Dilution Factor For Discharge Calculator

Work out dilution factor for discharge instantly with clear inputs, formula shown and shareable results.

About Dilution Factor For Discharge Calculator

The fully mixed concentration downstream is the flow-weighted average of effluent and river. Dilution factor is total flow divided by effluent flow. Consents are usually set against the low-flow condition — commonly the 95th percentile exceedance flow — because that is when dilution is least and the risk greatest.

Formulas

Mass balance mixing

C_mixed = (Q_effluent x C_effluent + Q_river x C_background) / (Q_effluent + Q_river)

Dilution factor

DF = (Q_effluent + Q_river) / Q_effluent

Worked example

With the default values below, Dilution Factor For Discharge Calculator returns Fully Mixed Concentration of 2.0266. Change any input above and every figure here is recalculated in your browser.

Inputs used

Effluent Flowminimum 0 m3/day: 5,000 m3/day
Receiving Water Flowminimum 0 m3/s: 50 m3/s
Effluent Concentrationminimum 0 mg/L: 25 mg/L
Background River Concentrationminimum 0 mg/L: 2 mg/L

Results

Fully Mixed Concentration: 2.0266
Dilution Factor: 865.000
Increase Above Background: 0.0266
Effluent Flow In Cumecs: 0.057870

Step by step

1Values used
2Mass balance mixing
3Dilution factor
4Fully Mixed Concentration= 2.0266
5Dilution Factor= 865.000
6Increase Above Background= 0.0266
7Effluent Flow In Cumecs= 0.057870

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, fees or charges or compounding frequency into account.
Figures are rounded for display to 6 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
Discharge impact assessment must use the statutory low-flow condition and environmental quality standards specified by the regulator.

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

Discharge impact assessment must use the statutory low-flow condition and environmental quality standards specified by the regulator.

Frequently Asked Questions

Which river flow should be used?

The statutory low flow, typically Q95 — the flow exceeded 95% of the time. Using average flow overstates dilution and produces consents that fail in summer.

Does full mixing happen immediately?

No. There is a near-field mixing zone downstream of the outfall where concentrations are much higher. Environmental quality standards may have to be met at the edge of a defined mixing zone.