Radionuclides in Drinking Water
Ion ExchangeDuring ion exchange treatment, water is passed through a resin containing exchangeable ions. Stronger binding ions displace weaker binding ions and are removed from the water. There are two types of ion exchange—anion exchange and cation exchange. Anion exchange resins generally exchange chloride for anionic contaminants, like uranium. Cation exchange resins generally exchange sodium or potassium for cationic contaminants, such as radium. Mixed bed resins with cation and anion exchange media in two layers are available for systems that need to remove both radium and uranium. Ion exchange is also effective for the removal of beta particles and photon emitters.
Ion exchange has been identified by EPA as a “best available technology”(BAT) and Small System Compliance Technology (SSCT) for radium, uranium, gross alpha, and beta particle and photon emmiters. It can remove up to 99 percent of these contaminants depending on the resin, pH, and competing ions. Ion exchange resins are regenerated by a series of steps, including backwashing, brining, and rinsing. Ion exchange vessels typically have a service capacity of 200 to 1,500 bed volumes (BV) for radium, as a function of water hardness, and 100,000 to 300,000 BV for uranium.
Ion exchange columns can be automated to require minimal operator attention making them appropriate selections for small systems. They can also be used as point-of-entry (POE) devices. Ion exchange columns can also remove other contaminants. Alkalinity, nitrate, and arsenic are removed by anion exchange. Cation exchange resins remove hardness constituents such as calcium, magnesium, iron, and manganese.
For ion exchange to work, the contaminant must be in the proper ionic form and must bind more strongly than the displaced ion. Competition by other ions (such as sulfate and hardness) can reduce the service capacity of the resin bed for the target radionuclide.
With ion exchange, a more preferred ion can cause an effect called chromatographic peaking of a less preferred ion. Chromatographic peaking causes the less preferred ion to exit the resin bed at a higher concentration than the influent concentration. This occurs if the bed is not regenerated at the proper time. The effects of chromatographic peaking or low pH with anion exchange can be minimized by operating two or more ion exchange beds in parallel. If chromatic peaking is a concern, parallel beds can be operated out-of-phase in their respective service cycles, so any breakthrough of arsenic, nitrate, and alkalinity in a single bed is diluted with the other treated streams from the remaining bed(s).
Anion resins can be damaged by oxidants such as chlorine. If pre-disinfection cannot be avoided, the chlorine should be dosed so that very little is left when it enters the ion exchange unit. Chlorine can also be removed before entering the ion exchange unit with dechlorination chemicals or activated carbon.
Resin fouling may also be a concern with ion exchange. Particulates and metals (e.g., iron, manganese, or total organic carbon [TOC]) in the source water can clog the media or precipitate and block exchange sites. Pre-filtration and pre-treatment can help to remove fouling compounds and preserve bed life; however, the pre-treatment process may also remove the radionuclide and create a disposal problem. Anion exchange can reduce the alkalinity of the water. Raising the alkalinity by adding carbonate or caustic may be necessary.
Although resins can be regenerated, the process produces waste solutions with elevated levels of the contaminants removed and the regenerant salt. Treatment residuals generated by ion exchange may include brine, backwash, rinse water, and aged/ineffective resins. Liquid disposal options may include direct discharge, discharge to a sewer system, discharge to a wastewater treatment plant, or disposal to an underground injection control (UIC) well. Aged/ineffective resin will need to be disposed of in an appropriate class of landfill. Some states may allow land application of treatment plant sludges and spray irrigation of wastewater. Radionuclides may become so concentrated in the brine and the resin that they may require special handling and disposal procedures. Systems may use disposable media that can be removed by a waste broker, and use the resin to exhaustion (rather than regenerating), especially if disposal of liquid residuals to a wastewater treatment plant is not an option. Refer to the disposal section of the Web site for more detailed information. Disposal Issues