The microbial zonation of SRB and soNRB enhanced the performance of SR-DSR process under the micro-aerobic condition

The micro-aerobic condition has proven to effectively enhance the COD removal and elemental sulfur (S⁰) transformation rate in the sulfate reduction-denitrifying sulfide removal (SR-DSR) process. However, the mechanisms of how micro-aerobic condition enhances S⁰ transformation remain largely unknown. Therefore in this work an integrated investigation was performed to document the mechanisms and the effect of different startup modes (micro-aerobic startup (termed as mSR-DSR) and anaerobic startup (termed as aSR-DSR)) on bioreactor performance and microbial community dynamics. The results showed that micro-aerobic startup achieved a shorter period to reach a stable performance for SR-DSR, which could be one of the factors affecting the choice of the bioreactor startup mode considering engineering application. For all the tested conditions, removal of nitrate, sulfate and lactate were 100%, >80% and 100%, respectively. The maximum transformation rate of elemental sulfur in mSR-DSR was 57%, which was higher than that in aSR-DSR. The mechanism explorations revealed that micro-aerobic condition not only particularly enriched the sulfide-oxidizing, nitrate-reducing bacteria (soNRB) but also promoted the microbial zonation of sulfate-reducing bacteria (SRB) and soNRB, thereby achieving more S⁰ transformation in the effluent. Under micro-aerobic condition, SRB were mainly distributed in the bottom and middle part of the reactor, while soNRB were assembled in the top. The relative abundance of soNRB in both aSR-DSR and mSR-DSR notably increased to 41.5% and 23.7% at the top when 5 mL air min^-1 L_reactor^-1 was applied. Furthermore, the degradation of organic carbon was also accelerated under micro-aerobic condition, possibly due to the enrichment of organic compounds degrading bacteria Bacteroidetes_vadin HA17.