Seasonal characterization of municipal wastewater and performance evaluation of a constructed wetland system in Addis Ababa, Ethiopia

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Rapid urbanization and industrialization are increasing fresh water consumption and lead to a corresponding rise in wastewater generation. Because of insufficient capacity of wastewater treatment, many of Addis Ababa’s residents are forced to dispose domestic waste into the environment. The purpose of this study was to characterize wastewater and evaluate the performance of constructed wetland. The physicochemical and bacteriological parameters of municipal wastewater was analysed in dry and wet seasons. Three parallel pilot horizontal subsurface flow constructed wetland was constructed, lined with polyethylene and filled with gravel. The two HSSFCW cells were planted with Vetiveria zizanioides and Phragmites karka while one cell was unplanted and used for control. The wastewater was loaded with 0.177 m3/day flow rate at hydraulic loading rate of 0.05 m/day with a hydraulic retention time of 3 days. Standing nutrient accumulation and aboveground biomass of plants were determined. The findings revealed a significant difference in concentration of COD, BOD5, NH4+ and temperature of the municipal wastewater between dry and wet seasons. The HSSFCW cell planted with V. zizanioides removed more TSS (96.7%), BOD5 (92%), COD (90.8%), TP (88.5%), TC (4.7 log units) and E. coli (4.1 log units) than that planted with P. karka. The maximum above-ground dry biomass and the maximum standing nutrient accumulation were achieved by P. karka than V. zizanioides at the end of experiment. It was concluded that the HSSFCW cells planted with V. zizanioides and P. karka have a potential of removing nutrients, organic matter and pathogen efficiently and was not affected by seasonal variation.