In recent years, authorities have issued more stringent COD regulations for industrial effluents in addition to BOD limitations. Optimization of operation in the activated sludge treatment plant can be an effective way to meet those regulations without increasing treatment cost. A well dimensioned aerobic or anaerobic plus aerobic treatment plant for pulp or paper mill waste waters is capable of reducing BOD effluent concentrations to values below 25 mg/l. If the aerobic plant is just big enough to meet this limit, COD elimination will be found to be in a certain ratio to BOD elimination, which has become well known in recent years. If this COD elimination is not sufficient to meet official requirements for COD concentration in the effluent, improvement of COD elimination by optimizing the operating conditions of the activated sludge plant may be possible. Lowering BOD sludge loading and increasing hydraulic retention time to more than 8 hours generally are the ways to do this. Concentration of dissolved oxygen has no effect on COD elimination above 1 mg/l. The best COD elimination can be obtained in optimized aeration cascade reactors and two stage aeration reactors.

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