The enrichment of more functional microbes induced by the increasing hydraulic retention time accounts for the increment of autotrophic denitrification performance

With pyrite (FeS2) and polycaprolactone (PCL) as electron donors, three denitrification systems, namely FeS2based autotrophic denitrification (PAD) system, PCL-supported heterotrophic denitrification (PHD) system and split-mixotrophic denitrification (PPMD) system, were constructed and operated under varying hydraulic retention times (HRT, 1-48 h). Compared with PAD or PHD, the PPMD system could achieve higher removals of NO3  -N and PO43--P, and the effluent SO42  concentration was greatly reduced to 7.28 mg/L. Similarly, the abundance of the dominant genera involved in the PAD (Thiobacillus, Sulfurimonas, and Ferritrophicum, etc.) or PHD (Syntrophomonas, Desulfomicrobium, and Desulfovibrio, etc.) process all increased in the PPMD system. Gene prediction completed by PICRUSt2 showed that the abundance of the functional genes involved in denitrification and sulfur oxidation all increased with the increase of HRT. This also accounted for the increased contribution of autotrophic denitrification to total nitrogen removal in the PPMD system. In addition, the analysis of metabolic pathways disclosed the specific conversion mechanisms of nitrogen and sulfur inside the reactor.

Automated summary

In this study, three denitrification systems were constructed using pyrite and polycaprolactone as electron donors and operated under varying hydraulic retention times. Compared with autotrophic denitrification (PAD) or heterotrophic denitrification (PHD), the split-mixotrophic denitrification (PPMD) system achieved higher removals of NO3-N and PO4-P, with a greatly reduced effluent SO4 concentration. Gene prediction and metabolic pathway analysis revealed the increase in functional gene abundance and specific conversion mechanisms in the PPMD system with the increase of HRT.