Table 1: U.S.P. Grade Water

In pharmaceutical industry, the water purification system must be definitive and consistent, particularly with respect to bio-purity of water.  The legendary pharmaceutical water system during the distillation era was a complex mixture of water softeners, carbon beds, IX resin beds, multi-stage distillation columns, polished stainless steel tanks, water heaters and coolers, distribution pumps, UV-light sanitizers, bacteria filters and monitoring instrumentation, all of which occupied space, and cost anywhere from $ 250,000 to $ 2 million which was a lot of money then. In comparison, the  modern pharmaceutical water purification system that achieves ten-fold improvement in performance, costs ten-fold less and occupies ten-fold less space is exemplied by Blue Spring System WFI-EDI which is a double-pass reverse osmosis system incorporating EDI enhancement.

Reverse osmosis (RO) offers simplicity, high output flow capacity, compactness, and low cost.  RO-based WFI installations also eliminate the need for storage tanks for WFI, re-circulation pumps, bacteria filters, UV-lights, and the associated maintenance costs.  RO process is also far more efficient in removal of pyrogenic endotoxins from water than distillation. One problem with the RO process is that with typical municipal water measuring 300-600 mg/L TDS, the RO membranes can barely achieve the older <10 mg/L TDS USP standard. To meet this standard on a consistent basis, a double-pass RO in which the output of the first RO is further purified by a second RO was commonly used in the industry. The best the double-pass RO could do was 1 - 2 mg/L for most city waters. But this is not good enough for the current USP XXVII which calls for electrical conductivity of  <1.3 mS/cm which is equivalent to TDS  of  <0.82 mg/L. Help is needed from another de-ionizing technology.

Electro-de-ionization (EDI) is the process of removal of ionic impurities from water by the passage of  electric current. It started in 1984 with O'Hare patent (U.S. 4,465,573) but it was not commercially developed until the late 1990s. Since then, there have been several exciting developments in the EDI technology that make EDI the path of the future for de-ionization of water. For example, the latest, spiral-wound EDI module design ² is leak-proof, serviceable and cost-effective in comparison with the original flat-stack design.

Modern EDI uses a pair of ion-permeable membranes ³  to separate the ions from the water, under the influence of DC electric current. The EDI cell is filled with ordinary mixed-bed IX resin to enhance  removal of ions and to lower electrical power consumption. The electric current also produces H⁺ and OH^- ions by splitting water molecules. These ions effect in situ, continuous regeneration of the IX resins. EDI is environmentally friendly because it uses no chemicals for regeneration. EDI is a continuous process which means the equipment is on-line 24/7. Conventional IX equipment is intermittent due to periodic regeneration requirements.  EDI provides water of consistent purity as opposed to conventional IX process which exhibits cyclic swings in water purity. The capital cost of the EDI equipment is higher, but the operating costs are lower.  EDI is more suitable for pharmaceutical water processing than conventional IX because EDI offers consistency of output water quality which the conventional IX process lacks. 

One big disadvantage of EDI process is that it requires pre-purification of water to a conductivity level below approximately 25  mS/cm.  It is common practice in the EDI industry to use a reverse osmosis (RO) unit as pre-treatment for EDI feed water, to meet this pre-requisite. Direct use of the EDI output is suitable as Purified Water category per USP, but it must undergo further purification either by  reverse osmosis or by distillation to qualify it as WFI. This is because EDI is not capable of removing organics and biological impurities with any consistency, and may actually add these impurities to water.

Combining EDI and RO technologies can overcome the limitations of either technology.  The most logical system configuration consists of two RO units and an EDI unit in-between. Initially, the municipal water is pre-treated to remove hardness and chlorine, before entering the RO/EDI system.  The first RO unit serves as pre-treatment for the EDI unit. The EDI output may be used directly in Purified Water applications. The second RO unit upgrades the output of the EDI unit to WFI level, per USP requirements. A typical RO/EDI system producing 6 gpm of WFI quality water occupies only 6'x7' space and costs an order of magnitude less than an equivalent distillation-based system, in terms of capital costs, operating costs, and maintenance costs.

Typical service requirements of the RO/EDI installation in the pharmaceutical industry include weekly replenishment of salt for the water softener, semi-annual replacement of  activated carbon beds, weekly cleaning and sanitizing of the RO membranes,  periodic changing of cartridge filter elements, replacement of RO membrane elements every 2-5 years and replacement or servicing of the EDI modules, when required.  There are also electro-mechanical service items like pumps, valves, piping, system instrumentation and EDI power supply.

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