Effect of Nitrate on Sulphur Transformations Depending on Carbon Load in Laboratory-Scale Wetlands Treating Artificial Sewage

Abstract

Two laboratory-scale constructed wetlands planted with Juncus effusus were used to investigate the dynamics of sulphur transformations under varying nitrate and organic carbon loads as well as its interactions with microbial carbon and nitrogen transformations. The removal of dissolved organic carbon was obtained to be around 65-87% with specific removal load of 1.40-2.63 g/m2 d. 94% of nitrate removal (under inflow concentration of 15 mg/L) irrespective of organic carbon loads indicated a highly active denitrification process in wetlands. Sulphate reduction was performed at a high level of 83% in a low redox potential (about -300 mV) under condition of inflow organic carbon concentration of 50 mg/L. The dosage of nitrate in the inflow can strongly hinder the process ofdissimilatory microbial sulphate. The coexist of sulphide with concentration of 1.65-2.65 mg/L and elemental sulphur of 0.17-2.18 mg/L in the pore water of wetlands demonstrated a simultaneous occurrence of microbial sulphate reduction and sulphide oxidation. A lower ammonium oxidation removal was initiated, which was probably caused by the toxic effect of sulphide with concentration of about 3 mg/L in the pore water. The sulphide concentration in the pore water was highly exponentially correlated with the redox potential, indicating the control of sulphide in wetlands could be performed by the adjustment of redox potential via aeration and/or nitrate dosage.