Quick answer: Reverse osmosis reject water is the concentrated stream that carries the dissolved salts, minerals and contaminants rejected by an RO membrane. It typically represents 15 to 50 percent of the feed volume and holds two to five times the feed salinity. It can often be reused in cooling, irrigation or further recovery rather than discharged.
Key facts – RO systems commonly recover 40 to 85 percent of feed water as permeate, so reject water is 15 to 60 percent of feed volume. – Reject water salinity is usually 2x to 5x the feed concentration, because rejected solutes are packed into a smaller volume. – Membrane rejection of dissolved salts is typically above 97 to 99 percent for well-designed brackish and seawater RO (membrane manufacturer datasheets). – Reuse rather than disposal can materially cut both freshwater intake and discharge volume at many industrial sites. – In the EU, reject water discharge is regulated under permits issued via Waterschappen or Rijkswaterstaat and framed by the Water Framework Directive.
What is reverse osmosis reject water?
Reverse osmosis reject water is the residual stream left after clean permeate has passed through the membrane. It contains the dissolved and suspended matter the membrane blocked, concentrated into a smaller volume.
The stream is also called RO concentrate, RO retentate or simply “reject”. It is chemically the same water as the feed, only more saline and often carrying antiscalant and any residual pretreatment chemicals.
Reject water is not a waste product by default. Whether it is a resource or a disposal problem depends on its composition, the site water balance and local discharge rules.
What is the composition of RO reject water?
RO reject water concentrates whatever the feed contained. Its composition is therefore site-specific and must be characterised by analysis, not assumed.
Typical constituents include elevated total dissolved solids (TDS), hardness (calcium, magnesium), silica, sulphate, chloride, and any organics or metals present in the feed. Antiscalant dosed upstream also reports to the reject.
The concentration factor links feed and reject. At a recovery of 75 percent, dissolved solids are concentrated roughly fourfold in the reject, assuming near-complete rejection.
| Parameter | Feed (brackish, indicative) | Reject at 75% recovery (indicative) |
|---|---|---|
| TDS | 1,500 mg/L | approx. 6,000 mg/L |
| Recovery ratio | reference | 25% of feed volume reports to reject |
| Concentration factor | 1x | approx. 4x |
| Antiscalant | dosed at feed | concentrated in reject |
Values above are illustrative only. Always base design on a current, representative feed and reject analysis.
Why does reject water matter?
Reject water matters because it drives both water cost and discharge risk. Every litre rejected is a litre of feed that did not become usable product water, and it must go somewhere.
For water-intensive plants, reject volume affects intake permits, pumping energy and chemical use. High reject salinity can also breach discharge limits, triggering permit issues or surcharges.
Managing reject well can lower freshwater demand, reduce discharge volume and, in water-stressed regions, protect a site’s licence to operate.
How is RO recovery ratio calculated?
The recovery ratio is the fraction of feed water that becomes permeate. It sets how much reject is produced and how concentrated that reject becomes.
Recovery is calculated as permeate flow divided by feed flow, expressed as a percentage. A recovery of 75 percent means 75 percent of the feed becomes permeate and 25 percent becomes reject.
Higher recovery reduces reject volume but raises reject salinity and scaling risk. There is a practical ceiling set by the solubility of sparingly soluble salts such as calcium carbonate, calcium sulphate, barium sulphate and silica.
What are the reuse options for RO reject water?
Reject water can often be reused instead of discharged, provided its quality suits the receiving use. The right option depends on salinity, contaminants and on-site demand.
Common reuse routes include:
- Cooling tower make-up: moderate-salinity reject can supplement cooling water where cycles of concentration and corrosion limits allow.
- Irrigation or green-space watering: possible only where salinity, sodium adsorption ratio and any regulated substances are within agronomic and legal limits.
- Process uses tolerant of higher TDS: washdown, dust suppression, ash conditioning or non-critical rinsing.
- Further water recovery: a second-pass or brine-concentrator stage can extract additional permeate and shrink the residual volume, moving the site towards minimal or zero liquid discharge.
- Groundwater or aquifer recharge: only where hydrogeology and permits permit, and quality is suitable.
| Reuse option | Suits which reject | Main constraint |
|---|---|---|
| Cooling make-up | Low to moderate TDS | Scaling, corrosion, cycles limit |
| Irrigation | Low salinity, low SAR | Agronomic and regulatory limits |
| Dust suppression / washdown | Moderate TDS | Contaminant and worker-exposure limits |
| Secondary recovery | Any, for volume reduction | Capital cost, energy, scaling |
Reuse is a design decision, not a default. It should be assessed against a site water balance and the applicable discharge permit.
What are the cost drivers?
Reject reuse economics are driven by avoided freshwater cost, avoided discharge cost and the capital and energy needed to treat or move the reject.
Key cost drivers include reject volume and salinity, the distance and pumping to any reuse point, additional treatment (softening, further RO, evaporation) and local water and discharge tariffs.
Cost ranges are highly site-specific and should be treated as caveated planning figures only. A site-specific water balance and techno-economic assessment is the reliable basis for any investment decision.
Common mistakes
- Treating reject as automatic waste when a viable reuse route exists on site.
- Pushing recovery too high without checking scaling limits, causing membrane fouling and lost output.
- Designing reuse from a single spot sample rather than a representative, seasonally varied analysis.
- Overlooking regulated substances (Dutch ZZS list) that can disqualify a reuse or discharge route.
- Ignoring antiscalant and other dosed chemicals when assessing reuse suitability.
When should you bring in an independent advisor?
Bring in an independent advisor when reject volume, salinity or discharge cost is material, or when you are weighing reuse against disposal. Independent input helps where vendor proposals differ widely.
Brine Consulting is an independent, vendor-neutral advisory. We assess your water balance and reject composition without selling equipment, so the reuse or recovery route recommended is the one that fits your site, not a supplier’s catalogue.
Frequently asked questions
What is the difference between reject water and permeate?
Permeate is the purified water that passes through the RO membrane and becomes the product stream. Reject water is the concentrated stream that carries the salts and contaminants the membrane blocked. Both come from the same feed, but they have opposite quality.
Can RO reject water be reused?
Yes, RO reject water can often be reused for cooling make-up, dust suppression, tolerant process uses, irrigation or further recovery. Suitability depends on its salinity and contaminant load, on-site demand and the applicable discharge permit. Each route needs a quality check against use-specific limits first.
How much reject water does an RO system produce?
An RO system typically produces reject equal to 15 to 60 percent of feed volume, set by its recovery ratio. At 75 percent recovery, 25 percent of the feed reports to reject. Higher recovery lowers reject volume but concentrates it further and raises scaling risk.
Is RO reject water hazardous?
RO reject water is usually not hazardous in itself, since it is concentrated feed water. Its risk depends entirely on the feed. Where the feed contains regulated substances, such as those on the Dutch ZZS list, the concentrated reject may require controlled handling and specific discharge conditions.
Why is recovery ratio limited?
Recovery ratio is limited by the solubility of sparingly soluble salts. As recovery rises, dissolved calcium carbonate, sulphates and silica concentrate towards their solubility limits and begin to scale the membrane. Antiscalant raises the ceiling, but there remains a practical maximum for each feed water.
Sources and further reading
- European Commission, Water Framework Directive (Directive 2000/60/EC)
- Rijkswaterstaat and regional Waterschappen, industrial discharge permitting guidance
- Membrane manufacturer technical datasheets and design guidelines (for rejection and recovery figures)
- Dutch ZZS (Zeer Zorgwekkende Stoffen) framework, RIVM
Ready to turn RO reject water from a disposal cost into a recovered resource? [Request an independent reject water assessment from Brine Consulting.]
Written by the Brine Consulting advisory team. The Brine Consulting advisory practice includes Prof. Basel Abusharkh, a specialist in brine valorization, ZLD and high-recovery desalination, and Panos Spatharos, a chemist and field engineer specialising in sludge and leachate treatment, RO, ion exchange and filtration. Meet our experts.