What’s the TDS of RO Waste Water? Let’s Find Out!
Written by: Gene Fitzgerald // Last Updated: May 3, 2023
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If you have been researching reverse osmosis systems, you’ve probably come across the term TDS – or total dissolved solids.
This is an important indicator for the overall composition of water, and you should have at least a basic understanding of what it means and what it implies for your water supply.
This article answers the question, what’s the TDS of RO waste water?
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Key Takeaways
- The TDS of RO waste water is a function of the TDS of the feed water, the salt rejection rate of the RO membrane, and the amount of waste water produced.
- Generally speaking, the TDS of reverse osmosis waste water is higher than the feed water TDS, and much higher if your reverse osmosis system wastes 1 gallon or less for each gallon purified.
What’s the TDS of RO Waste Water?
So, what’s the TDS of RO waste water?
The TDS of reverse osmosis waste water depends on three factors:
- Feed water TDS – some water sources have a TDS of 200 others 800 ppm.
- Your RO membrane’s salt rejection rate – different membranes have different rejection rates. This usually ranges between 95 and 99%.
- The amount of water that your reverse osmosis system wastes for each unit of purified water – highly efficient systems waste as little as 0.25 gallons per gallon purified. Less efficient ones send up to 5 gallons of waste water down the drain for each gallon of filtered water they provide.
How do these three factors work together?
Let’s say your feed water TDS is 800 ppm. And your RO membrane rejects 98% TDS. That means your purified RO water will read 16 ppm in TDS (800 ppm * 0.02 = 16 ppm). The rest ends up in the waste water.
That’s where your RO system’s pure-to-wastewater ratio comes into play. If the system wastes 1 gallon of water per 1 gallon purified, the RO waste water would read 800 ppm + 784 ppm = 1584 ppm in TDS. If it wastes 4 gallons, it would be 800 ppm + 784 ppm / 4 = 996 ppm. And so on. So a system wasting more water means lower waste water TDS as compared to a system that wastes less.
By the way, RO membrane salt rejection rate also varies over time as a membrane gets used more. If your membrane is older, it might have started clogging or fouling, resulting in a lower TDS rejection rate. You should avoid dropping down below 80% TDS rejection. If that’s the case, it’s recommended to replace your membrane immediately.
What Is TDS (Total Dissolved Solids)?
TDS stands for total dissolved solids, and it’s an indicator of the amount of dissolved particles in your water. Water is a pretty good solvent, and many impurities can completely dissolve in it. Even though you’re not able to see them, they will still be there.
Does TDS equal contamination? Absolutely not! Beneficial minerals like calcium, magnesium, potassium, and even carbonates all contribute to a water’s TDS reading. That’s why you should consider TDS more of an aesthetic factor. High-TDS water will have a “stronger” taste.
Still, when it comes to reverse osmosis water purification, measuring TDS is a good way to figure out how effective the filtration process is – you can compare feed water TDS to the TDS of the purified RO water.
Further Reading:
How Do You Measure TDS in Reverse Osmosis Waste Water?
You can measure TDS in your reverse osmosis waste water using a TDS meter.
Simply remove the meter’s cap and push the ON button. Then hold the tip into your water sample.
Uses for High TDS Waste Water from RO
The term waste water can be a bit misleading. Some people assume that there is nothing they can do with the waste water of their reverse osmosis systems. Truth is, there are various useful applications for that water. You should consider some of those if you’re concerned about wasting too much water in the purification process.
Household Cleaning Chores
You can use waste water from your reverse osmosis system to clean your house. You don’t need pure fresh water for mopping the floor, for example. You can even use this for wiping down countertops and other surfaces. Just make sure that you’re careful with any surfaces that will come in contact with food.
Watering Your Garden
Reverse osmosis waste water is also useful for watering gardens. The only downside here is that it might contain high levels of sodium, which could make it unsuitable for certain plants. Pay attention to the sodium contents of your reverse osmosis waste water, and know what your plants require.
Car Wash
Washing your car is another good option for using your RO waste. You should be careful about certain contaminants that can leave unsightly residue on your car, however. As long as you don’t have too many of those in your water, you’re good to use it for washing your car.
Laundry Pre-Rinse
You might not be a fan of using RO brine for doing your laundry. However, you can still use it for the pre-rinse. Any contaminants that make their way into your clothes will get washed away during the normal wash cycle anyway. There might be some issues with using RO waste water for doing your laundry related to the lifespan of your washing machine, though. You should be careful to avoid damaging the machine with heavily contaminated water.
Flushing the Toilet
Finally, you can consider something as simple as flushing your toilet. There are no downsides to doing that with reverse osmosis waste water. The only minor disadvantage is that you have to figure out a way to transport the water to your toilet, which usually involves carrying a bucket. This might not be very convenient in most cases.
What Is the Maximum TDS Level a Reverse Osmosis System Can Handle?
The maximum TDs level that can be handled by a reverse osmosis system varies according to several factors, mainly the construction of the membrane. In general, you should never filter water above 2,000 ppm. This could quickly clog up your filters and lead to various other issues. If your water is so heavily contaminated, you might want to look into using additional pre-filtration methods.
If you have any questions about the TDS of RO waste water please don’t hesitate to leave a comment below!
About the Author Gene Fitzgerald
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