Excess sludge reduction during activated sludge municipal wastewater treatment by integrating an anoxic holding tank and post-ultrasound treatment to enhanced biomass maintenance metabolism

The exchange to the aeration basin of activated sludge biomass previously exposed to a controlled anoxic environment (HT) followed by low intensity ultrasound post-treatment (USPT) enhanced excess sludge minimization during aerobic biological treatment of municipal wastewater. HT biomass sonicated at low ES inputs (< 56KJ/gTS) decreased floc size by 41% and enhanced its metabolic activity by 50-250% compared to control. ES inputs >118 KJ/gTS caused HT biomass solubilization and irreversible loss of its metabolic activity and reflocculation ability. Surface response methodology and USPT optimization indicated that HT biomass activity was only enhanced at ES inputs <17 KJ/gTS. During continuous activated sludge processing (ASP) of real primary effluent the observed yield (Yobs) decreased by 20% compared to control ASP at SF (stress factor) of 1 (biomass exchanged without USPT). At SF of 0.5, 1 and 1.5 (biomass exchanged with USPT) the Yobs further decreased by 33, 25 and 44% respectively as compared to control. This indicated that combining biomass anoxic exposure with USPT enhanced sludge reduction by increasing microbial maintenance metabolism likely in combination with microbial flora shift in the ASP depending on SF. ASP effluent quality remained constant for SF of 0.5 and 1 (88 and 91% total chemical oxygen demand (tCOD) and total suspended solids (TSS) respectively) but decreased by 12 and 41% respectively at SF of 1.5. The ASP biomass activity and microbial stress (specific oxygen uptake rate (SOUR) and dehydrogenase activity DHA)) increased in average by 23% and 39% respectively under all SF conditions tested, suggesting that the experimental conditions induced metabolic stress on the ASP biomass with concomitantly lower Yobs.