COAGULATION IN DRINKING-WATER TREATMENT - PARTICLES, ORGANICS AND COAGULANTS

A review of coagulation in drinking water treatment is presented. The paper emphasizes the importance of raw water chemistry, natural organi c matter (NOM) concentration and type, and the chemistry of coagulants . Mineral and organic particles may be stable in water due to electros tatic charge interactions, hydrophilic effects, or to steric interacti ons from adsorbed macromolecules. NOM rather than particles initially in water supplies can control coagulant dosages and selection. NOM con sists of a mixture of various organic compounds including hydrophobic (humic and fulvic acids) and hydrophilic fractions. The negative charg e and chemical structure of the hydrophobic acids affect chemical reac tions with coagulants, particularly metal based coagulants. The remova l of NOM with Al coagulants can involve hydrolysis, complexation, prec ipitation, and adsorption reactions. Specific ultraviolet absorbance ( SUVA) can be used to estimate whether the NOM of a water is high or lo w in hydrophobic acids and to estimate removals of DOC by coagulation. Preozonation of water supplies containing algae may lead to microfloc culation or impair coagulation depending on algae type, concentration and molecular weight of extracellular organic matter (EOM), and ozone dose.