Modelling bacterial removal in waste stabilization ponds in tropical climates.

The mechanisms of the reduction of faecal coliform and faecal streptococci were investigated in the University of Dar es Salaam waste stabilization ponds under tropical climatic conditions. Field studies were conducted in the ponds with pond depths ranging from 1.22 to 1.81m and hydraulic retention time of 8.1d to 15.3d. To assess the performance of the University of Dar es Salaam waste stabilization ponds, samples where collected at influent and effluent of each 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 coliform and the pour plate count method was used to enumerate faecal streptococci. The mortality rate constants, k were determined using the plug flow model. The observations made indicate that sunlight, pond depth, and pH were the key parameters to the model, leaving out temperature and dissolved oxygen due to the low correlation with mortality rate constant. In the model developed, bacterial mortality rate constant for pH, kpH was i.16 d -1 and the ratio of rate constant for light mortality term to light attenuation coefficient ks/K was 1.524x 10-4m cm2/cal and the model efficiency was 0.77. The model was successfully verified by simulation with data obtained from pilot scale pond systems at the University of Dar es Salaam and from data obtained from the Asian Institute of Technology ponds in Thailand. A brief model application was also given to aid designers and decision-makers. It was also found that the model can predict reasonably well faecal streptococci mortality rate and total coliform mortality rate. Therefore any of the above mentioned faecal bacteria can be used in the study of bacterial mortality in waste stabilization ponds. The model was also applied on data from the Alsamra ponds in Jordan and it was seen that it overestimates bacterial mortality rate. Over-estimation was because of high temperature in Tanzania compared with temperatures in Jordan. Therefore this model works well in tropical climates and should not be applied to temperate climates without the inclusion of the effect of temperature.