Using polymers with direct filtration

For several years, studies have been carried out by the East BayMunicipal Utility District, Calif., to evaluate the suitability of cationic polyelectrolytes as primary coagulants for use in direct filtration (inline) water treatment plants. After a brief description of the water system, some results of the studies on long-term use of polymer and clay/polymer coagulation are presented to confirm predictions based on short-term tests, to compare the effectiveness of polymer, clay/polymer, and alum coagulation, and to study the potential for formation of chlorinated organic compounds when polymers were used. Addition of clay was necessary when the turbidity of the raw water was less than 3.5 n.t.u. Clay /polymer coagulation gave higher filtration rates and longer filter runs than alum coagulation, reduced the amount of backwash water required, and reduced the amount of sludge produced. The cost was not significantly different from that for alum coagulation. It was estimated that cationic polymers had the potential for producing up to 13 ug trihalomethanes per litre.