Reverse Osmosis

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The United States Pharmacopeial Convention Inc. (USP) has proposed new specifications for Water for Injection (WFI) and Purified Water (PW). These specifications will include quantitative conductivity, and total organic carbon (TOC) measurements will replace 100-year-old qualitative wet chemistry tests. The impetus behind these changes is to create a means of analysis that is less labor-intensive, yet more reflective of the water quality. A comparison of the current and proposed specifications is given in Table A.

For the study summarized in this article, extensive testing was conducted to determine the optimum design and operation of a two-pass reverse osmosis (RO) machine to produce water meeting the requirements for United States Pharmacopeia 23 (USP 23) PW. The study included the development of high-performance, sanitary-design membrane elements; a comprehensive study of two-pass ROs currently in operation; and an intensive production-scale evaluation of a two-pass RO performance, examining various operational variables.

Carbon Dioxide and Conductivity

The proposed USP 23 PW monograph will call for on-line (or immediate off-line) conductivity at or below 1.3 microSiemens per centimeter (µS/cm) (when the temperature is at or above 25°C [77°F]) as Stage 1 testing. The second-stage testing calls for off-line analysis showing a conductivity of 2.4 µS/cm (at 25°C ± °C) This off-line conductivity requirement is higher than the on-line requirement, allowing for the increase in conductivity caused by the contribution of dissolved carbon dioxide (CO2) gas present in the water. Thus, a key to producing water meeting the on-line requirement is the removal of CO2 from the water.

When CO2 gas is dissolved in water, a portion reacts with the water (H2O) molecules to form carbonic acid. Being a dissolved gas, the CO2 passes completely through an RO membrane, and once the CO2 reassociates with water molecules to form bicarbonate (HCO3) in the RO product water, it contributes to the conductivity of the permeate water.

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