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RO membrane manufacturers are used to getting more complaints regarding issues with their products in winter. In fact, unnecessary membrane replacements are continuously made when the temperatures significantly drop.
That happens as a result of not taking into account the change in temperature of the feed water. Customers experience low water output and automatically assume that something is wrong.
To prevent you from making the same mistake, this guide will explain how exactly water temperatures affect an RO membrane, and in turn, the production of water.
We will also talk about RO membrane storage temperatures.
The temperature of the feed water is a crucial factor in the performance of a reverse osmosis membrane. During the winter season, when the temperature outside drops – and with that, the temperature of the water in your supply – changes in the production of purified water through an RO system can occur.
If everything else is functioning properly, an increase in water production will happen by the time spring rolls over. That is because warmer water temperatures allow for better membrane permeability.
RO membrane temperature limits also come into play when storing a membrane. But first things first…
RO membranes perform differently at different water temperatures. The recommended feed water temperature range is 40-100°F (5-35°C).
If your feed water is below 40°F (5°C), it means that the water has the highest density, and it is getting closer to the freezing point. That results in a serious drop in filtered water production.
When it comes to water temperatures higher than 100°F (35°C), they are also not recommended as they can impair the membrane’s performance. In fact, the elements of the membranes are not designed to withstand extremely high temperatures and will get destroyed above 113°F (45°C).
Of course, some high-temperature membranes can filter hot water up to 186°F (85°C), but these are designed for special circumstances and are not intended for household use.
The temperature for stored RO membranes depends on whether they are new or have been used.
The feed water temperature affects the production of filtered water through a reverse osmosis membrane inversely. That means that high temperatures increase permeability, while low temperatures decrease the production of filtered water.
Water becomes thicker at lower temperatures and therefore is not able to flow through the membrane at a high rate. On the other hand, when the water is warmer, it becomes thinner, and can easily permeate the RO membrane. This happens as a result of the viscosity of water.
In fact, for every °F that the feed water temperature drops, the production of filtered water decreases by roughly 3%. Inversely, when the water’s temperature rises by 1°F, the output of filtered water increases by around 3%.
This means that the diffusion rate of water increases with the temperature, but so does the salt passage. As the feed water gets warmer, the salt rejection rate decreases and more solids get pushed through the membrane.
Viscosity in water represents its inability to flow. To really grasp how the change in water temperature changes permeability of a reverse osmosis membrane, we need to understand how the viscosity changes at different temperatures first.
Of course, we cannot exactly measure the viscosity in water with our naked eye, so let’s use a more viscous liquid to paint a better picture. Let’s take some syrup or honey, for example. If you run warm honey through a sieve, it will flow much faster than if you were to pour it cold. That happens as a result of its viscosity.
The liquid’s inability to flow increases when the temperature is decreased. And that’s exactly why we may experience low water production through an RO membrane in winter.
The water production flow that is rated on RO systems and membranes typically assumes a water temperature of 77°F (25°C). If that temperature changes, so will the production rate of the filtered water.
The temperature correction factor is a great measuring gauge that can help you identify the membrane permeate rate based on the actual temperature of your feed water.
By knowing your membrane’s rated permeate flow (at 77°F) and the actual feed water temperature, you can use the TCF to determine the permeate flow rate for your temperature, or exactly how many gallons of filtered water can pass through the membrane given the temperature you’re dealing with.
Simply put, multiply the rated flow in gallons by the TCF for your feed water temperature taken from the table below.
For instance, let’s assume that your membrane’s permeate rate (at 77°F or 25°C) is 50 gallons. Your feed water’s actual temperature is 59°F (15°C), and given the TCF chart from below, the TCF for 59°F is 0.7. By multiplying 50 by 0.7, we see that the expected permeate rate for your feedwater is 35 gallons per day.
Keep in mind, though, that these calculations are not set in stone. Depending on the difference in the production process and chemistry, the temperature correction factors can vary for different membrane products. Nevertheless, the table below can help you calculate a rough estimate of the permeate flow rate you should expect.
|Water Temp °C||Water Temp °F||TCF|
If you’re dealing with low feed water temperature, you’re also dealing with decreased filtered water production. But can you raise the temperature to improve the permeate flow?
In theory, yes, you can. You can use water from a heater, but that’s neither advisable nor safe for drinking. You can try exposing the feed water tubing to some warmth to see if you can bring it to room temperature. Another idea is to wrap the water’s tubing around a heater or something else that’s warm, but that’s also unlikely to be that successful.
Your best bet for increasing water production is to try to increase the feed water pressure. You can do that with an electric booster pump, and that may bring you a better flow rate of filtered water during colder days.
If you have any questions about RO membrane temperature limits please don’t hesitate to leave a comment below!
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