Physico-chemical analysis of industrial wastewater pollution from kigali special economic zone (KSEZ) and the potential impacts in the downstream regions of Kigali City in Rwanda

Industrialisation is the utmost pursuit of developing countries of the world. In Rwanda, the increasing industrial activities have resulted in a new pollution problem due to the unabated release of untreated and incompletely treated industrial wastewater into the environment, causing high pollution loads in the receiving water bodies and the ecological environment. Kigali Special Economic Zone (KSEZ) is an industrial hub with over 60 industries. Except fo a handful of separate studies on industrial effluent a decade ago, no water pollution study was conducted after the hub formation. The present study aims to investigate the pollution impacts of wastewater effluents from selected industries in KSEZ and the main wastewater discharge channel (MWDC). Wastewater samples were collected weekly for three months (from February to March 2019) and analysed using standard instruments and laboratory procedures for selected water quality parameters. The results revealed that wastewater effluents from KSEZ comprise various hazardous pollutants beyond the maximum permissible limit of the Rwandan industrial effluent discharge standard and the World Health Organization maximum allowable limit for wastewater reuse for irrigation. The pH of the wastewater ranges from acidic to neutral (4.3–6.7), and there were drastic variations in electrical conductivity (EC) (394–1205.6 μS/cm), biological oxygen demand (BOD₅) (77.5–237.6 mg/L), chemical oxygen demand COD (240.5–735.6 mg/L), phosphate (2.6–6.1 mg/L), nitrate (1.01–3.2 mg/L), and the heavy metals including Cu⁺² (0.20–9.40 mg/L), Pb⁺²(0.30–1.35 mg/L), Mn⁺²(0.065–1.10 mg/L) and Zn⁺² (4.55–5.95 mg/L). The aforementioned variations were statistically significant (P < 0.05), and some parameters were manifold higher than the recommended limit, indicating the direct input of untreated wastewater from KSEZ industries, which may pose severe health impacts on the population. Similarly, the phosphate (2.6–4.7 mg/L) and nitrate (1.01–3.2 mg/L) concentrations suggest possible eutrophication in receiving waterbodies. Therefore, considerable attention should be given to wastewater discharges from KSEZ, with strict enforcement of existing legislations.