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.

Immobilized aerobic denitrifying bacteria (Marinobacter sp. strain B108) can solve the problem of bacterial retention and improve the nitrate removal efficiency under aerobic environment, thus removing nitrate from marine recirculating aquaculture systems (MRAS) economically and efficiently. Polyvinyl alcohol (PVA), sodium alginate (SA) and tea residue powder were used as support materials to immobilize Marinobacter sp. strain B108. The ratios of immobilized materials were explored and optimized, and the physicochemical characteristics and denitrification kinetics of immobilized aerobic denitrifying bacterial pellets were further investigated on the basis of the optimal immobilized materials ratio. The optimum ratios of support materials were 10.103% PVA, 1.548% SA, and 0.732% tea residue powder. Under the optimum conditions, the immobilized bacterial pellets had high mechanical strength and strong resistance to mechanical shear, and the removal efficiency of NO₃^--N within 7 h was 98.63% as initial influent NO₃^--N concentration was 100 mg/L. The kinetic process of NO₃^--N removal could be fitted by Monod model, and the V_max and K_m were 5.041 mg/(L·g·h) and 24.810 mg/L, respectively. The kinetic process of NO₂^--N removal could be fitted by Andrews model, and the V_max, K_m and K_i were 0.790 mg/(L·g·h), 23.783 mg/L, and 0.092 mg/L, respectively. The conclusions could lay a good foundation for aerobic denitrifying bacteria practical application in MRAS for nitrate removal treatment.