By Dave Fischer, Vice President of Technology, QED Environmental Systems
Special to The Digest
Numerous volatile organic compounds (VOCs) are on federal and state lists of toxic compounds that must be removed from water before it is used or discharged into the environment. A variety of treatment technologies are available, each with its own advantages and disadvantages. For many VOCs, air stripping has proven to be the best option, with the most favorable balance of capital and operation costs, ultimately leading to the lowest cost per volume of water treated. Now, new sliding tray air stripper technology is available that offers high removal efficiencies and further simplifies maintenance.
VOC removal process options
The most common technologies for removing VOCs from drinking water or contaminated groundwater and waste streams are:
Granular activated carbon (GAC) – Commonly used to adsorb natural organic compounds, taste and odor compounds, and synthetic organic chemicals in drinking water treatment, 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.  GAC is a “transfer” technology – contaminants are removed from water and moved to another media. In this case, contaminants adhere to the carbon, which must be disposed of or regenerated.
Advanced oxidation processes (AOPs) – A range of chemical treatment processes designed to remove organic (and sometimes inorganic) materials in water and wastewater by oxidation through reactions with hydroxyl radicals. Most wastewater treatment AOP processes use combinations of ozone (O3), hydrogen peroxide (H2O2) and/or ultraviolet (UV) light. 
Air stripping – A process that removes or “strips” VOCs from contaminated water by contacting clean air with contaminated water across a high surface area, causing the volatile compounds to transfer from the water to the air. Like GAC, air stripping is a transfer technology. Air stripping is governed by Henry’s law, which states that the amount of dissolved gas in a liquid is proportional to its partial pressure above the liquid. The proportionality factor is called Henry’s Law constant (H).  A VOC’s H determines how hard or easy it is to strip.
Reverse osmosis/ultrafiltration – Filtration is a process of removing particulate matter from water by a force differential across a porous membrane. Reverse osmosis (RO) filters have a pore size around 0.0001 microns. In addition to removing all organic molecules and viruses, RO also removes most minerals present in the water. An ultrafiltration filter (UF) has a pore size around 0.01 microns. Ultrafiltration removes larger particles, and may remove some viruses.  Both RO and UF are incapable of removing VOCs to very low levels.
Table 1 compares among the most common VOC removal processes. While each technology has pluses and minuses, and some can be used in combination, air stripping often has the lowest cost per volume treated. For example, GAC has low capital costs, but far higher ongoing costs for refreshing carbon as it gets used up. AOP has higher capital costs due to the need for more sophisticated controls and specialized materials, chemical feed equipment, and ongoing chemical feed and energy costs. While air stripping capital equipment costs are higher than that of GAC, operating costs will almost always be lower. Air stripping can also work with GAC to extend GAC bed life by lowering the mass of VOC through removal of the easily stripped portion.
Air stripping methods and their advantages and disadvantages
Air stripping is one type of a common chemical process known as mass transfer. VOCs from contaminated water contact clean air across a high surface area, causing the VOCs to transfer from the water to the air. Counter-current flow causes the cleanest air to contact the cleanest water. This ensures efficient mass transfer throughout the entire flow path. The air to water (A/W) ratio used is the primary physical driving parameter; increasing A/W increases removal efficiency.
Successful air stripping process requirements include:
- Dissolved VOCs in a water matrix
- Pretreat to remove any free-phase organics
- Clean air (concentration gradient driven)
- High surface area of contact between air and water
- Sufficient contact time
- No surfactants or other H-lowering factors (dissolved polar organics)
Figure 1 lists some common VOCs and how easy they are to strip.
Common air stripping methods include tower, stacking tray, and sliding tray designs. Table 2 provides a comparison among the methods.
Tower style air strippers use a tall column, filled with high surface area media for mass transfer. Plastic or ceramic mass transfer media features open area for air to flow. Inspection and cleaning can be an issue with tower type air strippers.
With tray designs, a high volume of air is mixed with the contaminated water, resulting in bubbles and froth that creates surface area for contaminants to move from the water into the air. A counter-current flow path most effectively uses the air volume.
Stacking tray designs tend to be heavy, with limited observation possible through portholes. They require additional space on the side of the unit, plus piping disconnects. A multi-person crew is often required to disassemble the unit for cleaning.
Sliding tray designs have a front door that opens like a pizza oven and stripper trays that slide out on rails, much 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 chemicals like dissolved iron or calcium. Insoluble iron oxides or calcium scales can deposit on mass transfer packing or trays and restrict air passage, leading to lower A/W and less VOC removal. Figure 2 illustrates the different stripper types.
New sliding tray technology designed for easy cleaning
One example of sliding tray style air stripper technology is the patented E-Z Tray® air stripper from QED Environmental Systems. This technology uses froth and turbulent mixing, which may allow longer operation before cleaning is needed compared to tower packing.
The E-Z Tray 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.
Providing easy access for process monitoring and inspection, even while in operation, the design is less prone to fouling and can be cleaned by one person with a simple pressure wash. It is also less intrusive at a site and offers a wide turn-down range.
The newest model from QED, the EZR Tray® air stripper, was specifically designed with an emphasis on even easier ease of access for cleaning. The latest tray design is half the weight of older trays. This makes it less costly for users to purchase a spare set of trays. In addition, the trays take up less space, so soak tanks are much smaller and hold less volume of cleaning solution. Maximum flow capacity is expanded for many of the units in the new design, allowing the use of smaller models for easy to strip VOC cases.
For example, an older tray system required to treat 100 gallons/minute of flow for hexane cost around $40,000, while the new tray design could step down three model sizes at an expected savings of 26 percent.
Air stripping effective for removing VOCs
Air strippers are effective at removing dissolved VOCs and gases from water. Well maintained air stripping equipment will provide many years of efficient service. Of the available options, new sliding tray air stripper technology is a good choice that simplifies maintenance while providing high removal efficiencies.
1. EPA Drinking Water Treatability Database, Granular Activated Carbon, https://iaspub.epa.gov/tdb/pages/treatment/treatmentOverview.do?treatmentProcessId=2074826383, retrieved 7/3/19.
- United States Environmental Protection Agency, Advanced Photochemical Oxidation Processes Handbook, EPA/625/R-98/004 1998.
- Perry, R. H., and D. W. Green, Perry’s Chemical Engineer’s Handbook, 7th ed., McGraw-Hill, New York 1997.
- Ultrafiltration, Nanofiltration and Reverse Osmosis, Safe Drinking Water Foundation, https://www.hinesburg.org/water-project/safewaterdotorg-info-nano-and-ultrafiltration-reverse-osmosis.pdf.