Stormwater Best Management Practices in an Ultra-Urban Setting: Selection and Monitoring

3.6    Water Quality Inlets

3.6.1    Description and Purpose

Due to their subsurface application, water quality inlets are particularly suitable for the ultra-urban environment. Prefabricated water quality treatment devices have evolved considerably since the oil/grit separator was first introduced into the marketplace; however, the basic design objectives remain the same. The main purpose of a water quality device is to improve the quality of stormwater runoff by settling out fine and coarse sediment, trapping debris and trash, and separating oil and grease. In general, water quality devices contain mechanisms to enhance both particle settling and oil and water separation. Some devices use settling and surface oil separation mechanisms, whereas others use filtration or vortex motion settling and separating mechanisms.

Most prefabricated water quality treatment systems come in a range of sizes. The size of the unit depends to a great extent on the quantity of stormwater runoff that needs to be treated. Additional stormwater can be bypassed into the existing storm drain system. Water quality treatment devices can be either on-line or off-line depending on the design of the treatment mechanism.

Although well suited for the ultra-urban environment, water quality inlets must receive frequent cleaning and inspection to maintain their effectiveness. FHWA has previously not recommended the use of water quality inlet BMPs such as oil/grit separators for highway applications, although they may perform adequately in maintenance yards with proper maintenance after installation. Finally, many states recommend they be considered only for pretreatment applications or as a last alternative.

3.6.2    Design Alternatives

The conventional oil/grit separator (OGS) is a three-chambered concrete gravity separation system. Typically, OGSs are installed in highly impervious areas that generate sediment, debris, and petroleum hydrocarbons in the stormwater runoff. OGSs are ideal for an ultra-urban environment because they are installed below the ground and can be retrofitted to an existing storm drain system. Because of resuspension problems associated with this device, OGSs typically are designed off-line, and clean-out should be conducted after each storm. In reality, OGSs are rarely cleaned out after every storm.

The catch basin insert (CBI) unit is a gravity filtering system designed to trap coarse sediments (gravel-size), debris, and oil and grease from stormwater runoff. The basic design consists of some type of screening mechanism for debris and trash, and a filter medium for particle settling and oil absorption. CBIs can be either mounted beneath the catch basin grate or placed within the sump area. CBIs typically are installed in small unpaved land use areas that generate coarse sediments and debris in the stormwater runoff. These devices are not meant as stand-alone systems, but rather they are designed to supplement other BMPs such as OGSs, sand filter systems, ponds, and infiltration trenches. Clean-out frequency depends largely on the land use and size of the basin that drains to the insert.

Prefabricated access hole and multichamber water quality devices are designed to trap sediment and oil within a storage chamber area and release the treated stormwater through an outlet pipe back into the storm drain system. Sediment separation results from a vortex motion that is created within the storage chamber area. As stormwater runoff is diverted into the storage chamber, the velocity of the flow is decreased and sediment particles are encouraged to settle out. These devices are ideal for ultra-urban areas since they are installed underground. They can either be retrofitted to an existing storm drain system or used in the design of a new system. These devices typically are installed in areas of high impervious land use where sediment and petroleum hydrocarbons are found in the stormwater. The size of the unit is a function of the quantity of stormwater to be treated. These devices can be on-line or off-line. Clean-out generally occurs when 85 percent of the sediment and oil storage chamber is filled. Both the access hole and concrete chamber devices are equipped with bypasses for large storms.