In many cases, it will be advisable to enhance operational safety and increase degradation performance of existing activated sludge plants by inserting a high-capacity trickling filter with plastic media. Easily degradable carbohydrates are largely decomposed in the trickling filter at low energy requirements. This allows the subsequent activated sludge stage to be set to reduced BOD sludge load levels which are required for efficient COD elimination, because the volume load has been reduced and a higher biomass concentration can be obtained at a lower sludge volume index. In the light of several years' pilot testing of numerous different effluents, and of knowledge derived from observations of several industrial plants in the paper sector, precise rating directives can now be given for plants of this type. It has proven advantageous to discharge the trickling filter effluent directly into the activated sludge plant without intermediate clarification. The effluent temperature, which frequently exceeds 35°C, is reduced by up to 10°C in the trickling filter. This improves the conditions prevailing in the activated sludge plant, thus permitting limit values for the discharge of effluents into surface waters to be adhered to.

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