Dissolved Air Flotation (DAF) - Where Is Used In The Process

How does a dissolved air flotation system work? Dissolved Air Flotation (DAF) systems are designed to remove suspended solids, biochemical oxygen, and oils and greases from wastewater streams. Contaminants are removed through the use of a dissolved air-in-water solution produced by injecting air under pressure into a recycle stream of clarified DAF effluent.

This recycle stream is then combined and mixed with incoming wastewater in an internal contact chamber where the dissolved air comes out of solution in the form of micron-sized bubbles that attach to the contaminants. The bubbles and contaminants rise to the surface and form a floating bed of material that is removed by a surface skimmer into an internal hopper for further handling.

Summary of the DAF process:

In the feed stream, after saturation with dissolved air, the flow spreads uniformly across the tank.
The air bubbles adhere to the suspended particles creating a floating action on these particles. These solids are removed using a scum removal system.
Heavy solids which might have settled are conveyed into a sump by the bottom scraper.

Dissolved Air Flotation Process Steps

pH Correction and Flocculation The feed pump takes flooded suction from the raw water tank and pH (if required) of the raw water is adjusted inline, based on pH transmitter reading, so as to optimize conditions for downstream removal of contaminants in the flocculation and clarification stages. Typically ideal pH is in the slight alkali range (8-9) but the final set point is determined during commissioning. Once the pH correction process is completed, two flocculation chemicals, a coagulant and flocculant are injected into the wastewater in preparation for the clarification process. The flocculant retention tube is designed to allow sufficient mixing time before the water is discharged into the DAF. All chemical storage tanks are fitted with a low-level switch for auto-shutdown & to alert the operator of a low-level condition; the tank levels should be checked regularly and topped up as required

DAF Clarification The dissolved air flotation (DAF) clarification stage separates the flocculated particles (suspended solids) from the water. A pressurized stream of air-saturated water is mixed with the flocculated water and, with the release of pressure, a cloud of very fine air bubbles carries the contaminates to the tank surface. Clean water overflows from the DAF tank, via an under-weir, to the sewer or clarified water tank for the next treatment step. The overflow level is adjustable and controls the level in the DAF flotation section. Where practical (e.g. where additional filtration is included) treated water can be used for saturator feed water, otherwise, potable water is used.

Sludge Removal A low-speed scraper, mounted on top of the DAF, continually moves this surface sludge layer over a ‘beach’ plate and into a skip prior to removal and off-site disposal. If a sludge pump is installed this can transfer the sludge to a holding tank for liquid waste disposal. In this process, clarification and thickening happen at the same time. Separation of solids and FOG from the aqueous liquid stream occurs to produce a clarified effluent with low suspended solids & FOG levels.

Drinking water supplies that are particularly vulnerable to unicellular algal blooms and supplies with low turbidity and high colour often employ DAF. After coagulation and flocculation processes, water flows to DAF tanks where air diffusers on the tank bottom create fine bubbles that attach to floc resulting in a floating mass of concentrated floc. The floating floc blanket is removed from the surface and clarified water is withdrawn from the bottom of the DAF tank.