For development with greater than 40 parking stalls or where the potential for pollution by oil and/or grease exists, the first half inch of runoff must be treated using the best technology available. This requirement may be waived by the plan reviewer only when the applicant can demonstrate that installation of such practices is unnecessary.

Please see the Bioretention Device page for information on this practice.

Permanent inlet filters designed to trap oil and grease have sorbent media either as part of the filter bag, as a boom around the rim of the frame, or as a pouch that sits in the bag. The frames must be made of a durable material, typically stainless steel, that will not require frequent replacement.

Advantages

• Widely applicable
• Requires minimal land areas
• Compatible with most storm drain systems
• Can be used in retrofit applications

Disadvantage

• Limited effectiveness with large storm events
• Provides no other stormwater benefits
• Requires frequent maintenance

Proprietary inlet filters are available for many different inlet sizes from a variety of vendors.

Installation and maintenance of these inlet filters should should follow all manufacturers’ specifications. The frames, filter, and media should be inspected regularly to ensure that the practice is functioning properly and parts replaced when they become ineffective.

Filtering devices are designed to remove oil, grease, sediments, trash, and other debris from stormwater by passing them through a filtering device. Oil and grease filters are most often used at gas stations, industrial sites, parking lots, loading areas, and anywhere hydrocarbons are likely to be present in large quantities. Because they generally operate underground, they are often used in retrofit applications where other management practices are not practical. In high flow situations, the volume of water may exceed the capacity of the filter chamber and stormwater may bypass the device without treatment.

Advantages

• Widely applicable
• Requires minimal land areas
• Compatible with most storm drain systems
• Can be used in retrofit applications

Disadvantage

• Limited effectiveness with large storm events
• Provides little detention time
• Does not reduce peak flows
• Requires frequent maintenance

Proprietary filtering devices that are available in a wide array of configurations from a variety of vendors and should be custom designed to meet the individual needs of the site. The filtering media type must be specified in the plan.

Installation and maintenance of these devices should should follow all manufacturers’ specifications. They must be inspected regularly to ensure that the device is functioning properly and filters replaced when they become ineffective.

Stone or rock trenches are designed to trap oil and grease pollution on the surface and pores of limestone. These trenches will typically line the edges of parking areas to act as the first line of treatment, before the oil and grease is emulsified downstream.

Advantages

• Limestone widely available
• Relatively inexpensive

Disadvantage

• Applicability limited to sites without storm sewer
• Provides little other stormwater benefits
• Replacement of rock may be difficult

The trench needs to have the volume to treat the first half inch of runoff, either through total void space or through hydrologic modeling. The trench must also have a way to draw down between storms, typically with an underdrain, and be lined with geotextile fabric to prevent soil migration. Because this trench does not have pretreatment, it is assumed to be clogged and may not take credit for infiltration.

The trench must be inspected regularly to ensure that it's functioning properly. Replace the rock when it becomes coated with oil or grease (i.e. black-colored) or if it becomes filled with sediment.