Effects of HRT and water temperature on nitrogen removal in autotrophic gravel filter

• A gravel filter designed to simulate subsurface horizontal-flow constructed wetland.
• Sodium thiosulfate applied as an electron donor for autotrophic denitrification.
• 96% of added nitrate nitrogen (NO3−–N) was converted to nitrogen gas.
• 98.86% of thiosulfate was gradually converted to sulfate throughout the system.
• Utilization of Na2S2O3 could reduce operating cost and prevent secondary pollution.

Organic Carbon added to low ratio of carbon to nitrogen (C/N ratio) wastewater to enhance heterotrophic denitrification performance might lead to higher operating costs and secondary pollution. In this study, sodium thiosulfate (Na2S2O3) was applied as an electron donor for a gravel filter (one kind of constructed wetland) to investigate effects of hydraulic retention time (HRT) and water temperature on the nitrate removal efficiency. The results show that with an HRT of 12 h, the average total nitrogen (TN) removal efficiencies were 91% at 15–20 °C and 18% at 3–6 °C, respectively. When HRT increased to 24 h, the average TN removal increased accordingly to 41% at 3–6 °C, suggesting denitrification performance was improved by extended HRT at low water temperatures. Due to denitrification, 96% of added nitrate nitrogen (NO3−–N) was converted to nitrogen gas, with a mean flux of nitrous oxide (N2O) was 0.0268–0.1500 ug m−2 h−1, while 98.86% of thiosulfate was gradually converted to sulfate throughout the system. Thus, our results show that the sulfur driven autotrophic denitrification constructed wetland demonstrated an excellent removal efficiency of nitrate for wastewater treatment. The HRT and water temperature proved to be two influencing factors in this constructed wetland treatment system.