What is a DAF for Oily Water Treatment?
The Oil Water and Produced Water Dissolved air flotation (DAF) is a water treatment process that clarifies wastewater by removing oils and suspended solids simultaneously. The removal is achieved by introducing ambient air and dissolving it in the recycled wastewater stream by shear pumps and then releasing the air at atmospheric pressure in a flotation tank while creating nano and micro bubbles.
The released air forms nano and micro bubbles which adhere to the oil and suspended matter causing the it to float to the surface of the water where it is then removed by a chain skimming device. Chemicals can be added to the feed water to improve oil and solids removal. Heavy solids are collected in the sludge hoppers and removed frequently by pumps or manual draining.
The unit contains two stages for oil removal.
Stage one and contains parallel plates lamella packing that multiply the surface area for the settling of solids and floating of oils. It works by minimizing the height required for the solids to drop and oils to raise and attach to other drops of oils.
The floating of oil is assisted by micro bubbles rising through the water.
The water then passes to the second oil removal stage through parallel perforated collector pipes. The collector pipe assists in balancing water flow through the lamella plates.
The collected water then passes through two stages of coalescing packing that helps capture escaped oil droplets and make it collide with each other and grow in size so it can be collected to the surface and skimmed by second stage chain skimmer.
The treated water then passes through baffle and collected by weirs. The weir help maintaining stable water level inside the DAF.
The DAF has the following advantages:
Where and Why are DAFs used?
DAFs are widely used in treating industrial wastewater effluents from oil refineries, chemical plants and paper mills to the food & beverage industries. DAFs are used to removed suspended solids, such as Total Suspended Solids (TSS), Fats, Oil & Grease (FOG) and other pollutants from wastewater so that the water is suitable for re-use or discharge to a wastewater treatment facility.
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Dissolved air flotation (DAF) is a proven and effective physical/chemical technology for treating a variety of industrial and municipal process and wastewater streams. DAF systems are commonly used for the removal of oils & greases and suspended solids to meet a variety of treatment goals including:
The Flotation Tank is where the water separates from pollutants. The water is flowing through the flotation tank at a slow speed, giving time for heavy objects to settle down and lighter objects to float up to the top with help from bubbles and flocculators. The flotation tank can vary in depth, width and length based on the application and the time needed for pollutants to be removed.
The Dewatering Grid is a rectangular framework of angular steel plates that lock sludge in place as it rises to the surface. Only when sludge has thickened enough to rise above the top edge of the grid can it be skimmed and pushed to the float hopper. The Dewatering Grid helps:
While many DAF system designs push sludge across the entire length of the tank in the same direction as the wastewater flow called a Co-Current Skimmer, another design uses a skimmer assembly that rotates against the hydraulic flow of the water called a Counter-Current Skimmer. The Counter-Current Skimmer design shortens the sludge skimming distance and eliminates solids carry-over.
Flocculators are designed to provide the mixing action and retention time required to adequately coagulate and flocculate solids in wastewater to improve solids removal.
DAF pumps are a key component of all DAF Systems. On it rides some of the largest capital, operations and maintenance expenses involved in wastewater pre-treatment systems.
DAF manufacturer’s approach recycle pumps in two different manners.
The first way is to provide a specialty white water pump. This pump not only pumps the water but also dissolves the air into the water. These types of pumps are often more difficult to find and are more expensive. Also, with putting air in the pump, there is always a risk of cavitation, which causes internal damage and results in more-frequent-than-desired parts replacement.
The second way is a more efficient and cost effective approach. It is to combine a standard ANSI pump with an angled air dissolving tube. In this second case, the pump doesn’t do any air dissolving – it just pumps water. No air in the pump, means very little risk for cavitation. In this way, standard ANSI recycle pumps don’t do any air dissolving – it just pumps water. That means we can provide higher solids tolerances, use stronger pump materials, operate at lower pressures, and do it at a cost much lower than possible with a specialty whitewater pump.
Where many DAF system manufacturers use a mechanical means to dissolve air into water, i.e. a specialty “whitewater pump,” there is a more efficient and cost-effective approach.
The air dissolving tube is where whitewater is generated. This short expansion in the recirculation piping allows clarified effluent and a small volume of compressed air to mix until saturation is achieved. The angled configuration allows for increased water and air interface so saturation occurs almost instantly.
This design works so well that often a customer will change from their specialty whitewater pump to an ANSI pump and angled air dissolving tube.
United International Hydro Engineering