The authors have presented a new concept of excess sludge elimination treatment with recirculation of sludge via ozonation in the activated sludge process. This paper is intended to clarify the potential application of the process to municipal and industrial wastewater treatments. In a full-scale operational experiment lasting 10 months under 550 kg/d of BOD loading, no excess sludge was needed to be withdrawn and no significant accumulation of inorganic solids occurred in the aeration tank. Most of the inorganic compounds in the sludge were released to the soluble phase. Material balance indicated that one-third of ozonated sludge was mineralized via the recirculation treatment, and thereby the requirement of sludge mass to be treated was 3.3 times as much as sludge to be eliminated. Effluent TOC was slightly higher than under the conventional activated sludge process, indicating that refractory TOC was released from the sludge eliminated by treatment. The amount of released TOC corresponded to less than 2 weight % of eliminated sludge under recirculation rates below 30% of total biomass in the aeration tank in a day, but increased at higher recirculation rates. The operation costs associated with the process were estimated to be lower than those of conventional dewatering and disposal.

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