Reduction in bacterial densities of wastewater solids by three secondary treatment processes.

The objective of this research was to determine whether wastewater treatment plants which have no primary clarifiers but recir culate the sludge within the treatment system for long periods would reduce the densities of Salmonella sp. and fecal indicator bacteria in the sludge particles to a greater extent than would conventional primary settling and activated sludge treatment. Bacterial densities in the sus pended solids in the entering wastewater were compared with densities in the waste sludge. Reductions in Salmonella sp. densities (number/ gram) in the sludge were about 0.5 log greater for the five no-primary long-sludge-age plants than for the conventional plant. Similar differences were found for fecal indicator densities. Fecal indicator densities based on suspended solids content of the wastewater were nearly the same for all six plants. Since reductions were consistent for all plants, it appears possible to estimate performance of a treatment plant for reducing bac terial densities in sludge solids only by examining the final raw sludge product. Res. J. Water Pollut. Control Fed, 62, 177 (1990).