For every 1.0 mg/l of alum added, how much bicarbonate alkalinity needs to be available?

When alum is added to water as a coagulant, it reacts with the bicarbonate alkalinity present in the water. The stoichiometry of the reaction indicates that for every 1.0 mg/l of alum, approximately 0.45 mg/l of bicarbonate alkalinity is required to effectively neutralize the charges and facilitate floc formation. This relationship is crucial in treatment processes because insufficient alkalinity can lead to ineffective coagulation, resulting in suboptimal water clarity and higher concentrations of suspended solids.

The requirement for bicarbonate alkalinity is based on the molecular interactions occurring during the coagulation process. The alum breaks down and releases aluminum ions, which interact with the negative charges of suspended materials and require bicarbonate alkalinity to maintain the pH levels and stabilize the reaction. Therefore, having the appropriate balance of alum and bicarbonate is essential for efficient water treatment operations. Understanding this interaction helps operators ensure that sufficient alkalinity is available for optimal coagulation when alum is used.