VARIABILITY ANALYSIS
DURING BIOLOGICAL TREATABILITY
OF COMPLEX INDUSTRIAL WASTEWATERS FOR DESIGN
Enos L. Stover, Associate Professor
David E. McCartney, Graduate Student
Faramarz Dehkordi, Graduate Student
Don F. Kincannon, Professor
School of Civil Engineering
Oklahoma State University
Stillwater, Oklahoma 74078
The U.S. Environmental Protection Agency (EPA) has set standards for effluent quality
in order to meet the objectives of the Federal Water Pollution Control Act Amendments of
1972. The effluent limitations for secondary treatment as defined by EPA specify that
arithmetic means of total BOD5 concentrations shall not exceed 30 mg/1 in a period of
30 consecutive days, nor shall the total BOD5 exceed 45 mg/1 in a period of 7 consecutive
days. These requirements specify that 30 mg/1 effluent total BOD5 be achieved at least 50%
of the time and that 45 mg/1 effluent total BODs not be exceeded over about 25% of the
time. Actual municipal and industrial wastewater discharge requirements vary from plant
to plant, but in general are at least as stringent as the secondary treatment levels established
by EPA. Variations in wastewater characteristics, environmental conditions and biological
system performance must be evaluated to properly design a biological wastewater treatment system to reliably achieve the allowed effluent criteria.
The activated sludge process is the most commonly used process for treatment of industrial wastewaters. Reliable design of an activated sludge process to achieve a high effluent
quality at minimum cost requires the design engineer to estimate the expected effluent quality as well as the variability in effluent quality. Reliable design can be achieved by conducting
treatability studies to develop biokinetic constants and treatment performance data that can
be used in the various biokinetic predictive design models available. These kinetic design
models after Gaudy, Lawrence and McCarty, Eckenfelder, McKinney, and Weston represent the best available state-of-the-art technology available today.
Variability in wastewater characteristics (flows and pollutant concentrations) and variability in biological treatment performance are two of the most important considerations
during scale-up and design of industrial wastewater treatment facilities. It is extremely important during the experimental planning to develop wastewater survey and biological
treatability programs for collection of sufficient data to evaluate the variability and statistically analyze the data for design. The wastewater characterization requires knowledge of
the discharge characteristics due to production schedules, clean-up operations, etc. and
development of sufficient characterization data for determining the design conditions, for
example at the 50 and 90% probable flows and loadings. Consideration of the data variability from the biological treatability study due to the dynamic nature of the heterogeneous
microbial populations is more complicated and in most cases has not been properly addressed during design of industrial wastewater treatment facilities.
This chapter presents a systematic approach developed for evaluation of biological system process performance variability using methods of probability. This probabilistic design
approach provides a consistent basis for analysis of the inherent variability associated with
the biological kinetic constants or coefficients required for design of activated sludge systems. Then the variability of both the effluent quality and the descriptive biokinetic constants developed during the treatability study can be employed to predict the expected
variability of the full-scale system.
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