What Is Hydraulic Retention Time (HRT)?

Hydraulic Retention Time (HRT) refers to the average residence time required for wastewater to complete biochemical reactions within a reactor. It is calculated as $HRT=V/Q$ (where V is the effective volume and Q is the influent flow rate) [1] [4] [6]. In practical applications, a distinction must be made between nominal HRT and actual HRT; when calculating anoxic tanks, only the external recirculation ratio correction is considered [6]. Patent data show that in organic wastewater treatment, HRT is mostly controlled within 45 minutes to 3 hours, which can raise the BOD removal rate to above 80% [5].

Insufficient HRT reduces pollutant degradation efficiency. For example, an oxidation ditch experiment in 2020 showed that when HRT was too short, the TP removal rate fell below 66%. Excessively long HRT (e.g., 14 hours) leads to carbon source deficiency and inhibits denitrification. In the A2/O process, an HRT exceeding 8 hours lowers sludge activity and impairs TP removal [6].

HRT should be adjusted by changing the wastewater flow gradually to avoid shock loading [1]. Single-factor experiments are used to plot HRT-efficiency curves; for example, increasing the HRT of printing and dyeing wastewater from 1 hour to 3 hours significantly improves the COD removal rate. Shortening the HRT to 4.97-8.70 hours in the MBR process can enhance denitrification capacity [6].