Energy Optimization at Municipal Wastewater Treatment Plants

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Most wastewater treatment plants in North America, are approaching their design life necessitating significant capital investment for expansion and upgrade to meet increasingly stringent effluent quality criteria while addressing increased energy costs and heightened needs for economic and environmental sustainability. According to the US EPA, municipal wastewater treatment plants (MWWTP) consume about 75 billion KWH annually, or 3% of the US electricity generated. Aeration energy accounts for about 60% of the overall energy consumption at MWWTP. Furthermore, biosolids processing and disposal costs account for 50%-60% of overall wastewater treatment costs. The main technical challenges facing the municipal wastewater sector relate to nutrients and biosolids management, as well as optimization of energy, necessitating a paradigm shift towards energy neutrality and resource recovery. In contrast to the first generation of biological nutrient (nitrogen and phosphorous) removal (BNR) processes, which remove N by oxidation of ammonia and organic nitrogen to nitrates and then denitrifying to nitrogen gas, second generation BNR processes which nitrify and denitrify through nitrite as well as anaerobically oxidizing ammonia with nitrites, not only offer significant energy savings of up to 40%, but also reduce the addition of much-needed extraneous organic carbon to supplement the wastewater organics for denitrification by as much as 90%. All the second-generation BNR processes employ suspended growth systems that require much higher hydraulic retention times and much larger bioreactor volumes compared to biofilm or fixed-film processes. Energy generation at MWWTP primarily involves anaerobic digestion. However, the efficiency of volatile solids reduction (VSR) in conventional digesters with hydraulic retention times of around 15-30 days, is typically around 50%. While recently, the biogas production from anaerobic digesters has been enhanced through co-digestion of food wastes with municipal biosolids, the fundamentals of synergism are not well understood. The overall long-term objective of the proposed research program is to minimize energy consumption at MWWTP through reduction of aeration energy for nitrogen removal and enhanced conversion of municipal biosolids to methane in anaerobic digestion. The project has three short-term objectives: (1) development of an integrated biofilm process i.e. a fluidized bed bioreactor that combines short-cut nitrification with anaerobic ammonia oxidation in a single reactor; (2) development of a high-rate biofilm anaerobic fluidized bed digester with ultrasonication, and (3) investigation of the fundamental process parameters contributing to the observed synergistic effects of co-digestion of food wastes and wastewater biosolids.