Hydrodynamic separator

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Hydrodynamic separators (HDS) are stormwater management devices used to control water pollution. They are designed as flow-through structures with a settling or separation unit to remove sediment and other pollutants.[1] HDS are considered structural Best Management Practices (BMPs), and are used to treat and pre-treat stormwater runoff.

HDS systems use the physics of flowing water to remove a variety of pollutants and are characterized by an internal structure that creates a swirling vortex. By using the vortex flow of water, along with supplemental features to reduce velocity, an HDS system is designed to separate floatables (trash, debris and oil) and settle-able particles, like sediment, from stormwater. HDS systems are not effective for the removal of very fine solids or dissolved pollutants.

With the federal and state storm water management regulations, professionals may look to a broad array of BMPs for meeting stormwater treatment objectives. Hydrodynamic separation is one such method. There are a variety of HDS systems available, each unique and different in design.

Contents

[edit] Evaluating HDS Technologies

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Professionals should consider a number of factors when selecting an HDS product for a site:

1) Sizing & Treatment Performance[2] HDS systems should be sized based on treatment objectives including desired level of pollutant removal, the amount of flow to be treated, and particle size to be targeted. Performance is highly sensitive to hydraulic loading rate and care must be taken to avoid routing excess flow through the devise and compromising performance. Each vendor’s product has different pollutant removal rates that should be evaluated before selecting the system. Below are some articles to gauge an understanding of performance verification processes and evaluation methods.

TAPE and NJDEP/TARP processes are comprehensive evaluation programs in the U.S. that include lab and field along with specific sizing criteria for hydrodynamic separation systems. An article of reference to assist in performance evaluation is also available: A Comprehensive Approach to Performance Verification for Stormwater Treatment Systems. Links to this article are provided in the external links section of this page.

2) Maintenance & Inspection Requirements / Associated Cost HDS systems are not maintenance intensive, when compared with land-based BMP’s.[3] Each manufactured system is different, therefore maintenance and inspection requirements should be looked at closely when purchasing an HDS system. Vactor or vacuum trucks are typically used for maintenance, so unobstructed access to accumulated pollutants for removal is critical.[4]

3) Installation and operating costs[5] Costs for HDS systems depend on site-specific conditionse 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.

4) Land Costs [6]

According to the Environmental Protection Agency, “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.”[7] HDS systems may be ideal for areas where land is not readily available and/or tight retrofits are needed as they are installed underground.

[edit] Regulations and Approvals for Hydrodynamic Separation

As stormwater regulations become increasingly stringent, many states and municipalities have developed criteria to govern the use and sizing of HDS systems. 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. Many agencies maintain published lists that identify acceptable HDS systems and sizing criteria for that area.

[edit] References

  1. ^ United States Environmental Protection Agency. Washington, DC. "Storm Water Technology Fact Sheet: Hydrodynamic Separators." September 1999. Document No. EPA 832-F-99-017.
  2. ^ US EPA, op. cit.
  3. ^ US EPA, op. cit.
  4. ^ StormwaterAuthority.org "Best Management Practices."
  5. ^ US EPA, op. cit.
  6. ^ US EPA, op. cit.
  7. ^ US EPA, op. cit.

[edit] External links

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Stormwater structures
Containment
Flow control

Flow control structure  · Hydrodynamic separator  · Storm drain
Infiltration