Removal of faecal pathogens in constructed wetland Systems in tropical environment

Abstract

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.