Enter the volume of the aeration tank (m^3) and the influent flow rate (m^3/s) into the Calculator. The calculator will evaluate the Hydraulic Retention Time. 

Hydraulic Retention Time Formula



  • HRT is the Hydraulic Retention Time (sec)
  • V is the volume of the aeration tank (m^3)
  • Q is the influent flow rate (m^3/s)

To calculate Hydraulic Retention Time, divide the volume of the aeration tank by the influent flor rate.

How to Calculate Hydraulic Retention Time?

The following steps outline how to calculate the Hydraulic Retention Time.

  1. First, determine the volume of the aeration tank (m^3). 
  2. Next, determine the influent flow rate (m^3/s). 
  3. Next, gather the formula from above = HRT = V/Q.
  4. Finally, calculate the Hydraulic Retention Time.
  5. After inserting the variables and calculating the result, check your answer with the calculator above.

Example Problem : 

Use the following variables as an example problem to test your knowledge.

volume of the aeration tank (m^3) = 800

influent flow rate (m^3/s) = 20

FAQs on Hydraulic Retention Time

What is the significance of Hydraulic Retention Time in wastewater treatment?

Hydraulic Retention Time (HRT) is crucial in wastewater treatment as it determines the time the wastewater spends in a treatment process or tank. This duration is essential for effective biological treatment, sedimentation, and the removal of contaminants. A properly calculated HRT ensures that the wastewater has sufficient time for treatment before being discharged or moved to the next treatment stage.

How does the volume of the aeration tank affect the Hydraulic Retention Time?

The volume of the aeration tank directly influences the Hydraulic Retention Time. A larger tank volume increases the HRT, allowing more time for the biological treatment processes to occur. This can lead to more efficient removal of pollutants and improved water quality. Conversely, a smaller tank volume reduces the HRT, which might not provide adequate time for treatment, potentially leading to less effective contaminant removal.

Can the influent flow rate be adjusted to optimize Hydraulic Retention Time?

Yes, adjusting the influent flow rate can optimize Hydraulic Retention Time. Decreasing the flow rate increases the HRT, providing more time for treatment processes to occur. However, care must be taken to ensure that the flow rate is not too low, which could lead to settling of solids and other operational issues. Conversely, increasing the flow rate reduces the HRT, which might be necessary during peak loading conditions but can compromise treatment efficiency.

Why is it important to accurately calculate Hydraulic Retention Time?

Accurately calculating Hydraulic Retention Time is crucial for designing and operating efficient wastewater treatment systems. An accurate HRT calculation ensures that the system can effectively handle the expected influent load while providing adequate treatment. This prevents issues such as incomplete treatment, system overload, or the need for costly system modifications. Additionally, accurate HRT calculations are essential for regulatory compliance and ensuring the treated effluent meets environmental standards.