Effect of enhanced coagulation with Calcium Hypochlorite on an Algae Laden Drinking Water Supply

Abstract

Algae laden water is difficult to treat since it significantly impacts on finished water quality and treatment plant operations. The primary treatment plant operation issues are encountered in sedimentation since gravitational forces are largely ineffective due to algae instability and filtration processes due to increased filter clogging from algae carryover from sedimentation stage. The primary finished water quality issues are related to taste and odour and disinfection by-products (DBPs) resulting from disinfection. This research study investigates the effectiveness of pre-chlorination (enhanced coagulation with chlorination using calcium hypochlorite solution) on the removal of algae and algaederived organic matter and its influence on the coagulation and flocculation processes of water treatment.

Batch bench-scale experiments were carried out in the laboratory using algae laden water samples of predominantly blue-green algae collected from a river and cultured in an open pond to peak algal concentration. The experiments were carried out for varying algal concentrations measured through chlorophyll tests using a spectrophotometer and recorded as chlorophyll-a. Break-point chlorination tests using calcium hypochlorite were carried out to determine chlorine demand before a series of jar tests with and without chlorination were carried out to determine the optimum coagulant dose. The chlorine dose used for chlorination was equivalent to chlorine demand of each sample used.

Algae was found to significantly impact on the physical and chemical properties of water by increasing colour and turbidity, cause drop in pH, decrease in water hardness and increase in alkalinity. Algae was also found to cause an increase in chlorine demand. Experiments showed that pre-chlorination using calcium hypochlorite improves coagulation and flocculation processes and enhances algal removal through sedimentation. Coagulation without chlorination showed 74% algal (chlorophyll-a) removal, 62% colour improvement, 72% turbidity removal, 8.7% pH reduction, 13% total hardness reduction and 19% alkalinity reduction. Pre-chlorination (coagulation with chlorination using calcium hypochlorite) was observed to cause 5% saving on coagulant requirement, 90% algal (chlorophyll-a) removal, 74% colour improvement, 84% turbidity removal, 4.2% pH reduction, 18% total hardness reduction and 26% alkalinity reduction.