TABLE OF CONTENTS The previous chapter looked at some of the background theory to coagulation and flocculation; this chapter considers the chemicals used as coagulants and chemicals used to assist in coagulation.
Coagulation theory proposes four methods of destabilization of colloids. The theory suggests that coagulants should have properties to enable the following mechanisms of destabilization.
Double layer compression compounds forming trivalent cations (positively charged) will be very effective. In practice aluminium and ferric salts are used.
Charge neutralization compounds forming cations are required. Again aluminium and ferric salts are used.
Particle bridging simple theory suggests that long cationic molecules would be most appropriate. In practice both anionic and cationic polymers can often be used to destabilize negatively charged colloidal particles.
Enmeshment in a precipitate compounds forming a hydroxide floc or a carbonate precipitate may be used. Again aluminium and ferric salts are appropriate but other metal salts can be used (for example, calcium hydroxide or magnesium carbonate).
It is clear from the above that aluminium and ferric salts are suitable for three of the four methods of colloid destabilization identified. This has the great attraction that it is not necessary to know the precise nature of the colloids or the destabilization process, and that if the nature of the water changes it will normally be possible to deal with this by adjusting the coagulant dose and the coagulation pH value. This is not to say that all waters can be coagulated with a hefty dose of an aluminium or ferric salt;
