Optimization of on-site wastewater treatment efficiency and recovery based on nutrient mobility and adsorption kinetics modelling using HYDRUS-2D coupled with PHREEQC

The closed-loop on-site wastewater treatment system (OWT) comprising steps of septic tank (ST) to remove Biological Oxygen Demand (BOD) and suspended solids (SS), biofiltration (BF) clarifier for biological removal of nitrogen (N), phosphate (P), BOD and SS, reactive Polonite® filter for chemical adsorption and precipitation removal of dissolved P and tidal flow constructed wetland (TFCW) sand filters for polishing the effluent to low P and N effluent Swedish standards. The field experimental data that have been used to optimize TFCW design in the numerical modelling using HYDRUS-2D coupled with and without PHREEQC indicated that the adsorption efficiency of the reactive Polonite® adsorbent was nearly double to that obtained in TFCW sand filters for PO4-P (95 %) and Total-P (85 %) removal in summer at a high temperature range (15.4–18.8 °C) and pH range (9.9–10.8). The weaker PO4-P (53 %) and Total-P (25 %) removal efficiency in winter was due to a low temperature (1.5–8.1 °C) and low pH (7.2–7.9). This decrease in pH was attributed by salinity in the domestic wastewater and dilution of rainwater. Modelling results revealed that the rate of P adsorption kinetics in the TFCW sand filters enhanced with calcium flow from chemical dissolution in the preceding Polonite® adsorbent was increased with the increase in temperature. However, the P adsorption was less sensitive at high ferrihydrite (Fe(OH)3) dose, suggesting limited effects of cations dissolution and abundance of metal oxides and hydroxide ions at the mineral surface for anions exchange with phosphate for surface complexation. The strategy of combining field data and modelling provided valuable insights for assessing adaptability and optimizing TFCW design under variable fluxes and scenario effects of an insulating/uninsulating and dilution by rainwater in cold-climate regions.