Description:

Air stripping involves the mass transfer of volatile contaminants from water to air. For ground water remediation, this process is typically conducted in a packed tower or an aeration tank. The typical packed tower air stripper includes a spray nozzle at the top of the tower to distribute contaminated water over the packing in the column, a fan to force air countercurrent to the water flow, and a sump at the bottom of the tower to collect decontaminated water. Auxiliary equipment that can be added to the basic air stripper includes an air heater to improve removal efficiencies; automated control systems with sump level switches and safety features, such as differential pressure monitors, high sump level switches, and explosion-proof components; and air emission control and treatment systems, such as activated carbon units, catalytic oxidizers, or thermal oxidizers. Packed tower air strippers are installed either as permanent installations on concrete pads or on a skid or a trailer.

Aeration tanks strip volatile compounds by bubbling air into a tank through which contaminated water flows. A forced air blower and a distribution manifold are designed to ensure air-water contact without the need for any packing materials. The baffles and multiple units ensure adequate residence time for stripping to occur. Aeration tanks are typically sold as continuously operated skid-mounted units. The advantages offered by aeration tanks are considerably lower profiles (less than 2 meters or 6 feet high) than packed towers (5 to 12 meters or 15 to 40 feet high) where height may be a problem, and the ability to modify performance or adapt to changing feed composition by adding or removing trays or chambers. The discharge air from aeration tanks can be treated using the same technology as for packed tower air discharge treatment.

Modifying packing configurations greatly increase removal efficiency. A recent innovation is the so-called low-profile air stripper that is offered by several commercial vendors. This unit packs a number of trays in a very small chamber to maximize air-water contact while minimizing space. Because of the significant vertical and horizontal space savings, these units are increasingly being used for ground water treatment.

Air strippers can be operated continuously or in a batch mode where the air stripper is intermittently fed from a collection tank. The batch mode ensures consistent air stripper performance and greater energy efficiency than continuously operated units because mixing in the storage tanks eliminates any inconsistencies in feed water composition.

The eventual duration of cleanup using an air stripping system may be tens of years and depends on the capture of the entire plume from the ground water.

Synonyms:

DSERTS Code: F19 (Air Stripping)

Applicability:

Limitations:

• The potential exists for inorganic (e.g., iron greater than 5 ppm, hardness greater than 800 ppm) or biological fouling of the equipment, requiring pretreatment or periodic column cleaning.
• Effective only for contaminated water with VOC or semivolatile concentrations with a dimensionless Henry's constant greater than 0.01.
• Consideration should be given to the type and amount of packing used in the tower
• Process energy costs are high..
• Compounds with low volatility at ambient temperature may require preheating of the ground water.
• Off-gases may require treatment based on mass emission rate.

Data Needs:

Vendors require the following information to select the properly configured tower and packing for a specific application: range of feedwater flow rates; range of water and air temperatures; whether the tower will operate continuously or intermittently; tower feed and discharge systems (gravity feed or type and location of pumps); height restrictions on the tower; influent contaminant identification and concentrations; mineral content; pH; requirements for effluent water contaminant concentrations; and restrictions on air discharge from the tower.

Performance Data:

The major problem encountered with packed tower air strippers is fouling of the packing, which reduces the air flow rate. Fouling is caused by oxidation of minerals in the feed water, such as iron and manganese, by precipitation of calcium, and by biological growth on the packing material.

Cost:

The key cost driver information and cost analysis was developed in 2006 using the Remedial Action Cost Engineering and Requirements (RACER) software.

Low Profile Tray Stacks

Key Cost Drivers 

·        Influent flow rate

·        Relative contaminant volatility

·        Off-gas treatment (when necessary)

Cost Analysis

The following table represents estimated costs (by common unit of measure) to apply ex situ air stripping using low profile tray stack technology at sites of varying size and complexity.   A more detailed cost estimate table which includes specific site characteristics and significant cost elements that contributed to the final costs can be viewed by clicking on the link below.

Detailed Cost Estimate 

Packed Towers

Key Cost Drivers 

·        Influent flow rate

·        Relative contaminant volatility

·        Off-gas treatment (when necessary)

Cost Analysis

The following table represents estimated costs (by common unit of measure) to apply ex situ air stripping using packed tower technology at sites of varying size and complexity.   A more detailed cost estimate table which includes specific site characteristics and significant cost elements that contributed to the final costs can be viewed by clicking on the link below.

Detailed Cost Estimate

A major operating cost of air strippers is the electricity required for the ground water pump, the sump discharge pump, and the air blower. The power rating of the ground water pump depends on the pressure head and pressure drop across the column and should be obtained from pump curves. As a general rule, pumps in the 4 to 80 liters per minute (1 to 20-gpm) range require from 0.33 to 2 HP; from 80 to 290 liters per minute (20 to 75 gpm) power ratings are 1 to 5 HP; and from 380 to 2,270 liters per minute (100 to 600 gpm), power ratings range from 5 to 30 HP. A crude method of estimating blower motor power assumes that each foot of air stripper diameter requires 1.5 HP.

References:

Treatment Technologies for Site Cleanup: Annual Status Report (ASR), Tenth Edition, EPA 542-R-01-004

Innovative Remediation Technologies:  Field Scale Demonstration Project in North America, 2nd Edition

Abstracts of Remediation Case Studies, Volume 4, June 2000, EPA 542-R-00-006

DOE, 1994. Technology Application Analysis: In Situ Air Stripping of Contaminated Groundwater at U.S. Department of Energy Savannah River Site Aiken, South Carolina, prepared by Stone & Webster Environmental Technology & Services.

Elliott, M.G. and E.G. Marchand, 1990. "USAF Air Stripping and Emissions Control Research," in Proceedings of the 14th Annual Army Environmental Symposium, USATHAMA Report CETHA-TE-TR-90055.

Federal Remediation Technologies Roundtable, 1995. Remediation Case Studies: Groundwater Treatment, EPA/542/R-95/003.

• Recovery of Free Petroleum Product at Fort Drum, Fuel Dispensing Area 1595, Watertown, New York

Federal Remediation Technologies Roundtable, 1998. Remediation Case Studies: Groundwater Pump and Treat (Nonchlorinated Solvents), EPA/542/R-98/014

• Pump and Treat of Contaminated Groundwater at the City Industries Superfund Site, Orlando, Florida

Shukla, H.M. and R.E. Hicks, 1984. "Process Design Manual for Stripping of Organics", Water General Corporation for EPA, EPA/600/12-84/139, NTIS PB 84 232628.

Singh, S.P., 1989. "Air Stripping of Volatile Organic Compounds from Groundwater: An Evaluation of a Centrifugal Vapor-Liquid Contractor", USAF Environmental and Service Center Report ESL-TR-86-46.

Wilson, J.H., R.M. Counce, A.J. Lucero, H.L. Jennings, and S.P. Singh, 1991. "Air Stripping and Emissions Control Technologies: Field Testing of Counter Current Packings, Rotary Air Stripping, Catalytic Oxidation, and Adsorption Materials", ESL TR 90-51.

• Englin AFB
• DOE - Savannah River Site, SC
• Langley AFB, VA
• McClellan AFB, CA
• Army Demo Fort Dix, NJ
• Verona Wellfield Battle Creek, MI
• City Industries Superfund Site, Orlando, FL
• Fort Drum, Fuel Dispensing Area 1595, Watertown, NY
• 

General FRTR Agency Contacts

Government Web Sites

A list of vendors offering Ex Situ Physical/Chemical Water Treatment is available from  EPA REACH IT which combines information from three established EPA databases, the Vendor Information System for Innovative Treatment Technologies (VISITT), the Vendor Field Analytical and Characterization Technologies System (Vendor FACTS), and the Innovative Treatment Technologies (ITT), to give users access to comprehensive information about treatment and characterization technologies and their applications.

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