By Rehan Iqbal
Reverse osmosis doesn’t work like most filters. Instead of trapping contaminants, it separates them from part of the water supply, creating both clean and heavily contaminated water. That contaminated water goes down the drain. How much waste water do these systems generate, and is it enough to be a cause for concern? Are there ways you can reduce this waste, or capture waste water for other uses? We have the answers you’re looking for, whether you own an RO system, or you’re looking to install one in your home.
Most filtration systems use the filter media to capture contaminates. Sediment filters block large particles. Contaminates cling to the surface of carbon media. Water conditioners grab onto mineral ions and join them together to form crystals. With claims like “filters down to 0.0001 microns,” you may think reverse osmosis filters work the same way. However, all the RO membrane does is block these contaminants.
In osmosis, if you separate water with a permeable barrier, dissolved solids will pass through the barrier until both sides reach an equilibrium. For example, let's say you put drinking water on one side of the barrier, and sea water on the other side. The drinking water contains 3 ppt of salt, while the salt content in the sea water contains 36 ppt of salt. Over time, the salt passes through the barrier until both sides contain 18 ppt.
If you add pressure to one side, the process reverses. The solids pass through the barrier to the non-pressurized side. For example, if you apply pressure to the sea water side, the salt passes through the barrier. The salinity on the fresh water side goes up, while the salinity on the sea water side goes down. Eventually, the sea water will be salt free, while the fresh water will be salty.
While reverse osmosis was originally invented to filter salt from sea water for use in submarines, that’s just one of its uses. Reverse osmosis is the best water treatment method for removing lead, arsenic, nitrates, radium and chromium. However, due to how the way this system works, the waste water has higher levels of these contaminates, making it unsafe to drink. This contaminated water goes down the drain.
How Much Waste Water Does a Reverse Osmosis System Generate?
A standard under sink RO system in good working order generates around four gallons of waste water for every gallon of filtered water. High efficiency systems can generate as little as two gallons of waste water for every gallon of filtered water. However, performance depends on the condition of the system and available pressure to push water through the filter. If water pressure drops or clogs form in the system, efficiency can drop dramatically, generating as much as 30 gallons of waste water for every gallon of treated water.
Point of Entry Vs. Point of Use
Which Type of RO System Do You Need?
Do you need a whole house reverse osmosis system? Probably not. Prices for these systems are far higher than a complete well water treatment system, even if you need filters for heavy metals. The cost of these systems also extends past the filters themselves.
Reverse osmosis removes dissolved solids, creating processed water that readily absorbs minerals from anything it touches. This includes copper pipes and lead solder in your home’s plumbing system. To use a whole house system safely, you need to replace metal pipes with non-reactive plastic.
When people choose to use a reverse osmosis system, it’s mostly to remove contaminants that only cause harm if they enter your body. By choosing an under sink or countertop solution, you’re only treating the water you drink or use for cooking. This dramatically reduces the amount of waste water generated by the RO system.
Let’s say you have a family of four, and everyone drinks 64 ounces of water per day. That’s a total of two gallons of water. Having pasta for dinner? You’re using another one to two gallons of water to boil it. Since other cooking methods use less water, you’re usually using just two to four gallons per day for eating and drinking. Compare that to other water usage:
On average, each person uses 80-100 gallons of water per day, yet only a small fraction of that goes into our bodies. If you have a whole house water system, and it’s working perfectly, you’re using two to five times as much water than an unfiltered system. Instead of 320 to 400 gallons of water per day, you’re using 640 to 3,200 gallons, depending on individual water use and filter efficiency. However, if you use reverse osmosis for just drinking and cooking, you’re only increasing your water usage by a maximum of 8 gallons.
How Can I Change My Habits to Reduce Waste from Reverse Osmosis?
If you’re looking to cut down on waste water, the first step is easy: limit how much you use your reverse osmosis system. For the most part, the contaminates that a reverse osmosis system filters out don’t have a meaningful effect on water used for cleaning. That includes dishes and silverware, even though you eat with them.
If you use an RO system to remove contaminants that are dangerous to consume, like arsenic, you should use this filtered water for all of your drinking and cooking. If you only use your filter to improve the taste of your water, you may find that it doesn’t make much of a difference boiling food or making sauces. However, you will notice a difference making beverages like tea and coffee. Experiment and see where the flavor of RO water makes a difference.
How Can I Reduce the Amount of Waste Water Generated by My RO System?
The efficiency of your system depends on flow and pressure. Problems with either of these make your filter system send more water down the drain.
Reverse osmosis depends on water pressure to work properly. Ideally, the water going into the system should be at least 35 and 40 PSI, although a pressure of 60 psi offers the best performance in most systems. If you’re on well water, you may need a stronger pump to supply your RO system, not to mention the rest of your home.
Whether you’re on well or municipal water, you can improve performance by adding a booster pump to push water through the system. These can be set to turn on when the water pressure falls below a certain set point. That way, it’s only on when the RO system is in use. Of course, low water pressure might be due to leaks in your plumbing system. If you’ve noticed your home’s water pressure drop off recently, it may be time to call a plumber for an inspection.
Clogged filters reduce water pressure in the system. Manufacturers have recommendations for filter change intervals, but you may need to change cartridges sooner if you use more water than the system is rated for. Adding pressure gauges helps you monitor the health of your RO system.
A single pressure gauge at the start of the system will alert you to blockages and flow problems. Some systems have the option of adding gauges to each stage of the system. This lets you identify which filter is causing the blockage. Modern systems have an automatic shutoff valve. This redirects water, if there’s a blockage in the system. That way, water isn’t constantly ejected to the drain when it can’t fill the filters in your RO system.
Carbon filters are used in RO systems to remove chlorine, which damages the reverse osmosis membrane. These filters also remove contaminants before the reach the RO filter to take stress off of the system. You can take this a step further by adding a sediment filter before the RO system. This traps large particles that can clog both reverse osmosis and carbon filters.
Can I Do Anything With RO Waste Water?
It is possible to set up a recycling loop to capture and reprocess waste water. By letting some of the waste water go through another pass, allowing it to go into the clean water stream. However, directing too much water back to the RO system overloads it with contaminants.
This loop requires added plumbing and a pressure pump to get the recycled water into the supply line. This method isn’t practical with a point of use RO system, but it may be possible with a whole house system.
Since the water rejected by the RO filter contains increased levels of contaminates, it’s far worse health and taste-wise than the water that comes out of your tap. You wouldn’t want to drink it or bathe in it.
However, it can be used for things that don’t require potable water. All you need to do is connect it to a collection tank. You can use this water for cleaning things outdoors, like patio furniture and vehicles. It’s also fine to use this gray water to flush in toilets, although the additional plumbing required usually isn’t practical.
You may be able to use waste water on plants, as long as that water doesn’t have anything that leaches into the soil. Lead and arsenic can build up in soil and the plants that grow in it. This is especially dangerous for edible gardening, because you can end up with fruits and vegetables that contain high amounts of these contaminants.