Effect of intermittent operation model on the function of soil infiltration system

To enhance denitrification in a process of solute infiltration through a soil, a two-section mixed-medium soil infiltration system (TMSIS) for urban non-point pollution was developed. The artificial aerobic respiration and nitrification took place in the upper aerobic section (AES), while grass powders and sawdust were mixed in the bottom anaerobic section (ANS) to supply organic carbon source for denitrification bacteria, and the reduction was increased by iron addition in the ANS. Measured resident concentrations from the bottom of each ANS column were assumed to represent mean values averaged over the column cross-sectional area. The TMSIS with hydraulic loading rates (HLR) of 0.32, 0.24, and 0.16 m³ m^-2 day^-1 and with wetting-drying ratio (R_WD) of 1.0 showed remarkable removal efficiencies for chemical oxygen demand (COD), NH₄⁺-N, and TP, respectively. The hydraulic loading rate of 0.32 m³ m^-2 day^-1 was selected as the optimal HLR due to the high contaminated runoff treatment efficiency. When R_WD was 1.0, 0.5, or 0.2 with hydraulic loading rate of 0.32 m³ m^-2 day^-1, the TMSIS could treat synthetic urban runoff contaminants very well. The corresponding effluent water met the China's national quality standard for class V surface water. The wetting-drying ratio of 0.5 with hydraulic loading of 0.32 m³ m^-2 day^-1 was selected as the optimal operation conditions for the TMSIS. Aerobic respiration and nitrification mainly took place in the upper AES, in which most of the COD and the NH₄⁺-N were removed. Mixed sawdust and grass powders used as a carbon source and heterotrophic denitrification were put at the bottom of the ANS. The developed TMSIS has the potential to be applied for urban non-point pollution removal.