Influence of temperature and dissolved oxygen on nitrification in a membrane bioreactor treating urine

Abstract

The biological nitrification of source-separated urine in a membrane bioreactor (MBR) is recognised as a promising approach to transform it into liquid fertiliser. However, a major limitation is the prolonged hydraulic retention time (HRT), which increases the system footprint. Given the strong dependence of nitrification on temperature and dissolved oxygen (DO) conditions, this study investigated the effects of three different temperatures (10 °C, 20 °C, and 30 °C) and DO levels (2, 4, and 6 mg/L) to determine the optimal conditions for enhanced nitrification performance and the resulting minimised HRT. Concurrently, microbial analysis was conducted to gain a comprehensive understanding of the MBR system. Results indicated that high DO levels resulted in poor biomass growth and a higher abundance of ammonia-oxidising bacteria (AOB) compared to the nitrite-oxidising bacteria (NOB), Nitrospira. At 10 °C, Nitrosomonas, was more prevalent than Nitrosococcus as an AOB. Conversely, at 30 °C, the relative abundance of Nitrosococcus increased by up to three-fold, leading to higher nitrite concentrations. Overall, the optimal conditions were found to be a temperature of 20 °C and a DO level of 4 mg/L, achieving a nitrification rate of 201 ± 49 mgN/L·d and the shortest HRT of 8 ± 2 days, with Nitrosococcus and Nitrospira as the predominant AOB and NOB, respectively. The findings suggest that optimising DO at moderate temperatures enhances nitrification while reducing aeration energy. These results inform energy-efficient operational strategies for decentralised nutrient recovery systems from human urine.