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The 3 Most Overlooked Parameters in Reverse Osmosis Membrane Selection
Many users tend to focus on two key indicators—salt rejection rate and water production capacity—when selecting reverse osmosis (RO) membranes. However, in long‑term operation, the parameters that truly determine system stability and membrane element lifespan are often the ones that are easily ignored. This article reviews the three most overlooked parameters, drawing on Dalton Membrane’s practical experience in membrane selection, for reference by industry peers.
Parameter 1: Feed Water Temperature
In RO selection, feed water temperature is often simplified to a fixed value (typically 25°C). In actual operation, however, water temperature fluctuates significantly with the seasons. Studies show that for every 1°C drop in water temperature, membrane permeate flow decreases by approximately 2%–3%. As a result, the difference in production capacity between winter and summer can be as high as 15%–20%. If selection is based solely on the standard condition of 25°C, when winter temperatures drop to 10°C, the actual system output may be only 60%–70% of the design value, leading to insufficient water supply.
In many projects, problems are only discovered during commissioning or winter operation, forcing operators to add membrane elements or raise the feed temperature. At Dalton Membrane, we typically require that the permeate flow be verified against the local "most unfavourable temperature" during selection, ensuring that the system meets demand even in the coldest season—rather than using the optimal temperature as the design basis.
Parameter 2: Operating Pressure
Many users only focus on the "maximum allowable pressure" of the membrane element while seriously neglecting the "normal operating pressure range". A membrane system must operate within a stable pressure window, which is determined by factors such as membrane flux and feed salinity. If the design operating pressure is 15 bar, but the actual feed pressure stays at only 8 bar over long periods, the permeate flow will be far below the design value. Conversely, if the actual operating pressure consistently exceeds the design value, the excessive pressure will accelerate membrane compaction, causing irreversible loss of salt rejection and shortening membrane life. At the same time, some users mistakenly believe that choosing an ultra‑low‑pressure membrane is a one‑size‑fits‑all solution. However, if the feed water has high salinity, an ultra‑low‑pressure membrane may lack sufficient driving force to meet the required salt rejection. Dalton Membrane recommends that, during selection, the stable pressure range for normal system operation be clearly defined and comprehensively verified together with temperature and recovery rate, rather than merely looking at the nominal upper pressure limit of the membrane element.
Parameter 3: Concentrate Flow Rate
Concentrate flow rate is very easily overlooked during selection. It directly determines the cross‑flow velocity on the membrane surface. If the flow rate falls below the minimum allowed for the membrane element, insufficient velocity will aggravate concentration polarisation, leading to accelerated scaling by sparingly soluble salts, organic deposition, and rapid membrane fouling. On the other hand, if the concentrate flow rate is too high, although the risk of fouling is reduced, recovery rate decreases and both energy consumption and waste discharge increase. Each membrane element has a specified range for concentrate flow. During selection, it is essential to ensure that the flow in each stage stays within this range, with particular attention to the concentrate flow in the last‑stage elements, so as to avoid pushing the lower limit in pursuit of high recovery.
In Dalton Membrane’s project practice, we always check the concentrate flow rate against the technical manual requirements using design software during selection, and we reserve a safety margin between recovery rate and anti‑fouling performance to ensure long‑term system stability.
Conclusion
The key to long‑term system stability often lies in these easily overlooked parameters—feed temperature, operating pressure, and concentrate flow rate. Salt rejection and permeate flow are certainly important, but a little more rigour in selection—paying close attention to these details—will save many detours in future operation.
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