by Robert P. Donovan
No membrane role is larger or more important than its role in the operation and success of reverse osmosis (R/O) equipment, the workhorse of today's high-quality ultrapure water (UPW) systems.
R/Os generally remove more than 90 percent of the contaminants in the water fed to them and enable downstream ion-exchange columns to polish that water to the high quality needed by the users. R/Os also are used in smaller, less demanding roles such as purifiers for residential water. Their function remains the same-the removal of contaminants from a water feed stream.
I remember osmosis from high school biology class as the transport of liquid across a semipermeable membrane-a cell wall-so as to dilute the concentration of a solute-a salt solution-on the less-pure side of the membrane and lower its concentration to nearer that of the liquid on the purer side.
Membrane semipermeability is critical to osmosis so that pure liquid can pass through the membrane but the salt solution cannot. Thus, pure liquid can pass from the pure liquid side to the salty side but salty ions cannot pass through this membrane from the salty side to the pure liquid side.
This action increases the volume and pressure of the solution on the salty side while diluting the solute concentration. The liquid transfer process proceeds until the pressure increase on the salty side creates a back flow of pure liquid (not salt solution) through the semipermeable membrane that matches the forward flow of pure liquid driven by osmosis. The steady-state pressure difference established across the membrane is called the osmotic pressure.
While the R/O application represents a major success story for membrane filters, it is by no means the only membrane success story. Membranes are finding more and more uses throughout the UPW industry.
In the reverse osmosis units of UPW systems, an external pressure is applied to the contaminated water entering on the “salty” side of the membrane, increasing its pressure well above the osmotic pressure across the membrane that separates this water from the purer water exiting the unit.
This externally applied pressure increases the component of pure water flow from the contaminated side to the purer side well above that required for balancing the osmotic component flowing in the opposite direction. Thus the quantity of pure water on the pure water side-the R/O permeate-increases while that on the contaminated side-the reject water-decreases. At the same time the concentration of contaminants in the reject water increases.
No membrane is perfectly semipermeable, blocking 100 percent of all species except water. There will be some leakage of some contaminants from the contaminated water side through the membrane to the pure water side. And the membrane properties can be altered by the buildup of solids on its surface-membrane fouling.
Its consequence is reduced water flow through the membrane. Fouling represents a significant operational problem that can dominate an R/O operator's life. Keeping the contaminant concentration in the reject water below its solubility limits becomes important in avoiding precipitation of solute.
Most R/Os operate at 60 to 80 percent efficiency; the remainder becomes part of the reject water flow. There are exceptions, and with careful (often expensive) pretreatment of the feed water some R/O installations operate at efficiencies in excess of 90 percent.
While the R/O application represents a major success story for membrane filters, it is by no means the only membrane success story. Membranes are finding more uses throughout the UPW industry, including membrane degasifiers, which are now commonplace, membrane humidifiers and membrane distillation units.
Robert P. Donovan is a process engineer assigned to the Sandia National Laboratories as a contract employee by L & M Technologies Inc., Albuquerque, NM.
Next month, Robert Donovan takes an in-depth look at water conservation and reuse methods geared for the semiconductor industry as part of the CleanRooms Technical Feature line up.