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Hydraulic Retention Time Calculator

Table of contents

What is hydraulic retention time?How do I calculate hydraulic retention time?Hydraulic retention time in wastewater treatmentSolids retention time (SRT)Solids retention time vs. hydraulic retention time – Difference between HRT and SRTHow to use the hydraulic retention time calculatorFAQs

Omni's hydraulic retention time calculator is the tool that easily lets you determine the hydraulic retention time (HRT) of a reactor or a tank. If you are studying a reactor or the aeration tank of a wastewater treatment plant, you might find this calculator useful.

Here you'll find more about the HRT:

  • What is hydraulic retention time in wastewater treatment?
  • What's the formula for the hydraulic retention time?
  • What is the activated sludge process (ASP) in wastewater treatment?
  • We'll see what solid retention time (SRT) is; and
  • The difference between HRT and SRT.

What is hydraulic retention time?

The hydraulic retention time or HRT is defined as the average amount of time required for a molecule of water or any other liquid to pass through a bioreactor or a tank, allowing enough time to accomplish the removal of a specific organic pollutant.

The theoretical HRT is defined as the ratio between the reactor volume and the inlet flow rate and is usually expressed in hours.

How do I calculate hydraulic retention time?

To calculate hydraulic retention time (HRT), use the formula:

HRT = Volume / Q,


  • HRT – Hydraulic retention time [h];
  • Volume – Volume of the tank or reactor [m³]; and
  • Q – Influent volumetric flow rate [m³/h].

Let's say, for example, that you'd like to find the HRT of an aeration tank with a capacity of 3000 m3, and the plant's influent flow is 10,000 m3/d.

To obtain the HRT, we simply substitute these knowns in the hydraulic retention time formula from above:

HRT = Volume / Q
HRT = 3000 m³ / 10000 m³/d
HRT = 0.3 d = 7.2 h

Easy right? Notice we converted time units from days to hours in order to compare our result with the typical aeration tanks' HRT (1.5 to 24 hours), which are usually given in hours. As you can see, the hydraulic retention time for the aeration tank of our example, 7.2 hours, is within the usual range.

Hydraulic retention time in wastewater treatment

In the specific case of wastewater treatment, the HRT is the necessary time to complete the degradation of organic matter present in the wastewater. This organic matter or organic load is measured as BOD (biochemical oxygen demand). Depending on the requirements of the effluent water, other pollutants can be removed using this same process but with longer HRT.

In wastewater treatment plants (WWTP), the activated sludge process (ASP) is the most commonly used secondary treatment. This is the biological aerobic process that allows the removal of organic material by promoting the growth of microorganisms in wastewater.

The heart of this process takes place in the aeration tank. Here, the wastewater and activated sludge are mixed in the presence of dissolved oxygen. By recycling the activated sludge back into the aeration tank, the number of microorganisms can be increased significantly compared to the wastewater influent. The concentration of microorganisms in the activated sludge process consists of the amount of mixed liquor volatile suspended solids (MLVSS).

In wastewater treatment, aeration tanks' hydraulic retention time varies depending on the plant size, inlet flow rate, and requirements of the effluent water. It can range from as low as 1.5 hours up to 24 hours.

Relative low HRT can translate into the washout of the reactor, which means that all the active microorganisms escape from the reactor without enough exposure to the organic matter. Thus no degradation.

Solids retention time (SRT)

The age of the sludge, also known as solids retention time or SRT, corresponds to the average amount of time that solids and bacteria are kept in the activated sludge plant. This term is directly related to the possible generation time of the bacteria and the degradation processes that can occur. This means that the greater the SRT, the more bacteria can multiply to "eat" the organic matter present in the influent wastewater and degrade it. SRT is usually expressed in days.

The SRT, or mean cell residence time (MCRT), is also used to describe the specific load of the system. This is how much comes in and goes out and at which rate. Different pollutants present in the incoming wastewater (such as BOD, nitrogen, or phosphorus) require various processing times:



BOD removal

Can be achieved in 2 to 8 days.


Achieved at SRT of about 10 days or more.

Biological phosphorus removal

Between 10 to 15 days.

Stabilization of the activated sludge

At a very high SRT, over 20 days.

These time spans are significantly related to temperature. The higher the temperature, the shorter the values of SRT. That's because the degradation processes accelerate as temperature increases.

Solids retention time vs. hydraulic retention time – Difference between HRT and SRT

From the above, we can see that the main difference between SRT and HRT is that the former relates to how much time microorganisms need to reproduce. The latter describes how much time those microorganisms need to "eat" the organic matter present in the water.

But what about lengths of time? Could it be the case that the HRT and the SRT are set to the same value (HRT = SRT)? The answer to this is that it depends on the specific process and type of reactor.

In the case of wastewater treatment, these two values are kept different. While HRT can be from a few hours up to a day, the minimum SRT is about a few days. If SRT = HRT, the time of contact between bacteria and wastewater will be too short to enable bacteria to utilize the organic matter in the wastewater. To ensure bacteria degradation, SRT should be kept greater than HRT.

How to use the hydraulic retention time calculator

With the hydraulic retention time calculator, you'll be able to quickly find the retention time of any reactor or tank by following these steps:

  1. Enter the volume of the reactor or aeration tank (V).
  2. Input the value of the inlet flow rate in the field influent flow (Q).
  3. Once you've entered these two, the calculator will show you the hydraulic retention time (HRT).

What if you don't know the tank's volume? In that case, you can press the additional settings section of the calculator to enter the tank's dimensions.

🙋 You can click on the unit's drop-down menu associated with each variable to change your result to other units!


How do I calculate retention time on an aeration tank?

The retention time on an aeration tank is usually associated with the term hydraulic retention time (HRT). This one can be easily calculated, though:

  1. Find the volume of the aeration tank in cubic meters.
  2. Obtain the influent flow rate in cubic meters per hour.
  3. Divide the volume by the flow rate to calculate the HRT value.

Why is solid retention time is longer than hydraulic retention time?

The activated sludge process (ASP) is the most commonly used in wastewater treatment. In this process, an aeration tank (aerobic digester) is operated with relatively high solid retention time (SRT) and short hydraulic retention time (HRT). This ensures the biological degradation process that allows the removal of BOD (biochemical oxygen demand). In high-rate digesters, SRT is usually about three times higher than HRT.

What is the range of hydraulic retention time for wastewater treatment?

The HRT usually ranges between 5 to 24 hours in wastewater treatment. Longer or shorter values of HRT have different effects:

  • Longer values of HRT translate into an increase in the degree of organic matter degradation.
  • Shorter HRT could cause a washout. The microbial population does not have enough time to treat the organic matter, and the effluent leaves with pollutants.
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