How Does An Ro Membrane Work?

Reverse osmosis is a process which uses a membrane under pressure to separate relatively pure water (or other solvent) from a less pure solution. When two aqueous solutions of different concentrations are separated by a semi-permeable membrane, water passes through the membrane in the direction of the more concentrated solution as a result of osmotic pressure.

If enough counter pressure is applied to the concentrated solution to overcome the osmotic pressure, the flow of water will be reversed).

Water molecules can form hydrogen bonds in the RO membrane and fit into the membrane matrix. The water molecules that enter the membrane by hydrogen bonding can be pushed through under pressure. Most organic substances with a molecular weight over 100 are sieved out, i.e., oils, pyrogens and particulates including bacteria and viruses.

Salt ions, on the other hand, are rejected by a mechanism related to the valence of the ion. Ions are repelled by dielectric interactions; ions with higher charges are repelled to a greater distance from the membrane surface. Monovalent ions such as chloride ions will not be rejected as efficiently as, for example, divalent sulfate ions. The nominal rejection ratio of common ionic salts is 85 – 98%.

Quality of RO Membranes and Filters – They’re not all alike!

While one RO System may look just like the next in terms of design and components, the quality of those components can be very different. These differences can have a significant impact on the quality of the water the system produces.

Here are some examples of questions you might ask and consequences associated with “less than desirable” quality.

  • Has the manufacturer used sound methods? What types of welds have been used in these plastic products? Will they allow contaminated water to bypass the filtration system? Will they allow the system to leak?
  • How has this filter or membrane been created? Will it allow the water to ‘channel’ and, in effect, bypass the removal component of this device?
  • What about the quality of the ‘fill’? Are it’s contents of a high enough quality to produce the expected percentage of contaminant reduction? Carbon quality, for instance, can have huge variances in reduction capability, reduction capacity, and the sloughing of ‘fines’, which can prematurely clog or foul the RO Membrane.
  • What are the manufacturer’s controls on tolerances or variations in specifications? If this component is rated as a 1-micron filter will it truly filter out everything larger than 1 micron or will it only do the job 80% of the time? And, what if it actually filters at a .5-micron rate? That will stop the system from flowing — clogging it and forcing filter replacement? If this is a sediment filter and it fails the excess sediment will clog or foul the RO Membrane.
  • And in general – Are the materials used in this product FDA or NSF (National Safety Foundation) approved? If not, you might question their quality or performance ability.

So, it becomes clear that the quality of the components is the key to an optimal functioning RO System.