Hydrodynamic separators (HDS) are stormwater management devices that use cyclonic separation 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 either creates a swirling vortex or plunges the water into the main sump. [2] :1 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 or dissolved pollutants. [1] The systems are also subject to scour and sediment washout during large storm events, e.g. a 10-year storm. [2] :42
A number of factors are relevant in selecting a hydrodynamic separator product for a site.
 | This section needs to be updated.(February 2021) |
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.[ clarification needed ] 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. [1]
The Technology Assessment Protocol-Ecology (TAPE) and Technology Acceptance and Reciprocity Partnership (TARP) 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. [3] [4] [5]
Currently,[ when? ] the Environmental & Water Resources Institute (a component of the American Society of Civil Engineers) and ASTM International are developing comprehensive verification guidelines and standard test methods for assessing the performance of these devices.[ citation needed ]
HDS systems are not maintenance-intensive, when compared with land-based BMP’s. [1] 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.[ citation needed ]
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. [1]
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.” [1] HDS systems may be ideal for areas where land is not readily available and/or tight retrofits are needed as they are installed underground.
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. [6] Other jurisdictions evaluate the applicability of HDS on a site-specific basis. [7] 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.
Stormwater, also spelled storm water, is water that originates from rain, including snow and ice melt. Stormwater can soak into the soil (infiltrate), be stored on the land surface in ponds and puddles, evaporate, or contribute to surface runoff. Most runoff is conveyed directly to nearby streams, rivers, or other water bodies without treatment.
Water pollution is the contamination of water bodies, usually as a result of human activities. Water bodies include for example lakes, rivers, oceans, aquifers and groundwater. Water pollution results when contaminants are introduced into the natural environment. For example, releasing inadequately treated wastewater into natural water bodies can lead to degradation of aquatic ecosystems. In turn, this can lead to public health problems for people living downstream. They may use the same polluted river water for drinking or bathing or irrigation. Water pollution is the leading worldwide cause of death and disease, e.g. due to water-borne diseases.
A storm drain, storm sewer, surface water drain/sewer, or stormwater drain is infrastructure designed to drain excess rain and ground water from impervious surfaces such as paved streets, car parks, parking lots, footpaths, sidewalks, and roofs. Storm drains vary in design from small residential dry wells to large municipal systems.
Wastewater treatment is a process used to remove contaminants from wastewater or sewage and convert it into an effluent that can be returned to the water cycle with acceptable impact on the environment, or reused for various purposes. The treatment process takes place in a wastewater treatment plant (WWTP), also referred to as a Water Resource Recovery Facility (WRRF) or a Sewage Treatment Plant (STP) in the case of domestic wastewater. Pollutants in wastewater are removed, converted or broken down during the treatment process.
The Nationwide Urban Runoff Program (NURP) is a research project conducted by the United States Environmental Protection Agency (EPA) between 1979 and 1983. It was the first comprehensive study of urban stormwater pollution across the United States.
Industrial wastewater treatment describes the processes used for treating wastewater that is produced by industries as an undesirable by-product. After treatment, the treated industrial wastewater may be reused or released to a sanitary sewer or to a surface water in the environment.
A retention basin, sometimes called a wet pond,wet detention basin or stormwater management pond (SWMP), is an artificial pond with vegetation around the perimeter, and includes a permanent pool of water in its design. It is used to manage stormwater runoff to prevent flooding and downstream erosion, and improve water quality in an adjacent river, stream, lake or bay.
Bioswales are channels designed to concentrate and convey stormwater runoff while removing debris and pollution. Bioswales can also be beneficial in recharging groundwater.
First flush is the initial surface runoff of a rainstorm. During this phase, water pollution entering storm drains in areas with high proportions of impervious surfaces is typically more concentrated compared to the remainder of the storm. Consequently, these high concentrations of urban runoff result in high levels of pollutants discharged from storm sewers to surface waters.
Best management practices (BMPs) 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 and wetland management, BMPs may refer to a principal control or treatment technique as well.
Bioretention is the process in which contaminants and sedimentation are removed from stormwater runoff. Stormwater is collected into the treatment area which consists of a grass buffer strip, sand bed, ponding area, organic layer or mulch layer, planting soil, and plants. Runoff passes first over or through a sand bed, which slows the runoff's velocity, distributes it evenly along the length of the ponding area, which consists of a surface organic layer and/or groundcover and the underlying planting soil. The ponding area is graded, its center depressed. Water is ponded to a depth of 15 cm (5.9 in) and gradually infiltrates the bioretention area or is evapotranspired. The bioretention area is graded to divert excess runoff away from itself. Stored water in the bioretention area planting soil exfiltrates over a period of days into the underlying soils.
A sediment control is a practice or device designed to keep eroded soil on a construction site, so that it does not wash off and cause water pollution to a nearby stream, river, lake, or sea. Sediment controls are usually employed together with erosion controls, which are designed to prevent or minimize erosion and thus reduce the need for sediment controls. Sediment controls are generally designed to be temporary measures, however, some can be used for storm water management purposes.
A silt fence, sometimes (misleadingly) called a "filter fence," is a temporary sediment control device used on construction sites to protect water quality in nearby streams, rivers, lakes and seas from sediment in stormwater runoff. Silt fences are widely used on construction sites in North America and elsewhere, due to their low cost and simple design. However, their effectiveness in controlling sediment can be limited, due to problems with poor installation, proper placement, and/or inadequate maintenance.
Urban runoff is surface runoff of rainwater created by urbanization. This runoff is a major source of flooding and water pollution in urban communities worldwide.
A fiber roll is a temporary erosion control and sediment control device used on construction sites to protect water quality in nearby streams, rivers, lakes and seas from sediment erosion. It is made of straw, coconut fiber or similar material formed into a tubular roll.
Point source water pollution comes from discrete conveyances and alters the chemical, biological, and physical characteristics of water. In the United States, it is largely regulated by the Clean Water Act (CWA). Among other things, the Act requires dischargers to obtain a National Pollutant Discharge Elimination System (NPDES) permit to legally discharge pollutants into a water body. However, point source pollution remains an issue in some water bodies, due to some limitations of the Act. Consequently, other regulatory approaches have emerged, such as water quality trading and voluntary community-level efforts.
Water-sensitive urban design (WSUD) is a land planning and engineering design approach which integrates the urban water cycle, including stormwater, groundwater and wastewater management and water supply, into urban design to minimise environmental degradation and improve aesthetic and recreational appeal. WSUD is a term used in the Middle East and Australia and is similar to low-impact development (LID), a term used in the United States; and Sustainable Drainage System (SuDS), a term used in the United Kingdom.
Low-impact development (LID) is a term used in Canada and the United States to describe a land planning and engineering design approach to manage stormwater runoff as part of green infrastructure. LID emphasizes conservation and use of on-site natural features to protect water quality. This approach implements engineered small-scale hydrologic controls to replicate the pre-development hydrologic regime of watersheds through infiltrating, filtering, storing, evaporating, and detaining runoff close to its source. Green infrastructure investments are one approach that often yields multiple benefits and builds city resilience.
A tree box filter is a best management practice (BMP) or stormwater treatment system widely implemented along sidewalks, street curbs, and car parks. They are used to control the volume and amount of urban runoff pollutants entering into local waters, by providing areas where water can collect and naturally infiltrate or seep into the ground. Such systems usually consist of a tree planted in a soil media, contained in a small, square, concrete box. Tree box filters are popular bioretention and infiltration practices, as they collect, retain, and filter runoff as it passes through vegetation and microorganisms in the soil. The water is then either consumed by the tree or transferred into the storm drain system.
Industrial stormwater is runoff from precipitation that lands on industrial sites. This runoff is often polluted by materials that are handled or stored on the sites, and the facilities are subject to regulations to control the discharges.