When is it time to replace your RO Membrane?

It might appear that it is easy to know when to replace membrane elements in reverse osmosis systems (RO). Many companies will follow one or more of these guidelines to replace their membrane.

  • Time, usually after three years (the warranty period of the membrane manufacturer)
  • An increase in RO permeate conductivity as it relates to their water quality requirements
  • A reduction in the permeate/product rate is directly related to water demand.

Relying on these simple limits could lead to membrane replacement prematurely. RO membrane elements can last for more than ten years, yet still provide salt rejection and permeate flow rates comparable to those they used at their startup. The membrane’s performance deterioration due to membrane age happens so slowly that it rarely plays a significant role in its life expectancy. It is often more dependent on how well the RO system has been maintained than replacement.

The RO membrane’s life expectancy can be dramatically reduced if a strong oxidant such as free chlorine is in contact with it. The RO membrane will be damaged by any exposure to chlorine. This will depend on the chlorine concentration and how long it has been in contact with the membrane. The RO salt rejection will decrease with continued exposure. Permeate flow rates will also increase. It is best to normalize the permeate rate for changes in water temperature or operating pressures.

A decrease in salt rejection, accompanied by an increase in normalized flow rate, usually means that all membrane elements need to be replaced.

You can get more information about the problem by measuring the conductivity in each membrane vessel before you assume that all the membrane elements must be replaced. You can correct rejection issues by only replacing the membrane elements in certain locations in the membrane vessel array. For example, the lead end if there is excessive fouling from suspended solids in inlet water. Poor performance in concentrate-end vessels may be due to scale formation or simply a result of a higher water temperature when low energy RO membrane is used.

Profile the membrane vessel conductivities to determine the source of the problem. Replacement membrane elements may be necessary

The most common reason for a reduced membrane life is excessive fouling. This can be described as an increase in the RO feed-to concentration pressure drop of more than 15{0fdcb698a23929641504ab47cca9891ed2227c4ae96fba1d0275f88b8f46a986} or a decrease in the normalized permeate rate of more than 15{0fdcb698a23929641504ab47cca9891ed2227c4ae96fba1d0275f88b8f46a986}. Cleaning solutions will take longer to clean the membrane surface from biological particles and/or silicate-clay particles. The cleaning solution won’t be able to access the areas blocked by larger particles if they are allowed to accumulate and block the flow channels. Cleaning that does not result in a normalized flow rate or feed-to-concentrate pressure drop may need to be more thorough. This can be done with offsite cleaning.

Normalized permeate flow rates will decrease due to membrane surface fouling. The lead-end membrane elements are most affected by fouling because they produce more permeate. These elements lose permeation and downstream membrane elements produce more water, which leads to increased fouling.

Inability to restore normalized permeate flow rate using aggressive cleaning often means that all (first-pass membrane) elements will need replacing.

Biofilm in the piping or system components can release large biological particles into the RO feed water. These particles can become trapped in the membrane spacing material of lead-end membrane elements. This is more common for older RO membrane models, which use a thinner spacing material than the 34 mil spacing. These particles can cause the feed-to concentrate pressure drop of the lead elements to rise, and also across the elements in their first-stage pressure vessels. The stage can be smashed if the pressure drops across it exceed 60 PSI.

The problem can be fixed by replacing only the lead end membrane elements if the pressure drop in the first stage is not greater than 60 psi. Crushed concentrate-end membrane elements within vessels might also need to replace if there is an excessive pressure drop.

Scale formation can happen quickly due to many reasons. The pressure drop at the end of the scale/salt process will increase due to many factors. If the majority of the scale is composed of calcium carbonate, on-site cleaning can be successful. This will result in a significant increase in permeate conductivity due to the affected membrane vessels. Sulfate scale is less soluble than carbonates and may require the replacement of concentrate-end membrane elements.

Scale formation can cause an increase in RO pressure drop. This cannot be restored by cleaning. Only the concentrate-end membrane elements need to be replaced to restore original performance.

Silica scale may form in areas where the water source has a dissolved silica content greater than 40 mg/L, and the water temperature is slightly cooler. The normalized permeate flow rate will decrease as the silica scale blinds the membrane surface of downstream elements. As other elements produce more permeate, the scale will build up as the concentrate-end membrane elements become ineffective at permeating.

The concentrate-end membrane elements of the silica scale will need to replace. This percentage is roughly proportional to the normalized permeate flow rate decline.

It will help reduce the number of membrane elements that must be replaced by knowing why RO performance has dropped and which membrane elements are at risk. This will allow you to alter the RO pretreatment or the RO cleaning frequency, if necessary, to increase the lifespan of any remaining membrane elements. A membrane autopsy is likely to be justified if the exact cause of failure can’t be determined.

This post was written by a water treatment expert at Paragon Water Systems. Paragon Water Systems is a water filter supplier. We know that the best product comes from the best materials. We manufacture home water solutions such as reverse osmosis systems, under sink water filtration systems, showerhead filtration systems, carbon cartridges, and a wide variety of other products. Our focus is to provide Americans with safe and clean water throughout the home.