Enhancing low-temperature wastewater treatment by a novel localized magnetothermal strategy coupled with rotating biological contactors.

Low temperatures typically suppress microbial activity and enzyme kinetics, resulting in reduced nitrogen removal efficiency in biological wastewater treatment systems. A novel localized magnetothermal heating strategy was firstly developed to create a stable thermal micro-environment within the biofilm. Integration of a magnetothermal module into a rotating biological contactor (RBC) enhanced microbial activity and nitrogen removal under low-temperature conditions (≤12 °C), while reducing energy consumption by 43.62% compared with bulk-water heating. During 210 days of operation, the magnetothermal-enhanced reactor (M-RBC) achieved a total nitrogen removal efficiency of 87.62%, which was 35.24% higher than the control group (C-RBC). The bulk-liquid reoxygenation rate increased by 69.23% in the M-RBC, which markedly accelerated the ammonia oxidation process. The activities of key enzymes, including ammonia monooxygenase (AMO) and nitrite reductase (NIR), as well as the intracellular ATP content, were markedly increased, thereby improving nitrogen removal efficiency. Localized magnetothermal strategy also reshaped the microbial community structure by enriching nitrogen-transforming bacteria such as Neomegalonema and Tahibacter, while reducing the relative abundance of filamentous bacteria from 35.01% to 8.48%. Furthermore, functional gene analysis showed increased relative abundances of amoA and nirK in the M-RBC, suggesting that localized heating enhanced ammonia oxidation and nitrite-reduction potential and altered the nitrogen metabolism pathway under low-temperature conditions. This study achieved efficient and energy-saving nitrogen removal under low-temperature conditions, providing a sustainable and practical approach for biological wastewater treatment in cold regions.