If a water purification system is designed, operated, and maintained properly, it will not be plagued by microbial contamination. However, if special attention is not dedicated to the control of microbial growth, problems will very likely result. Neither the proposed (USP 23) nor the current (USPXXII) specifications for USP Purified Water have established a limit for microbial concentration. However, the General Information chapter in both cases gives a 'recommended action limit' of 500 colony-forming units per milliliter (cfu/mL) for the feedwater and 100 cfu/mL for the final product water. Despite the fact that these action limits are not necessarily required, most pharmaceutical manufacturers strive to maintain microbial concentrations below these recommended action limits. This article will address the methods for operating and maintaining a USP Purified Water system to continuously meet the objective of 100 cfu/mL.
Typical Purified Water Systems.
A typical USP Purified Water system may consist of several processing steps, each designed to provide further purification of the water. The first step of a typical system, based on two-pass reverse osmosis (RO) technology, is usually a multi-media filter followed by a water softener to remove insoluble particles and hardness ions, respectively. Next, a bisulfite (HSO3) injection system may be used to remove chlorine. The central purification process is then the two-pass RO system. The final step is to pump the purified water to a heated or ozonated recirculating loop (see Figure1).