WWTP retention times

Computes hydraulic retention times in anaerobic, denitrification, nitrification zones and secondary clarifier for average and maximum flow, without recirculation, with return sludge and with return sludge plus internal recirculation.

Bioreactor retention times

This calculator estimates hydraulic retention times in the biological stage (anaerobic, denitrification, nitrification zones and secondary clarifier) for average and maximum flow. It considers three variants: without recirculation, with return sludge (VK) and with return sludge plus internal recirculation (IC) from the end of nitrification back to the start of denitrification. Results are indicative and intended for design checks.

Average daily flow Q24 [m³/day]

Maximum flow Qmax [m³/day]

Tank volumes [m³]

Anaerobic zone volume [m³]

Denitrification zone volume [m³]

Nitrification zone volume [m³]

Secondary clarifier volume [m³]

Recirculation (ratios to Q24)

Return sludge ratio Q_VK / Q24 [–]

Internal recirculation ratio Q_IC / Q24 [–]

Flow scheme: return sludge (VK) is withdrawn from the secondary clarifier and fed to the head of the biological stage (affecting anaerobic + denitrification + nitrification zones). Internal recirculation (IC) is taken from the end of nitrification and fed to the start of denitrification (affecting denitrification and nitrification only; it does not affect the anaerobic zone nor the clarifier). The calculation is steady-state and intended for indicative design checks only.

Retention times for Q24 (average flow) [h]

Zone	no recirculation	with return sludge	with return sludge + IC
Anaerobic zone	–	–	–
Denitrification	–	–	–
Nitrification	–	–	–
Secondary clarifier	–	–	–

Retention times for Qmax (maximum flow) [h]

Zone	no recirculation	with return sludge	with return sludge + IC
Anaerobic zone	–	–	–
Denitrification	–	–	–
Nitrification	–	–	–
Secondary clarifier	–	–	–

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