<0.5 mg/L


<1.0 mg/L


<0.1 mg/L


<1.0 mg/L

Carbon Dioxide

<5.0 mg/L

Heavy Metals

<0.1 mg/L as Cu

Oxidizable Substances

Passes USP Permanganate Test

Total Solids

<10 mg/L

Total Bacterial Count

<50 cfu/ml


None Specified

USP XXII (published 1990) purified water standards remain the same as USP XXI. Purified water is essentially equal to deionized water, at least chemically (not necessarily biologically). Figures 1 and 2 outline the most common methods of purified water generation. After the deionization process, water is collected in a storage tank. A distribution loop takes water from the storage tank to all use points and then back to the storage tanks.

The purified water temperature is typically maintained at 60 to 80°C (hot), ambient, or 4°C (cold). A number of heat exchangers are located around the loop and after the DI system to achieve and maintain the desired temperature. Ifthe system is hot, point-of-use heat exchangers should be used to obtain ambient water. A design engineer would need to evaluate a given system, and strategically locate and size heat exchangers to both maintain the temperature in the loop and to provide water to the use points at the desired temperatures.

Regardless of the system temperature selected, the storage tank and the loop must be sanitized periodically. For the stainless steel system outlined above, sanitization implies raising the water temperature to 80°C (at a minimum) at the cold point and maintaining it for the validated time interval. This is often done automatically off shift.

Another commonly used approach to purified water generation, storage and distribution is RO/DI. Figure 4 is a schematic of an RO/DI approach. The components in this type of system are usually all plastic, therefore, sanitization is done chemically. Use of a sterilizing 0.2 micron filter in addition to, or instead of, the resin filter should be resisted. This practice may appear beneficial, but it is specifically prohibited by the proposed LVP GMP's (proposed CFR 212.49) and it is not recommended.

Hot Raw Cold

Hot Raw Cold

Figure 4. USP purified water (RO/DI water).

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