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432 Chapter 12 Urban Runoff and Combined Sewer Overflow Management

Geotextile fabric
Cleanout pipe

18 in. fine sand

8-in. perforated pipe Geomembrane
Figure 12.18 Conceptual Design of a Filtration Basin. Conversion factor: 1 in. 25.4 mm
(Source: City of Austin, TX, USA)

12.5.6.1 Filtration Basins Stormwater runoff diverted to a filtration basin can be de-
tained, allowed to percolate through filter media, and collected in perforated pipes as shown
in Figure 12.18. These perforated pipes then transport the filtered runoff to the receiving
water. These systems have been used extensively in Austin, Texas, showing good pollutant
removal efficiencies and low failure rates. One major question regarding filtration basins is
the effect of cold temperature and freezing conditions on the operation of these systems.

Pollutant Removal. Pollutant removal in filtration basins occurs because of settling dur-
ing the initial ponding time and filtering through the soil media. Removal efficiencies in
filtration basins depend on several factors, including the storage volume, detention time,
and filter media used. In general, longer detention times increase the system’s pollutant re-
moval efficiency. Increasing the detention time usually requires increasing the overall size
of the filtration basin. Reducing the size of the perforated pipe, increasing the depth of the
filter medium, or decreasing the percolation rate of the filter medium can be used to in-
crease the detention time. Filtration basins primarily use sand as the filtering medium.

Design Considerations. In Austin, Texas, sand filtration basins are typically designed to
provide a detention time of 4 to 6 h and have been used to treat runoff from drainage areas
from 3 to 80 acres (1.2 to 32.3 ha). A stormwater sand-peat filtration basin in Montgomery
County, Maryland, was designed to store the first 0.5 in. (13 mm) of rainfall from the im-
pervious land in the watershed. In the Maryland area, this sizing criterion results in the
treatment of 50% to 60% of the annual storm runoff volume. Runoff from larger storms
that exceeds the capacity of these filtration systems is diverted away from the filtration
basin or discharged through an emergency spillway. To improve the longevity of sand and
sand-peat filtration basins, runoff entering the systems is typically pretreated to remove
suspended solids. Such pretreatment techniques as the use of a wet pool or water quality
inlets can be used in conjunction with filtration basins.
Maintenance Requirements. Stormwater runoff filtration basins require extensive main-
tenance to remove accumulated sediments and prevent clogging of the filtering medium.
Maintenance requirements include inspecting the basin after every major storm event for
the first few months after construction and annually thereafter; removing litter and debris;
and revegetating eroded areas. In addition, the accumulated sediment should be removed
periodically and the filter medium, when clogged with sediment deposits, should be re-
moved and replaced.

Limitations on Use. Filtration basins can often be difficult to locate in highly urbanized
areas because of their large land requirements. In addition, high groundwater levels can re-
strict their use.