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Introduction to Designing and Sizing Hydrodynamic Separators

By Derek Berg and Pat Valentine, P.E. 

Introduction to Designing and Sizing Hydrodynamic Separators


For decades, hydrodynamic separators (HDS) have been used as a primary treatment device for stormwater. While these systems have been in use for many years, sizing and designing an HDS system can be challenging and confusing. Performance targets, sizing methodologies, confusing performance calculations from manufacturers, online vs. offline placement, and other factors must be considered for proper design and functionality of an HDS system.

Hydrodynamic Separators Defined

HDS systems—sometimes referred to as oil-grit separators, swirl devices or vortex separators—are structural best management practices (BMPs) used to treat and pretreat stormwater runoff. HDS systems are belowground structures, typically made of concrete, with internal components that create a swirling vortex to enhance particle settling. The most common mechanism behind particle separation is sedimentation as characterized by Stokes’ Law. To encourage the suspended particles to settle, HDS devices swirl the captured stormwater in order to extend the flow path as long as possible and provide the time required for floatables and hydrocarbons to rise while sediment and other pollutants settle in a sump.

HDS systems often contain internal components designed to reduce water velocity and prevent turbulence, which decreases resuspension of previously captured pollutants. Some HDS systems also incorporate screening technology to enhance removal of trash and debris. One of the primary benefits of HDS systems is their ease of maintenance. Pollutants are captured and retained in a single structure that can be easily cleaned without confined space entry using a vacuum truck.

Uses for Hydrodynamic Separators

Stormwater Quality Treatment

HDS systems are used frequently to provide stormwater quality treatment in places where space is limited for conventional land-based stormwater treatment practices such as green infrastructure (GI), ponds, wetlands and swales. HDS systems with screening capability also are commonly used as end-of-pipe solutions, helping to meet total maximum daily load requirements by removing trash and debris.

Pretreatment to Detention and Infiltration

HDS systems are frequently installed upstream of underground detention and infiltration systems as pretreatment devices. By capturing the majority of trash, total suspended solids (TSS) and hydrocarbons upstream of these systems, maintenance becomes easier as the material is congregated into a smaller device with clear access for cleanout. This also reduces the risk of outlet control structures clogging.

Figure 1. An HDS system pretreats a perforated CMP infiltration system.

The importance of pretreatment is even more crucial for infiltration systems or other systems that use the void space in the stone backfill for storage (see Figure 1). If sediment is not removed prior to reaching the stone, over time the voids within the stone can clog, storage space will be reduced, and the detention system will be under-designed. If this happens, the only way to restore functionality will be to remove and replace the system, which can be very costly and disruptive. By utilizing pretreatment with an HDS system upstream of infiltration systems, sediment that can clog the stone voids and native soils can be captured upstream, thus extending the longevity of the entire system.

Pretreatment for Low-Impact Development and Green Infrastructure

HDS systems are increasingly being used as pretreatment for rainwater harvesting and bioretention systems. Pretreating harvested water prior to storage in rainwater harvesting systems protects downstream pumps, filters and fixtures from damage or clogging, and it lowers cleaning and maintenance costs by keeping pollutants out of the cistern and mechanical system. It also reduces the amount of organic matter and biological oxygen demand in the cistern, decreasing the likelihood of creating anaerobic conditions and associated odors.

Although less common, HDS systems can also provide pretreatment to bioretention systems by removing unsightly trash that can distract from the aesthetic appeal of these systems as well as removing TSS that can lead to clogging and system failures. Local site conditions will determine if this is feasible, as HDS systems outlet from pipes that are below grade, while bioretention systems often accept influent at the surface. However, for sites with enough slope, the HDS system can be located upstream at a higher elevation so the water exiting from the outlet of the HDS systems will then flow into the top of the bioretention system.

HDS Specification Criteria

It is recommended that specifications for HDS systems include the following components:

• Target pollutant (TSS, trash, hydrocarbons)

• Pollutant Load Reduction Goal (50% removal, 80% removal, 100% removal)

• Sizing methodology (e.g., treatment flow rate, net annual pollutant removal)

Figure 2. Particle Size Distribution

Of particular importance to proper design and performance is the target partial size specification. TSS within stormwater runoff will typically have a wide range of particle sizes, ranging from fines such as clay that is less than 2 microns to medium gravel that can be 4,000-8,000 microns (see Figure 2). HDS systems are more effective at removing coarser materials; as settling velocity increases exponentially, particle diameter gets larger. The generally accepted range of effectiveness for HDS systems is 50 microns or larger. For applications that call for capturing of particles less than 50 microns, a filtration device is recommended.

Target particle size distribution will also affect the operating rate of an HDS system. In many jurisdictions, the performance target will be set via local regulations. If it is not, the engineer of record will need to decide which particle size to target.

Figure 3. Removal Efficiencies for Selected Particle Gradations

Figure 3 demonstrates the effect of selecting different particle size targets on a particular HDS system operating rate, and thus system size and costs. Note that each HDS manufacturer will have different removal efficiencies/operating rates for each of their HDS systems. In this example, if the target removal efficiency is 80%, the operating rate varies considerably, depending on which gradation is being targeted: 50-micron particles, 80-micron particles or 150-micron particles. The finer the target gradation, the lower the operating rate and the larger the system will need to be to achieve the targeted pollutant removal. For example, a 4-foot-diameter HDS manhole may remove 80% of 150-micron particles at 1 cfs, but a larger 6-foot-diameter HDS manhole will likely be required to remove 80% of 80-micron particles at 1 cfs. Targeting different particle sizes is one reason different manufacturers will often recommend different-sized HDS systems for the same site.

Common Sizing Methodologies for Hydrodynamic Separators

Proper sizing should start by checking local regulations. In many cases, local or state regulations will dictate how to size an HDS system and even provide a list of which products are approved for use. If there are no regulatory requirements, the engineer must identify which pollutant(s) need to be removed as well as their target removal percentage.

There are two primary methods of sizing an HDS system. The “Point On a Curve Method” determines which HDS model size provides the desired removal efficiency at a given flow for a defined particle size. The summation process of the “Net Annual Sizing Method” is used when a specific removal efficiency of the net annual sediment load is required and specific local sizing requirements are not prescribed.

Point On a Curve

The Point On a Curve method looks at the performance data of an HDS system at specific operating rates using a defined particle size distribution. In many cases, regulations require that a specific flow rate—often referred to as the water quality design flow (WQQ) or treatment flow rate—be treated. This WQQ represents the peak flow rate from either an event with a specific recurrence interval (i.e., the one- or two-year storm) or a water-quality depth (i.e., 1 inch of rainfall). Treatment flow rates are defined as the rate at which the HDS system will remove a specific gradation of sediment at a specific removal efficiency (e.g., 80% removal of 150 microns at 1 cfs). Therefore, they are variable based on the gradation and removal efficiency specified by the design engineer, and the system size is scaled according to the project goal. HDS systems should be designed to treat all flows up to the treatment flow rate.

Figure 4. Observed TSS removal efficiency of the CDS 2015 under maintained conditions (sediment storage at 0% of capacity) using OK-110 silica (median particle size of 110 microns).

Figure 4 shows the observed TSS removal efficiency of a four-foot-diameter HDS system at a range of flows using OK-110 silica, which has a median (d50) particle size of 110 microns. As expected, as the flow rate increases, the removal efficiency decreases, as there is less time for the particles to settle. In this example, if you were to size a system for 80% removal, the flow rate would be approximately 0.6 CFS.

The Point On a Curve Method methodology is conservative but simplistic; assuming all devices have been tested with the same PSD and methods, it makes it easy to compare HDS systems from different manufacturers at a specific removal goal.

Net Annual Sizing

Differences in local climate, topography and scale make every site hydraulically unique. A variety of Net Annual sizing calculations have been developed to estimate the net annual sediment load reduction for a particular HDS model based on site-specific information, regional rainfall intensity distribution, anticipated pollutant characteristics, and laboratory-generated performance data. Whereas the Point On a Curve method uses one specific rainfall event to design an HDS structure, the Net Annual approach sizes the HDS structure by looking at the pollutant removal efficiency during all rainfall events that occur in a typical year or by running continuous simulations using long term rainfall records. These removal efficiencies are then mathematically weighted based on the frequency with which each rainfall intensity occurs.

These methods account for the fact that the majority of rainfall events occur at lower intensities than a typical design storm, thus most of the time the HDS system will be functioning at a lower operating rate than its peak capacity and will remove TSS or other pollutants at a higher rate during these low-intensity events. Conversely, some of the storms on an annual basis will have higher intensities, and thus the system will have higher flow rates and the removal efficiency will be below the target. But these are infrequent events during a typical rainfall year and are weighted accordingly.

The storm intensities and how often each storm occurs within a typical year is determined by looking at the local historic rainfall data. These rainfall intensity values and the drainage information for the project (drainage area runoff coefficient and time of concentration) are used to determine a flow rate using the Rational Method. Flow rates are then used to calculate operating rates for a proposed system. Finally, operating rates are paired with their corresponding removal efficiencies. The net annual TSS removal efficiency is then calculated by summing the relative efficiencies at each rainfall intensity.

Additional Design Considerations

Offline vs. Online. Engineers should identify the maximum flow rate (peak flow rate) that could enter the HDS system. In order to ensure proper performance and eliminate the risk of scouring, previously captured pollutant flows in excess of a device’s treatment capacity are typically bypassed either upstream of the system or via an internal bypass. Some HDS systems incorporate an internal bypass, which allows flows above the treatment flow to be internally bypassed in the HDS system and the device to be installed online. These devices treat low flows, bypass peak flows around treatment components and discharge both through a single outlet. This feature reduces the complexity of storm sewer layout and significantly reduces cost, as the contractor can install one manhole instead of three.

There are additional benefits of online systems. They are ideal for retrofit applications where an existing drainage line, standard junction manhole or poorly performing device can be replaced without other site disturbance. There are also quality-control advantages to having the bypass weir integral to the treatment system’s internal components. In particular, the device would come with the weir set to the correct height with no opportunity for error by the specifier or installer. If an HDS system does not incorporate an internal bypass weir, the system should be placed offline.

Even with a well-designed online system, a designer might choose to place a treatment device offline for other reasons. In general, for very-high-flow rates, an external bypass structure will scale better than trying to upsize a treatment structure. Likewise, the weir crest length in a bypass structure is typically longer than in an online treatment system. As a result, this increases the available cross-sectional flow area, allowing more flow to bypass at a lower hydraulic grade line. This is beneficial on sites with hydraulic limitations.

External bypass structures are also more flexible in their configurations. For example, a bypass vault’s dimensions could be altered to accommodate a large pipe or odd pipe-entry angle. In contrast, online systems are often constrained by the restrictions of the devices’ internal components, which limit the number or locations of inlet pipes. Finally, external bypass structures can accommodate additional special features, like orifice plates and adjustable weirs. Not every project requires such appurtenances, but it is important to know which options are available if needed.

Assess Site Limitations. The engineer must identify the footprint and depth constraints where the HDS system is to be located. These attributes may sway the designer toward a deep treatment system with a smaller footprint. If utilities, bedrock or groundwater are in the way, a shallower vault system with a larger footprint may be more appropriate.

Above-Grade Installations. Traditional HDS systems are designed to be installed below grade with soil supporting the walls of the structure. When HDS systems are installed above grade, custom structural analysis is required.

Structural Loading. Most systems are designed to meet HS 20 loading unless otherwise specified. HDS systems can be designed to accommodate higher loadings (e.g., airport, fire lane, etc.), but this often will require a custom structural analysis to be performed and should be evaluated early in the design phase.

Inlet and Outlet Pipe Diameter. Inlet and outlet pipe size can often dictate which model size is needed for a specific project. Each HDS device has a maximum pipe diameter that can be accepted for each model size.

Pipe Orientation. All HDS systems have limitations on where the inlet and outlet can connect to the device. For example, some HDS systems (like the one in Figure 5) rely on the placement of the inlets to activate the swirl. In this example, there are four possible locations for the inlet pipe. If two inlet pipes are desired, their location must be so that they both introduce flow in the same direction. Additionally, pipe locations into the systems should be examined closely to confirm that the knockouts do not interfere with each other, the joints or the top slab. This could impede manufacturability or jeopardize structural integrity.

Figure 5. Example of HDS Pipe Orientation

Elevations. Each product and model size requires different minimum distances from grade to the outlet invert. It is important to keep in mind that the outlet pipe is typically what drives the layout of the system.

Head Loss. Considering the head loss added to your network by an HDS system is good engineering practice to ensure the network operates as expected. The head loss associated with different types and sizes of HDS devices can vary in magnitude; knowing the impact of the selected device can make or break your storm sewer design. Head loss considerations are particularly important if you have a shallow depth to outfall, prevalent tail-water conditions or other hydraulically sensitive considerations.


Derek Berg is Director, Stormwater Regulatory Management, for Contech Engineered Solutions. He assists regulators, engineers and environmental organizations in the development and implementation of stormwater regulations, and spends much of his time interfacing with regulators at all levels of government and other relevant stakeholders on stormwater policy matters. Berg holds a Bachelor’s degree in Environmental Science and Policy as well as an MBA from the University of Southern Maine; email:

Patrick Valentine, P.E., is a Stormwater Design Engineer for Contech Engineered Solutions. He graduated from Virginia Tech with a Bachelor of Science degree in Civil & Environmental Engineering. As a Stormwater Design Engineer, his role at Contech requires him to design stormwater solutions that address challenges, including treatment for water quality, underground detention storage, subsurface infiltration and rainwater harvesting; email:

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Learning Objectives

At the conclusion of this article, the reader should be able to:

  • Define what a hydrodynamic separator is and its basic functionality
  • Identify the most common uses for hydrodynamic separators
  • Explain the two primary sizing methods for hydrodynamic separators
  • Explain the factors affecting online vs. offline placement of hydrodynamic separators

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Click on the button below to start the quiz for this course. Your score will be tabulated while you wait, and you will receive your certificate upon completion if you correctly answer eight or more questions.

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