Laboratory-scale LID infrastructure schemes for highly polluted urban stormwater runoff improvement: setup and initial testing

Urban stormwater runoff in highly polluted environments contains elevated concentrations of nutrients, particularly nitrogen and phosphorus, as well as chemical oxygen demand (COD), posing significant challenges to environmental water quality. This study investigates the laboratory-scale setup and efficiency of three low-impact development (LID) systems, namely, rain gardens, filter strips, and vegetated swales, for treating synthetic stormwater mimicking highly polluted urban runoff. Experiments were conducted under controlled hydraulic loading rates (HLRs) and pollutant concentrations to evaluate pollutant removal efficiency. Rain gardens, constructed with layered engineered media, including sand, compost, and iron-enhanced biochar, achieved the highest pollutant removal, with average reductions of 70% for total phosphorus (TP), 65% for nitrate-N (NO₃⁻-N), and 85% for COD. Vegetated filter strips achieved moderate nutrient reductions of 45% for TP and 40% for NO₃⁻-N, and 60% for COD. Vegetated swales achieved average reductions of 60% for TP, 55% for NO₃⁻-N, and 65% for COD. The research compares the pollutant removal performance of three LID prototype types under controlled laboratory high-pollution conditions, while recognising that differences in media composition, vegetation, and hydraulic behaviour were not independently evaluated. The rain garden prototype demonstrated the highest overall pollutant removal efficiency, while the swale and filter strip showed comparatively lower performance under the tested conditions. These results suggest that combining different LID types in a treatment train may enhance overall system performance, particularly for pollutants such as dissolved organic nitrogen and COD in highly polluted runoff. The novelty of this study lies in the controlled laboratory comparison of multiple LID typologies under highly polluted conditions representative of poorly serviced urban environments, for which limited experimental data currently exist. The findings demonstrate the potential of these LID systems for treating highly polluted urban runoff under controlled laboratory conditions. However, further research is required to assess their performance under field conditions.