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The removal of faecal pathogens from municipal wastewater treatment facilities is necessary for
reduction of potential risk of contracting faecal infection among the population especially in the
developing countries. To remove pathogens in the wastewater, the Constructed Wetland (CW)
technology is advantageously applied among others. However little is known about how
environmental factors, selected wetland designs as well as operation variables affect the overall
removal of faecal pathogens in CWs. Using bucket experiments planted with Typha latifolia,
Cyperus papyrus, Cyperus alternifolius and Phragmites mauritianus the reduction of both
Salmonella spp. and E.coli were above 98%. The study also found a significant difference in
faecal bacteria removal between macrophytes (p < 0.05), whereas the first-order removal rate
constants revealed that the C. alternifolius and T. latifolia were the most effective followed by C.
papyrus and the least was P. mauritianus. The influence of dissolved oxygen, pH, temperature
and salinity contributed significantly to the observed differences. A different study investigating
the influence of water flow velocity on the removal of faecal bacteria from water column using
semi batch recycled Horizontal Subsurface Flow (HSSF) CW experiments found that the
concentration of E. coli and Salmonella spp. in both planted and unplanted systems were
decreasing over time for every tested flow velocity. However, influence of flow velocity on the
reduction of Salmonella spp. and E.coli was not significant in planted bed compared to unplanted
systems (p > 0.05). The presence of macrophytes and better aeration from increased water flow
velocities influence the insignificant change in the reduction of bacteria observed in planted
system. On the other hand, findings from field survey noted significant discharges (72.41%
increases) of Salmonella densities from the CW that were higher than its influent. Blockage of
the outlet channel to increase water for irrigation, resulting into system flooding was thought to
be the main cause of the inefficiency. The effluent water was currently being used by nearby
paddy farmers to irrigate their fields, thus exposing their health to risks. The study also found a
risky (33% error) of using E. coli as indicator of levels of pathogens in wetlands. Despite of the
low efficiency observed from field application of CWs, this study concludes that the CWs still
remain a viable technology for removing faecal bacteria from wastewater in developing
countries. |
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