Divalent manganese stimulates the removal of nitrate by anaerobic sludge

This study investigated the effect of different concentrations of Mn²⁺ on the removal of nitrate by anaerobic sludge and changes in the microbial communities through batch experiments. The results showed that the addition of Mn²⁺ promoted nitrate removal by anaerobic sludge; the nitrate was completely removed within 6 d in the treatment group with >5 mM Mn²⁺. With the increase in Mn²⁺, the concentration of nitrite and nitrous oxide increased in the first 4 d and then decreased to 0 μM after 8 d of incubation. The increasing tendency of ammonium increased firstly and then decreased with the addition of Mn²⁺ compared to A. Moreover, the Mn²⁺ removal efficiency gradually decreased with the increase of Mn²⁺ concentration. The changes of microflora structure in sludge before and after adding Mn²⁺ were analyzed, and the results revealed that the microbial communities in the sludge may have evolved towards an energy-efficient association of short-cut nitrification, denitrification, and anaerobic ammonia oxidation after adding Mn²⁺. Mn²⁺ stimulated the removal of nitrate by anaerobic sludge mainly by promoting the growth of PHOS-HE36.