Anammox bacteria (Planctomycetales)1 can use nitrite nitrogen (NO2−-N) as an electron acceptor to directly convert ammonia nitrogen (NH4+-N) into nitrogen under anaerobic conditions. Compared with traditional processes of nitrogen removal, Anammox process is more environmentally friendly and economical, making it suitable for wastewater treatment with high ammonia. However, anammox bacteria require a long generation time (9–11 days)2 and are sensitive to environmental factors, e.g., metals3,4, antibiotics5,6, dissolved oxygen7,8 and sulfides9. Thus, the application of anammox bacteria in wastewater treatment is limited. Some high ammonia nitrogen wastewaters10, such as, landfill leachate, metal smelting wastewater, and new energy production wastewater, often contain high concentrations of heavy metals (Cu(II), Zn(II), Pb(II), Ni(II), Co(II), or Mo(II)11–13. These metals may seriously inhibit the nitrogen removal performance of anammox systems by interference with anammox bacteria activity3,14,15. Therefore, it is necessary to clarify the effect of metals on anammox process.
Many studies have explored the effects of heavy metals during various nitrogen removal processes. As an important cofactor of metalloproteinases and some enzymes, Ni(II) plays an important role in the growth and metabolism of microorganisms16. However, excessive heavy metals combined with enzymes are inhibitory or even toxic to biochemical reactions and microorganisms, and cause the disruption of enzymatic structure and activities17,18. The concentration of Ni(II) in industrial wastewater and municipal sewage treatment plants typically ranges from 0.1 to 1000 mg/L19,20. The Ni(II) concentrations in some wastewater are much higher than the concentration required for microbial life activities. Therefore, it is important to study the effect of Ni(II) on the inhibition threshold and microbial activity in anammox systems for anammox application in practical sewage treatment process.
Gutwinski et al.21 investigated the effects of different mixed heavy metals on anammox process performance during long-term experiments, and pointed out the mixture of Zn2+, Cu2+, and Ni2+ at concentrations of 0.8, 0.075, and 0.04 mg/L caused a rapid inhibition in anammox process. Wu et al.22 found an anammox system could maintain a superior performance at 10 mg/L Ni(II) during a long operation, and Ni(II) had a greater impact on the microbial community composition. An IC50 of Ni(II) on anammox bacteria was determined as 48.6 mg/L in batch experiments23. Different IC50 values of Ni(II) on anammox bacteria were found in different biomass reactors or different operation process. Existing reports about the effects of Ni(II) on Anammox are most focus on single Ni(II) or mixed heavy metal on the inhibition phenomenon and inhibition degree about anammox activity. The inhibition mechanism is limited known. And also, less information can be found about adaptability and domestication of Anammox under Ni(II).
Hence, the main objectives of this study: (1) determine the dominant reaction in the anammox system under Ni(II) shock; (2) investigate the effects of Ni(II) concentration and exposure time on nitrogen removal by Anammox; (3) analyze Ni(II) inhibition mechanism according to anammox activity and the changes of microbial community under Ni(II) shock; and (4) explore the cumulative inhibitory effect of Ni(II) and recovery of anammox activity of anammox systems with different Ni(II) concentration.