Coagulation

Coagulation is the chemical water treatment process used to remove solids from water by manipulating electrostatic charges of particles suspended in water. This process introduces small, highly charged molecules into the water to destabilise the charges on particles, colloids, or oily materials in suspension. Selecting the right coagulant for a system will enhance overall system performance. It also improves solids removal efficiency by enhancing filter and clarifier performance.

Many wastewater treatment applications require coagulation reactions, such as removing colloidal solids from water, demulsifying oil emulsions, and paint detackification. There are also many types of coagulants available to meet specific needs of a treatment process. In general, coagulation precedes flocculation in a chemical water treatment process.

Particles in water carry an electrostatic charge on their surface. Examples include clay, silica, iron, paints, and even oil. These small, suspended particles are stabilised in suspension & difficult to remove via mechanical methods.

A suspension of solids in water typically contains a variety of particle sizes. A lab analysis of “particle size distribution” will help define the size of particles and the relative amount of each size particle in the suspension.

Particles over 100μm are generally considered “settleable solids” and readily settle out of suspension. Medium-sized particles sized 10-100μm are generally considered “turbidity” and are often addressed in a wastewater treatment system with coagulation. Particles smaller than 10μm are “colloidal particles”, which are almost always treated with coagulation. Removing small particles is expensive, using only mechanical water treatment like filtration.

In most chemical water treatment processes, coagulation should occur upstream of flocculation. With coagulation, destabilised particles collide and create small masses, often called “pin flocs” or “micro flocs”. They are called that because they are barely visible to the naked eye at around 50 μm in size.

Flocculation is the process of clumping particles together to build larger agglomerates.  This process introduces a large molecule with charged binding sites to attract oppositely-charged particles or micro flocs. The flocculation reaction is highly visible, as the resulting “flocs” readily separate from the water.