| dc.description.abstract | A number of studies have been conducted concerning wetlands systems in East Africa;
most of which focused on the performance of constructed wetlands in the removal of
TSS, Org-N, NH3-N, NO3-, NO2- and BOD5. However, little attention has been given to
the quantification of the contribution made by each process on the total removal of Faecal
Coliform (FC). A research study was conducted to investigate the possibilities of
optimising the FC Removal in Horizontal Sub-Surface Flow Constructed Wetland
(HSSFCW) at University of Dar Es Salaam in Tanzania. The HSSFCW polishes the
effluent from maturation ponds which form part of Waste Stabilization Pond (WSP).
Modelling of FC by using STELLA program was the methodology used to achieve the
main objective of this study. The modelling procedures that were considered in this study
were the problem definition, conceptualisation, mathematical equations formulation,
calibration, verification and sensitivity analysis. The validation was not considered
because of data deficit in this study. The processes that were considered in the model
were inflow, outflow, sedimentation, filtration, die-off of bacteria and growth. Also the
study determined the best removal mechanism and parameter which was very sensitive to
total removal of FC.
Grab sampling method was used for collecting samples from the inlet, middle section and
outlet and were analysed for FC (Counts per 100ml), BOD5 (mg/l), NH3-N (mg/l), DO
(mg/l) and pH. The temperature (0C) and flow rate (m3/d) were also determined for the
samples that were collected between February and April 2007. Other input data included
solar radiation (cal per m2 per d) which was obtained from literature. All parameters
mentioned above were used in the modelling processes.
The observed results were; FC: 1,163,636±268,675 (inlet), 884,636±344,564 (middle)
and 45,909±19,373 (outlet). BOD5: 141±35 (inlet), 119±25 (middle), and 68±11 (outlet).
NH3-N: 10.92±0.47 (inlet), 9.93±0.55 (middle), and 8.01±0.64 (outlet). DO: 5.91±0.23
(inlet), 5.76±0.20 (middle) and 5.59±0.26 (outlet). pH: 6.28±0.14 (inlet), 6.67±0.37
(middle) and 7.62±0.72 (outlet). However, the temperatures: 29.77±1.63 (inlet),
30.20±1.63 (middle), and 30.64±1.57 (outlet) and Flow rates: 1.7±0.003 (inlet),
1.42±0.100 (middle) and 1.05±0.106 (outlet). For the modelling results, the removal
efficiency of FC on those processes was; die-off processed: 44.164%:
3,422,203±1,129,699: Sedimentation; 5.826%: 451,433±149,914: Outflow; 0.053%:
4,075±1,796 and filtration; 0.001%: 94±31. However, the growth and inflow processes
were 47.836%: 3,706,710±1,285,500 and 5.593%: 473,347±403,928 respectively. The
study established that calibrated results were lower than compared to observed results by
91%. From this result, it can be concluded that the model had optimised the FC reduction
in HSSFCW. Also the study established that, the growth and inflow processes increased
the FC within the system while the rest reduced. On sensitivity analysis, the study noted
that solar radiation was very sensitive in decreasing the FC within the system. The study
further established that pH was very sensitive in increasing the FC within the system
while temperature, DO, BOD5 and NH3-N were sensitive within the relative ranges of
±10%. The optimal FC was observed at 0% of the relative change of temperature, DO,
BOD5 and NH3-N in this model. Hence it can be concluded that solar radiation was the
best parameter that reduced the FC concentration within the system. Therefore, it can be
recommended that the model be used as tool for designing and managing the HSSFCW
after being validated in order to optimise the effluents quality that is being discharged to
the receiving water bodies. | en_ZW |
