Study on Betaine's Effect on Anammox Denitrification Performance Under Nickel Stress

The inhibition of the nitrogen removal performance of the anaerobic ammonium oxidation (Anammox) process by heavy metals is one of the significant challenges faced in the application of Anammox technology. To enhance the nitrogen removal performance of Anammox under Ni(Ⅱ) stress, this research investigated the effect of betaine on the enhancement of Anammox nitrogen removal under Ni(Ⅱ) stress and the mechanisms for alleviating Ni(Ⅱ) toxicity. The results indicated that betaine significantly enhanced the nitrogen removal performance of Anammox under Ni(Ⅱ) stress. At Ni(Ⅱ) concentration of 0.5 mg·L^-1, the nitrogen removal efficiency (NRE) of the Anammox process with betaine addition can reach over 93.50%, while the NRE of the Anammox process without betaine addition is 87.43%. At Ni(Ⅱ) concentration of 2 mg·L^-1, the NRE of the Anammox process with betaine can remain above 83.00% for more than 80 days, while the NRE of the process without betaine addition was deteriorated significantly (< 50%). Betaine can significantly improve the resistance of the Anammox process to Ni(Ⅱ), increasing the tolerance of the Anammox process from 2 mg·L^-1 to 4 mg·L^-1. Betaine also promotes the secretion of LB-EPS in Anammox sludge and increases the heme c content. Compared to the Anammox process without betaine addition, the activities of nitrate reductase (NR), hydrazine oxidase (HZO), and hydrazine synthase (HZS) in the Anammox process with betaine supplementation were significantly elevated, with the maximum increases being 196.22%, 80.81%, and 195.87%, respectively. Moreover, betaine supplementation increased the abundance of Candidatus Brocadia in the Anammox sludge, with its abundance reaching 25.12% at a Ni(Ⅱ) concentration of 2 mg·L^-1. The research provides a theoretical foundation and technical support for mitigating the inhibition of heavy metals on Anammox process and enhancing nitrogen removal performance of Anammox in heavy metal-contaminated environments.