The common practice for cell culture on a production scale is to provide pharmacopoeia-grade Purified Water for site applications and use this as a feed to a still to provide WFI. This combination offers considerable regulatory convenience and Bergmann (1990) provides an example.
Purified water may be produced by ion-exchange, distillation, or reverse-osmosis-based systems. In practice, due to the advantages of low running costs, and operational convenience, over 90 % of new systems use primary stage reverse osmosis with final polishing by electrodeioniza-tion, ion-exchange or a second reverse osmosis stage (ISPE Guide 2001). An example of this type of system using RO and EDI is shown in Figure 2.2.
As described by Jordain (2002) and Lampard (2002), it combines, on a single 'skid', water softening and micro-filtration pretreatment, reverse osmosis and EDI. UV disinfection and ultrafiltration can be added if required. The stainless steel construction enables hot water sanitization at 85 °C to be used. A simplified flow schematic is given in Figure 2.3. The version shown includes an ultrafilter and is specified to meet the EP 'highly purified water' standard. Without the ultrafilter, performance is well within 'purified water' requirements. If distillation is needed for WFI then the product water can be fed to a still.
On a small scale, for example within laboratories, various other alternatives are possible. Stills can be used, often combined with pretreatment by reverse osmosis or ion-exchange to minimize maintenance. The more frequently used approach is a miniature version of the multiple technology systems described above. This is normally provided in two stages. An initial step involves pretreat-ment and reverse osmosis, sometimes combined with ion exchange or EDI to fill a reservoir with partially purified water. This water is then 'polished' to achieve its final purity by repeat treatment using a combination of ion exchange, ultraviolet exposure and ultra-filtration. Final, point-of-use filters can provide further protection against bacterial contamination. Such a polishing system is shown schematically in Figure 2.4.
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