Advancing the Hybrid Membrane Aerated Biofilm Reactor (MABR) Process for Sustainable Biological Nutrient Removal

Abstract

Anthropogenic activities trigger high concentrations of nutrients in municipal wastewater. To prevent eutrophication in aquatic systems, nitrogen and phosphorous in municipal wastewater must be efficiently removed before the treated effluent re-enters receiving waters. Biological nutrient removal (BNR) via the suspended growth process (activated sludge) has been an effective tool for water pollution control, but nowadays it faces challenges associated with more stringent discharge limits, increasing energy costs, and carbon neutrality goals. The hybrid membrane aerated biofilm reactor (MABR) process, coupling the MABR biofilm and conventional suspended growth (the basis for the term “hybrid”), has a unique capacity to develop solutions for those challenges. This dissertation addresses practical and fundamental questions about the hybrid MABR process for sustainable BNR practice. In recent years, commercial scale installations are increasing along with process modeling. The resulting accumulated knowledge has greatly improved understanding of the hybrid MABR process, with new challenges and opportunities arising. Therefore, this dissertation commences with a literature review that outlined the fundamental principles of MABR technology and lessons-learned from recent commercial hybrid MABR systems. Performance and existing operational challenges were evaluated. Modeling efforts and emerging applications of MABR technology were also discussed. Process control and optimization were identified as the future research needs to facilitate accelerated adaptation of the hybrid MABR process. A fully anoxic suspended growth process is an appealing alternative to conventional suspended growth due to considerable aeration reduction and improved carbon processing efficiency. With development of hybrid MABR technology, implementation of a fully anoxic suspended growth community in BNR facilities became practical. We carried out microscopic examination and 16S rRNA gene-based microbial community profiling to determine how an anoxic suspended growth would differ from the conventional aerobic process in floc characteristics, microbial diversity, microbial temporal dynamics, and community assembly pattern. Fewer filamentous populations were found in the anoxic mixed liquor, suggesting easily sheared flocs. Results show that the anoxic microbial community had a distinct composition and structure, but its diversity and temporal dynamics were similar to the conventional aerobic community. A variety of well-studied functional guilds were also identified in the anoxic community. The anoxic microbial community assembly was more stochastic than the conventional aerobic community, but deterministic assembly was still significant with a large core microbiome adapted to the anoxic condition. Finally, we used the plant-wide modelling approach with dynamic simulations to examine a novel hybrid MABR concept. The process implemented aeration controls to maximize the nitrification rate in the biofilm while the suspended growth remained anoxic for denitrification. Results show that the novel hybrid MABR process had resilient performance in response to diurnal loadings, achieving intensified nitrogen removal performance under both warm and cold temperature scenarios. Significant reductions in N2O emissions, energy consumption, and physical footprint from the hybrid MABR were confirmed in comparison to the conventional suspended growth process. The model predicted higher CH4 emissions from the hybrid MABR than the suspended growth process due to methanogen growth in the oxygen-depleted outer MABR biofilm layer. Future measurements for CH4 emission are needed to obtain a holistic picture of the carbon footprint of the hybrid MABR process. The knowledge gained from this dissertation will expand our knowledge about the hybrid MABR process as well as our ability to accelerate translation of this beneficial process to sustainable BNR practice.