Effect of hydraulic retention time on performance of autotrophic, heterotrophic, and split-mixotrophic denitrification systems supported by polycaprolactone/pyrite: Difference and potential explanation

Biological denitrification is still the most important pathway to purifying nitrate-containing wastewater. In this study, pyrite (FeS2) and polycaprolactone (PCL) were used as electron donors to construct sole or combined denitrification systems, that is, pyrite-based autotrophic denitrification (PAD) system, PCL-supported heterotrophic denitrification (PHD) system, and split-mixotrophic denitrification system (combined PAD + PHD), all of which were operated under five different hydraulic retention times (HRTs) for 150 days. The results showed that the removal rates (RE) of nitrate (NO3 (-)-N) and inorganic phosphorus (PO4 (3-)-P) by PAD were 91% and 94%, respectively, but the effluent sulfate (SO4 (2-)) concentration was as high as 168.2 mg/L; the removal rate of NO3 (-)-N by PHD was higher than 99%, but the PO4 (3-)-P could not be removed ideally; the removal rates of NO3 (-)-N and PO4 (3-)-P by PAD + PHD were higher than 95% and 99%, respectively, and the effluent SO4 (2-) concentration was only 7.2 mg/L. Through the analysis of the surface scanning electron microscope (SEM) images of the two kinds of media before and after use, it was found that the coupled mode of PAD + PHD was more favorable for biofilm formation than the sole PAD or PHD process, and the microorganisms in the PAD + PHD mode made more full use of electron donors. Moreover, the biomass of the PAD + PHD mode was lower than that of the PAD or PHD process, but the denitrification efficiency of the coupled mode was more efficient, indicating that the functional microorganisms in the PAD + PHD mode might have a certain synergistic effect. Practitioner Points Removal rates of NO3-, PO4 (3)-, and SO4 (2)- by PAD were 91%, 94%, and -233%, respectively. Removal rate of NO3- by PHD exceeded 99%, but PO4 (3)- could not be removed ideally. Removal rates of NO3-, PO4 (3)-, and SO4 (2)- by PAD + PHD were 95%, 99%, and 86%, respectively. The coupled mode was more favorable for biofilm formation than the sole PAD or PHD. The coupled mode had lower biomass but got more excellent denitrification efficiency.

Automated summary

Biological denitrification is crucial for purifying nitrate-containing wastewater, and pyrite and polycaprolactone can be used to construct different denitrification systems. The combined mode shows the best performance, achieving high removal rates for nitrate and phosphate while reducing sulfate concentration.