Environmental Water-Treatment Knowledge: What Is Cavitation Air Flotation (CAF) and How Does It Work
The cavitation air flotation (CAF) system is a patented water-treatment device that is unique in the world, and an export-recommended technology of the U.S. Department of Commerce and the Environmental Protection Agency. CAF is designed specifically to remove grease, colloids and suspended solids (SS) from industrial and municipal wastewater. The whole flotation system consists of five parts, as shown in the figure:
Pre-treated wastewater flows into a small aeration section equipped with a cavitation aerator. As the water rises, it mixes thoroughly with the microbubbles generated by the aerator; the aerator transfers the air above the water surface into the water through an air-intake duct. The aerator works by using the high-speed rotation of the bottom disperser impeller to form a vacuum zone in the water; the air above the water surface is fed into the water through the aerator to fill the vacuum, and microbubbles are thereby produced and rise spirally to the water surface, dissolving oxygen from the air into the water.
Because of the density imbalance between the gas-water mixture and the liquid, a vertically upward buoyancy force is generated, carrying SS to the surface. During flotation, microbubbles attach to the SS; once at the surface, the SS is supported and held there by these bubbles. The SS floating on the surface is intermittently removed by a chain sludge scraper. The scraper moves along the entire liquid surface and pushes the SS from the inlet end of the flotation tank into the sludge-discharge pipe at the outlet end. Inside the sludge-discharge pipe a horizontal screw conveyor sends the collected sludge into a sludge-collection pit. The purified wastewater flows into an overflow trough and then by gravity to the biological-treatment section.
4. The system can remove grease, colloids and fibrous suspended solids from wastewater, reducing BOD, COD, SS and other pollutant loads, while visibly improving water color.
Cavitation air flotation has no pressure vessel, air compressor, circulation pump or other equipment, so investment in equipment is low.
The equipment occupies a small area, reducing civil-construction investment. A CAF unit with Q=200 m3/h occupies only 36.15 m2.
Because the system has no pressure vessel, air compressor, circulation pump or other equipment, it saves power. The CAF system with Q=200 m3/h consumes only 5.435 kW, whereas a dissolved-air flotation (DAF) system consumes as much as 65 kW.
The CAF system is very easy to operate, with no complex equipment, so labor and maintenance workloads are minimal, reducing labor and maintenance costs.
The removal rates of oil and suspended solids (SS) exceed 80%.
BOD and COD removal rates can reach over 60%, whereas the DAF system can only achieve about 35% for BOD and COD.
It promotes the oxidation of sulfides and reduces the sulfur content of the wastewater.
This system is very easy to operate, with no complex equipment; the entire system consists of only two mechanical parts, unlike the DAF system which includes many essential devices such as a pressure vessel, air compressor and circulation pump.
Economic Comparison with DAF and IAF (Induced Air Flotation)
This comparison is based on 2000 t/d with no chemical feed, as shown in Table 1.
Retention time 15-20 min.
Surface loading 5-10 m3/(m2 h).
Working water depth in the tank no greater than 2.0 m; tank length-to-width ratio no less than 4.
CAF treats various wastewaters of different quality by combining mechanical aeration with chemical flotation, and is an ideal physico-chemical device for wastewater. In some applications with light pollutant content, the treated clear water can be recycled or discharged to standard when combined with biological treatment, fully meeting the standards.
The system is mainly applied to the purification of industrial wastewater from papermaking, chemicals, dyeing and printing, food, breweries, pharmaceuticals, slaughtering and other industries.
1. Low investment, low energy consumption, automatic slag discharge with no noise, convenient operation and management.
2. Mainly removes most of the non-dissolvable COD in wastewater and the SS that can float (specific gravity less than water) and the SS that can sink (specific gravity greater than water).
3. Compared with the domestic traditional pressurized dissolved-air flotation system, it eliminates the dissolved-air tank, air compressor, high-pressure water pump and other equipment, and its energy consumption is 1/8 to 1/10 of conventional flotation.
The open recirculation pipe extends from the aeration section along the bottom of the flotation tank. While generating microbubbles, the cavitation aerator forms a negative-pressure zone at the bottom of the tank where the recirculation pipe is. This negative-pressure action causes the wastewater to recirculate from the tank bottom back to the aeration zone, and then return to the flotation section. This process ensures about 40% of the wastewater is recirculated and that the flotation section can still operate without influent.
The HD-AF cavitation air flotation is an excellent innovative wastewater-treatment technology and device, reasonably designed, convenient to operate and economical to run; it removes suspended solids directly from wastewater, generating flotation by air. The cavitation aerator injects microbubbles directly into the wastewater without prior dissolved-air preparation, then distributes them evenly through a disperser impeller, so the whole process never clogs and needs no pressure dissolution, air compressor or circulation pump.
Untreated wastewater first enters the aeration-shell section and mixes thoroughly with microbubbles; as the microbubbles rise they carry the suspended solids to the surface, the scraper moves along the liquid surface and scrapes the suspended solids onto an inclined metal plate, then pushes them into the sludge-discharge pipe, inside which a horizontal screw conveyor sends the collected sludge into a sludge collector. The conveyor and scraper are driven by the same motor, with power of only 0.5 hp; after purification the wastewater passes the overflow trough below the inclined plate before discharge. The overflow trough controls the water level of the flotation tank, ensuring the liquid in the tank does not flow into the sludge-discharge pipe. The open bypass pipe extends from the aeration section along the bottom of the flotation tank; while generating microbubbles, the cavitation aerator forms a negative-pressure zone at the tank bottom where the recirculation pipe is, causing the wastewater to recirculate from the tank bottom back to the aeration zone and then return to the flotation section. This process ensures flotation continues even with inflow present.
