MICROBIOLOGICAL STABILIZATION OF SLUDGE BY AEROBIC DIGESTION AND STORAGE

A study is conducted to evaluate the pathogen and indicator bacterial reductions in the sludges of two wastewater-treatment plants. These pl ants are extended-aeration and oxidation-ditch-type plants that have n o aerobic or anaerobic digestion facilities following the secondary tr eatment except for sludge-storage facilities. Sludge samples are taken from various locations throughout the plant. Sludge samples from thes e two plants are also collected to run laboratory investigations to de termine the sludge pathogen and bacterial reductions in aerobic digest ion and in storage. Fecal coliform, fecal streptococcus, and Salmonell a are measured on all sludge samples. Total suspended solids, volatile suspended solids, pH, and temperature are also measured. These plants are meeting the proposed EPA's class B pathogen reduction criteria, a two-order reduction. The improved pathogen reduction is related to th e design conditions of the plant, large sludge age, and long detention times. Laboratory aeration and storage studies indicate further patho gen-indicator organisms reduction of at least one order.