Kinetics of bacterial mortality in surface constructed wetlands.

Faecal coliform and faecal streptococci die-off was investigated in the surface constructed wetland cell. The constructed wetland cell was 7.5 X 1.5 X 0.7 m. The cell was filled with aggregate size 19mm up to a depth of 0.35m. The remaining depth was filled with water. To assess the performance of the constructed wetland, samples were collected at the influent and effluent of the cell. Temperature and pH were measured in the field using a pH meter. Dissolved oxygen was also measured in the field using a dissolved oxygen meter. Solar intensity records were obtained from the Physics department at the University of Dar es Salaam. In this study the membrane filter technique was used to enumerate faecal streptococci and the pour plate count method was used to enumerate faecal coliform in accordance with the Standard methods (1992). The mortality rate constants, k were determined using plug flow model. The observation made indicated that sunlight was the key parameter to the model, leaving out interception, filtration, dissolved oxygen, temperature, sedimentation and pH due to the low correlation with mortality rate constant. In the model developed solar intensity contributed 72.6%, dissolved oxygen and pH 7%, sedimentation 0.44%. The ratio for the rate constant term to light attenuation coefficient (ks/K) was 1.281 x 10-3 m cm2/cal and the model efficiency was 0.80. The model was not verified due to lack of data from a similar constructed wetland.