Optimizing Coagulation Process for Low to High Turbidity Waters Using Aluminum and Iron Salts

Problem statement: Turbid waters, containing colloidal particles, are normally treated by coagulation-flocculation followed by clarification. Ferric chloride and alum, which are the most common types of coagulants in water treatment plants of Iran as well as many other countries, were investigated with the aim of determining their capabilities to reduce turbidity of drinking water. Turbidity was added as kaolin. Optimization of coagulation process may assure removal of turbidity to a level below water quality standards in most cases. Approach: In this study, the effectiveness of aluminum sulfate and ferric chloride was evaluated at different pH values and coagulant dosage to find optimal operational conditions for low to high turbidity waters. The influence of lime, as a coagulant aid, on coagulation process was also studied. A set of jar test experiments was conducted to find the optimal pH and coagulant dosage. Results: Results demonstrated that coagulation process can assure turbidity removal from low to medium turbidity waters effectively, using relatively low levels of aluminum sulfate and ferric chloride (10-20 mg L). Turbidity removal efficiency still remained high when the initial turbidities of water were increased to 500 and 1000 NTU. Conclusion/Recommendations: Results showed that turbidity removal is dependent on pH, coagulant dosage, as well as initial turbidity of water for both used coagulants. The highest turbidity removal efficiency was within 82.9-99.0% for alum and 92.9-99.4% for ferric chloride over the applied range of turbidity. Turbidity removal efficiency was higher for ferric chloride compared to aluminum sulfate at optimum conditions. Both applied coagulants demonstrated promising performance in turbidity removal from water. The results of the current study can be used as a baseline data for drinking water treatment facilities which uses these two types of coagulants.