How Much Water Does Reverse Osmosis Waste? Find Out Here!
Written by: Alexandra Uta // Last Updated: Jun 20, 2023
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Everyone who tries to purchase a reverse osmosis system is confronted with an uncomfortable truth: Reverse osmosis systems waste water.
But just how much water does reverse osmosis waste? And is there a way to salvage the situation? If you’re currently mulling these thoughts over, we’re here to help.
This article discusses how much water reverse osmosis systems waste on average and suggests ways to keep the waste at a minimum. Keep reading to learn all the details!
Contents
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Key Takeaways
- The amount of waste water produced by a reverse osmosis system depends on factors such as feed water pressure, condition of the RO membrane and pre-filters, contaminant levels in the water, water temperature, and membrane recovery ratio.
- A standard under sink RO system with no pump will waste around 3-5 gallons of water per 1 gallon of pure water produced.
- RO systems with internal pumps may waste no more than 0.25 gallons per each gallon of filtered water.
How Much Water Does Reverse Osmosis Waste?
Reverse osmosis systems are highly effective at what they do: Removing water contaminants. But they also produce waste water. How much? That depends on several factors:
- The first factor here is water pressure. The higher the water pressure at the RO membrane, the less water the RO system wastes. Hence, RO systems with an inbuilt pressure or permeate pump tend to waste less water than those without a pump. For instance, a standard under sink RO system without a pump will waste approximately 3-5 gallons for every gallon of filtered water it produces. On the other hand, a filter with an internal pressure pump (like the AquaTru) will waste as little as 0.25 gallons per gallon of filtered water.
- A clogged filter/membrane will also cause more waste water. Why? Reduced pressure!
- Other factors influencing the amount of waste water produced by a reverse osmosis system include the concentration of contaminants in the water, feed water temperature, membrane recovery ratio, and the overall condition of the membrane.
Below is a chart which lists the average pure-to-wastewater ratios of some of the current reverse osmosis systems:
| Model | Pure-to-Waste Ratio | Recovery Rate |
|---|---|---|
| AquaTru RO System | 1:0.25 | 80% |
| Waterdrop G3 Under Sink | 2:1 | 66% |
| SimPure Y7P-BW | 1:0.25 | 80% |
| APEC Essence ROES-50 | 1:3 | 25% |
| APEC Ultimate RO-90 | 1:3 | 25% |
| Pelican PRO-RO 6-Stage | 1:3 | 25% |
| iSpring RCC7AK | 1:3 | 25% |
| Home Master TMHP | 1:1 | 50% |
| ZIP AlcaPure | 1:1 | 50% |
RO Waste Water Ratio Explained
“Waste water ratio” comes up a lot in conversations about RO waste water. It may sound confusing, but it’s nothing too complicated.
Waste water ratio simply means the ratio of purified water to waste water in a reverse osmosis system. So, for example, a waste water ratio of 1:5 indicates that five gallons of waste water are produced for every gallon of filtered water.
This brings us to the term “recovery rate”, which is essentially the same thing, except recovery rate represents the percentage of input water that can be used after purification. A recovery rate of 25% means that at least 25% of the input water can be collected as purified water. A 20-25% recovery rate is the standard for under sink RO systems without pumps. Systems with pumps could have at least 50%.
Still, it’s important to note that many RO designs can’t handle high recovery rates because it’ll lead to membrane scaling and fouling. So beware of products that boast of an outrageously high recovery rate.
Why Does a Reverse Osmosis System Waste Water?
It may seem like RO systems aren’t sustainable or budget-friendly because they waste water, but it’s essential to understand why this feature exists. There are two major reasons why reverse osmosis systems waste water. We’ve discussed them below:
Working Principle of RO Process
Reverse osmosis systems filter water by forcing it through a semipermeable membrane under pressure. This membrane has tiny pores that block contaminants but allow pure water molecules to diffuse through them. The purified water is stored in the RO storage tank, where it can be taken up when needed.
But what happens to the contaminants on the entry side of the reverse osmosis membrane? Ordinarily, they’d pile up on the membrane’s surface and clog it completely. But the RO system stops this by continuously flushing the membrane with water. The flushed contaminants are sent down the drain and are referred to as waste water.
So, in summary, RO systems waste water because they constantly flush the impurities out of the membrane to maintain optimum working conditions.
Extend the Life of RO Membrane
For most traditional water filter systems that don’t backwash or clean their filter medium in some way, there’s no option to protect the filter element from fouling or degrading eventually.
In the case of RO systems, however, wasting water to clean the membrane helps to prevent it from getting damaged prematurely due to a build-up of contaminants. So ultimately, it extends the life of the membrane.
How Can You Minimize RO Waste Water Production?
Although RO systems are designed to waste water, there are things you can do to minimize the volume produced. Here are some ideas:
- Regulate water pressure: As mentioned earlier, your feed water pressure can significantly affect the volume of waste water your RO system produces. If the water pressure in your home is too low, your system will produce more waste water. So, check the pressure of your feed water using a pressure gauge. Most RO systems need at least 40-60 psi to work efficiently. If the water pressure in your home doesn’t measure up, you can increase it with a pump.
- Do regular maintenance: You may waste more water than usual if your reverse osmosis filter system hasn’t been maintained in a long time. Carrying out regular maintenance, like replacing the membrane and pre-filters, should help you reduce the amount of waste water being produced. On average, RO membranes need to be replaced every two to five years, while the other filters in the system should be replaced every 6-12 months.
- Choose your RO system carefully: The RO system you choose determines how much water will be wasted. Unlike older models, newer ones are often designed with inbuilt pumps that help to reduce waste water. So before settling for any RO system, do your research and pick the ones with reasonable recovery rates. That way, you’ll solve half the problem.
- Reuse discharged water: You don’t always have to let the reject water go to waste. You can reuse it for tasks that don’t require pure water, like flushing toilets or watering your garden.
Further Reading:
Do Zero-Waste Reverse Osmosis Systems Really Work and Waste No Water?
Zero waste RO systems are a new and increasingly popular trend in the water filter industry. There are two types of zero waste RO systems, but it’s questionable if they truly work as intended. Let’s study the concept of these zero waste RO systems:
The first type of zero waste system recycles the waste water or brine produced by redirecting it to the hot water line. One could argue that that this doesn’t cause waste water being sent down the drain. However, we must also consider that the brine produced by the RO system has a higher concentration of contaminants. And when it is channeled through the hot water line, it will be passed out through your faucet, possibly exposing you to the impurities it carries. This doesn’t seem like such a great idea.
The second type of zero waste RO system recycles its waste water by sending it back through the cold water line for another round of filtration. Granted, this cycle keeps you from coming in contact with impurities and doesn’t waste water, but there’s still a downside: Constantly sending waste water back into the RO system puts pressure on the membrane, forcing it to work harder from the overload of contaminants. This will cause it to degrade quickly, so it’s not the best idea either.
So yes, zero waste RO systems may eliminate waste water, but they also create other problems.
Reverse Osmosis Waste Water Uses
You can’t drink, cook with, or bathe in reverse osmosis waste water, but there are other ways to use it. You can use RO waste water for:
- Cleaning: You don’t need purified water for general cleaning in the home. Reverse osmosis reject water can be used to mop floors, clean windows, and wash utensils.
- Washing cars: Car washing consumes a lot of water, and since you don’t need perfectly clean water to wash cars, RO waste water is a great alternative.
- Flushing and cleaning toilets: RO waste water can be used to flush and clean toilets.
- Watering plants: You can also water your plants with RO waste water; plants don’t demand 100% pure water. However, if your waste water has a high concentration of contaminants, your plants may be affected negatively. If you’re unsure how your waste water might affect your plants, test it on a few plants for about 15 days to see if they experience any negative changes.
- Sewage pipe cleaning: RO waste water can be used to clean sewage pipes regardless of how much TDS it contains.
- Laundry pre-rinse: You can do your laundry pre-rinse with RO waste water. However, it’s not advisable to use waste water on delicate or soft clothing because high TDS levels can damage them.
If you have any questions about reverse osmosis waste water ratio please don’t hesitate to leave a comment below!
About the Author Alexandra Uta
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