Dissolved Air Flotation (DAF) is the use of micro-bubble flotation as a solid/liquid or liquid/liquid (or both) separation method that is widely used in industrial and domestic wastewater treatment processes. Dissolved Air Flotation, is a well-established separation process that uses micro-bubbles as a carrier phase to remove solids, ions, macromolecules, fibers and other materials, BOD, COD reduction and for sludge thickening.
Air Flotation offers process advantages over such processes as filtration, precipitation, or adsorption using natural and synthetic sorbents and medias. The advantages include better treated-water quality, rapid startup, high rate operation, and a thicker sludge, shorter sludge residence time in a secondary clarifier stage and flexibility in changing the operation parameters as the result of active sludge separation.
DAF bubbles are formed by a reduction of pressure of a super air saturated water stream at pressures usually higher than 3 atm. In industrial practice, the supersaturated water is forced through precise needle valves for air control, and clouds of 0.02–0.10 mm bubbles in diameter are subsequently produced and mixed with the incoming waste stream. Once the saturated stream pressure is reduced and the two streams mixed the bubbles attach to the materials being separated and then floats to the water surface. Modern flotation devices and schemes exhibit both high throughput and efficiency.
Products removed: solids, oils, fuels (vehicle/jet), fuel oils, hydraulic fluids, immiscible machining oils, lube oil, transmission fluid, vegetable based oils, animal/vegetable by-products & many other products.
We offer many sizes in our Dinky DAF products from 5 to 50 square feet and in our large DAF line from 60 up to 600 square feet and many types of chemical pretreatment systems to float many types of solids.
Dissolved Air Flotation (DAF) Theory
Is the process of removing suspended solids, oils and other contaminants via the use of air bubble flotation. Air is dissolved into water, mixed with the waste stream and released from solution while in intimate contact with the contaminants. Air bubbles form, attach to the solids, increase their buoyancy and float the solids to the water’s surface. A percentage of the clean effluent is recycled and super-saturated with air, mixed with the wastewater influent and injected into the DAF separation chamber.
The dissolved air comes out of solution, producing millions of microscopic bubbles. These bubbles attach to the solids and float them to the surface where they are mechanically skimmed and removed from the tank. The Pan America Environmental dissolved air flotation systems are designed to remove fats, oils & grease (FOG), suspended solids, food/animal production/processing wastes, industrial wastes, hydrocarbon oils/emulsions and many other contaminants. Clarification rates as high as 97% or more can be achieved using our dissolved air flotation systems. Chemical pretreatment can often help to improve the performance of contaminant removal.
Conventional DAF saturation design uses a recycle pump combined with a saturation vessel and air compressor to dissolve air into the water. Air transfer efficiency is approximately 12% @ 90% entrainment.
Pan America’s DAF design incorporates today’s “state of the art” technology in DAF design. This design simplifies the DAF process, requires less startup time, less capital cost, instrumentation, labor and maintenance. The design is process friendly, providing virtually instant saturation upon
system startup without equalization and complex startup procedures. Once the system is adjusted the system can be shutdown and started up again without any readjustment or equalization. Higher air transfer efficiencies are also realized due to higher saturation pressures with 12% @ 93% entrainment. Pan America can provide both styles of DAF design depending on application and customer preferences.
DAF sizing takes into consideration many criteria for sizing: Flow rate, Water temperature, Waste characteristics, Chemical pre-treatment, Solids loading (LBS/HR/Ft2) , Hydraulic loading (GPM/FT2), Air to solids ratio (LBS of air/LBS of Solids).
DAFs are designed on the basis of the peak flow rate expected. The flow can range from 1 to 5 gallons per minute per square foot of surface area (GPM/Ft2). Bench testing of waste stream samples is usually the preferred starting point when sizing equipment and determining proper chemical processes prior to the DAF. The chemical pretreatment will assist and improve the DAF separation process.
Chemical Pretreatment often improves DAF solids removal efficiencies. The use of chemical flocculants with DAF is based on system efficiency, application (use of DAF) and cost. Commonly used chemicals include trivalent metallic salts of iron, such as FeCI2 or FeSO4 or aluminum, such as AISO4. Organic and inorganic polymers (cationic or anionic) are often used to enhance the DAF process.
The most commonly used inorganic polymers are the polyacrylamides. Chemical flocculant concentrations used normally range from 100 to 500 mg/l. (One mg/l in 1 million gallons per day is 8.34 lbs of material.) The wastewater pH may need to be adjusted between 4.5 and 5.5 for the ferric compounds or between 5.5 and 6.5 for the aluminum compounds using an acid such as H2SO4 or a base such as NaOH. In many applications, the DAF effluent requires pH adjustment utilizing a base such as NaOH to assure the DAF effluent pH is within the limits specified by the POTW (6-9 typically).
Attachment of most of the bubbles to solid particles can be effected through surface energies while others are trapped by the solids or by hydrous oxide flocs as the floc spreads out in the water column. Colloidal solids are normally too small to allow formation of sufficient air-particle bonding. They must first be coagulated by a chemical such as the aluminum or iron compounds mentioned above and then absorbed by the hydrous metal oxide floc generated by these compounds. Frequently, a coagulant aid is required in combination with the flocculant to agglomerate the hydrous oxide flocc, increase particle size and improve the rate of flotation. Mechanical/chemical emulsions can also be broken through pH and polymer reactions.
Where the float is to be used to feed animals used for human consumption, organic compounds such as chitosan, carrageenan, and lignosulfonic acid, or their derivatives can be used. Use only compounds approved by the Food and Drug Administration (FDA) Office of Veterinary Medicine.
Float Dewatering DAF float often contains 2 to l0 percent solids. The solids may need to be dewatered before disposal to reduce the sludge volume by reducing water content. Float dewatering is usually performed by using one of the following technologies: Filter press, Belt filter press, Centrifuge, Drying bed or Vacuum precoat filter.
Actual flow rates vary based on waste characteristics and chemical pretreatment.
– Frac water
– Produced water
– Biodiesel plants
– Algae removal
– Petrochemical plants
– Ice cream production
– Cheese production
– Drinking water pretreatment
– Sewage treatment plants
– Slaughter houses
– Rendering plants
– Vegetable processing plants
– Poultry processing
– Pet food production
– Seafood production
– Pulp & paper mills
– Aircraft maintenance
– Ballast water
– Bilge water
– Latex removal
– Prepared foods
– Tank & truck wash
The Pan America Environmental DAF Series systems are designed to remove fats, oils & greases (FOG), petroleum oils and suspended solids. Clarification rates as high as 97% can be achieved using the DAF systems.
A percentage of the clean effluent is recycled and super-saturated with air, mixed with the wastewater influent and injected into the DAF separation chamber. The dissolved air comes out of solution, producing millions of microscopic bubbles. These bubbles attach to solids and float them to the surface where they are skimmed and removed from the tank.