Abstract
Abstract Raw water quality deterioration has affected capabilities of Conventional Water Treatment Processes (CWTP) in many countries. CWTP 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 the 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.
Highlights
There has been a global trend of deterioration of water quality for potable water sources (Biswas & Totajada 2019)
Raw water abstracted from Lake Chivero and Lake Manyame is treated at Morton Jaffray Water Treatment Works (MJWTW), as in Figure 2, which consists of three units; that is, Old Works or Number 1 (160 ML/day), Number 2 Works (227 ML/day) and Number 3 Works (227 ML/day) with a combined design capacity of 614 ML/day (Muisa et al 2011)
The raw water parameters had low variability except total algae, which had a coefficient of variation of 65%
Summary
There has been a global trend of deterioration of water quality for potable water sources (Biswas & Totajada 2019). The water passes through the chlorine and lime dosing chamber for post disinfection and pH correction This treatment process has not been able to deal with the high pollution load into the water sources, resulting in poor treated water quality (Dandadzi et al 2019). The current chemicals being used at MJWTW have failed to treat the water to drinking water standards (Dandadzi et al 2020) This has resulted in a water supply system characterized by poor water quality, severe water rationing and reoccurrence of water-borne diseases outbreaks such as cholera and typhoid (Chirisa et al 2015). It is important to study how pre-oxidation using chemicals such as chlorine dioxide can help to reduce the cost of water treatment and improve the quality of treated water. The study investigated opportunities for use of ClO2 as an alternative pre-oxidizing agent
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