Work out dilution factor for discharge instantly with clear inputs, formula shown and shareable results.
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.
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
Discharge impact assessment must use the statutory low-flow condition and environmental quality standards specified by the regulator.
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.
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.