An Innovative Airlift Bioreactor Design with a Baffled Anoxic Zone for Simultaneous Carbon and Nutrient Removal via Various Mechanisms from High-Strength Wastewater

Abstract

This study investigated the concurrent removal of carbon, nitrogen, and phosphorus (CNP) from milk processing wastewater (MPW). The evaluation was carried out using a newly developed single-stage bioreactor known as the baffled dual internal circulation airlift A2O (B-DCAL-A2O) bioreactor. The bioreactor functionality was monitored to determine the impact of three important factors, i.e., hydraulic retention time (HRT; 7-15 h), air flow rate (AFR; 1-3 L/min), and aerobic volume ratio (AVR, 0.324-0.464). The use of baffles in the anoxic zone identified improved nutrient removal. Specifically, the total chemical oxygen demand (TCOD), total nitrogen (TN), and phosphorus removals of 94.8, 80, and 80%, respectively, as well as a reduction in the effluent turbidity of 8 NTU at optimum circumstances (HRT, AFR, and AVR of 10 h, 3 L/min, and 0.464, respectively) were obtained. From polymerase chain reaction (PCR) analysis, the superior nitrogen removal was attributed to the combined effects of simultaneous nitrification and denitrification (SND), anaerobic ammonium oxidation (anammox), and denitrifying phosphorus-accumulating organisms (DPAOs) and glycogen-accumulating organisms (GAOs). PCR analysis validated the coexistence of diverse functional microbial groups, including ammonia-oxidizing bacteria (AOB: Nitrosospira sp., Nitrosomonas sp., Nitrosococcus), and nitrite-oxidizing bacteria (NOB: Nitrospira [23S rRNA]), denitrifiers (Pseudomonas), anammox bacteria (Candidatus Brocadia and Candidatus Kuenenia), PAOs (Tetrasphaera, Candidatus Microthrix [16S rRNA], and Candidatus Accumulibacter [ppk1]), DPAOs (Pseudomonas), and GAOs (Sphingomonas [16S rRNA]). The phosphorus removal was attributed to the existence of PAOs and DPAOs in a single bioreactor. The results confirmed improved sludge properties, verified with the best results obtained for the turbidity (6 NTU) and SVI (90 mL/g) at an HRT of 15 h, an AFR of 3 L/min, and an AVR of 0.464.