Hydrodynamic SeparatorsHydrodynamic separators (HDS), also known as oil/grit separators (OGS), are widely deployed as stormwater treatment BMPs, both in stand-alone applications and as pretreatment devices in a treatment train. HDS target the removal of high specific gravity suspended solids (such as sand, grit, and degraded asphalt), as well as free-floating oil and grease and trash and debris. HDS are typically vertically oriented cylinders (manholes) or multi-chambered rectangular vaults that contain a permanent pool of water in the treatment chamber. HDS may use special components such as baffles, weirs, and screens to direct the flow path, attenuate water velocity, and enhance the settling of particulates and the capture of oil and other floatables. Often HDS utilize internal bypass features to prevent or minimize resuspension and washout of previously captured pollutants. The primary treatments mechanisms used in HDS are gravity separation and spill capture. High specific gravity particulates settle by gravity to the sump of the device, with low specific gravity oil, trash, and debris float to the surface and are trapped. Normally there is sufficient floatables storage for the device to capture and retain an oil or fuel spill of substantial volume until maintenance can be performed to recover the spilled hydrocarbons. Maintenance of HDS is typically performed with a vacuum truck to evacuate captured sediment and floatables from the unit. Maintenance is normally performed from the surface, without need for confined space entry. Depending on the loading from the site, maintenance frequency will typically range from once per year to once every 3-5 years. For the longer maintenance intervals, it is important to ensure that there is adequate storage capacity for accumulated sediment, and annual inspection is highly recommended. The primary advantages HDS include good capture of sand and grit at relatively high surface loading rates, capture of floatable pollutants, capture of oil and fuel spills, and relatively simple and low cost maintenance. Additionally, since HDS are typically installed underground, treatment can be provided without consuming valuable developable land. Internal bypass features also reduce the total system footprint since additional manholes and diversions structures are not required for external bypassing of very high flow rates during the most intense storms. HDS are very effective pretreatment for other BMPs such as stormwater ponds, bioretention, filter devices, detention structures, and infiltration, and can significantly extend the maintenance interval for these downstream measures. There are limitations to HDS treatment effectiveness. TSS and floatables capture is sensitive to the flow rate and detention time within the device, as longer detention time results in better removal of pollutants. Generally, HDS provides relatively low and variable capture of fine particulates (< 50 microns) and particulate-bound pollutants (metals, nutrients, hydrocarbons, bacteria) that are concentrated on the fine particle fractions, except at low surface loading rates during low intensity storms and during inter-event settling periods. HDS can be conservatively sized with a larger structure to provide additional detention time and improved capture of the fine particulate fractions. Neutrally buoyant pollutants (specific gravity similar to water, typically organic particulates) are difficult to remove with HDS.
|
News
For our latest posts and news, visit our social media pages at the links below:
