MICROBIOLOGICAL STABILITY OF WASTE-WATER SLUDGES FROM ACTIVATED-SLUDGE SYSTEMS

Proposed United States Environmental Protection Agency (EPA) regulatio ns require that processed sludge, prior to disposal, must meet certain pathogen-reduction requirements. The present study was conducted to e valuate reductions in pathogen-indicator bacteria in the sludges of si x wastewater plants. Five of the plants use extended aeration and oxid ation ditches; the other is a trickling filter plant. Fecal coliform, fecal streptococcus, and Salmonella were measured on all sludge sample s. Temperature, pH, dissolved oxygen, volatile suspended solids (VSS), and total suspended solids were also measured. The fecal coliform log reductions/g VSS for all the extended aeration and oxidation ditch pl ants were greater than 1.76. They varied from a log reduction of 1.76 to 4.21. The fecal streptococci reductions for the same sludge were ge nerally lower than the fecal coliform reductions. The variations in th e fecal streptococci log reductions in different plant sludges were fr om 0.87 to 2.73. The Salmonella log reductions varied from 1.39 to 4.7 and paralleled the fecal coliform reductions. The bacterial reduction s were related to the plant design parameters. A longer storage of slu dge usually produced a larger log reduction of the indicator bacteria. Generally, the extended aeration and oxidation ditch plants were meet ing the proposed EPA class B Pathogen reduction criteria, a two-order reduction.