Reverse-osmosis (RO) systems send some incoming water through a membrane as treated water and discharge the rest as reject water, also called concentrate or brine. To reduce that waste, first improve the system’s water efficiency; do not assume the reject stream is clean or safe to reuse. It contains a higher concentration of substances removed from the feed water, and local plumbing rules may restrict collection or reuse.
This guide explains the difference between treated and reject water, what to check before considering reuse, and the most reliable ways to reduce the amount sent down the drain. For broader home sustainability ideas, see our Zero Waste Living guide.
Understand the two water streams
An RO unit uses pressure to push water through a semipermeable membrane. The product stream is called permeate; the reject stream carries concentrated dissolved substances away from the membrane. The exact volume and composition vary with the system, source water, pressure, maintenance and operating conditions. Reject water is therefore not the same as untreated tap water, and it should not automatically be used for drinking, cooking, pets or edible plants.
The U.S. EPA says a typical point-of-use RO system can generate five gallons or more of reject water for each gallon of treated water, while less efficient units may generate up to ten. EPA WaterSense-labeled point-of-use RO systems must meet an efficiency threshold of no more than 2.3 gallons of reject water per gallon of treated water. Check the unit’s certification and rated efficiency rather than relying on a product’s general “water-saving” claim.
Start by reducing reject water at the source
- Choose an efficient unit. Compare certified reject-water ratios and product performance before buying.
- Maintain the system. Replace filters and membranes on the schedule in the manual; clogged or poorly maintained parts can affect performance.
- Check the feed pressure and installation. Ask the manufacturer or a qualified installer whether the unit is operating within its specified range.
- Use RO only where it solves a real water-quality need. The EPA notes that point-of-use systems can reduce certain contaminants, but not every home needs RO; check your water report and the unit’s certification for the contaminants of concern.
- Measure actual output. Collect the treated water and reject stream briefly using a safe, clean method described by the manufacturer, then compare the ratio with the rating.
Do not bypass a flow restrictor, alter the drain connection or modify the membrane to force a lower reject flow. That can reduce treatment performance, damage the unit or create a plumbing problem. If the measured ratio is much worse than the rated value, contact the manufacturer or installer rather than guessing at a repair.
Can RO reject water be reused?
Sometimes a non-potable use may be possible, but safety depends on the source water, the contaminants concentrated by the membrane, the intended use and local code. A common suggestion online is to collect RO reject water for plants or cleaning. That advice is not universally safe: salinity and dissolved substances can harm sensitive plants, and reuse plumbing rules differ. The International Residential Code includes restrictions on collecting RO reject water within certain onsite non-potable reuse systems. Ask your local building or plumbing authority and read your system manual before designing a reuse setup.
If an authority and the manufacturer confirm a particular non-potable use is allowed, keep the water clearly labeled, do not connect it to drinking-water plumbing, do not use it on edible crops unless a qualified source confirms it is suitable, and do not store it in a way that creates a hygiene or child-safety risk. Do not use it for drinking, cooking, bathing, washing produce or filling a pet bowl. For a permanent collection system, use a qualified plumbing professional and approved materials.
Simple household habits that save water
Even if reuse is not suitable, you can reduce water demand elsewhere: fix leaks, run full laundry and dishwasher loads, install appropriate efficient fixtures, and follow your local watering schedule. The EPA’s WaterSense program is a useful place to compare water-efficient products. These measures do not make RO reject water potable; they simply lower total household water use.
Measure the system before changing anything. Use the manufacturer’s instructions to identify the reject-water tube, then collect and measure its output for a set period while the system makes treated water. Compare the result with the model’s published ratio under similar pressure and temperature conditions; a single household measurement can vary as the storage tank fills and the unit cycles. If the drain flow seems unusually high, the water tastes different, or production slows, check for a worn membrane, overdue filters, a pressure issue or a faulty shutoff valve. Ask a qualified installer to diagnose plumbing or performance problems. Do not store reject water for long periods, and keep any permitted non-potable collection covered and clearly separated from drinking water. Water chemistry differs by location, so local authorities and the system manufacturer are better guides than generic online reuse lists. These checks can reveal an efficiency problem without risking the treatment system.
Frequently asked questions
How much reject water does a home RO system make?
It varies by model and conditions. EPA says a typical point-of-use system may discharge five gallons or more per gallon of treated water, while WaterSense-labeled systems must meet a stricter efficiency standard of 2.3 gallons or less per gallon produced.
Can I water my garden with RO reject water?
Do not assume it is suitable. The concentrate may contain elevated dissolved substances, and plant tolerance varies. Check water quality and local guidance before considering any outdoor use, and avoid edible crops unless a qualified authority confirms safety.
Can I use reject water for drinking or cooking?
No. It is the concentrated reject stream, not the treated permeate. Do not use it for drinking or food preparation.
Prioritize efficiency and verify any reuse
The safest first step is usually an efficient, correctly maintained RO system—not an improvised plumbing modification. Confirm the system’s certified reject ratio, check local rules and get qualified help before planning a permanent reuse route. For more practical household sustainability guidance, visit our Zero Waste Living guide.
Further reading: U.S. EPA WaterSense: Point-of-Use Reverse Osmosis Systems · EPA: Treating Water With Less Waste · International Residential Code excerpt on onsite non-potable reuse sources.