What is Concentrate Flow Rate?
The Concentrate Flow Rate (Qc) (also known as reject or brine flow) is the volume of water per unit time discharged from the reverse osmosis pressure vessel array carrying dissolved mineral ions rejected by the membrane.
Governing Formula
Qc = Qf - Qp = Qf × (1 - Y / 100) Where:
-
Qc= Concentrate (brine / reject) discharge flow rate [m³/day, L/min, GPD] -
Qf= Feed water flow rate to high pressure pump [m³/day, L/min, GPD] -
Qp= Permeate product flow rate [m³/day, L/min, GPD] -
Y= System volumetric recovery percentage [%]
Derived Equations:
Qp = Qf × (Y / 100) Qc = Qf - Qp = Qf × (1 - Y / 100) How the Calculation Works
Reverse osmosis systems obey strict conservation of volume at steady state: every cubic meter of feed entering the train leaves either as permeate or as concentrate.
Qfeed = Qpermeate + Qconcentrate
Permeate volume is the feed flow multiplied by the system recovery (Qp = Qf × Y / 100). The concentrate flow is therefore the remainder that the membrane does not pass as product:
Qc = Qf × (1 − Y / 100)
Because recovery is a ratio, concentrate flow scales linearly with feed flow: doubling the feed at the same recovery doubles the brine discharge. This is the starting point for sizing brine disposal pipelines, energy recovery devices (ERDs), and concentrate evaporators.
Worked Engineering Example
Design Scenario: Feed Flow = 100 m³/day & Recovery = 40%
Permeate Flow (Qp) = 100 m³/day × (40 / 100) = 40 m³/dayConcentrate Flow (Qc) = 100 m³/day - 40 m³/day = 60 m³/day
The system discharges 60 m³/day of concentrate brine.
Engineering Notes & Design Benchmarks
Concentrate flow rate is a vital parameter for sizing brine disposal pipelines, energy recovery devices (ERDs), zero liquid discharge (ZLD) evaporators, and deep well injection systems. It also sets the minimum feed spacer velocity (typically 0.1–0.17 m/s) needed to limit concentration polarization inside the pressure vessel.
| Application Source | Typical Recovery | Concentrate Share of Feed | Design Consequence |
|---|---|---|---|
| Seawater Desalination (SWRO) | 35% – 50% | 50% – 65% | Returned to sea or fed to energy recovery + second stage |
| Brackish Groundwater (BWRO) | 65% – 85% | 15% – 35% | Brine disposal, ponds, or deep well injection |
| Wastewater Water Reuse (MBR-RO) | 70% – 80% | 20% – 30% | Concentrate treatment / blending in reuse schemes |
| High Recovery / ZLD RO | 85% – 95% | 5% – 15% | Small but hypersaline stream feeding evaporators/crystallizers |
Assumptions & Limitations
Engineering Assumptions:
- Steady-state volumetric balance: feed = permeate + concentrate (Qf = Qp + Qc) with negligible density differences between streams.
- Uniform recovery: the specified system recovery is applied across the whole train, not per pressure vessel or element.
Design Limitations:
- Does not account for brine density and viscosity changes at very high total dissolved solids in seawater and ZLD applications (>35,000 mg/L), where volumetric and mass balances diverge.
- Does not predict brine salinity. Use the RO mass balance calculator with feed TDS and salt rejection to resolve concentrate TDS and dissolved-solids load.
- Does not replace membrane projection software (e.g., DuPont WAVE, Hydranautics IMSDesign, Toray DS2) for element-level flux and concentration polarization modeling.
Continue in the RO Engineering Workbench
Carry your feed flow and recovery into the RO Engineering Workbench to extend this volumetric balance with feed TDS and rejection — resolving concentrate salinity and dissolved-solids load alongside permeate quality in a single saved project.
Frequently Asked Questions
Why is minimum concentrate flow critical in RO pressure vessels?
Sufficient concentrate flow velocity inside the membrane feed spacer is required to scour membrane surfaces, control concentration polarization (β), and prevent localized precipitation of mineral scale.
How is concentrate flow related to recovery?
Concentrate flow equals feed flow times (1 − Recovery). At 40% recovery, 60% of the feed leaves as concentrate; at 80% recovery, only 20% does. Higher recovery produces less brine, but the brine becomes far more concentrated.
Why is concentrate flow important for ZLD design?
Zero liquid discharge systems must evaporate every liter of concentrate. The concentrate flow directly sizes the brine concentrator, evaporator, and crystallizer train, while its salinity sets the thermal energy demand.
Engineering Disclaimer
Engineering Note: This calculator provides simplified engineering estimates for preliminary analysis and educational use. Actual RO system performance depends on membrane type, feedwater chemistry, temperature, pressure, recovery, concentration polarization, pretreatment, and operating conditions. Final system design should be verified using manufacturer data, validated design software, applicable standards, and qualified engineering review.
Technical References
- DuPont FilmTec™ Reverse Osmosis and Nanofiltration Technical Manual (Form No. 45-D01504-en).
- AWWA Manual M46: Reverse Osmosis and Nanofiltration, American Water Works Association.