Reverse Osmosis Efficiency – How Efficient Is RO?
Written by: Gene Fitzgerald // Last Updated: Jul 21, 2023
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When talking about reverse osmosis efficiency, this usually means how much purified water you receive compared to how much is wasted.
Reverse osmosis systems are generally considered more wasteful than other types of water filtration, so just how efficient can they be?
Let’s find out!
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Key Takeaways
- The wastewater produced by a reverse osmosis system depends on factors such as feed water pressure and temperature, condition of filters and membranes, and TDS levels.
- A conventional RO system without a pump wastes around 3-5 gallons of water for every gallon of pure water, whereas RO systems with internal pumps waste as little as 0.25 gallons per gallon of filtered water.
Just How Efficient Is Reverse Osmosis?
So, how efficient is reverse osmosis? Well, reverse osmosis can be highly efficient but in many cases it’s not.
The wastewater ratio in reverse osmosis systems refers to the amount of filtered water produced compared to the amount of waste or reject water. For example, a 1:4 ratio means 4 gallons of wastewater are created for every gallon of filtered water.
This is linked to the “recovery rate”, the percentage of pure water produced. A pure-to-drain ratio of 1:4 equals a recovery rate of 20%, meaning 20% of the input water can later be used as purified water, while the remaining 80% go down the drain.
Typical under sink RO systems without pumps have a recovery rate of about 20%, while systems with pumps can achieve at least 50%. This means that your RO system will vary in its efficiency at producing clear water depending on various factors.
How Much Water Reverse Osmosis Wastes
The volume of wastewater generated by a reverse osmosis system is contingent on elements like the pressure of the feed water, the state of the RO membrane and pre-filters, the level of pollutants in the water, the temperature of the water, and the recovery ratio of the membrane.
A conventional RO system positioned under the sink and without a pump typically discards about 3-5 gallons of water for each gallon of pure water it creates. Conversely, RO systems equipped with internal pumps may discard as little as 0.25 gallons for every gallon of filtered water.
The efficiency of water usage in an RO system increases with water pressure at the RO membrane. RO systems that incorporate a pressure or permeate pump tend to be much more water-efficient than those lacking a pump.
A blocked filter/membrane will increase waste water due to the resultant decrease in pressure. Speaking of, lower feed water pressure and temperature contribute to more water being wasted. The same goes for higher TDS levels.
Real-Life Examples
Here are some of the average ratios of pure-to-waste water for several current reverse osmosis systems:
- AquaTru RO System
- Pure-to-Waste Ratio: 1:0.25
- Recovery Rate: 80%
- Waterdrop G3 Under Sink
- Pure-to-Waste Ratio: 2:1
- Recovery Rate: 66%
- SimPure Y7P-BW
- Pure-to-Waste Ratio: 1:0.25
- Recovery Rate: 80%
- APEC Essence ROES-50
- Pure-to-Waste Ratio: 1:3
- Recovery Rate: 25%
- APEC Ultimate RO-90
- Pure-to-Waste Ratio: 1:3
- Recovery Rate: 25%
- Pelican PRO-RO 6-Stage
- Pure-to-Waste Ratio: 1:3
- Recovery Rate: 25%
- iSpring RCC7AK
- Pure-to-Waste Ratio: 1:3
- Recovery Rate: 25%
- Home Master TMHP
- Pure-to-Waste Ratio: 1:1
- Recovery Rate: 50%
- ZIP AlcaPure
- Pure-to-Waste Ratio: 1:1
- Recovery Rate: 50%
What Does RO Membrane Recovery Ratio Mean?
The term “wastewater ratio” often pops up when discussing wastewater from reverse osmosis systems. Simply put, the waste water ratio compares the amount of clean water and wastewater produced in an RO system. For instance, a 1:5 waste water ratio means that five gallons of wastewater are created for every gallon of clean water.
This leads us to another term, “recovery rate”. It’s basically the same concept, but it shows the percentage of the original water that can be used after being purified. If the recovery rate is 25%, it means that 25% of the original water can be gathered as clean water. The usual recovery rate for RO systems without pumps, like those under sinks, is around 20-25%. Systems with pumps could reach at least 50%.
Why Does Reverse Osmosis Waste Water?
The creation of wastewater in reverse osmosis is unavoidable because of how the system needs to function. In reverse osmosis, water is pushed under high pressure through a membrane with microscopic pores. These pores are small enough to prevent pollutants from passing through, resulting in purified water that collects in a storage tank.
But, as this process continues, the contaminants stick to the membrane and could potentially block it. To prevent this blockage, the RO system constantly rinses the membrane, carrying away the mix of rinsed-off water and pollutants into the drain – this is known as wastewater.
Tips for Maintaining a Reverse Osmosis System So That It’s More Efficient
Optimize Feed Water Pressure
The pressure of the water that goes into the filter system for purification is called feed water pressure. More water gets purified, and less gets wasted when this pressure is high. If the pressure in your filtration system is low, you can minimize waste by increasing it.
Here are some ways to increase your feed water pressure:
- Install a booster pump on your filtration system. This can decrease wastewater by more than 90%.
- Increase the pressure in your house’s plumbing.
- If there’s a pressure valve in your system, boost the pressure there.
The ideal pressure for a reverse osmosis system should be around 60 psi. Anything less than 40 psi or significantly more than 60 can cause issues.
Recirculate and Repurify Reject Water
An alternative method to cut down on reverse osmosis waste water is sending it back through the RO system for further purification. However, this is generally only possible in certain whole house reverse osmosis systems that employ recirculation valves or have multiple RO membranes lined up sequentially.
Proper and Timely Maintenance of the RO System
When a reverse osmosis system is adequately maintained, the generation of excessive waste water is minimized. Keeping the system in good condition is not just a choice but necessary to decrease wastewater production and achieve optimal performance.
If you observe changes in the amount of wastewater generated by your RO system or in the quality of the purified water, it could indicate the need to replace filters or the RO membrane. Congested membranes raise backpressure, leading to an increase in wastewater. The same applies to old pre-filters.
How Often to Replace an RO Membrane
On average, reverse osmosis membranes should be replaced every 2 to 5 years, while pre-filters should be changed every 6 to 12 months, depending on your feed water quality.
Use an Up-to-Date Reverse Osmosis System
Minimizing wastewater from your RO system begins with the initial purchase. Certain systems exhibit higher efficiency than others. Modern systems are often specifically designed to address wastewater concerns, offering improved wastewater ratios.
Install a Pressure Pump
By directly connecting a pressure pump to your RO system, you can boost the water pressure flowing through it. This, in turn, guarantees optimal speed for the water passing through the RO membrane, effectively reducing wastewater production.
Install a Permeate Pump
A permeate pump operates without electricity and can significantly decrease water wastage by around 80 or 90%. However, it’s important to note that not all reverse osmosis system models support permeate pump integration.
Reuse the Waste Water
You can collect the wastewater from your RO system by connecting the drain pipe to a separate storage tank or bucket. This water can be utilized for various purposes, such as watering your garden or cleaning the house and car.
Further Reading:
About Zero-Waste Reverse Osmosis
Zero-waste reverse osmosis water filtration systems are engineered to minimize waste water. While they do technically function, their overall worthiness is debatable. There are two primary categories of zero-waste RO systems.
- The first kind recycles the wastewater by sending it to the hot water supply. However, this wastewater could be laden with various contaminants, and using it might expose you to these impurities, which can be harsh on your skin.
- The second category re-purifies wastewater by rerouting it back into the RO system. While this sounds ideal, there’s a drawback: The constant flow of wastewater back into the system eventually overloads the membrane with contaminants. These contaminants start to block the membrane, causing it to degrade rapidly and disrupt the RO process.
What Is Reverse Osmosis Filtration and How Does It Work Exactly?
Reverse osmosis water filtration is a process that uses a special membrane to separate clean water from wastewater filled with contaminants. Water is pushed through a very fine membrane under pressure. This membrane can catch even the smallest contaminants, anything bigger than .0001 microns, effectively removing bacteria, heavy metals, pesticides, herbicides, salts, and more from the water.
As the name suggests, reverse osmosis is the reverse of the osmosis process you may have learnt at school. Reverse osmosis needs external pressure to push water from a more concentrated solution (the unfiltered water) through the membrane to a less concentrated solution (the filtered water).
The RO membrane uses this principle to split water into two streams: clean water and wastewater filled with contaminants. Contaminants larger than the membrane’s tiny pores are trapped on one side, while only clean water passes through. The wastewater is discarded, and the filtered water moves on to the next stage of purification.
If you have any questions about RO efficiency please don’t hesitate to leave a comment below!
About the Author Gene Fitzgerald
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