Summary: Elimination of volatile organic compounds (VOCs) can be achieved in varying degrees using different approaches. The following article discusses activated carbon and aeration as well as the importance of pre-treatment.
The world of volatile organic compounds—or chemicals such as herbicides, pesticides, gasoline compounds (BTEX, MTBE), perchlorate, TCE/PCE, etc.—is vast and varied. Many of these compounds are regularly used in industry and the home. As with any potentially polluting materials, the most effective treatment is to handle and dispose of them properly. Since it’s inevitable that—through careless handling or accidents—some portion of these compounds will make their way into our groundwater, we must determine the most effective ways to treat them.
With most water treatment, we can consider treatment as either point-of-use (POU) or point-of-entry (POE). The POU market is flooded with devices—usually carbon cartridges—rated for VOC removal. Most should be used for treating water that’s already considered potable, but only where there may be a concern over the potential for contamination. Be sure to look for devices that are NSF certified for VOC removal and not just for taste and odor. Choose devices that will shut the water off after the rated number of gallons have passed through the filter. We recommend these devices for two reasons. First, this provides the best protection for the customer against bleed-through of the contaminant. Second, it will provide you with a recurring revenue stream for replacement cartridges. Carbon is a potential breeding ground for bacteria. If carbon is used on a non-chlorinated supply, extreme care must be used to sanitize the system at start up and during carbon changes. Some manufacturers provide cartridges that come in sealed containers, which are better choices because the water contact surfaces don’t need to be handled and, therefore, contamination potential is reduced.
Many of your POE applications will be the result of a local governmental agency request. These treatment systems are usually funded by insurance policies paid for by the consumer of the hazardous product in the form of a use tax. The treatment will be requested for a water supply that’s already contaminated or is in an area likely to be contaminated by a nearby VOC spill. Usually, the agency will dictate what gets put in for treatment—carbon, aeration or a combination. This is why we recommend you find the agency that’s responsible for this type of request and take time to meet the people making these decisions. Often, these agencies will have a list of potential water treatment specialists they use in a given situation. These jobs aren’t usually bid situation as the need is typically time sensitive. You must be ready to go when they call. Service is everything and, if you’re not willing to provide it, you’ll be dropped from the list.
Most of the time, equipment is requested on a rental basis. Rental costs are usually broken down as follows:
- Installation cost,
- Rental fee on the equipment,
- Bed change costs if carbon is used (on a per change basis), and
- Removal and disposal cost when treatment is terminated.
Treatment enhancement, carbon changes and treatment termination are usually dictated by the testing results. Testing is usually done by the agency that requested the treatment. From time to time, you’ll be asked to make recommendations as to the most cost-effective way to treat the problem—carbon alone, aeration, or a combination. You’ll need to have a good understanding of the adsorption and aeration processes and good support from your suppliers. Your supplier can tell you what to expect for the performance, but you’ll need to do calculations to apply their products. Be helpful by giving expected performance data. It’s important to demonstrate your knowledge and a willingness to cooperate but, at the same time, be mindful of the fact you’re a professional and your knowledge and time are valuable.
It has been our experience that most of the treatment for VOCs in residential potable water applications is done with activated carbon. This is usually because of the low capital costs and typically low concentrations being treated. A typical carbon tank installation will require only two, 10-inch×54-inch tanks with simple in-and-out, non-backwashing heads, like those used in exchange tank systems.
When installing carbon systems, there may be a concern over radiation exposure due to adsorption of radon (radium or uranium) from water. There’s also risk of exposure to gamma radiation from the system while it’s in service, and the possibility of creating low level radioactive waste if left in service too long. The U.S. Environmental Protection Agency (USEPA) recommends carbon not be used on water with radon concentrations above 5,000 picocuries per liter (pCi/L). Free software available on the USEPA website (see www.epa.gov/ne/eco/software/index.html) calculates exposures and waste disposal limits for carbon.
Another concern with using carbon is potential bacteria growth in the bed. The best solution is to use carbon on chlorinated supplies. If a chlorinator is installed, care must be taken to ensure the chlorine won’t be precipitating minerals that will be filtered out in the carbon bed. If chlorine isn’t going to be used, water should be shown to be biologically safe to drink before installing the system.
Preparation of the media tanks is important to the final system performance. The tanks should be filled, soaked and backwashed before going to the site. If the tanks are filled before going to the site, the water will need to be blown out of the tanks to reduce the potential for lifting-related injuries. Once at the site, the tanks should be filled with water and then backwashed briefly to re-classify the beds. This can be accomplished by connecting the tank(s) in up-flow configuration, allowing the discharge run to drain. The final connection of the tanks needs to be in a down-flow configuration.
The tanks should be installed in series. Often, only two tanks are required. Be sure to place three sample ports—one for raw water, one between the tanks to detect bleed-through, and one after both tanks. You’ll also want to place a cartridge filter on the raw water to keep sediment from entering the tanks. It’s also a good idea to place pressure gauges on each side of the cartridge filter, and carbon tanks to monitor pressure losses throughout the system (see Figure 1).
Some people place cartridge filters after the carbon tanks to catch carbon fines. This shouldn’t be necessary in a down-flow installation. If black material is coming through, it isn’t necessarily carbon fines but some other contaminant—like manganese—that builds up on the bed and is pushed through during peak flows. This contaminant could affect the performance of the carbon. Thus, it’s important to address the problem by treating the contaminant instead of treating the symptom.
If you need to predict life expectancy of the carbon, you’ll need to know the concentration of the contaminant(s) in the water, expected daily water use, and capacity of the carbon in gallons per cubic foot (gal/ft3). If you provide your supplier with the type of contaminant and the concentration, they can tell you the expected life of the carbon in gal/ft3. To calculate the life expectancy of a system that consists of two 2-ft3 tanks placed in series, an estimated water use of 300 gallons per day (gpd) and carbon rated at a capacity of 10,000 gal/ft3, see the formula in Figure 2.
Carbon suppliers will provide you with the recommended empty bed contact time (EBCT) for proper flow rates. You’ll need to size the system so the lead tank will provide the EBCT. The goal is to have the lag or “guard” tank in the series only provide backup protection. This allows samples to be taken between the tanks that show adequate contaminant removal. If the standard-size tanks don’t provide for adequate EBCT, make sure the customer understands why you need to increase the size of the system.
Aeration will only be requested on the most difficult-to-remove contaminants or where radon build-up on the carbon is a concern. A common problem contaminant that will require aeration is MTBE. This compound—a fuel oxygenate to make gasoline burn more cleanly—has a very high solubility and can be found in high concentrations that make it impractical to remove with carbon alone. There are very few choices for residential aeration systems. We know of only one manufacturer of residential-sized systems that provides extensive modeling data to allow prediction of removal efficiencies. This data have been collected through actual testing on full-sized units. It’s critical to know what the removal rates are to size the system. We recommend you only work with units that have this type of data available.
Even very good aeration units will need to have flows reduced to provide for adequate removal of many of the more soluble compounds. You need to be prepared to store water for later use just as you would for whole house, reverse osmosis (RO) systems. Depending on the requesting agency’s requirements, you may have to show the concentration of the contaminants in the exhaust from the unit. The supplier of the equipment can help you with this calculation.
You’ll also need to consider the concentration of other contaminants like iron and manganese that will precipitate once aerated. Some aeration units will handle these precipitated contaminants better than others. Because of the difficulty of handling the contaminated wastewater from pre-treatment, you may choose to pass these contaminants through the aeration unit and follow it up with filtration. Provisions need to be made for the eventual periodic cleaning of the aeration unit.
All aeration units will store the treated water at atmospheric pressure. This means “breaking pressure” will have to occur. Breaking pressure allows the water to be exposed to the atmosphere. When water is exposed to the atmosphere, there’s a potential for introduction of biological contamination. We suggest you consider provisions for some type of disinfection.
Elevated concentrations of iron, manganese and hydrogen sulfide are common problems that may require pre-treatment. A pre-treatment system will require design of a clean water regeneration/backwash system to prevent VOCs from being released. Clean water can be stored in an atmospheric storage tank and automatic controls can provide the clean water for regeneration/backwash.
So now what?
Go out and find those suppliers that will support you and provide the right information. Use what you’ve learned here to test their knowledge. The good ones will take the time to get you what you need. Be loyal and they’ll continue to be helpful. Then, find out who’s responsible for the treatment of contaminated supplies in your area. Show them your credentials and willingness to work with them. Offer to meet with their staff to introduce your company’s capabilities and experience. Watch industry magazines, papers and other news outlets for information on areas that have VOC problems. Encourage your customers in the area to request that you do the work on their system if they need this type of treatment. This is all about relationships and knowledge.
Go make it happen!
About the author
Jeff Twitchell is vice president of Air & Water Quality, of Windham, Maine. The company provides water treatment and radon mitigation for both the residential and commercial markets. He has bachelor’s degrees in chemistry, physics and mathematics. He has and maintains a Class II operator’s license for both distribution and treatment and has NEHA certification for Radon Service Provider and Tester. He can be reached at (800) 698-9655, email: email@example.com or website: http://www.awqinc.com.