Removal efficiency of organic matter and pathogens for horizontal flow subsurface constructed wetlands treating domestic wastewater in Kigali

Abstract

Now days the world is facing a big problem of getting sufficient quality water. This is due to the increase of the world population and industrialization as well as climate change which are affecting water resources due to the discharge of untreated wastewater and causes an impact on the environment and public health mainly in developing countries such as Rwanda. Constructed wetlands (CWs) utilization can provide sustainable domestic wastewater treatment because they rely on natural processes, less expensive to build, operate and maintain, low or no energy usage, no chemical consumption, provide habitat for wildlife and organisms and eco-friendly compared with conventional wastewater-treatment systems and the purified water with CWs can be suitable for reuse in irrigation and harvested plants can have economic value. In Rwanda there is few CWs for treating wastewater but there are no appropriate management mechanisms for those treatment systems in order to keep them in a sustainable manner. Therefore, the main objective of this study was to evaluate the removal efficiency of organic matter and pathogens of local full scale horizontal sub-surface flow constructed wetlands at Excella School in Kigali. During the monitoring period of 6 months, the average removal efficiencies of analysed pollutants parameters of the effluent were: COD (56.8 %), BOD5 (54.9 %), TN (46 %), TP (47.9 %), TC (94.58 %), FC (95.60 %) and E. coli (98.89 %). The average Biodegradability Index of the effluent was 0.78. Pollutants removal efficiency was increased as the macrophytes grew and the effluent from the studied constructed wetlands didn’t meet the standards of water for irrigation. Generally, the removal efficiencies of the pollutants were not high due to the fact that the wetlands macrophytes were still growing and macrophytes are the main in pollutants removal. Also effluent parameters such as pH, Temperature, Electric conductivity, TDS and TSS were analysed and they met the standards of the domestic wastewater to be released in the environment. The results showed that the pathogens were highly removed but the effluent did not comply the standards as well as BOD5 while TN, TP and COD comply with the standards. Inspecting the effluent quality regularly, introducing new microorganisms into CWs, further research, planting other macrophytes, trained staffs to be in charge of the CWs monitoring, operation and maintenance are recommended