DISSOLVED AIR FLOTATION (DAF) A DAF pilot study was conducted by EE&T personnel in the winter and spring of 1995 in Port Hawksbury, NS. The City was faced with increasing water demands as a result of distribution system expansions and were interested in adding a high-rate process train that would replace the existing conventional plant for day-to-day operation. One of the City's requests was that the conventional treatment train be left intact to serve as a back-up. The pilot DAF unit selected for this investigation had flow rate ranges from 36 to 54 gpm and hydraulic loading rates of 4 to 6 gpm/ft2. The pilot was equipped with two dual media pressure filters containing different filter media combinations. Flocculation times used ranged from 8 to 20 min. Results showed that excellent treatment of this low turbidity, low alkalinity reservoir water could be achieved, even at water temperatures as low as 3 C and with flocculation times as low as 8 min. Low filtered water turbidities, long filter run times, and very good removals of organics and disinfection byproduct formation potentials were achieved with both alum and polyaluminum chloride (PACl), although performance in cold water was superior with PACl. Based on the results of pilot testing, a 1.5 mgd DAF plant was designed and constructed. The existing treatment plant facility was expanded to enclose the new DAF process train. The clearwell capacity was expanded during construction and new backwash equipment and air-water underdrains were installed. Existing chemical feed equipment, storage areas, high and low lift pumping facilities, and personnel areas were used. The plant was designed for hydraulic loading rates of up to 6 gpm/ft2, with flocculation times of about 15 min.