Optimisation of the algal control process at Morton Jaffray water works, Harare

Abstract

Water quality deterioration in Harare is an urgent problem and some of the causes of this deterioration have been the regular inflows of poorly treated sewage effluents into the major source of drinking water, Lake Chivero, and the proliferation of algae (blue-green algae). The occurrence of algae in raw water from Lake Chivero has caused problems in water purification at Morton Jaffray (MJ) water works, Harare’s major water treatment works. At the plant, the filtration process has been the most affected as evidenced by frequent filter clogging and frequent backwashing now reported to be once every 4-8 hours compared to the acceptable frequency of once every 24-36 hours. The increased backwashing frequency has also resulted in reduced plant output and consequently water shortages in Harare. The high levels of algae have resulted in increased water treatment costs as more chemicals at high dosages are required for algae removal. Algae has also been detected in the treated water hence posing a health hazard for consumers. The study was carried out at MJ water works from January to May 2007.The objectives of this study were to assess the effects of algae on water treatment processes, identify factors contributing to effective removal of algae and determine the optimum values of contact time, coagulant dose, and algaecide dose for removal of algae. Methods of study included literature research and water treatment process monitoring. Jar tests, simulating coagulation, flocculation and sedimentation, to determine the optimum conditions for removal of algal cells, were done. Simulations of dosing different doses of algaecide at Lake Chivero intake tower were also carried out using jar test. Results showed that the water treatment works was hydraulically overloaded by 20 to 35%. The most abundant algae were found to be blue-green algae, particularly Anabaena, Microcystis and Chlamydomonas. The presence of algae was related to filter clogging, and the potential of causing taste and odours, as well as possibilities of toxin production in the final treated water. The concentration of algae in the raw water ranged from 875- 6000 cells/ml. Algae removal by coagulation, flocculation and sedimentation ranged from 50-94%. Removal of algae by the filtration process ranged from -93 to 50%. Jar simulations showed that at algaecide dose of 0.8mg/l, applied 30 minutes before coagulation, in combination with coagulant dose ranging from 80 to 110mg/l, at pH 7, algae removal reached 99%. Lethal doses, for algae, of copper as copper sulphate was found to be in the range of 0.8 and 1mg/l if the algaecide was added at coagulation It was concluded that there is a considerable variation in the effect of the algaecide at different contact times. It was shown that the filter clogging Anabaena was more susceptible to the algaecide when the contact time between the Anabaena and the algaecide was increased, and when the algaecide was dosed before addition of GAC and alum. Chemical doses and their application should be optimized; under-dosing results in poor removal of algae in clarification and problematic filtration, with the risk of breakthrough of algal cells containing toxins. Overdosing could also have a negative impact on the final water quality.