Why Does Reverse Osmosis Waste Water? Find Out Here!
Written by: Gene Fitzgerald // Last Updated: Apr 20, 2023
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Reverse osmosis systems are widely considered one of the most effective modern water filtration methods available – but they do have one downside: As a consequence of the reverse osmosis process, a sometimes significant proportion of waste water is generated for each gallon of filtered water produced.
Now, whether or not this “waste water” is a waste or a significant problem is the subject of some debate, and there are reasonable points on both sides.
Still, let’s take a closer look at why reverse osmosis systems waste water, and what you can do to mitigate this problem.
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Key Takeaways
- As pressurized water passes through an RO membrane it becomes separated into two streams: One is the filtered water that passes onto the next stages, and the other is the contaminant-filled waste water that is sent down the drain.
- RO membranes require flushing away the waste water to extend their lifespan. If contaminants were to build up on or inside the membrane, it would lead to fouling and premature membrane expiration.
Why Does Reverse Osmosis Waste Water?
So, why does reverse osmosis waste water?
Reverse osmosis systems generate waste water as a direct consequence of the filtration process. When water passes through a reverse osmosis membrane, it is separated into two streams:
- One is filtered water sent onto the RO storage tank.
- And the other is the contaminant-filled waste water sent down the drain.
Side note: A better way to describe the byproduct of excess water, according to some experts, is “water usage” rather than “water wastage”.
1. Due to the Working Principle of RO Membranes (System Design)
RO membranes work differently than other types of water filter elements. Most filters trap contaminants that are larger than their pores. Other types bind, adsorb, or absorb impurities. Clean water is allowed to flow onward.
Reverse osmosis systems, on the other hand, use water pressure to force water to diffuse through their membranes. But not all of the water can diffuse through. Some remains on the feed water side together with almost all contaminants which are being rejected. The contaminant-filled waste water must be drained out of the system to prevent contaminants from building up inside the RO chamber.
2. Protecting the Membrane from Fouling
If contaminants were allowed to build up on or inside an RO membrane, over time it would lead to fouling of said membrane – think clogging – damaging the membrane and rendering it prematurely ineffective.
How Much Water Does Reverse Osmosis Waste?
No matter how efficient RO systems are, some degree of excess water will be used as part of the filtration process.
Most standard RO systems for home use create somewhere in the range of 4 parts wastewater to 1 part filtered water. This has been the standard for RO systems for some time, but newer innovations and improvements have improved this ratio significantly in many of today’s models.
Using various methods of increasing water pressure, recycling, etc., many modern RO systems feature wastewater to filtered water ratios of 1:1, 1:2, and even 1:4.
RO Membrane Recovery Rates
RO membrane recovery rates refer to the ratio of permeate flow (filtered water) to feed water flow – which is a good way to benchmark the overall efficiency of the system. If the system is operating at a 60% recovery rate, then 60% of the feed water will be converted into permeate, with 40% being turned into waste water.
How You Can Reduce Waste Water in Your Reverse Osmosis System
Waste water generated by a RO system is an avoidable part of the process, but that doesn’t mean there aren’t steps you can take to reduce the amount of waste water generated.
Use an Up-to-Date System
This is an easy one to overlook, but an up-to-date RO system will often have a better membrane recovery rate than an older one.
Increase Feed Water Pressure
Water pressure highly affects how much waste water is produced. The membrane requires a certain level of pressure to push feed water through, and if that level is too low more water will be wasted in the process.
Typically, the minimum pressure for an RO system to work is 40 psi. To work optimally, they typically need about 60, sometimes even 80 psi. If your feed water pressure is too low, you can increase it throughout your home’s plumbing system or by installing a booster pump.
Booster Pumps
Booster pumps are required if your feed water pressure falls below the required threshold. These pumps use a motor and an electrical hookup to work and will boost the water pressure entering the RO system into the optimal range.
Permeate Pumps
Permeate pumps are different from booster pumps. They reduce wastewater from an RO system by an impressive 75% to 80% without using electricity whatsoever!
These pumps store some of the reject water to power a piston which pushes filtered water from the membrane housing into the storage tank. This prevents the water pressure in the tank from pushing against the RO membrane – increasing the system’s performance and efficiency.
Unlike a booster pump, a permeate pump doesn’t require any sort of motor or power source to operate.
One thing to note is that not every RO system is designed to work with a permeate pump, so make sure your system can handle one before purchase.
Proper Maintenance
Of course, performing proper maintenance on your RO system is the key to getting the best overall performance out of it. This includes minimizing waste water production.
Maintenance includes regularly changing pre and post-filters and, most importantly, the RO membrane itself. Consult your manufacturer’s recommendations for the exact filter change schedule.
Generally speaking, used membranes are less efficient than new ones. That’s due to clogging, meaning accumulation of dirt on and inside the membrane.
Additionally, regular cleaning and sanitizing of an RO system including the tank are necessary to keep everything running smoothly. This won’t necessarily impact the wastewater ratio directly but will keep everything clean and free of microbial contamination.
How Often Should I Replace My RO Membrane?
RO membranes should be replaced every 2 to 5 years, depending on the specific RO system. Once again, consult your manufacturer’s recommendations for the precise replacement schedule.
Wastewater Recycling
One more way to reduce or eliminate wastewater from your RO system is to collect it and reuse it for other household purposes. Of course, you don’t want to use this water for drinking as that would defeat the purpose of the RO system in the first place.
To recycle your wastewater, you can run the drain line to a second tank, bucket, or even an external faucet.
Further Reading:
How to Use RO Reject Water So That It Doesn’t Go to Waste
Household Cleaning
Household cleaning is an excellent use for RO waste water, as this water won’t be ingested in any way. You can use this collected waste water for washing floors, doing dishes, cleaning kitchens or bathrooms, or outdoors to wash patios, decks, or driveways.
Laundry Pre-Rinse
Using RO waste water as your primary laundry water is generally not a great idea, as water with high TDS (total dissolved solids) can harm delicate fabrics. Using it as a laundry pre-rinse, on the other hand, is a great way to use up excess waste water.
Watering Plants + Gardening
Using waste water to water both indoor and outdoor plants is another great way to recycle it. Some plants may be sensitive to water with a high level of TDS, so it’s a good idea to test water the plant for a few days and check if there are any negative impacts.
Washing Your Car
Washing your car uses a lot of water – somewhere in the neighborhood of 65 to 80 gallons – so using your collected waste water for this purpose is an ideal way to recycle it.
Can You Drink RO Waste Water?
Seeing as waste water is the contaminant-filled reject water that is generated as part of the RO process – it’s definitely not something you should be drinking.
Will it kill you? Not likely. But drinking wastewater defeats the purpose of investing in a quality reverse osmosis system in the first place.
Do All Reverse Osmosis Filter Systems Waste Water?
Zero-Waste Systems
Zero-waste systems reduce waste water by recirculating it back into your plumbing system. Typically, these systems will send water into the hot water plumbing – meaning your faucet will dispense waste water when you use hot water to wash your hands or do dishes etc.
Less advanced versions will send water into the cold-water line supplying feed water to the system. This type of system will force the RO system to work harder than necessary and wear down components more quickly than normal RO systems. That translates into more frequent filter changes and a shorter filter lifespan.
Typical reverse osmosis systems send waste water down a drain line and into a drain saddle connected to your sink’s drain line.
If you have any thoughts about the question, why do RO systems waste water, please don’t hesitate to leave a comment below!
About the Author Gene Fitzgerald
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