Hydrodynamic separators are flow-through, often proprietary structures that establish a high-speed rotating flow within a cylindrical chamber. Larger particles in the rotating stream have greater density than smaller particles to follow the tight curve of the stream, and thus strike the outside wall, falling to the bottom of the chamber where they can be later removed. While this means of settling does not always occur in this manner, generally the larger, more dense particles well settle to the bottom of these devices.
Some devices include other treatment and removal mechanisms such as the ability to trap floatables and/or provide a higher level of filtration or screening. These units are typically installed underground, frequently in existing sump manhole.
Many hydrodynamic separators are well-suited for use in or retrofitting existing manholes that service small to moderate-sized catchments (<5 acres). Devices are available in a variety of sizes and corresponding flow capacities. For some devices, the different models and corresponding dimensions can be accessed using the hyperlinks shown in this table.
The sizing of a hydrodynamic separation system can be estimated using various tools, formulae, or guidance documents. Most manufacturers have a design tool or calculator for determining the size of device that may be needed; however modeled flow rates or the Rationale Method can be used to estimate the runoff flow rates. Additionally, guidance from the State of New Jersey Department of Environmental Protection and Washington State Department of Ecology provide additional considerations based on performance of these devices. In addition, SHSAM (Sizing Hydrodynamic Separators And Manholes) is a computer program for predicting the amount of suspended sediments removed from stormwater runoff by a given hydrodynamic separator/standard sump over a given period of time. SHSAM uses a continuous runoff model and sediment removal responses to predict the removal efficiency of the hydrodynamic separator device.
Most important for the sizing is the anticipated frequency of operation and maintenance as well as the protection of downstream BMPs and receiving water. Sizing is usually based on design flow rather than design flow. Most have configurations to bypass additional flow without resuspending solids. Some devices have “enhancements” that can provide other treatment mechanisms beyond pretreatment. These can provide an additional level of treatment such as filtration, skimming (in addition to settling or screening), chemical treatment/ sorption, or biodegradation. This is denoted table on system and component sizing.
Proprietary hydrodynamic separators are generally designed to be easy to measure solids accumulation with access for maintenance from the street level. Manufactures often recommend more frequent inspections for a period after installation in order to gauge sediment and debris accumulation rates that may depend on factors such as nearby construction, winter or rainy conditions, street sanding, or a highly erosive drainage area. The information gained during this time is the primary indicator for how often inspections and cleanings may require to be scheduled. An initial estimate is usually available from the manufacturer. Manufacturers also have maintenance manuals or guides available, which can be accessed in the hyperlink within the table on maintenance recommendations. Generally, hydrodynamic separators are designed for quick and simple, but more frequent, maintenance than other practices. An advantage of hydrodynamic separators is that they are designed to minimize washout and sediment scouring within the system. In many locations, disposal of sediments removed may be handled in the same manner as disposal of sediments from catch basins (see here). Hydrodynamic separators generally are for areas without steep grading.
Below are links to several tables containing information on hydrodynamic separators. To see all tables on a single page, link here. To see the information in a single spreadsheet click on the following link: File:Hydronamic separation table all data.xlsx