Abstract:

Gaba II, a conventional water treatment plant in Uganda, draws its raw water from the Inner Murchison Bay of Lake Victoria. Over the last six years (1995-2000), the water quality has changed as evidenced by increase in colour, turbidity and algae (chlorophyll-a) concentrations. Increased algal blooms in the Bay have had negative effects on the water treatment especially the clarification and filtration processes. In this paper, the performance of the treatment plant in terms of organic matter and algae removal was assessed using colour, turbidity, permanganate value and chlorophyll-a as indicator parameters. Identification and quantification ofthe dominant algae groups in the treatment process was done, and pilot clarification tests were carried out to identify options for algal removal. The removal of the colour, turbidity, permanganate value and chlorophyll-a to the levels of 88%, 71 %, 65% and 85% respectively was found to take place mostly at the clarification stage. The results show that Gaba II water treatment plant does not perform well in terms of chlorophyll-a removal, especially at the filtration stage. At all the treatment stages, green and blue-green algae were found to be the dominant algal groups. The clarification efficiencies achieved in the pilot tests were 60-80% when using alum alone, 45-50% for alum with lime, 45-75% for alum with copper sulphate and 50-75% for alum with chlorine. This showed that none ofthe combinations was a better algae removal option than the presently practised clarification using alum alone.