Raw water quality deterioration has affected capabilities of Conventional Water Treatment Methods (CWTM) in many countries. CWTM used at Morton Jaffray Water Treatment Works (MJWTW) in Harare have proven ineffective due to pollution. The study investigated the use of Chlorine Dioxide (ClO2) as an alternative pre-treatment chemical. Its effectiveness was compared to that of Calcium Hypochlorite (Ca(CIO)2) used at MJWTW. Grab raw water samples from MJWTW were collected between January and March 2020 and pre-treated with ClO2 and Ca(CIO)2 followed by jar tests with alum to determine pre-treatment effectiveness. Parameters analyzed included Electrical Conductivity (EC), Total Dissolved Solids (TDS), Total Solids (TS), turbidity, Chemical Oxygen Demand (COD), pH and total algae counts (TAC). The raw water had mean TDS (264 mg/L), TS (440 mg/L), turbidity (7.1 NTU), COD (85 mg/L), pH (7.9) and TAC (28.4 × 106 cells/mL). The optimum alum dosage without pre-oxidation was 80 mg/L. Pre-oxidation with 0.075 mg/L ClO2 reduced alum dosage to 60 mg/L. This ClO2 dosage was compared to a Ca(CIO)2 dosage of same concentration and strength at 5 and 15-minutes contact time before alum dosage at 60 mg/L. The treated water quality parameter levels proved better performance for ClO2 compared to calcium hypochlorite.

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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.

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