Water purification is an essential but expensive process for manufacturers. Steve Willis, of Ionpure, outlines recent developments in continuous electrodeionisation that increase efficiency and cut costs for water purification
Innovation in water purification technology will undoubtedly offer future reductions in process costs and chemical waste. Leading this development, continuous electrodeionisation (CEDI) is already widely adopted by the international pharmaceutical industry, with more than 1,000 installations in operation. Yet it is a relatively young technology, commercially established as recently as the late 1990s, and its designers are taking significant steps that will benefit pure water system users.
Every major pharmaceutical manufacturer now uses CEDI polishing, following reverse osmosis (RO), at the final stage of water treatment and purification before storage. The purified water can be used to produce steam for sterilisation, for washing components and containers, or as an ingredient in products.
The main impurities present in water can be divided into three categories: organic material, measured as total organic carbon (TOC); ionic material, measured by conductivity or the level of individual ions by high performance liquid chromatography; and bacteria. The popular water grades for pharmaceutical use are "Purified Water" for products not coming into contact with blood and "Water-for-Injection (WFI), which demands biological ultra-purity.
While industrial users of pure water, such as microelectronics producers and power generators, set water standards to meet commercial and practical requirements, the pharma industry alone has standards defined by legal specifications. The pharmacopoeial specifications evolve along with purification methods. Taking WFI as an example, purification should incorporate distillation or RO, largely because it would be difficult to achieve the required limits for bacteria and endotoxins without these techniques.
reduced footprint
When distillation was the predominant technology, the water treatment and purification process featured a lengthy sequence of water softening, carbon filtration, ion exchange resin beds, UV treatment, numerous stainless steel vessels and pumping equipment, in addition to the multiple distillation columns, heater and coolers. All this came at considerable expense and sacrifice of floor space.
In contrast, CEDI is part of a purification system that is smaller in terms of both plant cost and footprint (Fig. 1). Following pretreatment, for instance by ultrafiltration, water reaching the purification stage almost invariably undergoes RO before polishing by CEDI (Fig. 2). The key stage in most pure water systems, RO membrane technology is very adept at removing a high percentage of all contaminants present. It typically removes 95-99% of ions and 99.99+% of organic and bacterial contaminants. CEDI is not yet a viable technology without RO but together they now constitute the standard process for producing purified water, or for pretreating a WFI still.
The predecessor to CEDI is mixed bed deionisation (MBDI). Conventional MBDI has its drawbacks, mainly because exhaustion and regeneration of ion exchange beds causes fluctuations in produced water quality. As CEDI, by definition, is free from regeneration, it is not subject to these fluctuations. As CEDI generates no chemical waste and has low running costs (typically <0.5kWh/1000 l of product water) it can be a clean, green alternative to MBDI plant, while matching it in performance. Designers have focused on making CEDI more competitive by improving module performance, increasing module reliability and lowering the overall cost of ownership.
CEDI works by removing ionised or ionisable substances from water using ion exchange membranes, electrically active media (typically ion exchange resins), and a DC electric potential. Unlike the conventional MBDI systems it is replacing, CEDI uses no chemicals, as no regeneration is required.
Most commercial CEDI devices have layers comprising alternating cation- and anion-permeable membranes with spaces in between, configured to create liquid flow compartments with inlets and outlets. The compartments, bound by an anion membrane (facing the positively charged anode) and a cation membrane (facing the negatively charged cathode), are termed diluting compartments. The compartments bound by an anion membrane facing the cathode and a cation membrane facing the anode are "concentrating compartments". To facilitate ion transfer in low ionic strength solutions, the dilute compartments are filled with ion exchange resins.
A transverse DC electrical field is applied by an external power source using electrodes at the boundaries of the membranes and compartments. The electric field attracts ions in the liquid to their respective counter-electrodes, which gathers the ions in the concentrating compartments (Fig. 3).
early process limitations
Much CEDI development that today benefits pharmaceutical applications was spurred by other industries - silicon wafer fabrication, solar cell and microelectronics production. In such applications the significant ionic impurities can be measured in parts per trillion.
At the early development stage, product water specifications for CEDI technologies were typically in the range 10-16 megohm-cm with removal of weakly dissociated species, such as silica and boron in the 90-98% range. This is sufficient for many industrial uses but not in microelectronics or many power applications where a stringent silica requirement means CEDI was followed by ion exchange polishing.
While this approach greatly extended the ion exchange service cycle and reduced operating costs, issues remained with regeneration and ionic breakthrough of the ion exchange resin. The ideal for CEDI was to produce water directly of a quality suitable for microelectronics production.
In Ionpure CEDI modules, the goal of the individual layers is to remove their respective counter ions, i.e. the anion exchange resin layers remove anions and the cation resin layers remove cations. To maintain electro-neutrality in a layer where cations are being transferred to the concentrate, water splitting must take place at the anion exchange membrane to provide the hydrogen ion. A similar process happens in the anion layer at the cation membrane, providing the hydroxide ion to replace removed anions. In the layered bed, the acid or base created through water splitting regenerates some of the ion exchange resin. It can also change the bulk pH in that particular layer.
This is critical to the removal of species that are very weakly ionised at neutral or slightly acidic conditions, typical of CEDI feed waters. Acids of silica and boron have pKa values between 9-10. This means the pH must be increased to this range to remove the ions. Dissolved CO2, the predominant species in most RO permeates, is also effectively converted to ions in this pH range.
Just as the pH is elevated in the enhanced anion layer, the pH can be reduced or neutralised in an enhanced cation or mixed bed layer. So it is necessary to pass through different types of layers to produce high quality water. The problem is that the electrical resistance of the different layers can vary, potentially allowing preferential current flow. Also the resistance of each layer can change due to the form of the resin or the bulk pH. This issue was overcome in standard single pass modules by doping certain layers to equalise resistance.
The importance of bio-purity is paramount in pharmaceutical applications. While CEDI cannot consistently remove bacteria and other organics, designers have introduced a module construction that can be sanitised by hot water (Fig. 4).
In the past, the process would take more than three hours to complete, in order to allow for gradual temperature change. However, with an Ionpure module, instant temperature switching and simpler process control permits water at 85°C and 2bar to be fed instantaneously and quickly switched back to cool feed. The time now required for module sanitisations is approximately 90 min and the ability to accept thermal expansion and contraction, leak-free, negates any need to tighten the module tie rods. The reduced complexity of controls and equipment at the CEDI system level also means simplified IQ and OQ during system validation.
latest developments
The most recent step forward in CEDI is the arrival of new modules that can treat water containing up to 2ppm of CaCO3 - a hardness level that is four times the typical industrial limit. Previous systems claimed to have high feed water hardness tolerance have been problematic - but that challenge has now been overcome.
Although not so applicable to the pharmaceutical industry, this development demonstrates the growing capability of CEDI. A typical treatment train for boiler feedwater would now include anti-scalant prior to a single-pass RO system, followed by a high-hardness CEDI module, to produce high-purity water without hardness scaling.
Another recent innovation reduces power consumption of certain modules by up to 33%, having a substantial impact on operating and system costs.
In conclusion, modern water systems that combine tech-nologies can reliably achieve high levels of water quality. In some applications the level of purity is limited only by the accuracy of the monitoring devices available.
For pharmaceutical water treatment, standard skid-mounted systems are now available that can be fully factory tested and most of the validation work completed prior to arriving at site. This makes them quick and easy to connect and commission on site. Already chemical-free, compact and with very low maintenance requirements, CEDI modules are developing in ways that are reducing pure water system costs and environmental impact even further