Coagulation

 

Coagulants

Coagulants are defined as the substances which are capable of removing colloidal impurities from water. With this definition in mind, it is well to have some understanding of the nature of these colloidal impurities.

Colloids

Classification and Properties - The colloidal phase refers to any form of matter in a certain state of sub-division or, better, within certain limits of particle size. The unit of measurement to be used is the millimicron, or mu. This is one millionth (0.000001) of a millimeter, which is approximately one twenty-five millionth of an inch. While no definite limits may be set within which various phases of matter may be gouped, it is possible to state that the molecules and ions of true solutions usually vary in diameter from 0.1 to 1 mu. The particles in the colloidal phase will usually fall between 1 mu and 100 mu above the higher figure they merge into the suspensions and emulsions which may be separated more or less readily by physical means. A solid of particle size between 1 and 100 mu when dispersed in a liquid is called a suspensoid, and a liquid the droplets of which vary in diameter from 1 to 100 mu when dispersed in another liquid is called emulsoid. Both are colloids. In most respects colloids resemble true solutions; in ordinary lighting they are clear and transparent, they will not settle, and they may be filtered without removal of the colloidal material. Under proper lighting conditions, however, a bright beam of light passed through a colloidal solution produces what is called a "Tyndall Cone," which results from the reflection of part of the transmitted light by the tiny particles. Under the ultra microscope colloidal particles are found to be in constant motion. This motion, termed the "Brownian Movement," results from the bombardment of the tiny colloidal particles of the colloid by the rapidly moving molecules of the solvent. The boiling point, freezing point, osmotic pressure and vapor pressure of colloidal solutions are essentially identical with those of the pure solvent. Colloids may be classified in a number of ways. In the first place, they may be classified as sols or gels. Sols are, of course, colloids which have the appearance of true solutions; whereas gels have a jelly-like structure from which the name colloid was derived. Colloids may be also classified as reversible and non-reversible. Ordinary starch solution is an excellent example of the former. If a little starch is added to boiling water and the mixture is boiled for a few minutes, a clear, faintly bluish solution results. When allowed to cool it sets to what is termed "starch paste." However, if reheated it will again assume the appearance of a solution. Colloids may be classified as lyophobic and lyophyllic. This classification has to do with the affinity of the colloidal particles for the dispersing medium. If the attraction between the colloidal particles and the dispersing medium is small, the colloid is termed lyophobic. If, on the other hand, the attraction between the colloidal particles and the dispersing medium is great the colloid is termed lyophyllic. Finally, the most important of all, colloids may be classified as positive or negative since it may readily be shown that they bear electrical charges.