The technology of reverse osmosis water systems is ever expanding but we mustn’t forget that for any equipment to function efficiently and for as long a time as possible, we need to maintain it.
On the surface, when to change the membrane of a reverse osmosis water system might seem obvious and most companies make it simple by relying on the following three guidelines:

The three year cut off (manufacturers usually put this time period as their warranty). An increase in the RO (reverse osmosis) permeate conductivity, related to water quality requirement. Designated reduction in permeate or product flow rate when related to water demand.

However, there is more to it than first meets the eye and the reliance on simple parameters can actually be counterproductive and lead to premature membrane replacement. So, what should you look at?

Is Age Important

RO membranes can last up to ten years and still be functioning with the utmost efficiency. Age associated performance deterioration occurs very slowly and it is much more likely that a replacement is needed for an entirely different reason than age, one most often related to how well the reverse osmosis water system is maintained.

Oxidant Exposure

Free chlorine is a strong oxidant that can have a huge negative impact on membrane life. The extent of damage will correlate with the chlorine concentration and the amount of time the chlorine has been in contact with the element. Any contact, however, causes issues and can lead to the need to prematurely take membrane replacement action.

Salt Rejection

Decline in salt rejection can vary, but if you decide that the decline is unacceptable when occurring in sync with an increase in normal permeate flow, you will need to consider replacing the membrane elements in their entirety. Before you do, however, you need to gain a little more knowledge about the problem. Measure the conductivity of the permeate water from each membrane vessel, for example, and you will be able to determine exactly which element needs changing and the exact location of that element. A complete replacement may not be necessary.

Fouling

The primary cause of membrane deterioration is fouling, which is succinctly defined as the increase in RO feed-to-concentrate pressure drop of more than 15% (or the permeate flow declining more than 15%). Particles collecting and impacting on the surface of the element make the job of cleaning solutions much tougher, and in some cases impossible. More aggressive cleaning can be attempted effectively, but if it is unsuccessful membranes will need to be replaced.

Scale

The formation of scale can be a problem for a few reasons. Onsite cleaning is normally effective in solving this issue with no membrane replacement necessary, but if the scale is caused by sulfate you may end up having to go for a replacement of the concentrate-end membranes.

When you know why your reverse osmosis water system is not performing as it should, you can begin to discern whether or not you need to replace the membrane in its entirety or just a certain element. Understanding the causes of a problem and the potential extent of it might save you unnecessary time and money in getting it fixed.

Author Plate Sean Clifford is an advisor at AllWater Technologies Ltd, a wholly independent company providing consultation and water treatment equipment, including a reverse osmosis water system and effluent treatment plant. Bringing together a host of experience and specialist knowledge, the company is committed to building and maintaining long-term relationships and creating maximum value and benefit for their customers.