Effect of sawdust dosage and hydraulic retention time (HRT) on nitrate removal in sawdust/pyrite mixotrophic denitrification (SPMD) systems

Pyrite plays an important role in nitrate-contaminated aquifer remediation. Sawdust is also widely used as a solid carbon source for nitrate removal. This research investigated the effect of sawdust dosage and hydraulic retention time (HRT) on sawdust/pyrite mixotrophic denitrification (SPMD) performance in in situ groundwater remediation. The results showed that the total organic carbon (TOC) concentrations in three bioreactors (20 g: SinPMD (Sin: insufficient sawdust); 40 g: SmoPMD (Smo: moderate sawdust); 80 g: SexPMD (Sex: excess sawdust)) were maintained at 16.1 ± 5.7, 12.8 ± 4.0, and 12.5 ± 3.8 mg L−1 when HRT was fixed at 12 h, respectively, whereas prolonged HRT resulted in a slight decrease in TOC concentration. The highest NO3−-N removal rates were 15.8 ± 1.5, 19.1 ± 0.9 and 19.3 ± 1.8 g-N m−3 d−1 for SinPMD, SmoPMD and SexPMD, respectively. Higher NO2−-N and NH4+-N accumulations only occurred in the SexPMD bioreactor. In the SPMD systems, autotrophic denitrification accounted for 20–30% when HRT was 12 h, which greatly increased from 30.0% to 57.2% in SinPMD and from 23.6% to 52.2% in SmoPMD when HRT was extended to 24 h. However, this contribution only increased from 21.6% to 31.6% in the SexPMD bioreactor due to an over-dosage of sawdust. Moreover, this study also found that the SPMD systems achieved low SO42− by-product production (<33 mg L−1), which was beneficial for maintaining a neutral pH for the sake of microorganisms (7.4–8.0). Autotrophic denitrifiers such as Thiobacillus denitrificans flourished in the SPMD systems and had a highest relative abundance of 30.3% in SinPMD, 18.7% in SmoPMD and 20.3% in SexPMD.