The heart of any dissolved air flotation (DAF) system is the pump that dissolves air into the wastewater. This is because DAFs work by adding whitewater to the wastewater to create microbubbles, which attach to the suspended solids and other contaminants. These bubbles lower the particles’ density, causing the contaminants to rise to the surface and be easily skimmed off.
At World Water Works, we incorporate our proprietary AirWhip™ pump technology into our WWW™ DAF systems to achieve efficient flotation, improving the quality of the dissolved air and allowing maximum adhesion to the floc particles.
How It Works
The AirWhip system draws treated water out of the DAF’s influent chamber and then dissolves compressed air into the flow to send back to the DAF header box. When dealing with dissolved air generation pumps, oftentimes some of the air will fail to coalesce with the water. Should this happen, we’ve designed an escape for those large bubbles to exit the aerated streams and return to the DAF influent chamber. The aerated water combines with the wastewater stream and enters into the DAF header box, where it spreads along the length of the DAF. At this point, the separation process can begin.
In terms of how to incorporate the AirWhip pump into a DAF system, there are a few options. The first one is the standard recycle layout, which uses 20 to 50 percent of the DAF system’s capacity. Commonly used for newer DAF systems, this layout uses clarified water from the DAF discharge, features efficient power usage and has a low risk of oil emulsification. Pressures are normally 95 to 105 psi but can be decreased by impeller trim to accommodate available plant air pressures or if smaller pumps with lower maximum pressure capabilities must be used.
A second option is the split flow aeration layout, a newer concept that doesn’t use any of the DAF system’s capacity. It uses 25 to 50 percent of influent flow, can dramatically improve DAF separation performance and will increase DAF throughput compared to the standard recycle layout. It also consumes roughly the same amount of power and has the same pressure requirements as the standard recycle layout.
A third option is the full flow aeration layout, which pressurizes and aerates the full raw flow and is commonly used in older DAF systems. Like the split flow aeration layout, this option doesn’t use any of the DAF system’s capacity and will dramatically improve DAF separation performance. However, it is the least energy efficient out of the three layouts. In terms of pressure requirements, this method does not require high pressures to achieve sufficient aeration and will normally use 40 to 60 psi. Higher pressure can be used if more air solution is desired, but energy requirements will rise significantly.
More AirWhip Features
Here’s an overview of some additional features and considerations of the AirWhip pump.
Materials. The AirWhip pump incorporates 316L stainless steel parts. Nickel aluminum bronze (NAB) is available for seawater applications, as is Duplex 2205 Stainless Steel for more difficult applications involving problematic abrasion. We also offer various seal and fitting options.
Loop back considerations. AirWhip pumps are operated on the high end of the pump curve for maximum air, making it advantageous to provide a “loop back” flow path to increase the flow through the pump. This option allows the pump to operate further out on the curve, avoiding the issues that often come with operating centrifugal pumps at high pressures and low flows. Providing a loop back flow may be necessary when installing an AirWhip to smaller DAF systems, where the pressure requirements force the use of a pump with a large impeller diameter.
Calculating cubic feet of air per hour (SFCH). Air flow readings for the AirWhip pump are not the same as other DAF pumps. Whereas other DAF pumps inject air in the low or negative pressure point of the pump flow, the AirWhip pump injects air at the highest internal pressure area of the pump casing, requiring an air pressure supply that’s greater or equal to the pump’s discharge pressure. Because the air is compressed as it passes through the flow meter, the reading will indicate a lower flow than if the air is not compressed. Actual air flow to the AirWhip is roughly double the indicated flow on the rotameter due to this compression.
Air flow supply. The actual air flow supplied to the AirWhip is directly related to the condition of the water. The amount of air that can be introduced to the pump depends on the water’s surface tension, which will govern the behavior of the air in the pump. Tap water, for instance, will have a high surface tension. Fine air bubbles tend to rapidly coalesce and will migrate to the center of the pump, creating a hydraulic imbalance. As a result, only a very small amount of air should be introduced to the pump. On the other hand, dirty water will have a much lower surface tension, impeding the rate of coalescence. In this case, a larger amount of air can be introduced to the pump.
To learn more about AirWhip pumps and our WWW™ DAF systems, visit our website.
