Effect of trace hydrazine addition on the functional bacterial community of a sequencing batch reactor performing completely autotrophic nitrogen removal over nitrite

A sequencing batch reactor (SBR) was conducted to perform completely autotrophic nitrogen removal over nitrite (CANON). The effect of long-term trace N2H4 addition on ammonium oxidizing bacteria (AOB) and anaerobic AOB (AnAOB) in the CANON system was investigated. AOB and AnAOB primarily related to Nitrosococcus, Nitrosomonas and Candidatus scalindua, respectively. Before and after trace N2H4 addition, the estimates of AOB population decreased from 1.03 × 107 to 6.25 × 104 copies/g (dry sludge), but that of AnAOB increased from 3.14 × 109 to 5.86 × 1010 copies/g (dry sludge). Despite there was a partially negative impact on AOB growth, the trace N2H4 addition exerted a stronger inhibition on nitrite oxidizing bacteria (NOB) and promoted AnAOB growth, which improved the nitrogen removal of the CANON system. Sludge granules enriched under long-term trace N2H4 addition were spherical and ellipsoidal, and the aerobic AOB were mainly located on the outer layers while AnAOB occupied most of the interior parts.