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What Is Hydraulic Retention Time and How to Optimize for Better Efficiency

By: Tom Frankel
Post Date: April 26th 2022

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What Is Hydraulic Retention Time and How to Optimize for Better Efficiency

Wastewater treatment facilities are an essential part of communities and businesses around the world. Cities with high populations and industries that generate large amounts of wastewater can harm the environment without properly treating the wastewater they produce. Only after receiving treatment at a wastewater facility can this water be introduced back into the environment in a safe manner.

Good effluent water quality is safe for rerelease into nature, but how does wastewater get to that point? Many biological treatment processes come into play, including the activated sludge process and anaerobic digestion.

Read on to answer the question of what is retention time in wastewater treatment and discover how you can experience the optimization of hydraulic retention time (HRT) in your facility.


Table of Contents


What Is Hydraulic Retention Time?

It is crucial to understand the importance of HRT and how it relates to facility processes. Note that hydraulic retention time is also different than solid retention time. At its simplest definition, HRT is the average amount of time a gallon of wastewater will remain in a particular basin during the wastewater treatment process.

For a more technical definition, think of HRT as the measure of a soluble compound’s average time remaining in a constructed biogas reactor or aeration tank. This process goes by other names, including hydraulic residence time, and has a huge impact on effluent water quality.

Here’s one way to picture hydraulic retention time. Wastewater enters a confined tank, which allows sedimentation to occur. Sedimentation is the process of waste matter in the water settling to the bottom of the tank. Eventually, the water on top of the settled particles will move to another tank in the wastewater management process, allowing more wastewater to take its place. The length of time it takes for the water on top to leave the tank is the hydraulic retention time of that tank.

Knowing the exact hydraulic retention time of a facility’s wastewater tanks is important to operations and efficiency. This is why facility managers seek to learn more about it, including its different types, how it compares to similar terms and how they can calculate their exact HRT.

Formula and Calculations for Hydraulic Retention Time

Formula and Calculations for Hydraulic Retention Time

Since hydraulic retention time deals with the average time that soluble and liquid compounds are in a specific wastewater tank, operators need to know the hydraulic retention time formula they can use to calculate that average.

Every facility will be different in terms of the size of its tanks and the efficiency of its system, and will have varying amounts of wastewater coming in that they must process to release safely back into the environment. Thus, knowing your facility’s unique HRT is paramount to reaching maximum efficiency.

HRT Formula

When learning how to calculate retention time in aeration tanks, identify the volume of the aeration tank in question. Represent this value as “V.” The volume of the aeration tank may be listed on the tank itself or on corresponding paperwork or documentation. If necessary, reach out to the manufacturer of the aeration tank for specific volume identification.

The next value to determine is the flow rate of the influent, a value which you may identify as “Q.” The influent is the wastewater in need of treatment that comes into the aeration tank. Thus, the influent flow rate is the rate at which new wastewater enters the aeration tank, and since the tank is of a specific size, the rate at which the influent enters is also the rate at which effluent exits.

So, for accurate HRT calculation, use the hydraulic retention time formula “HRT = V/Q.” Here, you are dividing the volume of the aeration tank, which is a value of meters cubed, by the influent flow rate. Note that the influent flow rate value can be meters cubed per hour or per day, depending on the specific information you wish to discover.

Additionally, consider using liters or other volumetric identifiers in the formula. For instance, when using liters, the formula would be “HRT = V in liters/flowrate in liters per hour.” Some may prefer this formula, as it allows them to skip cubic meters in their calculations and instead focus on the liquid volume in liters that passes through their wastewater treatment system.

Use whichever formula makes the most sense for your understanding and benefits your facility’s operations the most.

What Is the Importance of Hydraulic Retention Time?

What Is the Importance of Hydraulic Retention Time?

HRT is important because it influences the efficiency of wastewater treatment. This makes it a critical parameter of design for wastewater production facilities. In other words, HRT is more than a stat or simple reading of the state at which a facility is functioning. HRT gives insight into the overall efficiency of wastewater treatment operations. With control over a facility’s HRT, operators also have control over its efficiency.

Another reason HRT is curcial is that it has a strong, direct connection with biogas and biomass production. The breaking down of organic solids in wastewater produces biomass energy, which can be used for the distribution and use of electricity. However, it is key to realize that HRT correlates with how much biogas a system produces. HRT informs you when productivity may begin to decline as the organic solids remain in the anaerobic digestion tank.

When calculating a wastewater facility’s HRT, operators are taking the first step toward greater productivity and efficiency. They can have more control over how they reach goals as a facility, ensuring operations stay on pace with trajectory. It is smart to remember this to ensure your wastewater treatment plant is doing all it can do to have the most efficient and high-quality results possible.

How Does Hydraulic Retention Time Affect a Wastewater Treatment System?

As mentioned above, HRT is essential because it has an effect on wastewater treatment systems. A facility’s HRT may change depending on whether it has a high rate of influent, but it may also change if operators are looking for different results in their effluent. There are two separate concepts here — short HRT and long HRT.

Short HRT and long HRT are the terms used to describe the two main ways a facility can approach hydraulic retention time. Short HRT has to do with shorter average lengths of time that wastewater spends in a holding tank, while long HRT describes keeping the wastewater in the tank for a longer average period.

Short HRT

A short HRT in a wastewater treatment facility may initially seem like a time-saver, but the results may be less than ideal. Volatile fatty acids (VFAs) have a tendency to accumulate in anaerobic digestion systems in wastewater treatment facilities. This leads to a decrease in the pH of the wastewater being treated, causing it to become more acidic.

Clearly, acidic conditions can be damaging to a system, leading to the unstable performance of a wastewater treatment operation. In the end, this could create poor biogas or biomass production. The wastewater may travel through the system at a faster rate because of the shorter HRT, but efficiency will actually decrease because the final product is low-quality.

Long HRT

After discovering the reasons to avoid too short of an HRT, operators may think that a long HRT will solve all of their answers. While you may find success with some amount of greater length in the HRT, too long of a hydraulic retention time will also lead to poor results.

If a system’s hydraulic retention time goes too long, facilities could also experience a decrease in biogas production. While this may seem surprising at first, there is the reason why this occurs. Long HRT results in ineffective utilization of biogas components within the tank. It is as if you miss the golden opportunity for peak biogas production efficiency because the wastewater spent too long in the tank before exiting and allowing more wastewater to enter.

What Is the Difference Between Retention Time and Detention Time?

Retention time and detention time may seem like similar terms, but their definitions are far different. It is important to understand the difference between them, as they both can have an impact on wastewater treatment operations. That said, the processes they define are quite opposite from each other, leading to differing approaches to handling and treating water.

Retention Time

Retention Time

To reiterate, HRT refers to the average time a certain amount of wastewater stays in a holding tank before exiting the tank. Where much of the confusion between retention and detention comes from is that these terms can describe different types of ponds that store water before the water moves on to the next phase of its journey. Knowing the definitions of these terms as they relate to ponds can help operators understand them in a greater context.

A retention pond can also go by the name “wet pond” because it is a permanent pool of water and holds water regardless of rainfall or snowmelt. There may be a nearby stream feeding the retention pond. The purpose of a retention pond is to hold water for a period of time — the retention time — before it moves on. The retention time allows sediment and other materials to separate from the water and sink to the bottom, reducing resuspension and sediment overflow.

In a sense, retention time in a pond is similar to hydraulic retention time in a wastewater treatment facility. In both instances, there is a body of water present that acts as the temporary home for water on its journey. While in an aeration tank or the wet pond, materials can separate from the water and sink to the bottom. The treated water can then continue to its next destination at a higher quality than before it entered the body of water.

Detention Time

Detention time is more closely associated with detention ponds, which are also known as dry ponds. The term “dry pond” should give away what these ponds are like — they have no water in them until water flows in from the outside. These ponds are common in areas with dry climates and provide flood protection in the event of a storm or extreme snowmelt.

Detention ponds will hold water for a certain period of time, depending on their size, slope and location. The time the water spends in the dry pond is its detention time. Thanks to a dry pond’s detention time, communities and cities can experience less flooding and less particle runoff. During the pond’s detention time, sediments will settle to the bottom, like in retention ponds. However, since the pond starts off empty, it can avert flooding in areas where excess water is unwanted.

How to Optimize Hydraulic Retention Time for Better Efficiency

There are several ways to optimize hydraulic retention time for a wastewater treatment facility. Every facility will have unique operation statistics and goals, making it impossible to determine a one-size-fits-all kind of approach to optimization.

Still, operators can find many examples of HRT optimization, including research on how optimized HRT can improve energy-neutral wastewater treatment facility operations. Note that this particular study focuses on pilot-scale tests of a microbial desalination cell system.

Understand your facility’s goals and compile the data necessary to measure productivity. After finding those key performance indicators, you can better know the areas in which you’re succeeding and the areas that can use some improvement.

From there, raise or lower your facility’s HRT or acquire holding tanks that will better suit your wastewater treatment facility’s needs. Use the formulas in this article to determine a theoretical calculation of the required tank size that will help you reach your HRT goals.

Learn More About Our Wastewater Treatment Solutions

Learn More About Our Wastewater Treatment Solutions

At SSI Aeration, Inc., we understand wastewater treatment. We’re a global leader in the manufacturing and design of equipment for wastewater treatment facilities, including energy-efficient, innovative membrane diffusers. All of our products are high quality and sure to meet the needs of wastewater treatment plants across industries.

We also offer a full range of services, including field services, essential lab services and design and manufacturing services. We invite you to contact us today if you have any questions about our wastewater treatment solutions and how we can help you achieve peak optimization.