DAF system achieves highly efficient solids separation

An innovative DAF (dissolved air flotation) system for treating or refining wastewater, engineered in Europe to EU standards, has been introduced to Australia by CST Wastewater Solutions.

Applications for the DAF system include dairy and cheese factories, the paper industry, food industry, oil industry, wineries, tanneries, textile industry, large laundries, chemical industry and refineries, and for sludge thickening, and primary and secondary clarification in municipal and industrial biological wastewater plants. Each DAF system incorporates a CST-supplied SAT air dissolving reactor, which is the key to the dissolved air flotation process. The SAT generates the microbubbles that separate and thicken the suspended solids. Its double-injector air dispersal method results in high dissolving air efficiency up to 90%.

The first stage of air dissolving, up to a theoretical maximum saturation of 50%, occurs in the high-turbulence, high-intensity mixing zone of the central tube inside the vessel.

The second stage occurs in the larger volume of this vessel, where longer air-liquid contact provides a large exchange with very fine bubbles and a high exchange surface. The added 30-40% dissolution takes places during this second stage.

“Our SAT is also capable of improving the performance of existing flotation systems that employ low-efficiency microbubble generators,” said Michael Bambridge, Managing Director of CST Wastewater Solutions.

“The unique part of the flotation unit is its multifunctional central column, through which the raw wastewater and saturated recycle water is fed and uniformly distributed into the flotation cell; the central column also collects the clarified water and thickened sludge,” he said.

Raw wastewater, mixed with air saturated recycled water, is introduced into the main body of the flotation tank through a series of openings located around the circumference of the lower part of the central column. Solids are rapidly carried to the surface by the microbubbles and are evenly distributed as sludge on the surface of the flotation tank in a uniform hydrodynamic condition.

The sludge float is then removed by a rotating adjustable scroll scoop or by a surface skimmer, without disturbing the sludge blanket.

The DAF system’s designer, Aqua Engineering, has built more than 400 wastewater treatment units over a 30-year period. Apart from the high saturation efficiency, key characteristics of the DAF system include complete dissolution of injected air, automatic control of the injected air, and automatic control of the sludge blanket.

Access and maintenance are facilitated by using a low-head circular tank, a bottom sediment removal system, a continuous cleaning system for the inside wall, and an inspection window for visual control of the flotation process. There is no requirement for purging of excess air (and therefore no purge valve).

DAF models available from CST Wastewater Solutions range in size from 1.48 to 15.4 m in diameter and 1 to 2.58 m in height.

The SAT air dissolving reactor operates automatically, requires no maintenance and has a unique non-clogging design. It is manufactured entirely from AISI 316 stainless steel and is tested and certified to gas pressure vessel codes.

SAT models available range in diameter from 0.114 to 1.6 m and from 1.48 to 3.84 m in height.