Membrane and System Design Considerations in Producing High Purity Water
The use of membrane technology to produce water of greater and greater purity is rapidly evolving under the pressure of new product quality specifications, including those of the pharmaceutical and beverage industries. Membrane technology is well suited to achieving multi-component water specifications, given the fundamental nature of the separation process. Meeting a resistivity or conductivity specification as the sole gauge of water quality, however, can be more challenging. Membrane technology is capable of producing water with resistivity greater than 1 megohm when applied in a 2-pass RO system that is properly designed and operated.
A thorough study of polyamide (PA) membrane performance variations and possible interactions with feedstream components provides insight into the parameters which are most important in reducing permeate conductivity. This insight can also be useful in troubleshooting systems in the field, when unexplained performance fluctuations cannot be resolved by examining only the more common performance variables.
A series of controlled experiments has shown that membrane rejection will fluctuate in response to feed TDS and pH values, crossflow rates and element recovery levels. The performance of elements in the second-pass of a reverse osmosis (RO) system can be most dramatically affected. These variations, while not significant in the majority of applications, become crucial to the success of high-purity water processing. In addition, the effect of minor feedwater constituents, such as alkalinity and ammonia, are seen to play a dominant role in achieving high-purity permeate.
Polyamide (PA) thin-film composite membranes have a surface charge that plays a role in their separation ability, and the nature of this charge can be altered by the pH of the feedwater pH. The majority of PA (RO) membranes are negatively charged when operated on the pH levels most commonly encountered in water applications.
When the pH drops below a membrane's isoelectric point (generally between pH 4 and 5), these membranes become positively charged. The isoelectric point is that pH point at which the membrane has no net charge. This substantially decreases their performance when the permeate quality is being measured by conductivity. Acid transport through the membrane accounts for much of this apparent fall-off in performance. The effect is completely reversible when the pH is returned to near-neutral levels. The acid transport is facilitated by the presence of unreacted 'end' groups (amines) in the polyamide barrier layer. Depending on the amount of unreacted groups present in a particular membrane, different responses to pH changes may be seen.