Reverse osmosis (RO) systems produce two streams: treated water for drinking and cooking, and a concentrate (reject) stream that carries removed contaminants away. That reject stream is often called “waste water.” Here’s how much to expect, what affects it, and whether reuse is practical.
Why RO Produces a Reject Stream
RO works by pushing water through a membrane under pressure. Water molecules pass through; many dissolved solids and contaminants are held back. That concentrated portion must go somewhere—typically the drain. So “waste” is a normal byproduct of the process, not a malfunction.
How Much “Waste” Water?
The ratio of treated water to reject water depends on water pressure, temperature, membrane condition, and system design. Older or less efficient systems might be 1:4 or worse (one gallon treated for every four to the drain). Many modern residential RO systems are in the range of 1:1 to 1:3 (one gallon treated per one to three gallons reject). Some high-efficiency units claim better ratios. Over time, as the membrane gets older or prefilters clog, the ratio can worsen until maintenance is done.
So for a household that uses 3 gallons of RO water per day, reject might be roughly 3–9 gallons per day depending on the unit—small compared with total household use (toilets, showers, irrigation), but not zero.
Can You Reuse RO Reject Water?
Reject water contains the concentrated contaminants the RO removed. It is not safe to drink or use for cooking. Some homeowners reuse it for:
- Irrigation (lawns, non-edible plants), if local codes and soil conditions allow and you’re not growing food with it.
- Flushing toilets or mopping, if you plumb it that way and accept the complexity and cost.
- Washing cars or outdoor surfaces, again with the understanding it’s not for human consumption.
Reuse has downsides: plumbing changes, possible mineral buildup if used in appliances, and the need to avoid contact with drinking or food prep. Not all jurisdictions allow gray-water-style reuse; check local rules. For most people, sending reject to the drain is the simplest and most common approach.
Improving Efficiency
- Choose an efficient unit when buying; compare stated recovery or waste ratios.
- Keep prefilters and membrane on schedule so the system doesn’t work harder than necessary.
- Use RO water only where it matters (drinking, cooking, ice) rather than for every tap if you’re concerned about total water use.
RO waste water is a tradeoff for high-quality drinking water. Knowing the approximate ratio, maintaining the system, and optionally reusing reject where safe and legal can make the tradeoff easier to live with.