Gravity Flotation and Dissolved Air Flotation
Gravity flotation is used, sometimes in combination with sedimentation and sometimes alone, to remove oils, greases, and other flotables such as solids that have a low specific weight. Various types of “skimmers” have been developed to harvest floated materials, and the collection device to which the skimmers transport these materials must be properly designed. Figures 8-98(a)–(f) are photographs of different types of gravity flotation and harvesting equipment.
Dissolved Air Flotation
Dissolved air flotation (DAF) is a solids separation process, similar to plain sedimentation. The force that drives DAF is gravity, and the force that retards the process is hydrodynamic drag. Dissolved air flotation involves the use of pressure to dissolve more air into wastewater than can be dissolved under normal atmospheric pressure, then releasing the pressure. The “dissolved” air, now in a supersaturated state, comes out of solution, or “precipitates,” in the form of tiny bubbles. As these tiny bubbles form, they become attached to solid particles within the wastewater, driven by their hydrophobic nature. When sufficient air bubbles attach to a particle to make the conglomerate (particle plus air bubbles) lighter than water (specific gravity less than one), the particle is carried to the water surface.
A familiar example of this phenomenon is a straw in a freshly opened bottle of a carbonated beverage. Before the bottle is opened, its contents are under pressure, having been pressurized with carbon dioxide gas at the time of bottling. When the cap is taken off, the pressure is released, and carbon dioxide precipitates from solution in the form of small bubbles. The bubbles attach to any solid surface, including a straw, if one has been placed in the bottle. Soon, the straw rises up in the bottle.
In a manner similar to the straw, solids having a specific gravity greater than one can be caused to rise to the surface of a volume of wastewater. Solids having a specific gravity less than one can also be caused to rise to the surface at a faster rate by using DAF than without it. Often, chemical coagulation of the solids can significantly enhance the process, and in some cases, dissolved solids can be precipitated, chemically, then separated from the bulk solution by DAF.
“Dissolution” of Air in Water Examination of the molecular structures of both oxygen and nitrogen reveals that neither would be expected to be polar, therefore, neither would be expected to be soluble in water.
Dalton’s law of partial pressures states further that, in a mixture of gases, each gas exerts pressure independently of the others, and the pressure exerted by each individual gas, referred to as its “partial pressure,” is the same as it would be if it were the only gas in the entire volume. The pressure exerted by the mixture, therefore, is the sum of all the partial pressures. Conversely, the partial pressure of any individual gas in a mixture, such as air, is equal to the pressure of the mixture multiplied by the fraction, by volume, of that gas in the mixture.
The consequence of this equation is that, by way of the process of diffusion, molecules of any gas, in contact with a given volume of water, will diffuse into that volume to an extent that is described by Henry’s Law, as long as the quantity of dissolved gas is relatively small. For higher concentrations, Henry’s constant changes somewhat. This principle holds for any substance in the gaseous state, including volatilized organics. The molecules that are forced into the water by this diffusion process exhibit properties that are essentially identical to those that are truly dissolved. In conformance with the second law of thermodynamics, they distribute themselves uniformly throughout the liquid volume (maximum disorganization), and they will react with substances that are dissolved.
An example is the reaction of molecular oxygen with ferrous ions. Unlike dissolved substances, however, they will be replenished from the gas phase with which they are in contact, up to the extent described by Henry’s law, if they are depleted by way of reaction with other substances, or by biological metabolism. The difference between a substance existing in water solution as the result of diffusion and one that is truly dissolved can be illustrated by the following example.
Consider a beaker of water in a closed space—a small, airtight room, for instance. An amount of sodium chloride is dissolved in the water, and the water is saturated with oxygen; that is, it is in equilibrium with the air in the closed space. Now, a container of sodium chloride is opened, and at the same time, a pressurized cylinder of oxygen is released.
The concentration of sodium chloride will not change, but, because the quantity of oxygen in the air within the closed space increases (partial pressure of oxygen increases), the concentration of dissolved oxygen in the water increases. The oxygen molecules are not truly dissolved; that is, they are not held in solution by the forces of solvation, or hydrogen bonding by the water molecules. Rather, they are forced into the volume of water by diffusion, which is to say, by the second law of thermodynamics. The molecules of gas are constantly passing through the water-air interface in both directions. Those that are in the water are constantly breaking through the surface to return to the gas phase, and they are continually being replaced by diffusion from the air into the water. An equilibrium concentration becomes established, described by Henry’s law. All species of gas that happen to exist in the “air” participate in this process: nitrogen, oxygen, water vapor, volatilized organics, or whatever other gases are included in the given volume of air.
The concentration, in terms of mass of any particular gas that will be forced into the water phase until equilibrium becomes established, depends on the temperature and the concentration of dissolved substances such as salts and the “partial pressure” of the gas in the gas phase.
As the temperature of the water increases, the random vibration activity, “Brownian motion,” of the water increases. This results in less room between water molecules for the molecules of gas to “fit into.” The result is that the equilibrium concentration of the gas decreases. This is opposite to the effect of temperature on dissolution of truly soluble substances in water, or other liquids, where increasing temperature results in increasing solubility.
Some gasses are truly soluble in water because their molecules are polar, and these gases exhibit behavior of both solubility and diffusivity. Carbon dioxide and hydrogen sulfide are examples. As the temperature of water increases, solubility increases, but diffusivity decreases. Also, because each of these two gases exists in equilibrium with hydrogen ion when in water solution, the pH of the water medium has a dominant effect on their solubility, or rather, their equilibrium concentration, in water.
In the previous example, where a beaker of water is in a closed space, if a flame burning in the closed space depletes the oxygen in the air, oxygen will come out of the water solution. If all of the oxygen is removed from the air, the concentration of “dissolved oxygen” in the beaker of water will eventually go to zero (or close to it), and the time of this occurrence will coincide with the flame extinguishing because of lack of oxygen in the air.
Dissolved Air Flotation Equipment The dissolved air flotation (DAF) process takes advantage of the principles described earlier. Figure 8-99 presents a diagram of a DAF system, complete with chemical coagulation and sludge handling equipment. As shown in Figure 8-99, raw (or pretreated) wastewater receives a dose of a chemical coagulant (metal salt, for instance), then proceeds to a coagulation-flocculation tank. After coagulation of the target substances, the mixture is conveyed to the flotation tank, where it is released in the presence of recycled effluent that has just been saturated with air under several atmospheres of pressure in the pressurization system shown. An anionic polymer (coagulant aid) is injected into the coagulated wastewater just as it enters the flotation tank.
The recycled effluent is saturated with air under pressure as follows: A suitable centrifugal pump forces a portion of the treated effluent into a pressure-holding tank. A valve at the outlet from the pressure-holding tank regulates the pressure in the tank, the flow rate through the tank, and the retention time in the tank, simultaneously. An air compressor maintains an appropriate flow of air into the pressure-holding tank. Under the pressure in the tank, air from the compressor is diffused into the water to a concentration higher than its saturation value under normal atmospheric pressure. In other words, about 23 ppm of “air” (nitrogen plus oxygen) can be “dissolved” in water under normal atmospheric pressure (14.7 psig). At a pressure of six atmospheres, for instance, (6 × 14.7 = about 90 psig), Henry’s law would predict that about 6 × 23, or about 130 ppm, of air can be diffused into the water.
In practice, dissolution of air into the water in the pressurized holding tank is less than 100% efficient, and a correction factor, f, which varies between 0.5 and 0.8, is used to calculate the actual concentration.
After being held in the pressure-holding tank in the presence of pressurized air, the recycled effluent is released at the bottom of the flotation tank, in close proximity to where the coagulated wastewater is being released. The pressure to which the recycled effluent is subjected has now been reduced to one atmosphere, plus the pressure caused by the depth of water in the flotation tank. Here, the “solubility” of the air is less, by a factor of slightly less than the number of atmospheres of pressure in the pressurization system, but the quantity of water available for the air to diffuse into has increased by a factor equal to the inverse of the recycle ratio.
Practically, however, the wastewater will already be saturated with respect to nitrogen but may have no oxygen because of biological activity. Therefore, the “solubility” of air at the bottom of the flotation tank is about 25 ppm, and the excess air from the pressurized, recycled effluent precipitates from “solution.” As this air precipitates in the form of tiny, almost microscopic, bubbles, the bubbles attach to the coagulated solids. The presence of the anionic polymer (coagulant aid), plus the continued action of the coagulant, causes the building of larger solid conglomerates, entrapping many of the adsorbed air bubbles. The net effect is that the solids are floated to the surface of the flotation tank, where they can be collected by some means, thus removed from the wastewater.
Some DAF systems do not have a pressurized recycle system, but rather, the entire forward flow on its way to the flotation tank is pressurized. This type of DAF is referred to as “direct pressurization” and is not widely used for treating industrial wastewaters because of undesirable shearing of chemical flocs by the pump and valve.