Water Treatment > Quality Water Solutions > Operational Issues > Coagulation Review

Coagulation in potable water treatment is a primary barrier to waterborne diseases and an essential way to ensure quality. It has one basic purpose: to remove solid contaminants ranging from visible particles to colloidal suspensions. In water, these solids exist in a cloud of negative ions, so the particles repel each other and remain suspended. Coagulants supply positive charges that neutralize the negative ones. This destabilizes the suspension so particulates agglomerate into entities large enough to be settled and filtered.

Aluminum chemicals and iron salts are the most common coagulants. Most treatment is done with aluminum sulfate (alum). In water, the aluminum content dissociates into ions that carry three positive charges (Al +3). Iron iron salts generate either two or three positive charges as ferrous (Fe+2) or ferric (Fe+3) ions.

The greater a coagulant's positive charge, the greater its efficiency. Enhanced coagulants like polyaluminum hydroxychloride (PACl) contain polyvalent species with many more than three positive charges, so they can be applied in smaller doses than alum or iron salts. For example, dosage for a typical high-basicity PACl can be less than one-fifth that of iron or alum.

Synthetic organic polylectrolytes and natural starch- or gum-based materials are often used as coagulation aids because they have high charge densities and yield a large, weighty floc. They may be less effective than alum in treating colloidal color and other difficult-to-remove materials.

Coagulant programs aim to use as much of the coagulant as possible to remove contaminants. But optimization is a moving target. Raw water and plant factors change, so treatment facilities control coagulation through laboratory evaluation, plant procedures and on-line instruments. Dosages to deal with water quality excursions can be derived from historical trends, routine turbidity tests supplemented by color, UV absorbence and odor tests. Fine tuning can be done by analyzing variations in chlorine and oxidant demand, TOC and NOM.

Potable water coagulation programs should be founded on the use of the highest quality coagulants by operators who know how to dose them. This can better be assured if product strength and quality are known. At a minimum, plants should require NSF/UL certification to NSF Standard 60/61 and adherence to AWWA standards.