Page 473 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

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12.5 Urban Runoff Control Practices 431

Water body

Pavement
Grassed area
(plants and trees)

Runoff flow

Stone trench
level spreader
Figure 12.17 Schematic Design of a Filter Strip
(Source: U.S. Environmental Protection Agency)

and soil and reduce the structure’s pollutant removal efficiencies. In addition to these consid-
erations, filter strips should be constructed in areas with porous soil to promote infiltration.

Maintenance Requirements. Filter strips must be mowed and weeded regularly—the same
maintenance practices as vegetated swales. In addition, the strip must be watered after ini-
tial seeding. In some cases, however, large filter strips can be “left on their own” so that
large vegetation can grow and create a natural filter strip. This option reduces the level of
maintenance required and can enhance the pollution removal of the strip.
Limitations on Use. The major limitation on the use of filter strips is the slope of the
land; these strips operate best when placed on flat surfaces that have permeable soils. Also,
filter strips treating large watersheds can have large land requirements that preclude their
location in urban areas.

12.5.6 Filtration Practices

Filtration practices provide runoff treatment through settling and filtering using a specially
placed layer of sand or other filtration medium. Flow enters the structure, ponds for a period
of time, and filters through the media to an underdrain that discharges to surface water. These
practices attempt to simulate the pollutant removal of infiltration practices using less land
area. Two different types of filtration practices in use are filtration basins and sand filters.