RO Membrane Troubleshooting Guide: Causes And Solutions for Low Salt Rejection

As the core of household and commercial water purification systems, reverse osmosis (RO) technology, with its high-precision filtration capability, effectively removes impurities, salts, microorganisms, and more from water, providing us with clean water quality. However, during use, many users encounter situations where the RO membrane permeate fails to meet the required standard, affecting both the user experience and water safety. What causes the RO membrane permeate to fall below standard? Behind what may seem like a complicated issue, there are usually traceable root causes—next, Umekon will help you analyze the problems and provide solutions.

Figure 1: RO Membrane

The first cause is that the feed water entering the RO membrane contains excessive organic matter and residual chlorine. Residual chlorine oxidizes the amide bonds on the membrane surface, causing polymer chain breakage and forming a large number of micropores or channels. As a result, salts that should have been rejected pass directly through the membrane, leading to a sharp drop in salt rejection. Even low concentrations of residual chlorine, if present for a long time, will accumulate oxidative damage, causing continuous deterioration of salt rejection until it ultimately fails to meet the standard. This situation arises due to insufficient pretreatment filtration, and more appropriate filters should be used.

Secondly, insufficient pump pressure leads to inadequate driving force for water molecules to pass through the membrane, preventing them from overcoming the osmotic pressure of salts. This causes reverse diffusion of salts to intensify, weakening the membrane’s ability to reject salts, and the salt rejection rate decreases as pressure drops. If the pressure is too high, exceeding the membrane element’s tolerance limit, it can cause physical deformation of the membrane sheets and failure of sealing gaskets, allowing high-salinity concentrate to directly leak into the permeate side, resulting in a sharp drop in salt rejection. Moreover, excessive pressure accelerates membrane aging, loosening the pore structure, which over long-term operation causes irreversible decline in salt rejection. Therefore, it is essential to ensure that pump pressure is both adequate and appropriate.

In addition, if the wastewater ratio is too small and the system recovery rate is too high (the proportion of permeate relative to feedwater is too large), the salt concentration on the concentrate side rises sharply. Salts on the membrane surface cannot be removed in time by the concentrate flow, forming a high-concentration boundary layer. This increases osmotic pressure across the membrane, raising resistance for water molecules to pass through, while salts, due to the concentration difference, more easily penetrate the membrane—directly leading to reduced salt rejection. At this point, the wastewater ratio should be adjusted to ensure that salt rejection meets the required standard.

Problems with the membrane element itself are also an important cause of substandard salt rejection. After long-term use, the membrane element may lose its original filtration performance due to fouling, clogging, or aging. In such cases, the membrane element needs to be cleaned or replaced. When cleaning, suitable cleaning agents and procedures should be used to effectively remove dirt and scaling from the membrane surface. If cleaning proves ineffective, the membrane element must be replaced with a new one to ensure that the permeate water quality meets requirements.

From pretreatment to final permeate, from one-time repair to lifelong maintenance, Umekon’s 28 years of industry expertise has already embedded the concept of “meeting standards” into every service detail. If you are troubled by RO membrane permeate issues, our technical team can provide on-site diagnosis—after all, the confidence to solve water quality problems comes from nearly three decades of practical experience accumulated with tens of thousands of customers.

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Email: sales@cnumek.com / sales@amandawatertech.com
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