Feasibility of dissolved air flotation for drinking water treatment for Harare

Lake Chivero, Harare's major raw water source, is eutrophic and algae-infested. Consequently, the conventional water treatment processes (CWTPs) that are used at Harare's main water treatment works, Morton Jaffray Water Treatment Works (MJWTW), have been ineffective in algae removal. This study was carried out to investigate the feasibility of using dissolved air flotation (DAF) to remove algae at MJWTW. Experiments were carried out using a 60-L DAF pilot system with a hydraulic loading rate of 7 m/h. Raw and treated water was characterised in terms of turbidity, pH and electrical conductivity (EC), and chlorophyll-a. Results showed reductions in turbidity, pH, and chlorophyll-a of 64, 27, and 95%, respectively. However, EC increased by 42% due to the addition of alum and acid. The DAF performed better than the CWTP currently used at MJWTW based on a comparison to a study carried out in the same period simulating conventional treatment of the same raw water and values reported in the literature for the CWTP. The adoption of DAF at MJWTW could potentially result in more efficient water treatment and better algae removal too. Further tests on optimum acid dosing for DAF are recommended while using alum as a coagulant.