What is the primary mechanism by which dissolved organic carbon (DOC) interferes with coagulation?

Dissolved organic carbon (DOC) primarily interferes with coagulation by competing with the coagulant for reaction sites on the surface of particles. Coagulation is the process of destabilizing small, suspended particles in water so they can clump together and be removed. Coagulants, such as alum (aluminum sulfate) or ferric chloride (iron chloride), are added to the water to neutralize the negative charges of these particles, allowing them to collide and form larger flocs. DOC consists of organic molecules that also carry negative charges. Because of their charge and abundance, DOC molecules can react with the coagulant, consuming it before it can effectively neutralize the charges on the target suspended particles. This is often referred to as coagulant demand. The DOC essentially scavenges the coagulant, forming complexes that hinder the formation of strong, settleable flocs. Consequently, a higher dose of coagulant is required to achieve the desired level of particle destabilization when DOC levels are high. If the coagulant dose is not adequately increased, the particles will remain dispersed, leading to poor settling and reduced water clarity. Some DOC can also coat the surface of the particles, physically blocking the coagulant from reaching the particle surface. Additionally, certain types of DOC can form stable complexes with metals like aluminum or iron, effectively rendering the coagulant ineffective. In some instances, DOC can react with chlorine during pre-disinfection, forming disinfection byproducts (DBPs), which is another reason controlling DOC is crucial for effective water treatment.