How does alkalinity influence the efficiency of coagulation using alum as a coagulant?

Alkalinity is crucial for the efficient use of alum (aluminum sulfate) as a coagulant because it buffers the pH and ensures the proper chemical reactions occur for effective floc formation. Coagulation is the process of destabilizing suspended particles in water so they can clump together and be removed. Alum, when added to water, reacts with alkalinity to form aluminum hydroxide [Al(OH)3], which is the primary floc-forming agent. Alkalinity is a measure of the water's capacity to neutralize acids. It is primarily composed of bicarbonate (HCO3-), carbonate (CO32-), and hydroxide (OH-) ions. The reaction between alum and alkalinity consumes alkalinity and releases hydrogen ions (H+), which lowers the pH of the water. If the water has insufficient alkalinity, the pH can drop too low, inhibiting the formation of aluminum hydroxide flocs. The optimal pH range for alum coagulation is typically between 6.0 and 7.5. If the pH drops below 6.0, the aluminum ions remain soluble and do not form effective flocs. Therefore, if the raw water has low alkalinity, it may be necessary to add lime (calcium hydroxide) or soda ash (sodium carbonate) to increase the alkalinity and buffer the pH during coagulation. The amount of alkalinity required depends on the alum dosage and the initial alkalinity of the water. Without sufficient alkalinity, higher alum dosages will be needed which is not cost effective, or the coagulation process will be impaired, leading to poor turbidity removal. For example, if a water source has a low alkalinity of 20 mg/L as CaCO3, adding alum will consume this alkalinity and significantly lower the pH. In this case, lime or soda ash must be added to increase the alkalinity to an optimal level, such as 80 mg/L as CaCO3, to ensure effective coagulation.