Three basic types of inlet inserts are available: tray, bag, and basket types. Each is installed in a drain inlet or catch basin to treat stormwater runoff. The tray type allows flow to pass through filter media contained in a tray located around the perimeter of the inlet. Runoff enters the tray and leaves via weir flow under design conditions. High flows pass over the tray and into the inlet unimpeded.
The bag-type insert is made of fabric and is placed in the drain inlet around the perimeter of the grate. Runoff passes through the bag before discharging into the drain outlet pipe. Overflow holes are usually provided to pass larger flows without causing a backwater at the grate.
The basket-type insert consists of wire mesh placed around the perimeter of the inlet. The wire screens larger materials from the runoff. Some basket-type inserts contain filter media similar to the tray type.
Drain inlet inserts of all types have generally performed poorly in tests for several reasons. First, contact time between the runoff and the filter media is very short. Second, little storage area is available for material that is removed from the flow. The insert acts as a temporary storage location, retaining solids as flow decreases, but may allow resuspension when flow and velocity subsequently increases. Third, inserts require high maintenance and must be closely monitored during rain events to ensure that they are not clogged or bypassing flow. Such a level of maintenance is impractical for most installations.
Bag- and basket-type drain inlet inserts can be effective in removing gross pollutants (trash) if they are well maintained. For areas with a limited number of inlets where trash removal is the desired objective, these types of inserts can be a useful BMP. Tray-type inserts are generally not effective in trash or solids removal.
