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| Applicability | ||
| The applicability of parking
lot bioswales in any given design situation can best be determined
by determining which pollutants (see Components) need to be treated.
Swales are relatively effective in capturing solids, oils and the
less soluble metals. However, they are not as effective at treating
more soluble metals, and less still in the case of phosphorus. Nitrogen
removal rates are insignificant. Biofilters are therefore advisable
only when solids and oils are to be removed. Research indicates that
they are a better choice than oil separators to remove low concentrations
of oil and grease from runoff (no separator can reduce hydrocarbons
to levels below 10 mg/L). A residence time of nine minutes (King County,
p. 125) is needed to achieve the best and most reliable performance,
and the use of vegetation as outlined in this paper can reduce concentrations
to even lower levels. Another argument in favor of such systems is
that vegetation is much less expensive than oil separators.
Choosing the appropriate type of vegetation will depend upon available land, the importance of preserving open space, and individual water quantity and quality goals. Biofilter systems treat water on site rather than collecting it later in a water quality pond; such in situ treatment can have aesthetic, financial, educational, and ecological benefits. |
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