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Multiphase Pumps In Use: Dissolved Air Flotation

Dissolved air flotation represents an approved procedure in the field of wastewater treatment: liquids are separated from emulsified substances or substances floating therein. To do so, by using a special centrifugal pump, water is first saturated under high pressure with air and then, reduced to atmospheric pressure, pumped into the wastewater tank. During pressure release, micro bubbles are generated and accumulate on the suspended substances in the wastewater, carrying them to the water surface. By means of a skimming device the suspended substances finally can be removed.

Due to saturation with air, the centrifugal pump must be able to handle a second phase which can seriously influence the pump capabilities by different flow characteristics. In impellers of conventional centrifugal pumps, the gas bubbles often form larger layers and plugs due to the appearing centrifugal forces. Instabilities in the pumping characteristics are the consequence here, because they generate barricades in the flow area. Therefore conventional impellers are only used to handle gas contents of up to 6%.

Multiphase pumps however are designed in a different way by using open impellers and this does allow the use in applications with gas contents up to 30%. For this reason they have the required capabilities for the use in dissolved air flotation systems. These open impellers used do not have any back-plates or cover plates. Strong shearing forces are generated by relative movement between the rotating impeller blades and the liquid above and below the blades. Figuratively speaking, larger gas accumulations thus are “cutted” and work against the narrowing of the flow channels.

 

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In plants with conventional pumps, additional plant components such as compressors, pressure vessels, control units and valves usually are required for the gas enrichment of liquids. Because of direct gas input into the suction line when using multiphase pumps with open impellers, an additional use of compressed air, pressure tanks and sophisticated control technology is not required anymore. Typical fields of application for multiphase pumps in dissolved air flotation systems are water and waste water treatment, treatment of oil-water emulsions, post-treatment in biological sewage plants, heavy metal precipitation, denitrification as well as ozonation of tap water, process water, leachate and waste water.

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