Complete decomposition of biological waste sludge by thermophilic aerobic bacteria

Conventional activated sludge (AS) process is an economical and effective biooxidation process although a large amount of excess sludge is necessarily generated. We have developed a new zero-discharge AS process, in which no excess sludge is generated. It was formed by combining the conventional AS process with thermophilic aerobic sludge digester in which excess sludge is solubilized by thermophilic enzyme, so we call it hereafter the digester S-TE reactor. The excess sludge withdrawn from the AS step is subjected to the S-TE reactor, followed by its circulation to an aeration tank. Complete decomposition of the excess sludge is carried out when the sludge is solubilized by thermophilic aerobic bacteria (e.g. Bacillus sp.) and mineralized by mesophilic bacteria. The S-TE reactor is operated at 65°C with hydraulic retention time of 1 day. Bench-scale test facilities of both the new process and conventional AS process were comparatively operated under the conditions of BOD-SS loading of 0.3 kg/kg/dand MLSS of 2,000 mg/L using artificial wastewater for over 100 days. The S-TE reactor was initially inoculated with the seedling culture of isolated Bacillus sp. bacteria. No additional inoculation seem to be needed under the steady condition since these bacteria can form spores and survive even under mesophilic conditions. The average rates of VSS solubilization and VM removal in the S-TE reactor were 40% and 15%, respectively. No excess sludge was generated when 3-fold amounts of the excess sludge generated in the conventional AS process were subjected to the S-TE reactor. In the new process, BOD was removed with high efficiency. Slight increase of SS and TOC was observed when compared with those of the AS process. A pilot-scale facility of the process has been operated with a petrochemical plant wastewater for a year. MLSS concentration was continuously kept around 3,000 mg/L without withdrawing the excess sludge. Inorganic salts seemed not to accumulate in the process. Therefore, it was demonstrated that excess sludge generation was completely reduced in the new process. According to our estimates, total operating costs for the new process are reduced to 40-50% of those of the conventional wastewater dewatering process. An application to a municipal sewage sludge treatment is currently performed in its full-scale facility.