Coagulation is a foundational step in the purification of drinking water, serving as a preparatory phase that makes subsequent treatment stages possible. This initial process addresses the microscopic materials present in raw source water, which are too small to be removed effectively by simple physical methods like filtration alone. Without this preliminary step, the fine particles and impurities would pass through the system, leaving the water cloudy and difficult to disinfect. Therefore, coagulation is the chemical trigger that transforms many non-settleable contaminants into a form that can be physically separated from the water supply.
Understanding Raw Water Impurities
Raw water from sources like rivers and lakes contains vast quantities of suspended solids and colloidal particles, typically between 1 nanometer and 1 micrometer in size. These microscopic contaminants include fine clays, silts, organic matter, and microorganisms, and they are the primary source of water turbidity. Turbidity refers to the cloudiness or haziness of the water caused by these particles scattering light.
The reason these particles remain suspended indefinitely is due to their surface chemistry. Most fine particles naturally acquire a negative electrical surface charge when dispersed in water. This charge creates a repulsive force between the particles, preventing them from colliding and sticking together.
This state of electrostatic repulsion is quantified by the zeta potential. Because the negative charges repel each other, the particles stay dispersed in a stable suspension, defying the force of gravity and making physical removal impractical without chemical intervention.
How Chemical Addition Destabilizes Particles
Coagulation is the immediate process of chemically destabilizing suspended particles to overcome their natural repulsion. This is achieved by adding specific chemicals, known as coagulants, which carry a strong positive charge into the rapidly mixed water. The most common coagulants are positively charged metal salts, such as aluminum sulfate (alum) and ferric chloride.
When introduced, the positively charged ions from the coagulants quickly neutralize the negative surface charges of the microscopic impurities. This charge neutralization eliminates the repulsive forces between particles, causing the suspension to become unstable. Once destabilized, the particles collide and stick together to form tiny, sticky clusters known as microflocs.
The process may also involve “sweep flocculation,” especially at higher coagulant doses. In this scenario, the added metal salts react with the water’s alkalinity to form a voluminous, gelatinous precipitate, such as aluminum hydroxide. This sticky precipitate acts like a net, physically sweeping through the water to entrap and adsorb the destabilized particles as it forms.
Flocculation and Sedimentation
Following the rapid chemical destabilization of coagulation, the process transitions into the physical phase of flocculation. Flocculation involves a period of gentle mixing that encourages the newly formed microflocs to aggregate further. The water is moved through a basin where slow, deliberate agitation promotes collisions between these microflocs.
These repeated collisions cause the small, unstable particles to bind together, growing into much larger, visible masses called macroflocs, or simply “floc.” Coagulant aids, often long-chain polymers, may be added during this stage to help bridge the particles and strengthen the floc structure. The goal is to create dense, robust clusters large enough to be easily separated.
Once the floc has reached its optimal size and density, the water enters the sedimentation phase in a large basin or clarifier. Here, the water is held relatively still, and the force of gravity takes over. The dense, heavy flocs settle out of the water column and collect at the bottom of the tank as sludge, allowing clarified water to flow out from the top for further treatment.
Specific Contaminants Targeted
The practical result of the coagulation-flocculation-sedimentation sequence is the effective removal of various contaminants that impact water quality. This process significantly reduces turbidity and removes the particles responsible for unwanted color and odor.
Coagulation is particularly effective at removing Natural Organic Matter (NOM), a complex mixture of organic compounds found in source water. The removal of NOM is a high priority because it is a precursor to the formation of harmful disinfection by-products (DBPs) later in the treatment process. When chlorine is used for disinfection, it can react with residual NOM to create potentially carcinogenic compounds like trihalomethanes.
Finally, while disinfection handles pathogens directly, coagulation helps remove viruses and bacteria physically attached to suspended solid particles. By removing these host particles, the overall load on the final disinfection stage is reduced. This makes the entire purification process more efficient and ensures the production of safer drinking water.