Effect of hydraulic loading and plants in pollutant removal of subsurface flow wetlands

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University of Dar es Salaam
The effect of hydraulic loading rate and planting on removal of suspended solids and BOD5 from maturation pond effluents was carried out at the University of Dar es Salaam. In this study, pilot scale subsurface flow constructed wetland consisting of four units each of size 0.3 x 1.5 x 0.6 metres were designed and constructed. The units were filled with limestone gravel 6 - 25 mm in diameter and porosity of 35 %. The pilot beds were located at latitude 6° 46' S and longitude 39`13' E and receive University of Dar es Salaam maturation pond effluent (tertiary treatment). Three of the units were planted each with Phragmite mauritianus, Typha domingensis and Cyperus dubius respectively.The fourth cell was left unplanted so as to serve as a control. The plant density was 35 plants per square metre for each species. Stems and and rhizomes were used in planting which was done in July 1999. Sampling started three months after planting. In this study, all held and laboratory examinations were carried out according to standard methods (1992) for the examination of wastewater. Performance comparisons for the four-wetland cells were carried out using statistical decision theory based on hypotheses and level of significance. The influent water quality varied slightly over the trial period (suspended solids 22.04 - 48.48 mg/l; BOD5 18.04 - 38.9 mg/1; dissolved oxygen 2.82 - 6.72 mg/1; pH 7.54 - 9.51; and temperature 28.10 - 31.35 °C). With the exception of one day with heavy rain fall (180 mm) influent water quality parameters were consistently lowered in all the four wetland effluent (suspended solids 7.93 - 12.19 mg/l; BOD5 8.03 - 16.35 mg/l; dissolved oxygen 1.66 - 4.15 mg/1; pH 7.12 - 8.91; and temperature 27.00 - 30.87 °C). Mean suspended solids removal of 69.08 - 74.59 % were recorded regardless of the hydraulic loading rate and a little difference in performance between the planted and unplanted filter beds (p-value > 0.005). Mean removal for BOD5 was between 56.75 - 67.29% with a slight increase in removal efficiency the lower the hydraulic load; and as in the case of suspended solids, little difference was found between the performance of the planted and non planted wetland units (p-value > 0.005). Mathematical models for both BOD5 and suspended solid concentrations were developed to help predict the expected performance of wetlands units with similar characteristics as the one's in this study. In all, the results revealed that horizontal subsurface flow wetlands can be loaded hydraulically at much higher rates than reported in most literature and that the effect of plants on wetland performance was less visible during the study period
Available in print form, East Africana Collection, Dr. Wilbert Chagula Library, Class mark (THS EAF TJ851.T34F63)
Water power, Hydraulic engineering, Wast removal
Fofanah, A. (2000) Effect of hydraulic loading and plants in pollutant removal of subsurface flow wetlands, Master dissertation, University of Dar es Salaam. Dar es Salaam