PhD Theses

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Browsing PhD Theses by Author "Kaseva, Mengiseny"

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    Modelling of constructed wetlands for polishing on-site anaerobically pre-treated domestic wastewater.
    (University of Dar es Salaam, 2002) Kaseva, Mengiseny
    This research work has been carried out to assess the performance of the Horizontal Sub-Surface Flow Constructed Wetland (HSSFCW) system in polishing pre-treated wastewater from the Upward Flow Anaerobic Sludge Blanket (UASB) and Septic Tank (ST) systems. Four different experimental set-ups of serially operated pretreatment and tertiary treatment systems were carried out. The HSSFCW units were designed and operated with a hydraulic loading of approximately 0.1 m3/m2/day and a hydraulic retention time (HRT) of approximately two days. The first experimental set up (UASB-HSSFCW) consisted of three CW units (A, B and C). The first unit was used as a control while the second and third units were planted with Phragmites mauritianus and Typha latifolia, respectively. Results obtained in this experimental set-up indicated a final BODs effluent quality of 29.2 mg/1 in unit B that corresponds to a removal of 73.8%, while in the second experimental set up (ST-HSSFCW) a BOD5 effluent quality of 27.45 mg/1, or equivalent to a removal of 74%, was obtained. In the UASB-HSSFCW set-up, slightly higher TC (56.35%) and FC (67.9%) removal rates were-recorded in wetland units B and C compared to the control unit A. Wetland unit B performed better in terms of FC removal while unit C had a better TC removal rate. Results obtained in ST-HSSFCW planted with Typha latifolia indicated a mean TC removal of 75% while the mean FC removal was 66.1 %. In the third experimental set up (UASB-VRF-HSSFCW), the mean FC removal was 98.8%, while the TC mean removal was 98.3% and COD removal was 74.8%. In the STHSSFCW the COD removal was 70%. In the UASB-HSSFCW system, NH4-N removal rates of 1.0, 0.91 and 0.45 g/m2/day for units B, C and A; respectively were achieved during the initial six months (phase 1) of the study. These removal rates were higher than those obtained during the last six months (phase II) of the study (0.44, 0.35 and 0.43 g/m2/day for units B, C and A). In the UASB-VRF-HSSFCW system, NH4-N mass loading removal rate in VRF was 0.91 g/m2/day, which was comparable to the removal rates obtained in the UASBHSSFCW system but was higher than the 0.6 g/m2/day obtained in ST-HSSFCW. However, a much higher removal rate (1.54 g/m /day) was obtained in UASB-VRF'HSSFCW system suggesting further that a serially operated UASB-VRF-HSSFCW can perform better than a UASB-HSSFCW and ST-HSSFCW system alone. In the fourth experimental set-up, which consisted of the UASB and HSSFCW, packed with fine gravel and sand FC in the influent which was in the magnitude of 7 logs, was reduced to 3 logs, which were within the WHO and FAO guidelines for wastewater recycle and re-use in agriculture and aquaculture. Model results for BOD5, and bacteria dynamics in the first experimental set up indicated that out flow BOD5 and BOD decomposition plays an important role in the removal of BOD5. The obtained results suggest that the developed model has been able to simulate and can thus be used to predict the removal mechanisms of BOD5, and bacteria in the HSSFCW. The model can also be used for designing HSSFCW systems.