Deionised water, or DI water, has had the majority of the ions removed through a purification process. Clinical laboratories use deionised water for a variety of purposes, each requiring different levels of purity. For example, to produce deionised water to meet the current requirements of the common standards used by many laboratories1 a typical clinical water purification system may include the following technologies either individually or in combination.
- Deionisation (DI): using mixed bed ion exchange resins to exchange dissolved ionic contaminants
- Electro-deionisation (EDI): incorporating ion-selective membranes, ion-exchange resins and electricity to separate out and remove dissolved ionic contaminants
- Reverse osmosis (RO): using a semipermeable membrane to reject particulates, bacteria, dissolved ions and more
- Filtration: using sub-micron membrane filters to catch microscopic contaminants, such as bacteria and endotoxins
- UV radiation: using germicidal wavelength UV light at 254nm to inactivate, ‘kill’, water borne bacteria.
How does EDI or CEDI work?
In a basic deionisation system water is deionised by passing it through a bed or cartridge containing mixed ion-exchange resins. These resins have a finite life and must be replaced and safely disposed of once they are exhausted.
Resin replacement can lead to significant on-going costs, which may not have been considered when the clinical water purification system was originally purchased. For example, a typical bench top unit purifying 40 litres per day from a typical London water source, with a high level of total dissolved solids (TDS), could require replacement of the deionisation cartridge resin every two to three months.
By comparison, EDI technology or, to give its correct name, continuous electro-deionisation (CEDI), eliminates the need for replacement cartridges by combining ion exchange resins and ion-selective membranes with a direct electrical current to remove the ionised species from the source water. The CEDI stack is constructed from alternating concentrate compartments and feed/dilution chambers filled with mixed bed ion-exchange resin.
Each compartment or chamber being separated from one another by cation and anion permeable membranes; these membranes selectively allow the passage of either cations or anions across the cell. Cathode and anode electrodes are placed on the outer sides of the stack. Feedwater flows into the stack and is split, with approximately 90 percent passing into the feed/dilution chamber, 5-10 percent into the concentration chamber and less than 5 percent into the electrolyte compartment.
The cations and anions in the feedwater are drawn towards their respective electrode, cations being drawn to the cathode and anions towards the anode. Mixed bed ion exchange resin in the feed/dilution chamber assist in the migration progress and reduce the amount of electrical power needed to attract the ions. As cations move towards the cathode, they pass through cation permeable membranes and into one of the concentration compartments. Further movement towards the cathode is halted by anion permeable membranes.
The rejected ions being flushed from the system in the concentrate stream. The same process of rejection applies to the anions.
Although CEDI is an effective solution, it does have several drawbacks. In particular, the CEDI cell must be protected by pre-treating the water supply. If this is achieved correctly, then an operational life for the CEDI cell of between three and five years is possible; in systems where pre-treatment is inefficient or has failed the life of the cell will rapidly diminish to months or even weeks. Cell replacement is expensive, while the addition of a pre-treatment unit adds to the overall purchase cost of the system.
EDI/CEDI systems are particularly sensitive to the presence of calcium and magnesium ions in the source water – generally referred to as hard water. Dissolved carbon dioxide can also dramatically affect the treated water quality. If the level of total dissolved solids (TDS) is too high the cell can quickly become saturated in dissolved minerals, leading to a significant decline in deionisation efficiency.
Utilising effective EDI technology in water treatment
The first challenge is to reduce water hardness. Ideally the feed water to the EDI cell should have a total hardness level in the region of less than 1 part per million (ppm). Across the UK, for example, water hardness can vary dramatically from region to region, ranging from extremely soft at approximately less than 50 ppm, to extremely hard, up to 300-500 ppm.
Many existing bench top EDI water systems use a conditioning cartridge to reduce water hardness. These cartridges often contain ion exchange resins identical to those used in household domestic softeners; unlike domestic systems, however, which can be regenerated with brine (sodium chloride solution), when the conditioning cartridge has consumed its capacity for calcium and magnesium hardness it has to be replaced. The service life of these cartridges will vary considerably depending on the hardness of the incoming feed water hardness and the volume of water used.
Some manufacturers utilise sequestering agents, which complex-up the hardness into compounds that do not cause scaling, but again these have to be replaced once they have become exhausted. This can add significantly to the ongoing running costs if the unit is installed in an area of high water hardness.
A new approach developed by SUEZ Water Purification Systems (formerly Purite) eliminates the need for conditioning cartridges. Instead, two stage – or double pass – reverse osmosis (RO) technology is used. This double pass system has been engineered to operate efficiently with even the hardest of feed water sources, thereby offering far lower operating costs than water purification units incorporating conditioning cartridges or sequestering agents. The process of purification is continuous and consistently provides the correct quality, in terms of water hardness, to the EDI cell.
The adoption of double pass RO technology also offers further benefits. For example, by comparison with all other clinical water purification systems that have standardised on single pass RO, the double pass system reduces TDS to far lower levels; with a single pass (RO) there is typically a 96-98 percent reduction in TDS, while a double pass system the remaining impurities are further reduced by 96-98 percent to create an exceptionally high purity supply. This provides greater protection to the EDI cell, preventing scaling and helping to optimise its operating life, and results in higher overall quality of the purified output water.
A further advantage is protection from the effects of carbon dioxide on water quality; the higher the concentration, the greater the effect on the resistivity of the water – even levels as low as 5ppm can have a significant effect.
Although many clinical water purification systems use RO as part of the pre-treatment stage, this is only partially effective as a means of eliminating carbon dioxide. Carbon dioxide has a molecular weight of 44, while RO membranes have a molecular weight cut off (MWCO) of around 100-150. As a result, any carbon dioxide present in the feed water can pass through the system into the purified water stream, where it can potentially damage the EDI cell.
Typically, in a hard water region the free residual carbon dioxide levels downstream of the RO stage can be in the order of 20-50 ppm, requiring the installation of additional pre-treatment filters. By comparison, the technology developed by SUEZ uses the latest hollow fibre gas transfer membranes to strip carbon dioxide from the water. Based on Henry’s law of partial pressures, the inert hollow fibre membranes allow dissolved carbon dioxide to move from the water phase into the gaseous phase across the fibres. The process is extremely efficient and is driven by air provided by an on-board mini air pump.
This new technology is available in SUEZ’s Select edi 60 laboratory water purification system. The Select edi 60 is a compact and easy to use system that, thanks to the use of advanced manufacturing techniques, is competitively priced. Running costs are also low, due to the inherent reliability of the technology used, improved energy efficiency and low consumable demands. The Select edi 60 is ideal for use in all clinical laboratory applications, especially in hard water regions, or where there is a high concentration of dissolved carbon dioxide in the feedwater.
To learn more about the Select edi 60 visit: https://www.suezwatertechnologies.com/products/water-purification/select-edi-60
1E.g. BS 3696 Water for Laboratory use and ASTM D1193-06 Standard Specification for Reagent Water