What is a DAF?
Dissolved air flotation (DAF) is a water treatment process that clarifies wastewater by removing suspended solids. The removal is achieved by dissolving air in the water or wastewater under pressure and then releasing the air at atmospheric pressure in a flotation tank. The released air forms tiny bubbles which adhere to the suspended matter causing the suspended matter to float to the surface of the water where it is then removed by a skimming device. Chemicals can be added to the feed water to improve solids removal.
Dissolved Air Flotation (DAF) systems are designed to remove suspended solids (TSS), biochemical oxygen demand (BOD5), and oils and greases (O&G) from a wastewater stream. 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.
Dissolved air flotation (DAF) has proven to be very effective for the removal of biological solids from wastewater streams. Some of the advantages over traditional, gravity clarification include:
Some of the more common biological applications include:
One of the most common DAF applications is for the pretreatment of wastewater to remove suspended solids and oils and greases prior to discharge to a municipal sewer or a biological treatment system. For example:
In most cases, the wastewater contaminants must be chemically flocculated in order for the floatation system to remove them. Pretreatment DAF is used in about every industry class.
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.
How does a DAF work?
Wastewater is fed into a DAF system and hit with a stream of “whitewater”, which is recirculated clarified water from the DAF that’s super saturated with dissolved air. As these two mixtures blend together, microscopic bubbles attach to solid particulates, giving them enough buoyancy to surface in the DAF tank. As solids accumulate in a floating layer on the top of the DAF tank, a skimmer gently nudges the sludge toward a discharge hopper.
Any solids that don’t float will sink to the “V” bottom of the DAF tank. Settled solids are concentrated and discharged by an automatically controlled pneumatic drain valve. The clarified water flows out via an under-over weir on either side of the DAF unit. Some of this water is used in the recirculation loop while the rest flows out of the vessel.
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.
UIHE has been developing and continually improving dissolved air flotation (DAF) systems for decades, and has successfully implemented installations in a variety of applications.
DAF is a wastewater clarification process for the separation of solids, grease, and oils. The system helps concentrate sludge, removes a wide range of suspended solids (fats, oil, and grease) in wastewater and reduces COD and BOD. Our dissolved air flotation solutions provide high-quality, high-capacity, and efficient clarification.
DAF works by producing a stream of microfine bubbles that attach to solids and lift them to the surface, where they can be removed by a surface scraping mechanism.
DAF systems can be applied as a pretreatment to meet a municipality’s specific limits for oil, grease, or suspended solids, or to remove contaminants that would negatively impact an industrial biological wastewater treatment system downstream.
DAF technology is ideally suited to neutral-density particles and those that are buoyant or settle slowly. This is particularly relevant to the food industry and other businesses that produce light, high-volume solids that are resistant to settling. DAF also takes up about 25% of the equivalent surface area compared to conventional clarification technologies.
DAF is a proven, robust, and effective physical-chemical technology commonly used for many industrial and municipal applications, including:
DAF is used extensively in the following industries:
The flotation process is based on coagulation and flocculation mechanisms. By adding specific chemicals, such as iron chloride and aluminum chloride, colloidal particles are destabilized and floc formation takes place. Flocculation with other chemicals, such as polyelectrolyte, facilitates the collision between the destabilized suspended and colloidal particles to form larger clots that can be readily removed.
The process separates solid particles from the liquid. At first water is saturated with dissolved air under pressure. This recycle stream (pressurized from 4 to 6 bar), called the hypersaturated stream, is then mixed with the incoming wastewater in an internal contact chamber, where the microscopic air bubbles attach to solids and float them to the surface, forming a floating sludge bed.
A scraping assembly skims the sludge from the surface of the water and into a sump, from where it’s pumped to sludge treatment. The treated water is then discharged or undergoes other treatment processes.
At Extol Hydro, our experienced process designers and field personnel offer support during all stages of the project, from preliminary design through commissioning and operation. Our highly reliable solutions are designed and customized to handle a wide range of influent quality and flow rate. Our DAF solutions are robust and compact. They are easy to install, operate and maintain.
Contact our experts to learn more about dissolved air flotation, or to discuss your specific project details.
United International Hydro Engineering