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

Abstract

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: S_inPMD (S_in: insufficient sawdust); 40 g: S_moPMD (S_mo: moderate sawdust); 80 g: S_exPMD (S_ex: 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 S_inPMD, S_moPMD and S_exPMD, respectively. Higher NO₂⁻-N and NH₄⁺-N accumulations only occurred in the S_exPMD 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 S_inPMD and from 23.6% to 52.2% in S_moPMD when HRT was extended to 24 h. However, this contribution only increased from 21.6% to 31.6% in the S_exPMD 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 S_inPMD, 18.7% in S_moPMD and 20.3% in S_exPMD.