Hydraulic performance of biofilter systems for stormwater management

Table 5: Regression between K fs shallow and various parameters
Field method
K fs shallow
R² 0.52
Parameters
P value
(Constant) 0.00
K lab deep (ini)
0.00
Age 0.24
Ratio 0.34
Volume of water/year excluded
Total volume/m² 0.49
Achleitner et al. (2006) reported a similar lack of correlation between hydraulic
conductivity and site characteristics, and made the hypothesis that current K was mainly
governed by the initial value. This result is in some ways unfortunate, because it
provides little guidance to those charged with the maintenance of such systems, in being
able to predict their lifespan and maintenance requirements. However, it does show the
importance of correctly specifying the filter media during the biofilter design, and of
having appropriate quality control to ensure that the supplied and installed media meets
these specifications.
One approach is to use a “contingency factor” in the specification of hydraulic
conductivity for biofiltration systems. For example, where the design intent is to use a
soil media with a hydraulic conductivity of 180 mm/hr, sizing of the system should be
undertaken assuming a hydraulic conductivity of 50% of the design value (ie. 90
mm/hr). In this way, if the media does not meet specifications, or shows a decline in
hydraulic conductivity over time, the overall system performance will remain
satisfactory.
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