Dynamic response of pH to NO and S directional accumulation during sulfur autotrophic denitrification.

Double short-cut sulfur autotrophic denitrification (DSSADN) is capable of reconverting NO-N produced by partial nitrification -Anammox (PN/Anammox) into NO-N, while simultaneously reducing SO byproduct pollution. Understanding the nitrogen and sulfur transformation processes as well as the functional enzyme activity changes in DSSADN under varying pH conditions is crucial for achieving synchronous accumulation of S and NO-N. This study investigated the directional accumulation characteristics of S and NO-N in sulfur autotrophic denitrification (SADN) under different pH conditions by directionally adjusting the pH within the SADN system. The results showed that within the pH range of 7.5-8.5, the NO-N removal rate (NaRR) and S removal rate (SRR) increases with the increase of the growth of microorganisms. When the pH exceeded 8.5, NaRR and SRR were reduced, and the inhibitory effect intensified with increasing pH, this resulted in a decrease in NaRR and SRR from 1.76 kg/(m·d) and 4.47 kg/(m·d) (pH 8.5) to 0.60 kg/(m·d) and 1.36 kg/(m·d), respectively. As pH increased, the gene copy numbers of nirk and soxB gradually decreased, while the discrepancy between NaRR, SRR, and the removal rates of NO-N and S widened, resulting in gradual increases in NO-N and S accumulation efficiencies, which peaked at 96 % and 91 %, respectively. The accumulation rates of NO-N and S increased initially and then declined as pH rose from 7.5 to 10, reaching maximum values of 1.29 kg/(m·d) and 3.31 kg/(m·d) at pH 8.5. Meanwhile, increasing pH also induced significant changes in microbial community structure, the functional microflora shifted from the full SADN bacterial genus Sulfurimonas (pH 7.5-8.5) to the DSSADN common bacterial genus Thiobacillus (pH 9-10).