Manipulating dosing regimes to optimise nitrogen removal in partially-saturated vertical-flow treatment wetlands

Partially saturated vertical flow (PSVF) wetlands represent a promising, low-cost and easily implemented wastewater treatment technology for on-site treatment in small communities. However, to date, no studies have evaluated PSVF systems under realistic diurnal loading conditions which reflect the fluctuating inflows typical of small-scale household applications. Previous research has largely focused on flow-equalised dosing regimes and the influence of operational parameters, particularly dose volume, has received little attention. To address this gap, this study assesses the treatment performance of three PSVF wetland configurations (zeolite-woodchips, gravel-woodchips, and complete woodchips) subjected to diurnal loading. Three discrete dose volumes (2.75, 5.5, and 11 mm) were applied while maintaining a constant daily hydraulic loading rate of 110 mm d^-1, enabling systematic evaluation of how dosing strategies interact with media composition under realistic operating conditions. Organic matter and TSS performance under diurnal loading was similar to that under regular dosing with all PSVF systems achieving removals greater than 95% for cBOD₅ and ∼85% for TSS. Zeolite-woodchip systems consistently removed 97-99% of ammoniacal nitrogen irrespective of dose size, while removal for gravel-woodchip and complete woodchip systems declined with increasing dose size. Both the free-draining (FDL) and saturated layers (SL) contributed to overall TN removal. TN removal peaked at 93% in the complete woodchip system at the smallest diurnal dose but fell below 60% at larger doses due to ammonium breakthrough. In contrast, zeolite-woodchip systems provided 75% TN removal at the large diurnal dose size, comparable to regular hourly dosing operation. Denitrification rates in the woodchip SL were similar across systems and dosing regime falling within the range of 4-6.5 g m^-3 d^-1. Larger applied dose size increased breakthrough of E. coli and phosphorus, due to more rapid percolation of wastewater through the FDL.