Each reverse osmosis unit produces two streams which are clean permeate as well as an unrefined leftover, known as reject water. A majority of facilities will send the rejected water straight to the sewer and treat it as waste. But is it really required? Most of the time, not. If you have the proper treatment and final use the majority of RO reject water can be put back to be used instead of having to dispose of it. We will break down what constitutes reject water, where it can be reused, and what the limits are.
What Is RO Reject Water?
RO reject water — also called concentrate or brine — is the portion of feed water that never passes through the membrane. When reverse osmosis is in operation it is when pressure forces the water molecules across a semipermeable barrier, whereas minerals, dissolved salts and other impurities remain behind. The purified stream is transformed into permeate (product water) and everything that the membrane does not want transforms into the stream that is rejected.
It helps to think of it like brewing coffee: the cup you drink is the permeate, and the spent grounds left behind — carrying everything that didn’t pass through — are the reject. The reject isn’t “dirty” in the way wastewater from a drain is dirty. It’s simply water with a much higher concentration of whatever was already in the source water.
What’s Actually in RO Reject Water?
The precise composition of the water is determined by the source water as well as how the system is set up However, reject streams usually have a heavy load of total dissolved solids (TDS) as well as hardness of magnesium and calcium silica, and other organic matter. Since RO systems typically recuperate only 50-75% of the feed water to be permeable water, the remaining 25-50% of the water is considered a reject stream that carries nearly all the impurities that are concentrated from the water that was originally treated.
That concentration is exactly why reject water isn’t automatically safe for every downstream use — it needs to be matched to the right application, sometimes with light pretreatment first.
Why Not Just Discharge It?
The process of releasing water that has been rejected is easy; however, it’s not the least expensive or most environmentally responsible choice. Two pressures are pushing more facilities toward reuse:
- Regulatory and environmental cost — many regions restrict or tax the discharge of high-TDS wastewater, and permit renewals increasingly require documented water-reduction plans.
- Economic cost — the feed water in that reject stream was already pumped, filtered, and pretreated before it reached the membrane. Discharging it means paying for treatment twice: once to produce it, and again to dispose of it.
In high-consumption or water-scarce industries such as electronics, pharmaceuticals and power generation, the math can add up quickly, which is the reason recycling and recovery are a common component of RO system planning, rather than an afterthought.
Can RO Reject Water Be Reused? (Direct Answer)
Yes. In most instances, RO reject water can be reused; however, the appropriate method is contingent on the quality of water used and the intended use. Certain uses don’t require any treatment whatsoever, whereas others require filtration, softening or a second RO run prior to the water being appropriate. There isn’t a single solution to every situation. The best path to take is based on the source water TDS and your system’s recovery rate, and the purpose you’re planning to utilize the refuse for.
Practical Ways to Reuse RO Reject Water
Non-Potable Uses (Minimal or No Treatment)
The least-effort option for reuse is to apply reject water in areas where drinking-water quality isn’t needed, such as landscaping irrigation, washing down of equipment or flushing toilets. This can be a great option for short-term, low-volume usage but continuous irrigation with high-TDS water may gradually increase the salinity of soil, making it worthwhile to rotate with fresh water or keep track of the soil’s conductivity in the course of time.
Cooling Tower Makeup Water
Industrial facilities — particularly in electronics and power generation — commonly redirect RO reject into cooling towers as makeup water. Because cooling towers contain minerals via evaporation and reject water is a good source to be used to supplement or replace fresh makeup water if it is coupled with corrosion control and scaling treatment.
Second-Pass RO / RO-RO Systems
Feeding reject water into a second RO stage can meaningfully raise total system recovery. However, because the high salinity and TDS of a first-stage reject stream raises the risk of scaling or fouling in a second membrane, pretreatment such as softening or membrane filtration is generally needed first. With the right pretreatment step in place, some two-stage RO configurations can push overall recovery beyond 95%.
Zero Liquid Discharge (ZLD) Systems
Industries that have discharge restrictions that are strict (e.g., semiconductors, pharmaceuticals chemical processing, pharmaceuticals ZLD systems allow reuse until its ultimate goal of by vaporizing and crystallizing the rest of the concentrate, so that nothing is discharged as liquid waste. This is a costly process however, it’s becoming the norm for facilities that cannot discharge water with high TDS levels under local regulations.
What Are the Limitations and Risks?
Reuse isn’t automatically the right call for every reject stream. Common risks include:
- Scaling and fouling in downstream equipment or a second-pass membrane if the concentrate isn’t pretreated
- Soil salinization from prolonged, untreated irrigation use
- Inconsistent water quality, since reject composition shifts with source water and system recovery settings
A pilot-scale study of RO treatment on membrane bioreactor effluent found removal rates above 90% for key inorganics, meeting reuse standards for cooling and irrigation applications — but only under specific operating conditions for pH, flux, and membrane recovery. The takeaway: reuse works reliably when it’s engineered for the intended application, not applied as a blanket assumption.
How to Reduce RO Reject Water in the First Place
The most effective long-term strategy isn’t just finding uses for reject water — it’s generating less of it. The rate of recovery is mostly an effect of the design, which includes proper pretreatment (softening and antiscalant dosing) as well as multi-stage membrane arrays, as well as an operating pressure that is well-tuned to affect the amount of feed water you use is usable permeate instead of reject. A properly designed RO system with optimized recovery will reduce the volume of water produced and the burden of treatment on whichever process of reuse or disposal you decide to take.
FAQ
Are RO reject water suitable for plants?
It is generally a good choice for occasional or short-term use; however, its high mineral content may accumulate in the soil over time which is why it’s recommended to rotate it with fresh water instead of using it only.
Can I drink RO reject water?
No. It is a concentrated source of dissolved solids and the contaminants that which the RO system was created to eliminate, which is why it isn’t drinking water safe without additional treatment.
Does reusing reject water reduce operating costs?
Often, yes — it can lower both fresh water intake and wastewater discharge fees, though the savings depend on treatment costs for the specific reuse method chosen.
RO reject water doesn’t have to be a dead loss. Depending on your water quality and application, it can be redirected into irrigation, cooling towers, a second RO pass, or a zero-liquid-discharge process — cutting both water costs and environmental impact. The right approach starts with the system itself: a properly engineered RO plant with optimized recovery generates less reject in the first place and gives you more usable options for whatever concentrate remains.