Abstract
Partial ozonation of the return sludge in activated sludge processes has been employed to reduce the excess sludge production and related disposal costs. However, it may have an impact on the microbial community which might affect the effluent quality. While previous lab scale studies have reported that ozonation could partly inhibit nitrifying bacteria, a number of workers have reported no effect of ozonation on the nitrification capacity. This paper presents the measurements of biomass activity in a demonstration test at a full-scale plant, designed at half of its full-scale potentiality. The WWTP (25,000 m3/day), located near Como (Italy), treats wastewater characterized by a predominant industrial component. Ozonation permitted the reduction of as much as 40% of the excess sludge at ozone consumption values of 0.05-0.10 kg O3/kg TS removed. The COD and TN removal capacities of the plant were not affected. Microbiological studies indicate that the elimination of foaming by the sludge ozonation process is due to the reduction of foam causing filamentous bacteria. Results from the monitoring of biomass activity indicate that the effects on nitrite oxidizing bacteria and heterotrophic bacteria were negligible, while there was a marked reduction in the activity of the ammonia oxidizing bacteria which almost halved. Monitoring the nitrification activity allowed the plant to meet the desired sludge reduction and biological foam control goals without affecting the nitrification efficiency of the process