Constructed Wetland System for Domestic Wastewater Treatment: A Case Study in Addis Ababa, Ethiopia

Abstract

ABSTRACT During the last decades, constructed wetlands (CW) were very successful when used for treatment of wastewater from different sources such as municipal, domestic, industrial, agricultural and surface runoff. This new approach is designed based on natural processes involving complex and concerted interactions between the plants, the substrate and the inherent microbial community to accomplish wastewater treatment in a more controlled and predictable manner through physical, chemical and biological processes. In order to evaluate the performance of constructed wetland and generate information a total of 24 samples were collected and analyzed for selected wastewater quality parameters from a Jehovah Witnesses Branch Office wetland constructed to treat domestic wastewater. The parameters analyzed were biochemical oxygen demand, chemical oxygen demand, total suspended solids, ammonium N, nitrate N, total N, orthophosphate, total phosphorus, sulfate, sulfide, temperature, pH, total coliform and fecal coliform. They were all measured using standard methods. The treatment performance of JWBO wetland was evaluated based on the percentage removal efficiency of the above parameters. Within the study period, the mean removal efficiency of JWBO CW system was 99.3% (BOD5), 89% (COD), 85% (TSS), 28.1% (NH4 + -N), 64% (NO3-N), 61.5% (TN), 28% (orthophosphate), 22.7% (TP), 77.3% (Sulfate), 99% (Sulfide), 94.5% (TC) and 93.1% (FC).

Moreover, though the difference is not as such high, the result of this study indicated that wetland cells planted with Cyprus papyrus (cell 1 and 3) showed higher removal efficiency for NO3-N (82.4%), NH4 + -N (24.8%), TN (54.8%), PO4 3- (23.5%), and TSS (83.9%) than the other wetland cells. Similarly wetland cells planted with Phoenix canariensis (cell 4 and 6) showed higher removal efficiency for TP (17%), S2- (99%), BOD5 (98%), COD (90%), TC (94%) and FC (91%). While the other wetland cells planted with Cyprus alternifolia (cell 2 and 5) showed higher removal efficiency only for SO4 2- (82.2%) than the others. However, these differences were statistically significant (p<0.05) only for Sulfate and FC.The performance efficiency results indicated that, this wetland system has excellent removal capability for biochemical oxygen demand, chemical oxygen demand, total suspended solids, sulfate, sulfide, total and fecal coliform bacteria. However, since the HRT of JWBO CW was very short (2.16 days) the removal efficiency was low for nitrogen (especially ammonium nitrogen) and phosphorus. In general based on the overall results of the treatment performance of JWBO CW, the application of constructed wetland in Ethiopia can be considered as a technically as well as economically viable option for domestic wastewater treatment.