Enhanced Activated Sludge Minimization by Promoting Increased Maintenance Metabolism via Holding Tank Ultrasound Treatment

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The aerobic activated sludge process (ASP) is the most commonly used technology for biological treatment of municipal carbonaceous wastewaters. While aerobic treatment of municipal wastewater is robust and has been successfully applied at countless treatment facilities around the world its main drawback is the production of large quantities of excess sludge (EXS). On average a conventional ASP produces about 80 kg of dry EXS for every 1000 m3 of wastewater treated, with the EXS being traditionally disposed of by anaerobic digestion followed by land application or landfilling . However, modern capacity restraints and stringent environmental regulations as well as costs make these options unsustainable solutions moving forward. For example, City of Ottawa currently generates 43.6 dry tonnes of EXS per day every day, with the total cost for sludge treatment and disposal being as high as 40-65% of the total operating cost of conventional wastewater treatment plants (Perez-Elvira, et al., 2006). The ideal solution to alleviate both cost and sludge treatment capacity problems would be the direct reduction of EXS within the activated sludge process. The proposed study is unique and blends the benefits and potential synergistic effects of (1) promoting and selecting for lower biomass producing facultative microbes in the ASP via an anaerobic holding tank (HT), and (2) applying high or low intensity ultrasound (US) treatment to a portion of the HT sludge. High intensity US increases cell lysis and solubilisation to promote cryptic growth and lower biomass yields while low intensity US treatment will wound the microbial cells diverting energy to maintenance metabolisms and away from assimilative growth that will also lead to an overall reduction of new biomass sludge. The goal of combining these beneficial systems to reduce excess sludge will be assessed by investigating a novel modified version of the continuous ASP which integrates an anerobic HT with post-sonication treatment in the sludge recycle line prior to recycling back to the ASP aeration basin. Final effluent quality and important residual sludge characteristics such as dewaterability and settleability will be used to assess the quality of the sludge produced by the proposed treatment systems. Additionally, in order to understand the roles of microbial shift, cryptic growth and enhanced maintenance metabolism within the HT + US modified ASP the process will also be monitored at the microbial level at each step (HT, post US, ASP). A combination of microbial viability, activity and stress tests will be used, as well as molecular fingerprinting, population sequencing and scanning electron microscopy to characterize floc morphology will be applied to ultimately characterize microbial ecological changes.