A Pilot Study on Municipal Wastewater Treatment Using a Constructed Wetland in Uganda
Description:... The shoot density and size of open areas controlled the physical-chemical conditions in the wetland units. When the shoot density was greater than 50 shoots/m 2, low oxygen concentrations & lt; 2 mg/l, pH values of 7 - 7.5 and water temperatures of 22 ± 0.5 °C prevailed. In the open ponds with relatively larger open areas, peak oxygen concentration of 27 mg/l, pH of 10.4 and water temperature 29 °C were obtained. In the reduced open surface areas applied in the 2 nd and 3 rd phases, peak oxygen concentration of 12 mg/l, pH values of 7.5-8 and temperature of 23 °C were registered. These conditions influenced the extent of reduction of the pollutants achieved in each case. Removal efficiencies in both C. papyrus and P. mauritianus wetland units' of just over 70% of the input settled COD (maximum 350 kg ha -1) and settled BOD (maximum 100 kg ha -1) were obtained when the shoot cover was intense. It increased to over 80% in the third phase. TSS reduction above 80% of the input (maximum 250 kg ha -1) was obtained in all the vegetated wetland units and in all the phases. Similar findings were observed in the household wetland. A significant faecal coliform removal of 4 log units ivas obtained in the control ponds as compared to 3 log units derived trom the vegetated wetland units or 1.13 log units from the household wetland. The faecal coliform reduction in the planted units was correlated with TSS and particulate organic matter removal. The lethal impact of direct sunlight and its secondary effects such as high pH were considered more significant in ponds with open areas. All the results show that Ciperus papyrus planted units performed better than Phragmites mauritianus units. The low residual background BOD radiation of 12 mg/l in papyrus units as compared to 17 mg/l in Phragmites units further confirmed the observed trends. In addition, the rate of BOD decay in papyrus units was higher as indicated by the areal first order rate constant (0.084 m d -1) than in Phragmites (0.039 m d -1). Throughout, the effluent concentrations of these parameters from the vegetated wetland units were consistently below the Uganda wastewater discharge standards.
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