanticipated happens, the effluent concentrations will remain under the standard.