Can You Reuse Reverse Osmosis Wastewater?

Reverse-osmosis reject water can sometimes be reused for non-drinking purposes, but only when its source and quality make the proposed use safe. RO does not merely create “extra tap water.” The concentrate stream contains more of the salts and contaminants rejected by the membrane, so it should never be used for drinking, cooking, pet water or other uses that require potable water.

What is RO wastewater?

A reverse-osmosis membrane separates incoming water into two streams. Product water, also called permeate, passes through the membrane. The remaining concentrate or reject water carries a higher concentration of many dissolved substances.

The U.S. EPA explains that some typical point-of-use systems send five or more gallons of concentrate to the drain for every gallon of treated water. Current WaterSense-labelled RO systems must use no more than 2.3 gallons of concentrate per gallon of product water and meet independent efficiency and performance criteria.

The first sustainability step is therefore not finding ten ways to use an inefficient waste stream. It is choosing an appropriately sized, efficient system only when the source-water problem calls for RO.

When should RO reject water not be reused?

Send the concentrate to the approved drain or disposal point specified by the manufacturer and local rules when any of these applies:

• The system treats arsenic, nitrate, PFAS, radionuclides, high salinity or another health-related contaminant.
• The feed water is microbiologically unsafe or comes from an unverified source.
• A water softener supplies the RO unit, increasing sodium in the reject stream.
• Cleaning chemicals, membrane preservatives or sanitizers may be present.
• The proposed use could expose people, pets, food, waterways or sensitive plants.
• You do not know the source-water quality or cannot keep the collection system sanitary.

UMass Extension notes that RO reject water contains higher levels of contaminants than the incoming water. A normal-looking stream is not proof that it is suitable for reuse.

Safer household uses, when water quality allows

The following ideas are limited to reject water from a maintained household system treating an otherwise potable supply. Check the appliance manufacturer, local plumbing rules and water analysis first.

1. Toilet flushing

Manual bucket flushing can use a substantial volume without putting water on food or skin. Pour into the toilet bowl, not the toilet tank, unless the toilet manufacturer and a licensed plumber approve a purpose-built connection. Permanent reuse plumbing needs backflow and cross-connection controls and may require approval.

2. Initial cleaning of outdoor hard surfaces

Reject water may be used for a first rinse of muddy tools, bins or paved surfaces if runoff will not enter a storm drain, creek or garden bed. High-mineral water can leave spots or scale, so use potable or rainwater for a final rinse where appearance matters.

3. Pre-rinsing cleaning cloths or heavily soiled items

A small quantity can loosen dirt before a normal wash cycle. Do not use it in a washing machine unless the manufacturer permits the water quality, and do not use it for nappies, food-contact textiles or hygiene-sensitive cleaning.

4. Limited irrigation of suitable ornamental plants

Garden reuse requires the most caution. RO concentrate is saltier than the feed water, and salts can accumulate in soil, especially in dry climates or poorly drained beds. Irrigation-water quality is evaluated through electrical conductivity, total salts, sodium and specific ions—not TDS alone.

Colorado State University Extension explains how salinity and sodium affect irrigation suitability. If testing shows acceptable water quality, use small amounts on established, salt-tolerant ornamental plants, apply to the soil rather than leaves, rotate locations and watch for leaf burn, crusting or declining growth.

Avoid seedlings, salt-sensitive plants, potted plants with limited drainage and edible crops unless qualified local guidance based on the actual water supports that use. Do not discharge concentrate where it can reach waterways or neighbouring land.

Uses to avoid

• Drinking or cooking: rejected contaminants are more concentrated, not removed.
• Pet or livestock water: animals can also be harmed by unsuitable salinity or contaminants.
• Brushing teeth, bathing or handwashing: these are potable or hygiene uses.
• Humidifiers and evaporative coolers: minerals can aerosolize, form scale or damage equipment.
• Aquariums, ponds or wildlife water: rapid changes in salts and chemistry can harm aquatic life.
• Car batteries, steam irons or medical equipment: these often require distilled, demineralized or manufacturer-specified water.
• Food preparation or washing produce: use potable water.
• Uncontrolled storage: stagnant water can support microbial growth even when the incoming supply was safe.

How to assess garden reuse

1. Obtain the source-water report or laboratory results and identify what the RO system is removing.
2. Confirm whether softened water feeds the unit.
3. Measure the concentrate’s electrical conductivity and, where relevant, sodium adsorption ratio, chloride, boron and the target contaminant.
4. Compare the results with guidance for the soil and exact plant species.
5. Start with a small trial on non-edible, salt-tolerant landscaping.
6. Monitor soil and plant condition and stop if salt crust, leaf burn, poor infiltration or decline appears.

Australian Government guidance on household wastewater reuse emphasizes matching the amount and quality of reused water to what soil and plants can absorb. RO concentrate is not the same as ordinary greywater, but the same principles of salinity, drainage, storage and site-specific risk still matter.

Collect it without creating a new hazard

A loose hose feeding an open bucket can overflow, create a child or animal hazard and allow contaminated water to siphon into plumbing. Keep the manufacturer’s required air gap and approved drain connection. Any permanent diversion should be designed by a licensed plumber under local rules.

• Use a covered, labelled container reserved for non-potable water.
• Keep it inaccessible to children and pets.
• Use collected water promptly rather than storing it indefinitely.
• Clean the container regularly.
• Never connect concentrate piping to a potable line or appliance.
• Provide safe overflow to the approved sanitary drain.

Reduce reject water before trying to reuse it

Check feed pressure, prefilters, membrane condition and tank pressure because a poorly maintained system may run longer than necessary. Our guide to reverse-osmosis troubleshooting covers continuous draining and slow production. The RO tank-pressure guide explains how an undercharged tank can affect useful capacity.

When replacement is due, compare independently certified water-efficient models. Also reconsider whether RO is necessary for the diagnosed problem. A targeted carbon or sediment filter may treat some concerns without producing a concentrate stream.

The practical answer

Reuse is most defensible when an efficient household RO unit treats potable water, testing shows the concentrate suits a low-risk non-potable task, and collection does not create cross-connections or prolonged storage. Otherwise, follow the system’s approved drain arrangement. Water conservation should never move concentrated contaminants into soil, food, pets or waterways.