For more than ten years, the US Environmental Protection Agency (EPA) has been studying radon in drinking water. Because of the cost of removing radon from water, there has debate between the EPA and public water system operators about what is an acceptable maximum contaminant level (MCL).
In 1988, the standard was expected to be proposed at 10,000 picocuries per liter (pCi/L). In 1989, the expected standard was dropped to 200 pCi/L. In the past year, proposals have shifted to the 3,000 pCi/L range.
At this time, it appears that the EPA will propose a radon standard for municipal drinking water in April 2000. This standard will drive the residential radon removal market. The standard is now expected to be approximately 3000 pCi/L.
This moving target radon standard has caused manufacturers to develop many different types of aeration systems for radon removal.
Air stripping separates volatile compounds from water. Compounds that air strip well evaporate easily and are only slightly soluble in water. Gasoline components air strip well because gasoline is very volatile and only a small amount dissolves in water. Radon is so volatile that it is normally a gas and it too is only slightly soluble in water. Radon is one of the most easily air strippable compounds.
Air stripping is a transfer process. The chemical contaminant is transferred from the water phase to an air stream. The transfer of the contaminant takes place at the interface between the water and the air.
Therefore, an increase in the amount of liquid surface area in contact with the air stream will increase the rate of volatilization of the radon into the air. The major design variable for an air stripper is the production of large amounts of air/water surface area.
Five basic types of air strippers are available that can be used for radon removal. The goal of all these systems is to generate as much air and water surface area as efficiently as possible.
Recirculating sprays. These systems use the water pressure provided by the well pump to spray the radon-laden water into a tank (see Figure 1). Air passing through the tank contacts the small water droplets. The radon evaporates from the surface of the water droplets and is carried out of the system with the air stream.
The removal efficiency of a spray nozzle system is usually in the range of 50 to 70 percent. To improve this efficiency, a recirculation loop is sometimes added. This may increase the efficiency to as much as 85 percent.
Packed towers. Packed towers generate air/water surface area by trickling the water through a bed of packing material (see Figure 2). The packing material looks similar to wiffle balls. As the water flows down through the bed of packing, it coats the surface of the packing with a thin film of water.
A stream of air blown up through the packing material contacts the water film and volatilizes the radon. Typical efficiency of this type of residential system is approximately 92 percent.
Venturi nozzles. A venturi nozzle generates air/water surface area by forcing water through a nozzle at high velocity (see Figure 3). The passage of the high velocity water jet creates a suction and draws air into the water stream.
This is similar to what happens with a fire hose where the force of the water jet draws in air from behind the person holding the nozzle. A venturi nozzle can pull in about 1 cubic foot of air for every cubic foot of water that flows through the nozzle. Radon removal efficiency using a single venturi nozzle is about 70 percent.
Diffused bubble aeration. Diffused bubble systems are very simple in concept (see Figure 4). Bubble some air up through a tank of water and the radon will transfer from the water to the air. In practice it is a little more complicated than that.
In this process, the mass transfer area is limited to the surface area of the air bubbles. Therefore the more air bubbled through the water and the smaller the bubbles are, the more surface area will be generated.
Diffused bubble systems can have efficiencies from 70 to 99.9 percent.
Sieve trays. Sieve trays also bubble air through water to generate surface area (see Figure 5). However, they use a much larger volume of air to actually generate a froth of air and water. This creates a very turbulent volatilization area that generates very large amounts of mass transfer area. Sieve trays have removal efficiencies of 99.9 percent.
Another alternative radon removal technology is granular activated carbon (GAC). Water treatment dealers typically use GAC for taste and odor control and for chlorine removal. Radon also adsorbs onto GAC.
However, because radon is a radioactive element, it is constantly decaying and changing its form. In a little over 3 days, half of the radon adsorbed on GAC will have decayed to lead 210. Radioactive lead is a source of gamma radiation.
Gamma ray emission rates depend on the radon concentration and the water treatment rate.
GAC should not be used for high radon concentrations because of the possible gamma radiation problem that it will generate. Some authorities recommend that GAC only be used on radon concentrations less than 5,000 pCi/L. Gamma rays can be shielded with lead or big tanks of water.
The installation procedures for most aeration systems are quite similar.
Sizing the system is critical. This requires an accurate measurement of the flow rate the well pump is capable of producing. Use standard flow rate measurement techniques recommended by the well pump manufacturers. If the well pump produces too little water, it will starve the repressurization pump and cause the system to shut down.
Proper venting of the air stream from the stripper is essential. The exhaust air can contain several thousand picocuries per liter of radon in the air in some cases.
Air vent lines must be free of leaks, and be vented at a location where the radon will not be drawn back into the house. Typically the air vent should be located above the eve line of the house and as far away from the doors and windows as possible.
However, studies have shown that radon in air levels dissipate to background levels within 5 to 6 feet of the vent outlet. Be sure to follow the manufacturer's recommendation for sizing the vent line. Undersizing this line will cause high-pressure losses and significantly reduce the efficiency of any air strippers.
Interaction of the air stripper with other water contaminants and water treatment systems must be considered. Aeration of the groundwater will cause precipitation of iron, manganese and hardness. Filters may need to be installed following the air stripper. If other water treatment systems are already installed, it is important to consider how the installation of the air stripper may effect their operation. How the process train is arranged will depend on the specific water quality at each location.
System maintenance is essential for continued efficient operation. Regular testing of the radon removal efficiency is necessary since radon is tasteless, odorless and colorless. The homeowner has no other way to know whether the system is operating correctly or not. A minimum of one test annually of the untreated and treated water should be done. The system should also be cleaned and disinfected annually. Any air filters should also be checked, cleaned or replaced as needed annually.
Radon systems are generally sold during real estate transactions. Therefore, the seller typically wants to install the least expensive system possible. It is important that the water treatment dealer sell a cost effective program that will reliably meet the removal requirements and be easy to operate and maintain.
If equipment is chosen carefully and installed and operated correctly your customer can be satisfied with an air stripping system. Proper design, selection, sizing, installation and maintenance of air strippers provides a good answer to a difficult water treatment problem.
Bruce L. Lamarre is chief technical officer for North East Environmental Products, Inc., West Lebanon, NH
(whole house & well units)