John E. Tobiason
The West River Treatment Plant (WRTP) is a 10.2 million gallon per day inline direct filtration plant. Pilot scale studies have been performed at the WRTP since 1989 to evaluate treatment alternatives to meet changing regulations and to explore a possible plant expansion. Dissolved Air Flotation (DAF) was evaluated during 1992 in a pilot train consisting of ozonation (either pre- or intermediate-), two stage flocculation, flotation, and filtration. The DAF train was studied as a method of plant expansion whereby solids removal by DAF would allow for increased hydraulic loading rates to the filters. The effects of several DAF train process parameters on flotation and filtration performance were evaluated. DAF performance was compared in parallel to a direct filtration train consisting of pre-ozonation, rapid mix and filtration. Dissolved air flotation followed by filtration provided effective overall treatment with respect to finished water quality and filter performance. Finished water quality for the DAF train was similar to the parallel direct filtration train as measured by turbidity, ultraviolet absorbance at 254 mn, dissolved organic carbon, and particle counts. Solids removal by DAF resulted in filter run times three times longer than for the direct filtration train at the same filter hydraulic loading rate. DAF provided effective treatment with reasonable headloss development at increased filter hydraulic loading rates. Results are presented from studies evaluating variations in several DAF train process parameters. DAF provided effective treatment using both ferric chloride with a cationic polymer and ferric chloride alone as primary coagulants. DAF performed well at recycle ratios of 9.5 and 8.0 percent but clogging of the needle valves at 6.0 and 5.0 percent recycle resulted in less effective particulate removal. Intermittent operation of a full length mechanical scraper for sludge removal was effective at frequencies of one time per hour to one time per eight hours. Flocculation times of both 16 and 8 minutes resulted in effective flotation performance. Pre-ozonation resulted in effective treatment throughout 1992. However, intermediate ozonation (after flotation) on the DAF train produced mixed results with oxidation and subsequent precipitation of manganese producing high filtered water turbidity.