First employed as a way to remove hardness (calcium and magnesium) from domestic water supplies, ion exchange has been a method of removing dissolved ions from water for over 100 years.
In more modern history, ion exchange has been employed as a method to remove dissolved metal contaminants from wastewater streams prior to discharge to the sewer or the environment.
In this blog we’ll describe how ion exchange works, and some of its operating implications in a wastewater treatment context. Furthermore, we’ll compare this technology with ElectraMet’s electrochemical method of separating dissolved metals from waste streams.
Ion Exchange for Metals Removal Basics
The most common form of ion exchange for metals removal involves passing water through a deep bed of ion exchange beads inside a pressure vessel. The beads themselves are made of resin, and are often less than a millimeter in diameter. When passing through the bed of ion exchange beads the water must take an arduous path through the beads, putting all the water in close contact with beads.
The chemical structure of the ion exchange beads incorporates reaction sites, called functional groups, which are either negatively or positively charged. Attached to the functional groups are the ions which will be exchanged for the targeted contaminant.
Most metals are positively charged cations when dissolved, and are exchanged using cation resin. The cation resin has negatively charged reaction sites, which are loaded with positively charged hydrogen ions (protons) by conditioning the resin with either hydrochloric or sulfuric acid.
The negatively charged reaction sites will have a stronger affinity to most metals compared with the hydrogen ions. As the water makes contact with the resin beads hydrogen ions are exchanged for the metal ions leaving the metals ions bonded with the resin.
The bed of resin beads will have a capacity for holding metal ions limited by the number of reaction sites per bead, and the volume of the bed. When this capacity is reached the resin bed is said to be “exhausted,” and metal will no longer be removed.
When a bed of ion exchange resin is exhausted there are three basic options.
- Chemically regenerate the resin in place
- Remove the resin for regeneration offsite
- Remove the resin and send it to solid waste
Regenerating an ion exchange vessel in place is accomplished by flooding resin with a high concentration of the original ion that was exchanged. In the case of cation resin for metals this is most often ionic hydrogen. The source of the hydrogen is either hydrochloric acid or sulfuric acid.
Despite having a higher affinity for the metal removed, flowing a high concentration source of proton hydrogen over a long period of time will eventually strip the metal from the resin beads, recharging them with hydrogen.
In offsite regeneration this same regeneration process is employed, however, the resin is first removed from the vessels, and then shipped to a services site for regeneration there.
In some contexts simply removing the resin and sending it to a landfill may be appropriate.
Considerations for Metals Ion Exchange
Ion exchange excels in where there is a low concentration of a contaminant, and the goal is to make it close to zero. In this context resin replacements, or regeneration cycles will be infrequent.
Ion exchange resin will have a limited capacity to hold the target metals. As the concentration of target metals increases the resin becomes exhausted faster requiring more frequent regenerations or resin replacements.
Secondary Waste Streams
When regenerating resin in place liquid waste is created in the form of:
- Regeneration acid
- Dilution water (for some applications demineralized water is required)
- Backwash water
- Rinse water (water used to clear the acid from the resin after regeneration)
- Neutralization chemicals (used to pH adjust the waste stream)
In place regeneration of ion exchange resin can be problematic in a wastewater context. This process essentially replaces a liquid waste stream with a more concentrated liquid waste stream albeit a small one. A plan must be in place to deal with this liquid.
When disposing of the resin as solid waste, the resin itself becomes the waste stream. If loaded with metals this resin may have specific transportation protocols and require a specially permitted landfill.
The volume of these waste streams depends on the concentration of the target contaminant, and the capacity of the resin. High concentrations of target materials will exhaust resins faster. In some cases this makes the volume of regeneration waste, or disposed resin, unsustainable economically and operationally.
The ability for ion exchange resins to selectively target specific metals, or dissolved ions varies widely depending on the concentration of the target material, competing ions, and the overall background concentration of dissolved solids.
Wastewater streams often have elevated levels of dissolved solids. A facility may only be required to remove copper from the wastewater stream, however, high concentrations of calcium, sodium, and other cations may compete for reaction sites in the resin. This exhausts the resin faster with material that doesn’t require removal.
Process Assurance and Compliance
When designing an ion exchange system the bed size and regeneration frequency is calculated based on the incoming concentration of the target material. The bed must allow a sufficient contact time to reduce the target contaminant to the discharge limit, and a regeneration must occur before the bed is exhausted and contaminants break through the bed.
Wastewater streams are frequently subject to variability and process upsets. If a slug of water with a significantly higher concentration of contaminant moves into the system the bed size may not be sufficient to remove enough of the contaminant to achieve the discharge limitation. Additionally if there is an extended period of time where the concentration of the contaminant is elevated, the bed may become exhausted prior to a regeneration sequence discharging contaminated water until the regeneration sequence is queued.
ElectraMet’s Innovative Approach
ElectraMet offers a revolutionary approach to metal impurity removal for discharge compliance and material purity improvement. ElectraMet’s electrochemical process removes dissolved metals and recovers them as either solid metal or a highly concentrated recoverable sulfate solution.
Our solution offers a number of advantages as an alternative to ion exchange.
ElectraMet has three module designs to efficiently handle removal of metals from very low to very high concentrations and reduce them to low part per billion levels if required.
Secondary Waste Streams
In many configurations the ElectraMet solution recovers metals as high purity solid metal with resale value. There is no secondary waste stream.
In some circumstances a high concentration metal sulfate solution is produced. In the case that this cannot be off taken for value it is at least a smaller volume of liquid waste orders of magnitude smaller than other solutions.
The ElectraMet solution replaces waste and disposal costs with reusable materials and revenue.
ElectraMet’s solution works by the electrochemical reduction of specific metals by introducing an electric potential to the stream. Natural differences in the voltages at which metals are reduced to solids allows us selective targeting of specific metals even in streams with extreme background TDS.
Process Assurance and Compliance
The ElectraMet system uses electric current correlated in quantity with the mass of target material being removed. The system will automatically draw more current to adjust for increasing concentrations of target material entering the system. In this way the system can handle process upsets and alert operators that something may have changed in the upstream process.
By comparing the electrical current utilized at the beginning and end of processing the system provides continuous feedback confirming the target material has been reduced below the discharge limits.
As industry faces higher waste disposal costs and pressure for sustainable operation ElectraMet is positioned to provide metals wastewater treatment with lower cost, reusable materials instead of waste, and superior discharge compliance.