Volume 2-05, Chapter 3 - City of Wichita

Section 3.3.5 - Organic Filter
• Organic filters can utilize a variety of organic materials as the filtering media. Two typical
media bed configurations are the peat/sand filter and compost filter (see Figure 3-52). The
peat filter includes an 18-inch 50/50 peat/sand mix over a 6-inch sand layer and can be
optionally by 3 inches of topsoil and vegetation for aesthetic purposes. The compost filter
has an 18-inch compost layer. Both variants utilize a gravel underdrain system.
• The type of peat used in a peat/sand filter is critically important. Fibric peat in which
undecomposed fibrous organic material is readily identifiable is the preferred type. Hemic
peat containing more decomposed material may also be used. Sapric peat made up of
largely decomposed matter should not be used in an organic filter.
• Typically, organic filters are designed as "off-line" systems, meaning that the WQ v is
diverted to the filter facility through the use of a flow diversion structure and flow splitter.
Stormwater flows greater than the WQ v are diverted to other controls or downstream using
a diversion structure or flow splitter.
• Consult the design criteria for the surface sand filter (see Section 3.2.8) for the organic
filter siting requirements, and sizing/design steps. The following permeability values may
be used: sand = 3.5 ft/day, peat = 2.0 ft/day, and leaf compost = 8.5 ft/day. A porosity of
0.4 may be assumed.
• The local jurisdiction may require that the facility be placed in a reserve and/or
establishment of a drainage easement the facility, which is accessible from a public road
or other accessible easement. When required, the drainage easement should be at least
20 feet wide, provide a minimum traversable width of 15 feet, have a maximum slope of no
more than 10%, and be appropriately stabilized to withstand maintenance equipment and
vehicles.
• A minimum separation distance of 5 feet is required between the bottom of the filter and
the elevation of the historical high water table for filter without underdrains, 2 feet for filters
with underdrains.
3.3.5.4 Inspection and Maintenance Requirements
Regular inspection and maintenance is critical to the effective operation of stormwater
management facilities. An operation and maintenance plan is required and shall include:
1. “Covenants for Permanent Maintenance of Stormwater Management Facilities” (also
called the “Maintenance Covenants”). An example covenants document can be found in
Volume 3.
2. “Inspection Checklist and Maintenance Guidance” for each type of stormwater facility that
is located on the property. Templates for each stormwater management facility can be
found in Volume 3 of this manual. These templates can be amended slightly for use in
more customized O&M plans.
3. As-built drawings must accurately identify the location of the facility, and also clearly
identify reserves and access easements.
Volume 2, Technical Guidance Page 3 - 177