A sand filter does not reduce stormwater volume. While this BMP does not provide stormwater volume reduction toward the performance goal or on an annual basis, it does provide annual pollutant load reductions. Version 3 of the calculator provides an option to add an amendment to attenuate dissolved phosphorus.
For the sand filter BMP, BMP watershed areas is the only required design parameter.
The required treatment volume, or the volume of stormwater runoff delivered to the BMP, equals the performance goal (1.1 inches or user-specified performance goal) times the impervious area draining to the BMP. This stormwater is delivered to the BMP instantaneously.
This BMP does not provide volume reduction towards the performance goal.
Pollutant load reductions are calculated on an annual basis. A sand filter does not provide annual volume reduction. Fixed removal rates of 85 percent total suspended solids (TSS) and 85 percent particulate phosphorus are applied to stormwater that travels through the sand filter. If the user answers "No" to the question regarding adding amendments to attenuate phosphorus, a removal rate of 0 percent dissolved phosphorus is applied to stormwater that travels through the sand filter. If the user answers "Yes" to the question regarding adding amendments to attenuate phosphorus, a removal rate of 40 percent dissolved phosphorus is applied to stormwater that travels through the sand filter.
NOTE: The user can modify event mean concentrations (EMCs) on the Site Information tab in the calculator. Default concentrations are 54.5 milligrams per liter for total suspended solids (TSS) and 0.3 milligrams per liter for total phosphorus (particulate plus dissolved). The calculator will notify the user if the default is changed. Changing the default EMC will result in changes to the total pounds of pollutant reduced.
A sand filter can be routed to any other BMP, except for a green roof and a swale side slope or any BMP in a stormwater treatment sequence that would cause stormwater to be rerouted back to the sand filter already in the sequence. All BMPs can be routed to a sand filter except for a swale side slope.
The following general assumption applies in calculating the credits for a sand filter. If this assumption is not followed, the pollutant reduction credits cannot be applied.
On the watershed tab, the following warning is given:
“The calculator does not require sizing inputs for non-volume reducing BMPs. This BMP should be sized according to the guidelines in the MN stormwater manual.” Design and construction criteria can be found at this link.
A sand filter is to be constructed in a watershed that contains a 1.4 acre parking lot surrounded by 0.8 acres of pervious area (Turf area). All of the runoff from the watershed will be treated by the sand filter. The soils across the area have a unified soils classification of SM (HSG type B soil). The following steps detail how this system would be set up in the MIDS calculator.
Step 1: Determine the watershed characteristics of your entire site. For this example we have a 2.2 acre site with 1.4 acres of impervious area and 0.8 acres of pervious turf area in type B soils. The pervious area includes the area of the sand filter.
Step 2: Fill in the site specific information into the Site Information tab. This includes entering a Zip Code (55414 for this example) and the watershed information from Step 1. Zip code and impervious area must be filled in or an error message will be generated. Other fields on this screen are optional.
Step 3: Go to the Schematic tab and drag and drop the Sand Filter icon into the Schematic Window.
Step 4: Open the BMP properties for the Sand filter by right clicking on the Sand filter icon and selecting Edit BMP properties, or by double clicking on the Sand filter icon.
Step 5: If help is needed, click on the Minnesota Stormwater Manual Wiki link or the Help button to review input parameter specifications and calculation specific to the Sand filter BMP.
Step 6: Determine the watershed characteristic for the sand filter. For this example the entire site is draining to the sand filter. The watershed parameters therefore include a 2.2 acre site with 1.4 acres of impervious area and 0.8 acres of pervious turf area in type B soils. There is no routing for this BMP. Fill in the BMP specific watershed information (1.4 acres on impervious cover and 0.8 acres of Managed turf in B soils).
Step 7: Go to the BMP Parameters tab and answer the question "Have amendments to attenuate phosphorus been incorporated?". For this example, answer "No". Design parameters are not required for the sand filter BMP.
Step 8: Click on BMP Summary tab to view results for this BMP.
Step 9: Click on the OK button to exit the BMP properties window
Step 10: Click on the Results tab to see overall results for the site.
In Step 7 above, answer "Yes", then complete Steps 8, 9, and 10. Note the removal of dissolved phosphorus and the associated increased removal of total phosphorus.