Drawing of a typical AquaDAF� High-rate Dissolved Air Flotation (DAF) System. Click here to enlarge imageThe AquaDAF system is a natural fit when plant owners must comply with drinking water regulations such as the 2002 amendment to the Surface Water Treatment Rule. Operating on source waters of up to 30 NTU the process can consistently produce clarified effluent of less than 1 NTU. Furthermore the system achieves excellent total organic carbon (TOC) reduction which can enable compliance with the Disinfectants/ Disinfection By-products Rule (D/DBPR). Another potential application for the system is treatment plants that have algae in the source water. High-rate DAF removes 85% to 95% of algae and responds well to seasonal influent variations associated with the algae loading.
The operating principle behind dissolved air flotation is simple, yet effective for clarification. After addition of coagulant the raw water passes through two stages of low-energy, low residence time flocculation which creates a pinpoint floc, ideal for floating. Following the flocculation stages are the injection and flotation zones. In the injection zone, small microbubbles of air are injected into the flow. The microbubbles are created by saturating recycled effluent with air in a very efficient, unpacked saturator operating at 60 to 90 psi. The saturated mixture is forced through AquaDAF's proprietary nozzles which cause a pressure drop and subsequent release of microbubbles.
The microbubbles and solid particles agglomerate and the particles are floated to the surface in the flotation zone. The float is periodically removed either hydraulically by flooding the basin or mechanically using a scraper system. Hydraulic desludging results in a net production for the system of 98% to 99% and yields a sludge of approximately 0.5%. Mechanical desludging results in 99.5+% net production and a sludge of 2% to 4%.
In 2003, Infilco Degremont's first high-rate potable water DAF system in the United States was commissioned in West Nyack, NY. There at the 20 mgd Lake DeForest Water Treatment Plant, a 1000-acre reservoir provides approximately one-third of Rockland County's drinking water.
The owners sought an upgrade of the plant's aeration and sedimentation basins to a higher-rate clarifier in order to improve overall water quality and taste, as well as filter run times, plant efficiency and overall production. Specifically, the owners were looking for efficient removal of solids, TOC, algae and color, with a consistent effluent turbidity of less than 1 NTU. In its first year of operation, the AquaDAF system is producing three times lower turbidity than the plant's previous conventional treatment system, while operating at a loading rate of 12 gpm/ft2.
Infilco's second DAF system will serve the residents of Central California. The 48 mgd South San Joaquin Irrigation District (SSJID) plant will incorporate DAF as a pretreatment to submerged membrane filtration and is scheduled for completion in the summer of 2005. Studies have shown that membrane systems benefit from higher flux rates, less fouling and longer membrane life when pretreatment is employed. Since high rate DAF does not need to use polymers, the AquaDAF is an ideal membrane pretreatment system and its use in this type of application is likely to grow.
In Seneca, SC, small particles such as those found in Seneca's source water, Lake Keowee, do not flocculate or settle well in conventional coagulation basins. After successful piloting, Seneca's John F. Dean WTP is upgrading from 12 mgd to 18 mgd without having to construct any additional basins. In fact, the additional capacity will be achievable by retrofitting just three of the existing six sedimentation basins with Infilco Degremont's high-rate DAF clarifier. In addition, because sludge will be periodically removed during operation of the DAF system, the need to drain and clean the basins biannually is eliminated. Piloting at the site has already confirmed a substantial increase in filter run times, net production and overall water quality.
