Optimisation of substrate depth in shale-lined constructed wetlands for improved phosphorus removal and eutrophication control

• Shale appears to be a suitable substrate for constructed wetlands; • Increasing the depth of constructed wetlands improves their efficiency in removing pollutants; • The volume of treated wastewater from constructed wetlands is lower for deeper substrates; • The impact of eutrophication from wastewater is reduced by deeper constructed wetlands; • Greater plant growth is promoted by increasing depth in constructed wetlands. Eutrophication, driven by excessive nutrient and organic pollutant loads, has become a major environmental concern in many rapidly urbanising African cities, yet rural and peri-urban communities continue to lack cost-effective and energy-efficient wastewater treatment options. Constructed wetlands offer a low-cost, nature-based solution, but the influence of filter bed depth on treatment performance remains insufficiently studied, despite its implications for both construction cost and nutrient removal efficiency. This study therefore explored how variations in shale substrate thickness in pilot-scale vertical flow constructed wetlands (VFCWs) affect nutrient removal, organic matter degradation, morphological development of the emergent macrophyte, and the eutrophication potential of treated effluents. The experiment consisted of four wetland units, constructed with a 10 cm gravel drainage layer and shale depths of 55 cm (SchH55G1-3) and 70 cm (SchH70G1-3), planted with Pennisetum purpureum and loaded with domestic wastewater (6.43 L/day) for six months. The system with 70 cm shale showed slightly enhanced biomass production (15.9 ± 0.6 kg/m²) compared to the 55 cm bed (15.5 ± 0.9 kg/m²). Higher removal efficiencies were observed in the SchH70G1-3 unit for COD (89 ± 7%), BOD₅ (89.7%), nitrate (51.5%), ammonium (82.4%), TKN (81.3%), phosphate (87.5%), total phosphorus (90%), and eutrophication index (84.4%). The findings highlight shale as a promising substrate for wastewater treatment in VFCWs. The study further identifies 70 cm as the optimal substrate depth for enhanced treatment performance and environmental protection, underscoring the critical role of substrate thickness in optimising VFCW design for low-cost wastewater treatment in resource-limited communities. Shale-Lined Constructed Wetlands