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Developments in applications

Pretreatment systems
In pre-treatment systems ion exchange resins are used in water softeners and organic scavengers (organic traps) and the basic design and operation of these systems remains largely unchanged.

Water softening uses a cation exchange resin to exchange principally calcium and magnesium ions for sodium ions and so prevent the formation of calcium carbonate precipitates on reverse osmosis membranes, WFI stills and other down stream processes. Feed water is passed down through the resin bed which is in the sodium form. The polyvalent cations are absorbed onto the resin and the sodium ions are released. After a preset volume, or time, the system is taken off line and automatically regenerated with a brine solution.

Usually the residual hardness after a single pass through a softener is suitable but some modern systems require a lower level of hardness so one development has been to use serial softening with the 'lead' softener being regenerated based on a volume throughput and the 'polishing' softener being regenerated after a preset number of regenerations of the lead softener. This can produced water with < 1 ppm as calcium carbonate.

Organic scavengers (or organic traps) utilise a macroreticular anion exchange resin to remove a percentage of organic material from the feed water supply. As the feed water passes down through the bed of anion resin in the chloride form the negatively charged organics are preferentially absorbed onto the positively charged active sites on the resin beads to displace the chloride ions. The large molecules also get trapped in pores in the resin beads and are removed by a size exclusion process. The resin is regenerated using a brine solution with the excess chloride ions displacing the organic acids from the active sites. The change in osmotic pressure also causes the resin beads to shrinks and so squeezes out the trapped organic material.

This process works quite well but the percentage removal of organics is variable and non-predictable. No recent developments in resin technology which have improved this performance so this technology is being replaced by the use of ultrafiltration, particularly in locations which have a high organic content in the feed.

The most recent development in pre-treatment ion exchange systems has been the introduction of hot water sanitisation. A key objective in pharmaceutical water systems, is the control of microbiological levels in the water. Prior to the introduction of hot water sanitisable RO and CEDI systems (see below), a periodic chemical sanitisation of the whole pre-treatment system was sometimes carried out when the purification system was being sanitised. However, due to the down time and labour costs involved the frequency of chemical sanitisation varies significantly from site to site. With the development of hot water sanitisable purification equipment a demand was generated for hot water sanitisable pre-treatment systems.

The GRP vessels typical used for water softeners and scavengers are replaced with lined or coated metal vessels and the interconnecting ABS or PVC plastic pipework replaced with stainless steel pipework and fittings. Sanitisation temperatures up to 65 - 85 o C are possible which can achieve a good sanitisation effect. This has a substantial capital cost and operational cost impact for no improvement in the water softening or organic reduction performance of the system. This development is introduced only to provide a periodic sanitisation of the pre-treatment section of the system.

Purification systems
The most common method of producing Purified Water was twin bed deioniser systems and these could still easily meet the required water quality. A twin bed system uses a bed of cation resin, regenerated with hydrochloric or sulphuric acid, followed by a bed of anion resin, regenerated with sodium hydroxide. After either a preset throughput, or typically when a maximum conductivity value is reached, the system is taken off line and regenerated with acid and caustic. This technology works very well and twin bed units continue to be a common technology used to produce deionised water in other industries.

In 1986 the Permutit company introduced a development called Scion - short cycle ion exchange - which challenged the then current design philosophy for twin bed units by introducing a short service run (typically 6 - 8 hours compared with 1 - 2 days) and a short regeneration period (typically 2 hours compared with several hours) with counter current regeneration. A further development was the use of a third, polishing, column containing cation resin. This column removes the low level of cations which 'slip' off the first column and results in a lower product conductivity being possible. A variety of counter current regenerated systems have since been developed.

Recently, there have been attempts to minimise chemical consumption, reduce running costs and maximise water usage. This has been possible by incorporating the latest versions of the anion and cation resins combined with some interesting developments in the operation of the system. For example, Veolia recently launched an updated version of their short cycle system called RAPIDE Strata which incorporates the use of a stratified anion bed which incorporates both weak base anion resin (more efficient at organic removal) and strong base anion resin (more efficient removal of anions). A further development is the use of chemical 'pulses' rather than continuous injection when the acid and caustic regenerants are added. These changes are claimed to give better cleaning and regeneration with the typical regeneration time having been reduced to around 35 minutes.

A twin bed short cycle system can typically achieve around 5 µS/cm while with the inclusion of a second cation polishing bed qualities as good as 18 megOhm/cm can be achieved.

Development of these types of systems has resulted in significant reductions in the use of chemical regenerants per cubic meter of water produced, better water qualities and a system that is more tolerant of the organic content of the feed water. There are many justifications available on various web sites supporting the selection of ion exchange technology over reverse osmosis, and in many applications ion exchange is still the preferred option - but in pharmaceutical water systems this is not the case. Reverse Osmosis, combined with other technologies, is now the preferred option. This is probably due to a combination of reasons, for example the security of meeting the TOC limit, handling chemicals and effluent, and microbiological quality.

Another ion exchange treatment which has also significantly decreased in use is Service Deionisation. This involves using resin in portable cylinders which are shipped off site for regeneration when these are exhausted. There have not been many technical developments in this technology but it is still commonly used, particularly in non-pharmaceutical applications.

Arguably the main reason for the reduced usage of these technologies has been the development of Continuous Electrodeionisation technologies (CEDI). The first commercialized CEDI system was introduced in 1987 by Millipore and there are now many different companies manufacturing systems using a variety of system designs. All the systems are based on using cation and anion selective membranes to form 'compartments' in which ions are removed from the feed water (dilute or diluate) under the influence of an electric field. These ions are collected and removed from the system via waste 'compartments' (concentrate) which alternate with the purifying channels. Explanations of this technology are available from many sources but one useful on-line training resource can be found at www.cediuniversity.com.

The dilute compartments are filled with ion exchange resin and in some systems the concentrate compartments are also filled with resin. Some systems use mixed bed resins while some use layers of cation and anion resin combined with different cell geometries. The majority of systems utilise 'plate and frame' type designs while a few systems utilise a spiral configuration. All the systems are designed to removed dissolved salts, carbon dioxide, dissolved silica, ammonia and some organic material. Different manufacturers make different claims about the effect of their system on the microbiological content of the product water, however, generally most claim that their equipment does not add particles, organics, bacteria or endotoxins to the product water.

Although this technology has been in use for 20 years probably the most significant improvements in the technology have happened in the past 3 - 5 years. These have been:

. Resin in the concentrate compartments. This improves the removal performance and can remove the requirement to add or retain electrolytes in the concentrate flow.

. The selection of the ion exchange resin, the use of layers of resin rather than mixed resins and the use of resin doping.

. New sealing methods for the modules has reduced or prevented leakage from the modules which was a common feature of the systems available originally. (Spiral configured modules were originally developed in a sealed housing.)

. Hot water sanitisable modules. This made it possible to hot water sanitise the entire Purified Water generation system.

. Larger flow rate modules. This simplified the design of systems while also helping reduce the costs per cubic meter of water produced. (Some of modules are now 2/3 rd to 9/10 th cheaper.)

. Integration of a membrane system with a CEDI module. Christ recently introduced the Septron Bio-Safe to assist in the production of Highly Purified Water where a typical design configuration is to use RO and CEDI followed by a second membrane stage to ensure the Endotoxin specification of Highly Purified Water is always met.

With these product developments it is now possibly to continuously, reliably and without the use of regenerant chemicals, produce water which easily meets the requirements for Purified Water and with the option of automatic hot water sanitisation of the system. In fact, the water quality produced can be as good as 0.055 µS/cm conductivity, < 10 ppb TOC, < 10 cfu/100 ml and < 0.25 EU/ml. This improved water quality is arguably a less important benefit to the patient and to manufacturers than the improved consistency and reliability that the new generation of systems provides.

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