3 Protocolo de experimentación
3.7 Discusión
ator
Low Sediment Target Pollutants PollutantRemoval ReductionVolume Peak FlowReduction
Nutrients Organics
Trash Oil/Grease Metal
Bacteria
Moderate High Requires Pre-Treatment
Because swirl separators can be placed underground and come in a wide range of sizes, some small enough to fit within a standard manhole, they can be useful for removing settleables and floatables in areas where land availability is limited and tight retrofits are needed. The size of the drainage area that may be treated with swirl separators is limited by the maximum capacity of the units and by the available footprint.
beneFItS
Improves water quality by removing coarse sediment and floatables; some models
remove oil and grease.
Some models attenuate peak flows.
Low space requirements.
Good for sites where infiltration is not an option.
Ideal as part of a treatment train.
May be more cost-effective pre-treatment device than traditional wet or dry
detention basin.
May provide treatment in less space than wet vaults or detention basins.
Due to sealed top and inlet configuration, mosquito control may be less of an issue
than for wet detention basins. Low maintenance costs.
lImItatIonS
Swirl separators alone do not constitute compliance with San Francisco’s stormwater
requirements and are not considered treatment to the Maximum Extent Practicable (MEP) by the Regional Water Board.
Is not effective for removal of dissolved pollutants, fine sediments, and pollutants
that adhere to fine sediment.
Some systems have standing water between storms that could raise mosquito
breeding concerns.
Drainage area is limited by capacity of largest models.
Relatively high capital and installation costs.
No aesthetic value.
No habitat value.
SItInG
Soil depth and stability, site slopes, and groundwater depth may affect the applicability of swirl separators to different sites.
DeSIGn conSIDeratIonS
Design variations in commercial units: Commercially available swirl separators for stormwater treatment vary in their geometry, radial baffle design, and internal circular chambers. Some manufacturers include baffles or other devices, thereby creating a “dampened vortex,” to help particles settle out; others contend that free vortex creates less turbulence thereby increasing removal efficiency. Some include an inner chamber designed to enhance removal efficiency. Whether the inclusion of chambers and baffles gives better performance is unknown. Generally, a nonturbulent environment that allows particles to settle and floatables to rise, and protection against re-entrainment of settled particles, is considered an ideal condition for treatment. Depth of the units should also be considered; depth of units can extend from 3 feet below the inlet pipe up to 25 feet in depth for large units. Consult with manufacturers to determine the best product for the specific site conditions and treatment needs.
Treatment: Swirl separators are most effective where the pollutant load is primarily coarse sediment and floatables and where pretreatment of these pollutant types is desired to remove the treatment burden from downstream BMPs. Experience with swirl separators for treating Combined Sewer Overflows, the original use of the technology, suggests that the practical lower limit for removal is a particle diameter of 50-200 microns, depending on the specific gravity of the particle. For reference, it is generally believed that other treatment measures such as wet ponds and extended detention basins can remove smaller particles down to 10-20 microns. However, there is little data to support these numbers. Swirl separators may also be useful for pretreatment near stormwater “hotspots,” such as gas stations, where high concentrations of pollutants are likely to occur.
Sizing: Depending on the manufacturer, swirl separators can treat design flows from 0.75 to 300 cfs, with units ranging from 4 feet in diameter for the smallest precast units to 40 feet in diameter for custom units and may extend up to 25 feet or more in depth.
Swirl S
epar
ator
Separators can be configured as online or offline devices. If configured as an offline facility, the device is designed to treat the full peak flow of the inlet pipe (hydraulic capacity equals treatment capacity). If the separator is an online facility, however, commercially available units typically have a peak flow through the inlet pipe about four times greater than the flow the device is designed to treat (hydraulic capacity equals four times treatment capacity). Flows that exceed the treatment capacity but not the hydraulic capacity can still pass through the device; however less pollutant removal is achieved. Designers should refer to the manufacturer’s specifications to determine whether a product will be able to treat the desired flow.
operatIonS anD maIntenance
Because there are no moving parts, swirl separators are generally not considered to be maintenance intensive when compared with land-based BMPs such as swales and treatment ponds. However, without regular maintenance, these devices are prone to the following:
Accumulated sediment reducing available treatment volume;
Sediment resuspension during high flow storm events;
Accumulated floating material being released and discharged during high flow
events; and
Accumulation of pollutants to the point where contents are characterized as hazard-
ous for petroleum hydrocarbons or soluble metals.
The rate at which each system accumulates pollutants is site-specific, and most manufacturers recommend at least one inspection per month during the first year after installation. Vactor or vacuum trucks are typically used for maintenance, so unobstructed access to the treatment chambers is important to facilitate removal of accumulated pollutants. The table on the following page outlines typical post-construction inspection and maintenance activities and schedules for swirl separators, but these will vary by manufacturer. Project sponsors should adhere to the maintenance plan specified by the manufacturer of the chosen product.
coSt
Capitol costs for pre-cast units range from $10,000 to $60,000 depending on the size, manufacturer, and model. Custom units are generally more expensive. Installation costs are site-specific depending on the need to relocate utilities, working space, and depth of installation, but are typically approximately one-half to one times the unit cost. Several sources cite operations and maintenance costs of approximately $1,000 per year, varying from year to year. Maintenance costs include removal (with a vactor truck) and disposal of sediment and debris.
Typical Inspection Activities for Swirl Separators
Activity Schedule
Determine long-term maintenance schedule by visual
inspection of floatables and sediment accumulation and by measurement of sediment deposition rate using a probe or “dipstick.”
Monthly during first
year of operation
Check inlet and outlet pipes for obstructions.
Pump out unit and inspect for damage.
Annually
Typical Maintenance Activities for Swirl Separators
Activity Schedule
Remove accumulated sediment, debris, and floatables.
Special disposal of floatables may be necessary if petroleum
Swirl S
epar
ator
reFerenceS anD reSourceS
Barbaro, Henry L. and Clay Kurison, 2005. “Evaluating Hydrodynamic Separators.” Road Ecology Center eScholarship Repository, John Muir Institute of the
Environment, University of California, Davis.
California Stormwater Quality Association. 2003. “MP-51: Vortex Separator.” California
BMP Handbook- New Development and Redevelopment.
Environmental Protection Agency. 1999. Stormwater Technology Fact Sheet: Hydrodynamic Separators (EPA 832-F-99-017).
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Stormwater enters drain Treated outflow to collection system Debris is caught in polypropylene sack Debris is caught in plastic or metal mesh Debris is caught in various filter trays