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

Water quality improvement
Constructed filters are considered an excellent stormwater
treatment practice with the primary pollutant
removal mechanism being filtration and settling. Less
significant pollutant removal may result from evaporation,
transpiration, biological and microbiological
uptake, and soil adsorption.
Sand filters have been shown to have a high removal
efficiency of Total Suspended Solids (TSS), and
medium removal efficiencies for Total Nitrogen (TN)
and Total Phosphorus (TP) (Table 7.8). Organic filter
media also perform very well for TSS and standard for
TP, but perform relatively poorly for TN.
For filters that are also designed to infiltrate, see the
water quality summary in the subsurface infiltration
bed section, or in the infiltration BMP. For manufactured,
proprietary systems, see the manufacturer’s
information, as well as findings from independent studies
consolidated by EPA at: http://cfpub.epa.gov/npdes/
stormwater/urbanbmp/bmpeffectiveness.cfm. Also see
Chapter 9, LID Stormwater Calculations and Methodology,
which addresses pollutant removal effectiveness
of this BMP.
Construction Guidelines
1. Follow the recommended materials for constructed
filters listed in Appendix D.
2. Structures such as inlet boxes, reinforced concrete
boxes, etc. should be installed in accordance
with the guidance of the manufacturers or design
engineer.
Table 7.8
Pollutant removal efficiencies for sand filters
Studies
No. of
studies
TSS % Removal TN % Removal TP % Removal
Range Median Range Median Range Median
U.S.* 18 80 - 92 86 30-47 32 41-66 59
International** 38 75 44 45
Organic media* N/A 85-100 poor 50-85
3. Excavated or structural filters that infiltrate
should be excavated in such a manner as to avoid
compaction of the subbase. Structures may be set
on a layer of clean, lightly compacted gravel (such
as AASHTO #57).
4. Place underlying gravel/stone in maximum six-inch
lifts and lightly compact. Place underdrain pipes in
gravel during placement.
5. Wrap and secure gravel/stone with nonwoven
geotextile to prevent clogging with sediments.
6. Lay filtering material. Do not compact.
7. Saturate filter media with water and allow media to
drain to properly settle and distribute.
Maintenance
Filters require a regular inspection and maintenance
program to maintain the integrity of filtering systems
and pollutant removal mechanisms. Studies have shown
that filters are very effective upon installation, but
quickly decrease in efficiency as sediment accumulates
in the filter. Odor is also a concern for filters that are not
maintained. Inspection of the filter is recommended at
least four times a year.
When a filter has accumulated sediment in its pore
space, its hydraulic conductivity is reduced, and so is
its ability to removal pollutants. Inspection and maintenance
are essential for continued performance of a
filter. Based upon inspection, some or all portions of the
filter media may require replacement.
*The Center for Watershed Protection, in its National Pollutant Removal Performance Database – Version
3 (September 2007)
**The International Stormwater Best Management Practices (BMP) Database, October 2007
LID Manual for Michigan – Chapter 7 Page 165