Air stripping

Henry s constant is a function of temperature and a weak function of composition and pressure, however, the air stripping process is normally run at ambient conditions and published data within 20° and 30°C is available. 

Maximum pollutant removal by air stripping can be predicted by the following  

The packed tower is compact and efficient for air stripping. The waste water, in a lime slurry (for phosphate removal and pH control), is first sent to a mixing tank and then to a shell settling tank to settle out calcium phosphate and calcium carbonate. The clarified waste water enters near the top ofthe packed tower, while air is introduced countercurrently atthe bottom of the tower. Waste water product is then sent to a recarbonation basin so that calcium carbonate may be precipitated, removed, and reused. 

Air strippers are typically 15 to 60 feet in height, and have diameters ranging fi-om 1 to 10 feet. Water flow rates are typically in the range of 40 to 250 Ib/cfin air and air/water volumetric ratios are typically in the range of 10 to 3 00. Generally, contaminated air from the stripper may be routed to an incubator or vapor phase carbon adsorption. 

Countercurrent packed towers (in their various forms) appear to be the most appropriate equipment configuration for treating contaminated groundwaters for the following reasons  

Emission of stripped organics to the atmosphere may be environmentally unacceptable however, a countercurrent tower is relatively small and can be readily connected to vapor recovery equipment. 

FIGURE 8.1 Schematic showing conventional air stripping equipment configuration. (After Knox et al., 1986.) 

The oxygen based vapors that are produced can not be sent to the flare and must be incinerated locally. 

Good preliminary oil removal from effluents is also a must so that no pockets of aromatic or light HC are sent to stripping, as they can be dangerous in off-gases. This is why some refineries may prohibit air stripping. 

The advantages of the process are that it can be nm cold and that at a first glance it entails the least expensive construction and operating condltioits. It is designed to deal with very dilute condensates (S 0.5 g l M and to operate as follows  

The resulting residual content is about a few milligrams per liter. 

Because of a number of negative points, e.g. NH,j not eliminated, costly incineration of residual air, new standards on off-gas emissions and risks of explosion, this technique which used to be very widespread in Europe is now being phased out. 

---

Ammonium Ion Removal. A fixed-bed molecular-sieve ion-exchange process has been commercialized for the removal of ammonium ions from secondary wastewater treatment effluents. This application takes advantage of the superior selectivity of molecular-sieve ion exchangers for ammonium ions. The first plants employed clinoptilolite as a potentially low cost material because of its availability in natural deposits. The bed is regenerated with a lime-salt solution that can be reused after the ammonia is removed by pH adjustment and air stripping. The ammonia is subsequentiy removed from the air stream by acid scmbbing. 

Removal of Refractory Organics. Ozone reacts slowly or insignificantly with certain micropoUutants in some source waters such as carbon tetrachloride, trichlorethylene (TCE), and perchlorethylene (PCE), as well as in chlorinated waters, ie, ttihalomethanes, THMs (eg, chloroform and bromoform), and haloacetic acids (HAAs) (eg, trichloroacetic acid). Some removal of these compounds occurs in the ozone contactor as a result of volatilization (115). Air-stripping in a packed column is effective for removing some THMs, but not CHBr. THMs can be adsorbed on granular activated carbon (GAG) but the adsorption efficiency is low. 

Arc wire utilizes two continuously fed 1.6-mm dia intersecting wires with a d-c arc maintained between the wire tips as they meet. Compressed gas (usually air) strips the molten metal from the tips and forms a directional spray stream. This process is widely used to spray most metals. Arc wire is the most economical process because of the wire feedstock. Moreover, it utilizes - 10% of the thermal energy of the other spray processes (0.4 vs 6.6 kWh/kg using stainless steel) because of the direct arc heating of the wire tips. 

In Situ Air Stripping. An innovation to conventional pump and treat air stripping is in situ air stripping. Two horizontal wells are installed, one below the water table and one in the vadose zone. Air is injected in the lower well while contaminated soil vapor is extracted by vacuum through the upper well. 

U.S. EPA, "Case Study of Ground Water Extraction with Air Stripping, Des Moines TCE Site," in Innovative Operational Treatment Technologiesfor Application to Supefund Sites Nine Case Studies, prepared by T. Pheiffer, U.S. Environmental Protection Agency, Washington, D.C., 1990. 

Fig. 8. Illustrative relationships of air stripping removal efficiencies to media height and air-to-water ratios.

XAlD-4 for a group of chlorinated hydrocarb Dns is given in Table 8. The EPA may recommend a combination of air Stripping and carbon adsorption... 

Volatile organic contaminants occur primarily in groundwaters as a result of the disposal of industrial solvents on the ground or in soakage pits. The removal of these compounds has best been accompHshed by the use of air stripping or adsorption on activated carbon. 

The generated chlorine dioxide must be air stripped from the anode compartment in order to achieve high chlorite conversion efficiency. Sodium ions from the anode compartment are transported into the cathode compartment, forming sodium hydroxide [1310-73-2] and hydrogen gas coproducts ... 

Process and environmental air is compressed and passed through activated beds to reduce air emission levels to <5 ppm. Process wastewater is air stripped to remove CCl. The solvent containing air is also passed through the activated carbon beds. The total air flow through the beds averages about 3965 mVmin (140,000 SCFM). 

Some catalysts exposed to air stripping off-gas were subject to deactivation. However, using a catalytic oxidizer at a U.S. Coast Guard faciUty (Traverse City, Mich.) for the destmction of benzene, toluene, and xylene stripped from the groundwater, the catalytic oxidization unit operated at 260 to 315°C, and was able to achieve 90% destmction efficiency (see Groundwatermonitoring). 

Stripping Air stripping is applied for the removal of volatile substances from water. Henry s law is the key relationship for use in design of stripping systems. The minimum gas-to-liquid ratio required for stripping is given by ... 

Air stripping Solution containing ammonia high pH Ammonia vapor in air... 

---