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Reverse osmosis water purification is known for its high level of effectiveness. But it’s also known for wasting water.
If you’ve ever wondered what’s in your RO waste water, you’re going to love this article.
Here, we’ll talk about RO waste water composition and discuss why reverse osmosis wastes water in the first place.
Reverse osmosis waste water generally contains all the contaminants rejected or removed by the RO membrane. RO membranes have a TDS rejection rate of around 96-99%. This means all of the total dissolved solids in your feed water are rejected by the membrane and end up in your waste water.
Besides TDS, your RO membrane removes other contaminants in your water. We’ll see many of them below.
RO membranes have the ability to tackle a lot of contaminants. These are some of the contaminants removed:
Dissolved ions such as:
But a reverse osmosis membrane doesn’t stop at dissolved ions. It also removes radioisotopes like uranium and radium, volatile organic compounds, sediments, certain pesticides, microorganisms, asbestos, proteins and other organics, and more.
The TDS concentration of RO reject water depends on the RO membrane’s salt rejection rate, the amount of TDS in the feed water, and how much waste water the system produces.
For example, if your RO membrane has a TDS rejection rate of 97%, your feed water has a TDS of 500 ppm, and your system produces 1 gallon of waste water for every gallon of filtered water, your TDS waste water concentration will be 985 ppm.
(500 ppm in feed water at 97% rejection = 15 ppm in RO water and 985 ppm in reject water)
In a scenario where you have 1000 ppm TDS in the feed water, an RO membrane rejection rate of 99%, and 4 parts waste water per 1 part purified water, your TDS concentration will be derived as:
1000 ppm at 99% = 10 ppm in RO water and 1247.5 ppm in reject water.
In industrial settings that use waste water recycling etc., typical RO reject TDS concentrations range from 30,000 to 50,000 ppm.
The reverse osmosis process may seem flawed because of how it wastes water, but there are two main reasons why reverse osmosis waste water exists:
Reverse osmosis systems are designed to waste water, but this design is justified. RO systems filter water by forcing it through a semipermeable membrane under high pressure. The membrane has really tiny pores that are just small enough to prevent contaminants from passing but large enough to allow water molecules to diffuse through.
The filtered RO water goes to the storage tank, where it’s stored for later use.
With the filtered water stored away nicely, the system has to deal with the contaminants trapped on the membrane, plus the remaining water that hasn’t made its way to the other side of the membrane, yet. Both are flushed down the drain.
Wasting water – unpleasant as it sounds – helps to extend the RO membrane’s lifespan. Waste water ensures that the contaminants on the membrane do not overwhelm and damage it prematurely. This leads to increased efficiency and a longer lifespan.
Naturally, these contaminants would pile up on the membrane until they clog the pores completely. But the RO system prevents this by continuously flushing the membrane with water.
If you have any questions about RO waste water composition please don’t hesitate to leave a comment below!
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