Establishing Stable Nitritation in MABR through Aeration Control

Abstract

Nitrogen removal from municipal wastewater through partial nitritation-denitrification (nitritation) is challenging to accomplish in a membrane-aerated biofilm reactor (MABR) due to the reactor configuration, which potentially interferes with nitrite-oxidizing bacteria inhibition. This study investigated the impact of intermittent aeration on the development and sustenance of nitritation in a lab-scale MABR for the treatment of municipal wastewater. The study was accomplished in four phases (Phases I–IV) using a combination of continuous and intermittent aeration modes with aerated and nonaerated cycles of 10 min (5 on/5 off), 20 min (10 on/10 off), and 25 min (10 on/15 off), respectively, and a constant hydraulic retention time of 2.5 h. Biofilm development and stabilization were completed using a continuous aeration condition (Phase I). Nitrite accumulation rate, nitrate production rate, and ammonium nitrogen removal efficiency achieved in Phases II–IV were, 35%, 12%, and 99%; 76%, 3.4%, and 98%; and 94%, 1%, and 98%, respectively. Intermittent aeration significantly improved total inorganic nitrogen removal efficiency by ∼20%. Between the initiation of intermittent aeration and termination of the study, ammonia-oxidizing bacteria activities within the reactor increased by >150% from 4.53 to 12.6 mgN/h·g volatile suspended solids (VSS). In contrast, nitrite-oxidizing bacteria activities declined by >60% from 1.17 to 0.46 mgN/h·gVSS. The consistent lagging of nitrate production rate behind nitrite accumulation rate, increase in ammonia-oxidizing bacteria activities, and decline in nitrite-oxidizing bacteria activities over the operation period indicates the establishment of nitritation. This study demonstrates that using intermittent aeration, nitritation can be developed and sustained in MABR under mainstream conditions. Biological nitrogen removal from municipal wastewater is typically through the conventional activated sludge system using the traditional nitrification and denitrification pathway. However, three major issues abound with this pathway: high aeration energy, external organic carbon requirement, and the potential release of nitrous oxide. An alternative to the aforementioned process is the MABR. MABR is a low-energy technology with potential for pollutants removal through nitritation. Nitritation compares favorably to simultaneous nitrification-denitrification in terms of energy efficiency, sustainability, and operational costs. However, nitrite-oxidizing bacteria activities suppression is required for the successful establishment of nitritation. Studies have suggested that nitrite-oxidizing bacteria activity suppression can be accomplished in biological systems when operated under specific conditions, but although some of these conditions are inherent in some systems, they are absent in mainstream wastewater and MABR. Therefore, establishing nitritation in MABR for the treatment of conventionally collected sewage is challenging. This study recommends using continuous aeration to develop the biofilm and establish stable performance within the MABR, and subsequently switching the aeration mode to predetermined intermittent aeration cycles. This will reduce the dissolved oxygen concentration, suppress nitrite-oxidizing bacteria activities, and stabilize suppressed nitrite-oxidizing bacteria activities within the MABR biofilm.