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Huria: Journal of the Open University of Tanzania

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Nitrogen Transformation and Removal in Horizontal Surface Flow Constructed Mangroves Wetland

AS Mahenge

Abstract


The potential use of Constructed Mangrove Wetlands (CMWs) as a
cheaper, effective and appropriate method for Nitrogen removal from domestic
sewage of coastal zone in peri-urban cities was investigated from August 2007 to
September, 2008. Field investigations were made on horizontal surface flow
constructed mangrove wetland located at Jangwani beach in Dar es Salaam. A
wetland of 40m x 7m was constructed to receive domestic sewage from septic tank
of Belinda Beach Hotel and was operated in an intermittent continuous flow mode.
The wetland was loaded by sewage of strength 60% and 40% was seawater. The
wetland used the already existing mangrove plant specie Avicennia Marina. The
performance of the wetland in removal of nitrogen species was determined. The
observed removal rates of TKN, NH3-N and NO3-N, and were 61%, 85% and 76%,
respectively. The removal of TKN was contributed by mineralization of organic
nitrogen to NH3-N. The removal of NH3-N was contributed by nitrification,
volatilization and mangrove uptake processes. Nitrification process transformed
NH3-N to NO3-N at aerobic conditions while volatilization process transformed
NH3-N in gaseous form that finally escaped from a water phase to the atmosphere.
The removal of NO3-N was contributed by de-nitrification process that transformed
NO3-N to nitrogen gas which escaped to the atmosphere. The removal processes
were attributed by the forcing functions pH, temperature and DO with averages of
7.75, 29oC and 1.55 mg/L, respectively. Constructed Mangrove wetland has shown
high potential in Nitrogen removal from sewage, therefore it can be used for
sewage treatment.

Keywords: Constructed Wetlands, Mangroves, Sewage, Treatment Performance




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