[Conversion Pathways of Substrates in Sulfate-Reducing Ammonia Oxidation System]

The phenomenon of simultaneous transformation of ammonium and sulfate under the conditions of inoculating ANAMMOX culture has gotten the attention of researchers. However, there are some problems and doubts reported in the related literature. In this study, the characteristics of ammonium and sulfate synchronous transformation were investigated in a CFSTR via inoculation with ANAMMOX culture. Under the condition of oxygen removal and non-filling, in the unfilled sealed fermentation tank, the average conversion of NH₄⁺-N was 50.8 mg·L^-1 while that of sulfate-sulfur was 4.5 mg·L^-1. Elemental analysis results showed that the observed yellow solid was not elemental sulfur but rather iron-containing compounds. However, no obvious change of ammonium was observed when using a filled sealed batch reactor. Only sulfate transformed significantly, and the transformation rate was affected by the inoculation biomass. Under these two conditions, the ORP in the reactor was completely different. This is an indication that the synchronous transformation of ammonium and sulfate observed in both our study and other related studies is probably not a process mediated by ANAMMOX organisms, in which sulfate acted as the electron acceptor to oxidize ammonium. Actually, ammonium and sulfate transformation were completely independent: ammonium oxidation is due to the micro oxygen environment created by the reactor operation form, whereas sulfate conversion is attributed to the sulfate heterotrophic reduction that results from the organic matter release via microbial decay. This transformation can clarify and explain the problems and doubts reported in the related research.