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RO Reject Volume Calculator

Work out ro reject volume instantly with clear inputs, formula shown and shareable results.

About RO Reject Volume Calculator

Reject flow is feed flow times one minus recovery. At 75% recovery a quarter of the feed leaves as concentrate carrying four times the feed salinity. Disposal is often the binding constraint on inland desalination, since the concentrate cannot generally be discharged to a watercourse or sewer without consent.

Formulas

Reject flow

Reject = feed flow x (1 - recovery)

Disposal cost

Annual cost = reject flow x operating hours x 365 x disposal cost per m3

Worked example

With the default values below, RO Reject Volume Calculator returns Reject Flow of 25.000. Change any input above and every figure here is recalculated in your browser.

Inputs used

Feed Flowminimum 0 m3/h
100 m3/h
System Recoveryaccepts 0–99.9 %
75 %
Operating Hours Per Dayaccepts 0–24 hours
22 hours
Reject Disposal Costminimum 0 per m3
25 per m3

Results

Reject Flow
25.000
Daily Reject Volume
550.00
Permeate Flow
75.000
Annual Disposal Cost
$5,018,750

Step by step

  1. 1Values used
  2. 2Reject flow
  3. 3Disposal cost
  4. 4Reject Flow= 25.000
  5. 5Daily Reject Volume= 550.00
  6. 6Permeate Flow= 75.000
  7. 7Annual Disposal Cost= 5,018,750

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax or compounding frequency into account.
  • Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
  • RO concentrate discharge is subject to environmental permitting because of its salinity and antiscalant content. Confirm consent conditions before commissioning.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

RO concentrate discharge is subject to environmental permitting because of its salinity and antiscalant content. Confirm consent conditions before commissioning.

Frequently Asked Questions

How can reject volume be reduced?

Higher recovery with antiscalant and pH control, a second-pass brine concentrator, or zero liquid discharge with evaporation and crystallisation. Each step raises capital and energy cost sharply.

Can the reject be used?

Sometimes for cooling tower make-up, dust suppression, salt-tolerant irrigation or industrial washing, depending on its composition and local rules. Beneficial reuse usually needs a specific permit.

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