Effects of mixed liquor ozonation in an activated sludge system for sewage treatment

Sludge generation is a drawback of the application of activated sludge (AS) technology in sewage treatment. The reduction of sludge production by the ozonation of an AS system's mixed liquor is an approach to reduce excess sludge in the process. The aim of this study is to evaluate the application of four ozone doses to mixed liquor fractions from a laboratory-scale AS sewage treatment system. The effect of ozonation in the mixed liquor supernatant was also evaluated. The doses ranged from 0.35 to 0.80 gO(3)/gVSS. The results show that the applied ozone doses reduced the volatile suspended solids (VSS) concentrations by 25.5%-70.5%, which demonstrates the effectiveness of ozone in reducing the sludge concentration. In the supernatant of the ozonized mixed liquor, the chemical oxygen demand (COD) concentrations increased by 237%, 327%, 597% and 701% after the four doses. Moreover, the total nitrogen and total phosphorus concentrations in the supernatant increased by 9.4%-47% and 9.7%-8.2%, respectively, with respect to a mixed liquor without ozone application. The COD concentration in the effluent is affected by the ozonation. Thus, the COD removal efficiency of the AS system decreased by 6%-63%, whereas the nutrient removal efficiency was constant. In contrast, the specific oxygen uptake rate index was greatly inhibited (maximum inhibition of 31%) after the application of 0.50 gO(3)/gVSS.