Contamination assessment and optimization of coagulation-flocculation-sedimentation water treatment process for Kou river in northern Tanzania

Abstract

The Babati Urban Water Supply and Sanitation Authority (BAWASA) in northern Tanzania supplies water from the Kou River to Magugu town for domestic purpose, where its Fe (3.64 mg/L) and turbidity (87 NTU) concentrations were found in excess beyond their acceptable limits. A coagulation-flocculation-sedimentation water treatment plant was planned to be installed for treating the water, where the system’s performance is negatively influenced due to inadequate applications, hence requiring optimization. The quality of sediment and water of the Kou River was examined for physicochemical parameters and 9 metallic elements. The water did not meet the requirements for aquatic, biological life, and irrigation purposes for at least 1 metallic ion. Furthermore, pollution levels of the 9 metals in the river sediments were evaluated using 6 pollution indices and a moderate contamination was found, hardly posing ecological risk. A coagulation-flocculation-sedimentation water treatment system was optimized to respond to coagulant dose, pH, mixing rates, mixing time, and settling time in the removal of Fe and turbidity using jar-test. The optimum dose and conditions were: 30 mg/L alum, an initial mixing rate of 150 rpm for 5 minutes, slow mixing rate of 20 rpm for 30 minutes and settling time for 30 minutes which was validated using scale-up pilot test and resulted in 98.25% turbidity and 97.5% Fe removal. Furthe research on developing enhanced time-saving optimization method and developing a dose indicator technology to assign alum dose requirement based on the fluctuating characteristics of the raw waters (turbidity) is recommended.