Information
 Types of Membranes
 Temperature Effect
 RO Spirals Handling
 Storage
 Dry Membrane
 Gallons Per Day
 Bacterial Removal
 Ionic Solutes
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Reverse Osmosis Information

Dry Membrane

“Dry” membrane is a misnomer because reverse osmosis membrane has to retain a certain amount of moisture and or wetting agents to allow for complete wetting of the membrane. A fully desiccated membrane is usually too hydrophobic to rewet even under high pressures. It requires the action of an agent that can bring water into contact with the membrane surface. The agent is then exchanged out or flushed out of the porous interior of the membrane. If the moisture is lost there are no agents left to facilitate rewetting, so there can be no water flow through that section of membrane. Another effect that can take place during a drying cycle is related to the capillary forces acting on the extremely fine structure under the surface of the active layer. The force at the air liquid interface is so large that the walls of the capillary are pulled together so that the structure collapses on to itself. The effect is irreversible and will greatly restrict water flow through the membrane.

WHAT IS OUR “DRY” MEMBRANE? WHAT ADVANTAGE DOES IT PROVIDE?

By “Dry” membrane, we mean the membrane as received from the manufacturing area. The membrane has a modest amount of retained moisture and a residual amount of material that functions as a wetting agent. The wetting agents hold the water in the membrane very tightly to prevent easy drying and collapse of the membrane. The “as manufactured” membrane is very clean and nearly sterile. By careful manufacturing we can keep exposure to the environment to a minimum. The finished element has a minimum amount of free liquid, lower bacteria and mold exposure, a less hospitable growth environment, and a strong resistance to further moisture loss. All of these factors result in a much improved shelf life for the finished element. Under ideal conditions of storage, two plus years is reasonable. Since the element does not contain excess water, it is lighter and therefore less expensive to ship and more resistant to freezing. To summarize the advantages:

  • LONGER SHELF LIFE
  • MORE RESISTANCE TO FREEZING
  • LESS EXPENSIVE TO SHIP
  • LESS HANDLING AND TESTING = LOWER COST

WHAT DISADVANTAGES COME WITH “DRY” MEMBRANE?

Dry membranes can be integrity tested using air, but they can not be performance tested without losing the benefits summarized above. We handle the above disadvantage by doing sample testing and using statistics to give reasonable assurance of lot performance. The elements that are used in the wet testing procedure would be handled like standard product that does not need extended shelf life. The other disadvantage that comes with no wet testing is the transfer of that rinse time from our facility to the customer’s facility. In the process of testing an element, it will typically have a minimum of 30 minutes flush time before data is taken. Low productivity elements need about that much time to rinse down to our minimum TDS rejection standards. High performance membrane equilibrates much faster, and does not present any unreasonable delay in reaching TDS standards. In practical terms, the high productivity spirals will not require any additional time over what is currently done to performance test a system. Low productivity elements may need 30 minutes more test time. Higher test pressures can lessen extended test time requirements, as well as lower TDS feed water or additional pre-rinse time. To summarize disadvantages:

  • NO DIRECT PERFORMANCE DATA ONLY STATISTICAL ASSURANCE
  • ADDITIONAL TEST TIME REQUIREMENTS