Mass Transfer Units

The major purpose of dissolving air is to provide oxygen to be used by microorganism in the process of wastewater treatment. This is exemplified by the aeration employed in the activated sludge process. Aeration may also be employed for the removal of iron and manganese from groundwaters. In the removal of hardness, the presence of high concentrations of carbon dioxide may result in high cost for lime, as CO2 reacts with lime. Thus, excess concentrations of this gas may be removed from the water by stripping or spraying the water into the air. H2S is another compound that may be removed by stripping as benzene, carbon tetrachloride, p-dicholorobenzene, vinyl chloride, and trichloroethylene may also be removed by stripping. The discussions that follow address the units or method used in aeration, absorption, and stripping.

Figure 9.1 illustrates how a pollutant may be stripped by spraying the water into the air. As the water is sprayed, droplets are formed. This creates the condition for the pollutant to transfer from the droplet phase to the air phase, in addition to the direct liberation of the pollutant as the bulk mass of water breaks up into the smaller size droplets. Figures 9.2a through 9.2d show the various types of nozzles that may be used in sprays. Figure 9.2e is an inclined apron which may be studded with riffle plates. At the air-water interface at the surface of the flowing water, the air transfers between the water and air phases. The studding creates turbulence which, in aeration, transports the water exposed at the surface to the main body or bulk of the flowing water. The whole mass of water is aerated this way, because the mass of water transported to the main body carries with it any air that was dissolved when it was exposed at the surface. In stripping, the turbulent flowing water exposes the solute at the surface triggering the process of stripping. The rate of aeration or stripping depends upon how fast the surface is renewed (i.e., how fast the water mass from the main body is transported to the surface for exposure to the air).

Figure 9.2f is a stack of perforated plates. The water is introduced at the top and allowed to trickle down the plates; the trickling water is met by a countercurrent flow of air. The process creates a droplet phase and the air-gas phase inducing a mass transfer between the droplets and the air. Figure 9.2g is a spray tower. The water is sprayed using spray nozzles at the top of the tower forming droplets. These droplets are then met by a countercurrent flow of air creating the two phase for mass transfer as in the case of the perforated plates. Figure 9.2h is a cascade aerator or deaerator as the case may be. The cascade operates on the same principle as the inclined apron, only that this is more effective because of the steps.

FIGURE 9.1 Water spray.

Figures 9.3 and 9.4 show various types of aeration devices used in wastewater treatment plants. Figure 9.3a is a turbine aerator with an air sparger at the bottom. As the air emerges from the sparger, the larger bubbles that are formed are sheared into small pieces by the turbine blade above. Figure 9.3b is a porous ceramic diffuser. Because of the small openings through which the air passes, this type of diffuser creates tiny bubbles. Tiny bubbles are more effective for mass transfer, since the many bubbles produced create a large sum total areas for transfer. Figure 9.3c is a surface aerator. Water is drawn from the bottom of the aerator and sprayed into the air creating droplets, thus, aerating the water. Figure 9.4 shows a dome-type bubble diffuser. The dome is porous, can have a diameter of 18 cm, and may be constructed of

Perforated pipe discharging -'air or gas upward

Palm Beach nozzle

Collecting pan and outlet

Perforated

Palm Beach nozzle

Perforated pipe discharging -'air or gas upward

Sacramento nozzle (c)

Berlin nozzle (b)

Removable mouth piece

O Perforated j \ inlet pipe

Collecting pan and outlet

Vanes

Vanes

Berlin nozzle (b)

New York nozzle (d)

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