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Hydrodynamic Separators

Hydrodynamic separators are an important part of controlling water pollution from stormwater runoff and are a key tool in a stormwater manager’s stormwater treatment toolbox for meeting environmental regulations. Hydrodynamic separators (HDS) are primarily used to remove sediment from stormwater by taking advantage of the force of gravity.
Many HDS will also remove floatables such as oil and large items such as trash and debris.

There are three main types of HDS: vault systems, swirl concentrators and inclined plate separators. Vault systems are rectangular and contain baffles and weirs to control flow and retain solids, oil and debris. Swirl concentrators are cylindrical, and they direct the water in a circular motion in order to make efficient use of the available volume. Inclined plate separators take advantage of internal plates to provide additional area for settling solids.

All HDS store the captured material and thus require regular maintenance, typically by a vacuum truck. The units are designed so that the vacuum truck can remove the floatables such as trash and debris, then the water, and lastly, the captured sediment. Proper sizing of an HDS is critical to getting adequate removal performance and an acceptable maintenance interval.


How to Size a Hydrodynamic Separator

The first step is to determine the treatment requirements. Hydrodynamic separators remove suspended solids and floatables such as oil. This means they will capture other pollutants attached to the sediment or mixed in the oil. The capture of additional pollutants depends on many factors beyond the control of the stormwater manager, so it is not consistent from site to site. Specifiers should check with the local jurisdiction to verify what, if any, credit an HDS will be given for capturing pollutants other than sediment.

It may turn out that the overall treatment requirements for a project will require multiple treatment operations. In this case, it is essential to define the specific requirements for the HDS. Once the treatment goal of the HDS is defined, the sizing process can begin.

HDS are always rated in terms of the flow rate they can handle while delivering the claimed treatment performance. Rainfall is always reported in terms of depth falling over a certain time. This makes sizing an HDS a two-part process, first converting the available rainfall data to flow rate and then choosing an HDS with the desired performance at the given flow rate. Converting rainfall to flow rate is an entire field of study, and often a particular method is required by the jurisdiction.

Once the flow rate is known, the stormwater manager must determine what particle size to focus on treating. This will allow them to look at available performance data and determine how a system will perform on their site. With a list of potential solutions that will meet the treatment requirements, the stormwater manager must now consider additional factors before choosing the best solution.


Additional Factors in Choosing a Hydrodynamic Separator

A very important factor in choosing a hydrodynamic separator is whether it will be installed offline or online. Online design requires that the HDS be able to pass the maximum hydraulic flow in the storm sewer system, preferably without scouring out previously captured sediment.
This requires that the HDS be designed with some type of internal bypass. An internal bypass is an arrangement of weirs and walls that allow very large flows to pass through the system except around the area where the sediment is stored. Most HDS are designed with an internal bypass.

Maintenance is also a vital issue in the decision-making process. Maintenance frequency is generally a function of the size of the unit since all capture sediment is stored in the unit. Ease of maintenance depends on the design of the unit and how the internals are arranged. A larger unit will cost more upfront but will result in lower maintenance costs, so a life cycle cost analysis is advisable.


Applications of Hydrodynamic Separators

Hydrodynamic separators are designed to be standalone treatment systems and are capable of providing 80% removal of coarser particles if properly sized. Many jurisdictions require removal of finer particles, so HDS are often used as part of a treatment train designed to give 80% or more removal of sediment, including finer particles. HDS are particularly well suited to providing pretreatment ahead of a filter, either manufactured or natural.

Larger particles will more quickly clog a filter, preventing it from removing the smaller particles it is supposed to. This clogging leads to increased maintenance requirements. Installing an HDS as pretreatment for a filter optimizes system design by using the HDS to capture the coarse particles while the filter captures the fine particles.

As pretreatment for a natural system such as a pond or swale, the HDS reduces maintenance effort by concentrating the sediment in one easily accessible place.
It is much easier to vactor an HDS than it is to dredge a pond. Finally, HDS are also useful as pretreatment for infiltration systems. Once the larger particles are eliminated by the HDS, the water can be slowly and safely reintegrated with groundwater. Removing particles reduces clogging of the soil or other pervious surfaces that provide natural filtering in infiltration systems.

Most HDS also function as spill capture devices and will retain a certain quantity of oil or other floatable liquids. This is typically a secondary function, and sizing based on oil capture is rare. Most HDS will also capture trash and debris. Some will utilize nets or screens for trash capture and work as well as specialized trash capture devices, but again sizing based on trash capacity is rare. If trash is the only concern, then it may not be cost effective to pay for the sediment capture ability of an HDS, a dedicated trash capture device such as a TrashTrap may be a more efficient option.

HDS are designed to be installed underground, and as such, they use less land leaving more for development. While this doesn’t change the fundamental purpose of the separator, it does extend its value and purpose beyond the simple task of filtering water. It can simultaneously serve to improve land value, lower stormwater management costs and better support community needs.



The SiteSaver, by StormTrap, is an inclined plate separator with oil storage capacity and nets for capturing trash and debris. It does feature an internal bypass, allowing for online installation.
The inclined plate design, more commonly seen in wastewater treatment, allows for higher efficiency for a given size.
This advantage becomes particularly apparent at higher flow rates where competing systems become very large and expensive to install. It makes SiteSaver well suited as pretreatment for natural systems such as ponds, which are often designed to handle large flows.