Design methodology for sewage water treatment system comprised of Imhoff 's tank and a subsurface horizontal flow constructed wetland: a case study Dakhla Oasis, Egypt

In this study, in order to reuse the treated wastewater in irrigation in rural areas, a new sewage water treatment system comprised of primary treatment Imhoff's tank, Hama drying basins, inlet well, a subsurface horizontal flow constructed wetland (HSFW), water control device, and a groundwater tank is proposed and designed in Dakhla Oasis western desert of Egypt. The proposed system serves a population of 5000 capita with a designed discharge of 750 m³ d^-1. The kinetic parameters involved macrophyte organisms, media forms, water level, hydraulic retention time (HRT), and hydraulic loading rate (h_l) for the system were selected to achieve an efficient wastewater treatment system design. Imhoff's tank is sized as the primary sedimentation efficiency is 30%, and HSFW is sized based on the first-order kinetics (k-C∗) model, and total hydraulic design theory. The air temperatures were 29.7 °C and 13.8 °C in summer and winter respectively, influent pollutant concentrations after primary sedimentation for BOD, fecal coliforms (FC), total nitrogen (TN), and total phosphorus (TP) were 210 mg L^-1, 10⁸ CFU100 mL^-1, 30 and 7 mg L^-1, respectively. The expected designed effluent BOD and FC were 30 mg L^-1 and 1000 CFU100 mL^-1 respectively. The results show that FC removal controls the area of the HSFW (2.87 ha), 6 units of reed (Phragmites Australis and Papyrus) plants each one is 66$\times 72.6$ m with h_l of 2.6 cm d^-1 and HRT of 6.91 d. The expected overall removal efficiencies for BOD and FC were 85.7%, and 99.9% respectively.