Conventional ion exchange is a viable treatment option for applications where high-flow and high-conductivity requirements are not critical. In other situations, however, cost comparisons between ion exchange and continuous deionization may reveal potential economic advantages.
The biggest difference is that continuous electrodeionization eliminates all chemical regeneration and waste neutralization steps. While capital equipment costs may be higher with continuous deionization systems, operating expenses are lower because there are no regeneration chemicals, and labor or maintenance costs are less.
Electrical requirements are nominal. A typical system uses one kilowatt-hour (KwH) of electricity to deionize 1,000 gallons, based on a feed conductivity of 50 micromhos/cm and a product water resistivity of 10 megohm-cm.
Continuous electrodeionization systems do not require duplexing (two separate treatment units) which can increase the cost, complexity and size of the system.
Both conventional ion exchange and continuous electrodeionization may require pretreated feedwater to prevent scale formation and plugging by colloids and particles. Pretreatment is also required to reduce high levels of free chlorine and organic foulants. The type of pretreatment required is determined by product water quality requirements. For most high-purity water needs, a reverse osmosis (RO) pretreatment step is sufficient.
With RO pretreatment, continuous electrodeionization systems can achieve better than 99.5 percent salt removal, reduce the levels of individual ionic species to parts-per-billion or even parts-per-trillion levels, and produce high-purity water with resistivities of 10 to 18 megohm-cm (or 0.1 to 0.055 micromho/cm conductivity).
Continuously regenerating the ion exchange resins also removes the possibility of exhausted or improperly regenerated resins contaminating the product water.
Continuous electrodeionization systems typically convert 80 to 95 percent of the feedwater into product water. The waste stream can be discharged without treatment or recycled back to the RO pretreatment unit.
It can also reduce facilities costs because waste neutralization equipment and hazardous fumes ventilation equipment are not required. The elimination of regeneration chemicals can help improve workplace health and safety, as well as prevent corrosion from hydrochloric acid fumes. The costs of meeting EPA and OSHA requirements and "right to know" laws are also reduced.
There are significant tangible cost benefits associated with the elimination of regeneration. The costs of regeneration labor and chemicals are replaced with a small amount of electrical consumption. A typical EDI system will use approximately 1 kW-hr of electricity to deionize 1000 gallons from a feed conductivity of 50 microsiemen /cm to 0.1 µS/cm product conductivity. Since the EDI concentrate (or reject) stream contains only the feed water contaminants at 5-20 times higher concentration, it can usually be discharged without treatment, or used for another process. Thus facility costs can also be reduced since waste neutralization equipment and ventilation for hazardous fumes are not necessary.
There are also less tangible cost reductions, which are harder to quantify, but usually favor the use of EDI systems. By eliminating hazardous chemicals wherever possible, workplace health and safety conditions can be improved. With today's increasing regulatory influence on the workplace, the storage, use, neutralization, and disposal of hazardous chemicals result in hidden costs associated with monitoring and paperwork to conform to safety and environmental requirements (such as the US EPA and OSHA laws). In addition, the fumes, particularly from acid, often cause corrosive structural damage to facilities and equipment.
For the most part the elimination of regenerant chemicals is considered advantageous, but the chemicals do offer at least one benefit. In conventional demineralizers, acid and caustic is typically applied to the ion exchange resins at concentrations of 2-8% by weight. At these concentrations the chemicals not only regenerate the resins but clean them as well. The electrochemical regeneration that occurs in a EDI device does not provide the same level of resin cleaning. Therefore proper pretreatment is even more important with a EDI device, in order to prevent fouling or scaling. This is one of the reasons that RO pretreatment is normally required upstream of a EDI system. In general, the feed water requirements for EDI systems are more stringent than for chemically regenerated demineralizers.
- EDI does not require chemicals (as does DI resin regeneration)
- EDI does not require shutdowns
- EDI modules are the smallest and lightest per unit flow on the market. EDI systems are compact as well.
- Consistent, continuous ultrapure water
- Requires little energy
- Economic use of capital, no operating expenses, just the cost of power.
- Reduced facility requirements
- Minimal operator supervision required