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.
There are three reactions that govern the chemistry of CO2 in water (Equations 1-3):
CO2 + H2O <---> H2CO3
(carbonic acid) Eq. 1
H2CO3 <---> H+ + HCO3-
(bicarbonate ion) Eq. 2
where PKa = 6.38
HCO3- <---> H + CO32-
(carbonate ion) Eq. 3
where PKa = 10.37
When gaseous CO2 is dissolved in water, a portion is hydrated to form carbonic acid (Equation 1). This carbonic acid dissociates into bicarbonate and hydrogen ions. At a pH of 4.3, very little of the carbonic acid is dissociated. At a pH of 6.38, the molar concentration of carbonic acid equals that of the bicarbonate and hydrogen ions. At a pH of 8.3, there is no longer an appreciable amount of CO2 or H2CO3 present in the water. Above this pH, the bicarbonate ion is converted to carbonate H+, as shown in Equation 3.
As the pH increases, all three equations are driven to the right and there is less CO2 available in the gaseous form. Since RO membranes are unable to reject gaseous CO2, the permeate conductivity is lowest when the feed pH is near or above 8.3. When the pH is above 8.3, the CO2 is found in the form of the carbonate and bicarbonate ions, which are easily rejected by RO membranes.
As a part of the study to determine the optimum performance of two-pass RO, four different types of spiral-wound elements manufactured with polyamide (PA) membrane were compared to identify the optimum element type for the production of USP 23 PW. Several membrane elements of each type were evaluated in first- and second-pass simulation tests to determine their flow and rejection performance on softened, dechlorinated feedwater with an average conductivity of 350 µS/cm, a pH of 8 to 8.5, and an alkalinity of 250 ppm as CaCO3. Pretesting verified that all elements showed rejection of 99% + 1% at a high conductivity (2,000 µS/cm) made up of monovalent salts (sodium chloride [NaCl]). First- and Second-pass performances were evaluated based on conductivity measurements. Conductivity and pH were measured off-line using calibrated instruments, and flowrates were measured using a timed volumetric method.
During first-pass tests, salt passage ranged from 0.27% to 0.43%. Permeate from the first-pass test was then collected and used as feed for the same membranes, creating a second-pass simulation. Two tests were conducted; the first at the natural pH of the permeate, and the second at a pH of 8.0 to 8.5. The latter pH was achieved by addition of sodium hydroxide (NaOH caustic). This pH was selected to minimize the effects of CO2 on the conductivity of the second-pass permeate. The percentage passing averaged 6.5% prior to NaOH addition and 3.5% after NaOH addition. Even though the addition of NaOH increased the conductivity of the feedwater, the apparent rejection of all elements increased. This is attributable to the removal of dissolved CO2.