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

During inspection the following conditions should be
considered:
• Standing water – any water left in a surface filter
after the design drain down time indicates the filter
is not functioning according to design criteria.
• Film or discoloration of any surface filter material
– this indicates organics or debris have clogged the
filter surface.
A discolored film on top of a sand filter indicates the need
for maintenance
Source: California Stormwater BMP Handbook, New
Development and Redevelopment, 2003
• Remove trash and debris as necessary
• Scrape silt with rakes, if collected on top of the
filter
• Till and aerate filter area
• Replenish filtering medium if scraping/removal has
reduced depth of filtering media
• Repair leaks from the sedimentation chamber or
deterioration of structural components
• Clean out accumulated sediment from filter bed
chamber and/or sedimentation chamber
• Clean out accumulated sediment from underdrains
In areas where the potential exists for the discharge
and accumulation of toxic pollutants (such as metals),
filter media removed from filters must be handled and
disposed of in accordance with all state and federal
regulations.
Winter Considerations
Michigan’s winter temperatures can go below freezing
four to five months out of every year and surface filtration
does not work as well in the winter. Peat and compost
may hold water freeze, and become relatively impervious
on the surface. Design options that allow directly
for subsurface discharge into the filter media during
cold weather may overcome this condition. Otherwise,
the reduced performance when the filter media may be
temporarily frozen should be considered.
There are various filtration options available for treating
snowmelt runoff. In some cases, installations are
built below the frost line (trenches, subgrade proprietary
chambers) and do not need further adaptation for
the cold. However, some special consideration is highly
recommended for surface systems.
The main problem with filtration in cold weather is
the ice that forms both over the top of the facility and
within the soil. To avoid these problems to the extent
possible, it is recommended that the facility be actively
managed to keep it dry before it freezes in the late fall.
Additional modifications, such as increasing the size of
underdrains to eight inches, increasing the slope of the
underdrains to one percent, and increasing the thickness
of the gravel layer to at least 12 inches can prevent
freezing and are recommended by EPA.
Proprietary, subsurface filter systems provide an alternative
to standard surface-based systems. Essentially,
these systems provide an insulated (i.e., subsurface)
location for pre-treated snowmelt to be filtered. The
insulating value of these systems adds to their appeal as
land conserving alternatives to ponds and surface infiltration
basins.
Cost
Filter costs vary according to the filtering media (sand,
peat, compost), land clearing, excavation, grading,
inlet and outlet structures, perforated pipes, encasing
structure (if used), and maintenance cost. Underground
structures may contribute significantly to the cost of a
filter. In general, filters are relatively costly and maintenance-intensive
BMPs.
Underground sand filters are generally considered to
be a high-cost option for water quality management. In
1994, the construction cost was estimated from $10,000
to $14,000 per impervious acre served, excluding real
estate, design, and contingency costs (Schueler, 1994).
LID Manual for Michigan – Chapter 7 Page 166