Dissolved Air Flotation Thickeners
Waste primary and secondary sludges are combined into a homogeneous mixture and thickened in three 45-foot diameter dissolved air flotation tanks. The DAFT tanks abut against an integral equipment gallery which houses mechanical support systems. Mixed sludge is distributed to the DAFTs through a system of overflow weirs. As the sludge enters the DAFT center column it contacts air bubbles less than 100 µm in diameter released from a pressurized recycled effluent stream. The resulting air/solids aggregate floats to the surface of the tank. Concentrated float is skimmed from the DAFT water surface and pumped to the anaerobic digestion process. The DAFT system removes about 90 percent of the water in the waste sludge, increasing the float concentration to approximately 5 percent solids.
The thickener liquid effluent (subnatant) flows out of the tank via a submerged orifice collection system. This feature, coupled with a level controlled modulating valve, allows the liquid level within the thickener to be adjusted. By varying the liquid level, the float depth and concentration can be optimized before skimming into the collection troughs. a portion of the subnatant is recycled through the pressurization system.
Using the recycle pressurization method, a portion of the subnatant is mixed with air under pressure in the saturation vessels. The pressurization system drives the air into solution in the recycle flow. As the air saturated recycle is mixed with the unthickened sludge inside the DAFT tank center column, a decrease in pressure releases fine bubbles into the sludge flow. The bubbles contact and adhere to the solid particles creating a buoyant solid-gas complex which floats to the liquid surface. The float particles accumulate and concentrate until they are removed from the DAFT tank by surface skimmers.
The DAFT tanks are covered to contuain odorous foul air emissons. The exhaust air is dicharged to the NTF process for biological foul air treatment prior to release to the atmosphere.