RO Reject Recovery: How to Recapture Reverse Osmosis Waste Water
Written by: Gene Fitzgerald // Last Updated: Aug 7, 2023
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Recovering reverse osmosis reject water is commonly done to improve the efficiency of the filtration process. Otherwise, a lot of water gets wasted.
However, RO reject recovery is typically limited to industrial settings. It’s rare to see someone doing it in their household.
More often, people prefer to reuse their RO reject water for other purposes.
Still, let’s look into RO reject recovery and what you need to know.
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Key Takeaways
- Reverse osmosis reject water can be recovered in various ways such as dewatering, lime softening, chemical treatment, and brine crystallization.
- This is typically reserved for industrial applications.
- Domestic users can look into repurposing their reject water.
RO Reject Recovery – How to Recapture Reverse Osmosis Waste Water?
There are several methods available for recapturing reverse osmosis reject water. Most of them are not feasible for a regular domestic user. We’ve still covered those processes below if you’re curious how it’s done in industrial environments. But for the most part, you would probably be more interested in the section about reusing reject water.
For Residential Homes: Store and Reuse RO Reject Water
The best thing you can do as a residential user is to store your reverse osmosis reject water so you can reuse it for other purposes later on. These can include:
- Cleaning your home
- Watering your plants
- Flushing your toilet
- Cleaning your car
- Doing your laundry
Generally, you can use reverse osmosis reject water for anything that doesn’t require pure water. Take a look around your home and you will probably see at least a few opportunities.
Industrial Applications
On the industrial side, there are some much more advanced methods for recovering RO reject water. Some of those could theoretically be applied in a domestic environment, but they usually don’t make sense financially.
2 or More Reverse Osmosis Processes
The most basic way to recover reverse osmosis reject water is to connect two reverse osmosis membranes to each other. Water passes through the first membrane, and the reject water is then filtered again in the second one. Depending on the quality of the water, you might need to install additional treatment elements between the two reverse osmosis processes.
Dewatering/Evaporation
Evaporation is another simple process that can allow you to recover some of your reject water. Water is simply evaporated and then condensed into a separate container. If this sounds familiar, that’s because the process is pretty much like distillation, another commonly used technique for purifying water.
Lime Softening + Membrane Filtration
This process is typically used in a 2-stage reverse osmosis setup. After passing through the first reverse osmosis filter, water is specifically treated to remove certain contaminants which would make it impact the second filter more heavily. Typically this involves adding a softening agent to the water. This causes all solids to settle in a separate container.
Chemical Treatment
Chemicals can be added to RO reject water in order to treat it for specific contaminants. This is a complicated process that requires in-depth knowledge of the contaminants you’re dealing with, as well as the appropriate chemicals for removing them.
Brine Crystallization
Crystallization involves turning the brine into a solid material. This allows for much easier disposal. However, the main downside to crystallization is that it can be very expensive. This is why it’s usually kept as a last resort in case other options aren’t available or disposal costs are prohibitively high.
Electrodialysis (ED) and Electrodialysis Reversal (EDR)
Electrodialysis is a process which allows you to selectively target ions in the reject water. It can be highly effective, but also comes with some limitations, mainly due to the way certain salts affect the process in the long term.
What Is RO Reject Water?
What is reverse osmosis reject water in the first place? Once water passes through a reverse osmosis filter, it’s split into two streams. One is pure fresh water, while the other contains all the original contaminants, now in a much higher concentration. There is no way to avoid producing waste water when using reverse osmosis. That’s why it’s important to study the various options for recovering it, especially if you plan on installing a reverse osmosis system in an industrial setting.
As we mentioned above, it’s also possible to reuse reverse osmosis reject water for other purposes instead of trying to recover it directly. Many households can benefit from the reject water in various ways.
Further Reading:
TDS Concentrations of RO Reject Water
The TDS concentration of reverse osmosis reject water depends mainly on the TDS of the original input water as well as the rejection rate of the membrane. Another factor is the membranes recovery rate.
For example, if your feed water has a TDS of 500 ppm, you’re using a membrane with a 97% rejection rate, and you get one gallon of reject water for each gallon of purified water, you can expect to have a TDS of around 15 ppm in your RO water, and 985 ppm in the reject water.
Other example: 1000 ppm TDS in feed water, RO membrane salt rejection rate of 99%, 4 parts wastewater per 1 part purified water. 1000 ppm at 99% rejection = 10 ppm in RO water and 1247.5 ppm in reject water.
When using RO for waste water applications, generally, you can expect your reject water to have a TDS of around 30,000-50,000 ppm.
If you have any questions about RO reject recovery please don’t hesitate to leave a comment below!
About the Author Gene Fitzgerald
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