Performance of a bench-scale evapotranspiration tank under changes in flow rate and organic loading rate during domestic dark greywater treatment

ABSTRACT Unplanted tevap treating dishwashing greywater across loading phases, with COD removal of 91.4-100%, P removal of 97.7-99.7%, and variable NH4-N removal. The Evapotranspiration Tank (TEvap) is a nature-based solution for decentralized greywater treatment. This study evaluated the physicochemical performance and operational recovery of an unplanted bench-scale TEvap treating dishwashing dark greywater under five hydraulic and organic loading rates (OLR). Across five operational phases, phase-average OLR increased from 6.3 to 59.2 gCOD m−3 day−1. The system maintained high removal efficiencies, achieving 91.4% COD removal at a phase-average OLR of 59.2 gCOD m−3 day−1, while mean turbidity removal ranged from 92.35 to 99.66% along all operational phases. After the 55-day non-operational period, the TEvap returned to high COD and turbidity removal levels, reaching 99.19 and 99.57%, respectively. Nutrient monitoring showed consistently high phosphorus removal, with efficiencies remaining above 97% across the evaluated phases for quantifiable paired observations. In contrast, nitrogen behaviour was more variable, with negative ammoniacal-nitrogen removal observed during Phase V, while the dominant nitrogen transformation pathway remained unresolved. These results demonstrated the strong capacity of the TEvap for organic matter and turbidity removal during high-strength dishwashing dark greywater treatment. These findings significantly expand the limited literature on high-load dark greywater management and provide a quantitative foundation for future research to investigate microbial pathways, full-scale design parameters, and effluent reuse feasibility.