Hydrodynamic separators are stormwater

Stormwater

Stormwater is water that originates during precipitation events. It may also be used to apply to water that originates with snowmelt that enters the stormwater system...

 management devices used to control water pollution

Water pollution

Water pollution is the contamination of water bodies . Water pollution occurs when pollutants are discharged directly or indirectly into water bodies without adequate treatment to remove harmful compounds....

. They are designed as flow-through structures with a settling or separation unit to remove sediment

Sediment

Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....

 and other pollutants. HDS are considered structural best management practices

Best management practice for water pollution

Best Management Practices is a term used in the United States and Canada to describe a type of water pollution control. Historically the term has referred to auxiliary pollution controls in the fields of industrial wastewater control and municipal sewage control, while in stormwater management ...

 (BMPs), and are used to treat and pre-treat stormwater runoff

Surface runoff

Surface runoff is the water flow that occurs when soil is infiltrated to full capacity and excess water from rain, meltwater, or other sources flows over the land. This is a major component of the water cycle. Runoff that occurs on surfaces before reaching a channel is also called a nonpoint source...

.

Design and applications

HDS systems use the physics of flowing water to remove a variety of pollutants and are characterized by an internal structure that either creates a swirling vortex

Vortex

A vortex is a spinning, often turbulent,flow of fluid. Any spiral motion with closed streamlines is vortex flow. The motion of the fluid swirling rapidly around a center is called a vortex...

 or plunges the water into the main sump. Along with supplemental features to reduce velocity, an HDS system is designed to separate floatables (trash, debris and oil) and settleable particles, like sediment, from stormwater. HDS systems are not effective for the removal of very fine solids

Suspended solids

Suspended solids refers to small solid particles which remain in suspension in water as a colloid or due to the motion of the water. It is used as one indicator of water quality....

 or dissolved pollutants. The systems are also subject to scour and sediment washout during large storm events, e.g. a 10-year storm.

Evaluating HDS technologies

A number of factors are relevant in selecting an HDS product for a site.

Sizing & treatment performance

HDS systems should be sized based on treatment objectives including desired level of pollutant removal, drainage basin characteristics, climate of the region, and particle size to be targeted. Performance is also sensitive to water temperature, i.e. season. Care must be taken to avoid routing excess flow through the device and compromising performance. Each vendor’s product has different pollutant removal rates that should be evaluated before selecting the system.

The TAPE and TARP programs are evaluation programs sponsored by several state agencies in the U.S. These programs include lab and field testing and provide specific sizing criteria for hydrodynamic separation systems.

Currently, the EWRI-ASCE and the ASTM International are developing comprehensive verification guidelines and standard test methods for assessing the performance of these devices.

Maintenance & inspection requirements

HDS systems are not maintenance-intensive, when compared with land-based BMP’s. Each manufactured system is different, therefore maintenance and inspection requirements should be looked at closely when purchasing an HDS system. Vacuum trucks are typically used for maintenance, so unobstructed access to accumulated pollutants for removal is critical.

Installation and operating costs

Costs for HDS systems depend on site-specific conditions such as land characteristics, amount of runoff to be treated, system depth and performance requirements. Be aware that not all HDS systems are alike in treatment performance, and basing a decision solely on the installation and operating cost of a system may compromise system performance and the environment. Long-term maintenance costs should also be considered with overall costs when purchasing or selecting a stormwater BMP as initial installation and operating costs may not reflect the long-term investment needed to maintain the system.

Land costs

According to the U.S. Environmental Protection Agency (EPA), “Using structural BMPs that can be placed underground and are design to withstand site specific soil, groundwater and traffic loading conditions provide valuable savings in land area compared to conventional volume-based stormwater treatment practices such as ponds, wetlands, and swales.” HDS systems may be ideal for areas where land is not readily available and/or tight retrofits are needed as they are installed underground.

Regulations and approvals for hydrodynamic separators

As stormwater regulations become increasingly stringent, many states and municipalities have developed criteria to govern the use and sizing of HDS systems, and publish lists that identify acceptable HDS systems. Other jurisdictions evaluate the applicability of HDS on a site-specific basis. It is increasingly common to use HDS as the first component of a treatment train, a combination of BMPs in series, to remove coarse solids and floatable pollutants that can rapidly clog other BMPs thus prolonging their maintenance cycle.

External links

• Stormwater Technologies Clearinghouse - University of Massachusetts
• Environmental Protection Agency - Stormwater Program
• Technology Acceptance and Reciprocity Partnership (TARP) - Pennsylvania Dept. of Environmental Protection
• NJCAT Technology Verification Program - Verification program for hydrodynamic separator systems. Provides list of approved systems.