The ion exchange treatment process works by exchanging ions in well water with ions from a salt brine solution. Many of the naturally occurring contaminants to remove from well water have both positive and negative charges. Knowing the electrical charge of the contaminant in well water will determine what type of ion exchange treatment system will be most effective for their removal.
Ion exchange treatment systems use resins which are plastic beads made from polystyrene or acrylic. They measure 0.5 to 1.0 mm in diameter and are used in ion exchange filter tanks where the resin beds are typically about three feet deep. Water passes through the bed of resin where the ion exchange takes place. The resin is regenerated with a salt brine solution, acid or caustic solution depending upon the treatment process. There are four primary types of ion exchange resins: strong acid cation, strong base anion, weak acid cation, and weak base anion.
How Cation and Anion Exchange Units Work
Cation exchange treatment is effective for treating contaminants in your well water with a positive charge. Cation exchange is also commonly referred to as water softening. Cation exchange is commonly used for the removal of the hardness cations, magnesium and calcium. However, cation exchange treatment systems are also often used to remove other cations such as iron and manganese. A brine solution commonly consisting of potassium chloride or sodium chloride is used to regenerate the resin beads in the filter tank. The treated water from a cation exchange treatment system will usually have higher levels of sodium or potassium, depending on the type of brine salt solution that is used. DPH has a guidance-based health level for sodium in drinking water at 100 mg/L; therefore, it is recommended you have your treated water tested if you are on a salt restricted diet.
Anion exchange treatment units work virtually the same as cation exchange devices. In anion exchange, the resin beads are saturated with negatively charged ions that exchange places with the negatively charged contaminants in your well water that have a negative charge. Chloride ions are commonly used in these treatment systems. Anion exchange units are commonly used to reduce or remove nitrate and arsenic from water. The treated water will have higher levels of chloride (the anion) from the brine solution; therefore, it is important to test the treated water to ensure your chloride levels are not too high. The maximum contaminant level for chloride in drinking water is 250 milligrams per liter.
Types of Units
Ion exchange treatment systems consist of a filter vessel that contains the ion exchange resin. In most cases a control head that automates the backwash and rinse cycle is mounted on top of the filter vessel. Some ion exchange units do not have a control head to automate the backwash and rinse cycle; therefore, the filter vessel is removed and replaced with a filter vessel with clean ion exchange resin. Some companies may offer delivery of portable exchange units that are replaced at periodic intervals.
Ion exchange treatment systems that are equipped with a control head that will establish the frequency of the backwash and rinse cycle based on time or a volume of water. Older style ion exchange treatment units that operate on a timer are being replaced with those that operate based on a volume of water passing through the control head. The timer-based backwash control treatment units will typically use more salt brine solution and will backwash more frequently than needed. The newer on-demand style backwash control treatment units will backwash based on the volume of water treated, resulting in less salt brine solution being used and fewer backwash treatment cycles being discharged to the environment.
Ion exchange treatment equipment requires the incoming water to be at a minimum flow rate and pressure. Check the treatment system manufacturers’ requirements for flow rate and pressure and consult with your plumbing contractor to determine if the capacity of your existing water supply system is adequate. Upgrades may be needed to your water system to meet the ion exchange treatment equipment flow and pressure requirements.
Ion exchange units are usually rated according to the maximum flow rate of water they can accommodate, their total softening capacity, and the gallons of treated water produced between regenerations. Many factors affect the selection of an ion exchange unit, including:
- Concentration of the brine solution used for regeneration.
- Concentration of other minerals (such as sodium or sulfate) in the raw water.
- Presence of impurities (such as suspended particles) in the raw water.
- Presence of oxidized iron and manganese in the raw water.
It is important to have your untreated water tested before installing any treatment system so you know the level of minerals in the raw water that will impact the sizing and design selection of your treatment system.
Maintenance
Regardless of the quality of the equipment purchased, it will not perform satisfactorily unless it is maintained and installed in accordance with the manufacturer’s recommendations. Keep a logbook to record equipment maintenance and repairs.
Maintenance includes restocking the brine solution for the treatment system’s regenerating process. The frequency depends on whether the unit is a one or two tank system, the size of the brine storage compartment, if the treatment unit is equipped with a timer-based or flow-based controller, and how much treated water is needed for the home.
Exchange resins wear out over time. Check with the manufacturer to determine the general life span of the resin bed and its service frequency. If the resin bed is not regenerated on a regular basis with a good backwash and at proper intervals, it may become contaminated with slime or impurities from the raw water and become unusable. If raw water contains iron or manganese bacteria this is more likely to occur. If this happens, the resin must be replaced with new material. The effectiveness of this system may also require that raw water be pre-treated to adjust pH levels and filter sediments or other impurities before entering the ion exchange system.
Other Considerations
After installation, retest both the untreated water from the well and treated water at a state certified laboratory. Compare the results of the treated and untreated water to determine if the ion exchange unit is properly removing contaminants. Test untreated and treated water annually or more frequently if high levels of contaminants are present in the untreated water. Frequent testing will help you determine how well your treatment system is working and whether maintenance or replacement of components may be necessary.
Note: If treating to reduce hardness in the water, homeowners may want to consider limiting water softening to their hot water system lines only. This could help reduce the size of equipment needed; potentially the frequency of regeneration required and eliminate concerns regarding added sodium to the cold-water lines used for drinking and cooking. Also, homeowners can select units that exchange potassium instead of sodium if anybody in their home is on a salt restricted diet.
Backwash Wastewater Generated
Treatment backwash is generally necessary to lift, clean, and regenerate the filter media for ion exchange treatment devices. This process may also help eliminate any channels that may have formed in the filter media.
If you have an on-site septic system, water treatment wastewater must be discharged in accordance with current criteria established in the CT Department of Public Health, On-site Disposal Regulations and Technical Standards for Subsurface Sewage Disposal Systems. Check with your Local Health Department on any permitting or approval process in your town.
Contact CT Department of Energy and Environmental Protection if you are connected to municipal sewer and the WPCA does not allow discharge of treatment backwash water to the sewer.
Questions to Ask Before You Buy
Before purchasing a water treatment device, be sure to have your water tested at a state certified laboratory to accurately determine what contaminants are present and at what concentrations. This will help you determine if ion exchange is the most effective treatment device for your water. Refer to our guidance Questions to Ask When Purchasing Water Treatment Equipment for your Home, for more information.
For More Information & Additional Resources:
For a full list of private well related publications and fact sheets, regarding contaminants and treatment technologies: CT DPH Private Well Program, Publications and Fact Sheets
For general questions regarding testing, corrective action, treatment, etc. please contact: CT DPH, Private Well Program, 860-509-8401 or by email: DPH.PrivateWellProgram@ct.gov
For questions regarding health based concerns please contact: CT DPH, Toxicology Program at 860-509-7356 or by email: DPH.Tox@ct.gov