Insights into the response of anammox sludge to the combined stress of nickel and salinity.

Anaerobic ammonium oxidation (anammox) is a promising technology applied to treat industrial wastewater, while the commonly coexistent heavy metals and salinity usually become a challenging issue to be addressed. In this study, the responses of anammox sludge in terms of performance, activity, functional enzyme and extracellular polymeric substance (EPS) to the combined stress of Ni(II) and salinity (20 ‰) were investigated holistically. It turned out that low Ni(II) concentration (0.2 mg·L) together with salinity (20 ‰) showed an insignificant effect on the anammox performance, while a decreased nitrogen removal by 46.96 % was observed with the increased Ni(II) concentration to 1 mg·L. It should be pointed out that the anammox system exhibited good robustness evidenced by rapid recovery to achieve 89.13 % of nitrogen removal efficiency and 1.21 kg·m·d of nitrogen removal rate after the elimination of stress factors within 40 days. Ni(II) concentration was revealed to play a more important role in the specific activity of anammox sludge. The functional enzymes related to nitrogen removal, e.g. nitrite reductase (NIR), hydrazine oxidase (HZO) and heme c were found to be inhibited by the combined stress of Ni(II) and salinity, with decreased activity by 49.54 %, 39.39 % and 45.88 %, respectively. However, the enzyme related to assimilation, e.g. alkaline phosphatase (AKP) and nitrate reductase (NAR) appeared to be enhanced. The EPS content was found to decrease by 55.19 % under the combined stress. Detailed analysis of 3D-EEM and FTIR spectra further revealed that the combined stress of Ni(II) and salinity could change both the quantity and composition of EPS in anammox sludge. These results are expected to offer insights into the combined effect of nickel and salinity on the anammox system, and benefit the application of anammox technology for industrial metal-rich saline wastewater treatment.