Various treatment technologies are available for removing volatile organic compounds from water before it is used or discharged. Each one has its own pros and cons.

For many VOCs, air stripping, including newly introduced sliding-tray technology, has proven to provide the optimum balance of capital and operation costs and therefore the lowest cost per volume treated.

Basic VOC removal options

Four common technologies are used to remove VOCs from drinking water, contaminated groundwater and waste streams.

Granular activated carbon. Used to adsorb natural organic compounds, taste and odor compounds, and synthetic organic chemicals in drinking water, GAC uses a form of carbon processed to create a large surface area in pores available for adsorption. GAC is made from high-carbon organic materials such as wood, coconut shells, lignite and coal. The contaminants adhere to the carbon, which must be disposed of or regenerated.

Advanced oxidation processes. These chemical treatment processes remove organic (and sometimes inorganic) materials by oxidation through reactions with hydroxyl radicals. Most wastewater treatment AOPs use combinations of ozone, hydrogen peroxide and UV light.

Air stripping. This process strips VOCs by way of contact with clean air across a high surface area, causing the volatiles to transfer from the water to the air. 

Reverse osmosis/ultrafiltration. These technologies remove particulate matter by a force differential across a porous membrane. Reverse osmosis filters have a pore size of about 0.0001 micron. In addition to removing all organic molecules and viruses, it removes most minerals. Ultrafiltration uses a pore size of about 0.01 micron. Both types of filtration can remove VOCs to very low levels.

Table 1 compares these processes, some of which can be used in combination. Air stripping often has the lowest cost per volume. For example, GAC has low capital costs but higher ongoing costs for refreshing carbon. AOP has higher capital costs for sophisticated controls and specialized materials, chemical feed equipment, and chemical feed and energy costs.

While air stripping capital costs are higher than for GAC, operating costs are almost always lower. Air stripping can also work with GAC to extend carbon bed life.

 Air stripping methods

In air stripping processes, countercurrent flow causes the cleanest air to contact the cleanest water. This ensures efficient mass transfer throughout the flow path. The air-to-water (A/W) ratio is the primary physical driving parameter; increasing A/W increases removal efficiency. Requirements for successful air stripping include:

• Dissolved VOCs in a water matrix
• Pretreatment to remove any free-phase organics
• Clean air
• High surface-area contact between air and water
• Sufficient contact time
• No surfactants or dissolved polar organics present.

Common air stripping methods include tower, stacking tray and sliding tray designs. Table 2 compares these methods.

Tower-style air strippers use a tall column filled with high-surface-area media for mass transfer. Plastic or ceramic media feature open area for airflow. Inspection and cleaning can be an issue.

With tray designs, a high volume of air is mixed with the water, resulting in bubbles and froth that create surface area for contaminants to move from the water into the air. A countercurrent flow uses the air volume for maximum effectiveness. Sliding-tray designs have a front door that opens like a pizza oven and stripper trays that slide out on rails, like oven racks. The door can be easily removed and sprayed down for cleaning.

All stripper types are prone to fouling from inorganic compounds, especially scale-forming compounds. For example, insoluble iron oxides and calcium scales can deposit on mass transfer packing or trays and restrict air passage, leading to lower A/W and less VOC removal. 

Sliding tray technology

One sliding tray air stripper technology, which uses the froth and turbulent mixing approach, can enable long operation before cleaning is needed. The design uses a sealed stripper box with a removable end door. With no external tray seals and no need to disconnect piping for cleaning, leaks are virtually eliminated.

The design provides easy access for process monitoring and inspection, even while in operation. It is resistant to fouling and can be cleaned by one person with a simple pressure washer. It is also easy to accommodate on sites and offers a wide turndown range.

Air strippers are effective at removing dissolved VOCs and gases from water. Well-maintained air stripping equipment will provide many years of efficient service.

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About the author

Dave Fischer is vice president of technology with QED Environmental Systems, a manufacturer of innovative environmental products based in Dexter, Michigan.