Low Impact Development Manual for Michigan - OSEH - University ...

Description and Function
A constructed filter is a structure or excavation filled
with material that filters stormwater runoff to remove
particulate matter and the pollutants attached to it. The
filter media may be comprised of materials such as
sand, peat, compost, granular activated carbon (GAC),
perlite, or inorganic materials. In some applications the
stormwater runoff flows through an unfilled “pretreatment”
chamber to allow the large particles and debris to
settle out. Surface vegetation is another good option for
pretreatment, as long as it is extensive enough to protect
the filter from sediment during large storm events. The
runoff then passes through the filter media where additional
pollutants are filtered out, and is collected in an
underdrain and returned to the conveyance system,
receiving waters, or infiltrated into the soil. In general,
constructed filters are best applied at sites without sufficient
surface area available for bioretention.
Variations
There are a wide variety of constructed filter applications,
including surface and subsurface, vegetated, and
with or without infiltration. There are also a variety of
manufactured filter products that may be purchased (see
water quality devices BMP). In general, constructed
filters consist of some, if not all, of the following components:
excavation or container for media, pretreatment,
flow entrance/inlet, surface storage (ponding area), filter
media, underdrain (if necessary), and positive overflow.
Examples of these variations include:
• Surface non-vegetated filter,
• Surface vegetated filter,
Figure 7.17
Vegetated peat filter adjacent to a parking lot
Source: Pennsylvania Stormwater BMP Manual
• Surface contained filter,
• Surface linear “perimeter” filter,
• Small subsurface filter, and
• Large subsurface filter.
Surface Infiltration Filter
Filters may be designed to allow some or all of the treated
water to infiltrate. Infiltration design criteria apply for
all filters designed (Figure 7.18) with infiltration. In all
cases, a positive overflow system is recommended.
Figure 7.18
Filter with infiltration to subsoils
Source: Pennsylvania Stormwater BMP Manual
LID Manual for Michigan – Chapter 7 Page 159