Dissolved Air Flotation as We Know It
The ensuing need to be competitive has spawned many dissolved air flotation (DAF) manufacturers and end-users to seek alternative technologies to be not only cost competitive but to be able to provide a DAF system with better bells and whistles.
Over the years, there have been several pump manufacturers who stepped up to the challenge of providing such a device that would feature the bells and whistles that the water and wastewater industry has been longing for. This device is a pump that eliminates the need for the ancillary components required by conventional DAF systems. It is known as an air-dissolving pump that excludes the need for a saturation tank, air compressor and air control panel.
Equally important is its ability to efficiently dissolve air into solution via the pump’s volute, where pressure and shear forces facilitate the dissolution process. If the demand for such a device continues on the course of quicker, faster, smarter, it may only be a matter of time for air dissolving pumps to become the standard.
What is DAF?
DAF is a method for separating a liquid or a solid from a wastewater. The driving force behind DAF is the micron-sized bubbles, which are generated by dissolving a gas into a liquid under pressure. Pressure is maintained by a backpressure valve under a prevailing pressure of 40 to 100 psi. As the saturated liquid is injected into a separation vessel, the pressure drop at the discharge of the backpressure valve precipitates a swarm of micro-bubbles ranging in size from 10 to 50 microns in diameter. As the saturated liquid and influent are introduced into the feedwell, intimate mixing occurs between the bubbles and the solids. As the solids rise to the surface, a float layer forms. This is where the solids become concentrated by allowing free liquid to drain.
The objective of DAF is to not only minimize odor, but also to allow for higher process loading rates, increased dewatering efficiency, efficient separation of water from a solid or liquid, and greater solids concentration over gravity settling.
The principal behind DAF is based on Henry’s Law, which states that the amount of dissolved gas in the liquid is directly proportional to pressure and inversely proportional to temperature. Table 1 illustrates Henry’s Law.
How does Henry’s Law apply?
Henry’s Law clearly illustrates how pressure and temperature affect the dissolution process as it relates to air dissolving pumps. As stated by Henry’s Law, the amount of gas is directly proportional to pressure, meaning that the higher the pumps pressure the greater amount of gas that can be dissolved into a solution. Readers will note there are three pressure levels reported in Figure 1. Each level represents the maximum pressure from various air dissolving pump manufacturers.
In reviewing the saturation efficiency (amount of air dissolved in water at a given pressure and temperature) of each pump, actual saturation efficiency reports were not readily available, as this information appears to be confidential and proprietary.
Therefore, information on the deliverable amount of gas was based on theoretical calculations using solubility tables. If the deliverable amount of air is based on pressure, according to Henry’s Law, the pumps at 68 and 58 psi would not provide the same amount of deliverable air at 100 psi.
Figure 1 illustrates the relationship of the deliverable amount of air with respect to temperature and pressure.
For example, a pressure level of 100 psi will deliver 60% and 39% more gas over a pressure level of 58 and 68 psi, respectively.
Another case in point is the relationship between pressure and bubble size. As pressure is increased, bubble size decreases. This is important to note because the objective of good separation is the relationship of both the amount of deliverable air and the size of the bubbles. As the bubble size decreases, the collision efficiency of bubble-particle contact increases. Figure 2 illustrates the relationship of bubble size and absolute pressure.
A review of the deliverable amount of air (gas) clearly illustrates the effect of pressure and temperature of each pump. Equally important is the mass transfer of air (gas) into the liquid phase. This is where shear forces and mixing are important for efficient mass transfer.
PUMP A (100 psi-Two Stage Open Impeller)
Design features of Pump A are in the mixing and shearing action at each impeller stage (two stages of mixing/shearing). As the air and water are introduced into the pump suction, the first impeller breaks down the large air bubbles and subsequently passes through the second impeller. As the air and water pass through each stage, added shear and pressure are applied, enabling the air to be broken down into further smaller bubbles for more efficient saturation. Typical air bubbles generated at higher pressures are on the order of 20 to 30 microns in diameter.
PUMP B (100 psi-Single Stage Semi-Closed Impeller)
Pump B is similar in features to Pump A, except that it does not have the added mixing and shearing that Pump A provides. As discussed, it is the shear forces and mixing that creates ideal conditions for mass transfer of the air into the liquid. Pump B has one impeller where air and water are in contact. This, in turn, requires downstream added detention time. Although the downstream detention equipment is small, it is mirroring a standard saturation system on a smaller scale.
PUMP C (68 psi-Single Stage Closed Impeller)
Pump C is a single stage centrifugal pump that utilizes a closed impeller on the fluid side and an open impeller on the backside for air aspiration. The back impeller is slightly larger in diameter than the front impeller, allowing a balance between the air and the water.
In addition, water is taken from the volute of the pump to the back impeller to act as a motive force for increasing the air transfer. As the air and water mix inside the pump, shear from the impeller creates small bubbles, resulting in a greater contact area for gas to liquid transfer. Based on the theoretical air provided (assuming saturation efficiency is 100%), Pump B at 68 psi will provide 38% less deliverable air in comparison to both Pump A and Pump B at 100 psi.
PUMP D (58 psi-Single Stage Semi-Open Impeller)
Pump D is a single stage centrifugal pump that utilizes a semi-open impeller. Air is aspirated into the volute of the pump under a pressure of 58 psi where the air and water mix inside the pump via a single semi-open impeller. Based on the theoretical air provided (assuming saturation efficiency is 100%), Pump D at 58 psi will provide 60% less deliverable air in comparison to both Pump A and Pump B at 100 psi.
Now that the theoretical air solubility and pump design features have been evaluated, how does the air dissolving pump compare to conventional saturation systems in terms of ancillary equipment elimination and process performance.
The DAF system consists of a separation tank, skimmer, feed distribution well, effluent draw-off (effluent launder), back pressure valve and saturation system. The heart of any DAF system will be the bubbles generation (saturation) system. Figures 3, 4, and 5 illustrate the DAF unit with an air dissolving pump and conventional saturation system, respectively.
Because air-dissolving pumps aspirate air into the volute and promote efficient mixing/shear, equipment components like the air compressor, air control panel and saturation tank can be eliminated.
In December of 2003, the Meridian WWTP, located in Meridian, Idaho, expanded their plant process train with another 20 ft. diameter DAF unit which included one 220 gpm air dissolving pump system. The plant was designed to treat an influent flow of 5 mgd. Waste activated sludge from the secondary clarifier reported to the DAF unit at a design solids loading rate of 1.8 lbs/hr/ft2. Performance testing showed the DAF unit was capable of achieving an average effluent quality of 65 mg/L total suspended solids concentration (95% solids removal without the use of chemicals) and an average float solids concentration of greater than 4 wt%.