Optimization of algal removal process at Morton Jaffray water works, Harare, Zimbabwe

This study was carried out at Morton Jaffray (MJ) water works to identify the most abundant algae species and their concentrations, and to assess the effects of algae on water treatment processes. The study also sought to determine the optimum values of contact time, coagulant and algaecide doses for the removal of algae. Sampling and analysis of parameters studied was according to APHA standards. Jar tests simulated coagulation, flocculation and sedimentation to determine the optimum coagulant and algaecide dose, and contact time for removal of algae. The most abundant algae were blue–green algae. The concentration of all algae in the raw water from Lake Chivero ranged from 875 to 6000 cells/ml. It was found out that performance of MJ in removing algae was poor and the filtration stage appeared to be the most affected. The optimum contact time for the algaecide was 30 min. The contact time and settling time were the most sensitive parameters. Higher algae removal was achieved at lower pH values. It is recommended that the algaecide should be dosed at or near the raw water intake tower to increase the contact time unlike the present practice where it is added at the plant just before sedimentation. The application of granular activated carbon and post-chlorination currently practiced at the water works need optimization to remove/neutralize the toxins released as a result of the effect of the algaecide.

► This paper investigated the impacts of algae on water treatment processes.
► The most abundant algae species were blue green algae.
► The optimum contact time between the algae and algaecide was 30 min.
► Settling time and pH were also found to be critical in algal removal.
► Algal removal increased with increasing contact time and algaecide.