PRETREATMENT CONSIDERATIONS FOR DISSOLVED AIR FLOTATION - WATER TYPE,COAGULANTS AND FLOCCULATION

Pretreatment considerations of coagulant selection, flocculation time and flocculation mixing intensity were studied for two different water supply types, aquatic humic and non-aquatic humic waters of low turbi dity. Alum, ferric salts, and various polyaluminium chlorides (PACs) w ith different chemical properties were all effective in dissolved air flotation (DAF) when used under favorable conditions of dosage, pH, an d flocculation time. A stoichiometry was found for the aluminium coagu lants between coagulant dose and raw water DOC. Si and sulfate PACs we re effective in treating cold waters with short flocculation times of 2.5 to 5 minutes. Flotation performance improved slightly with increas ing flocculation mixing intensity. The results suggest that flocculati on tanks be designed to produce strong, ''pinpoint'' size flee particl es.