How does the ionic strength of raw water impact the efficiency of charge neutralization during coagulation?

The ionic strength of raw water affects the efficiency of charge neutralization during coagulation by influencing the compression of the electrical double layer surrounding colloidal particles. Coagulation relies on destabilizing these particles, which are typically negatively charged, to allow them to aggregate and form flocs. These particles stay suspended in water because of their surface charge, surrounded by an electrical double layer. This double layer consists of an inner Stern layer, where ions are tightly bound to the particle surface, and a diffuse layer, where ions are more loosely distributed. The ionic strength of water refers to the concentration of ions in the solution. Higher ionic strength compresses the diffuse layer of the electrical double layer. This compression reduces the electrostatic repulsion between colloidal particles, making them more likely to collide and aggregate when a coagulant is added. In other words, at higher ionic strength, a lower dose of coagulant may be required to achieve the same degree of destabilization because the particles are already closer to aggregating. Conversely, at lower ionic strength, the diffuse layer is more extended, increasing the electrostatic repulsion and requiring a higher coagulant dose to overcome this repulsion and achieve effective charge neutralization. Therefore, ionic strength plays a critical role in determining the optimal coagulant dosage for effective water treatment. For example, seawater has a high ionic strength, which can influence the coagulation process differently than freshwater with low ionic strength.