Recommended pollutant removal efficiencies, in percent, for iron enhanced sand filters. Note that the values given for metals and hydrocarbons are the same as those for non-enhanced sand filters. Though iron enhanced systems may have increased removal efficiencies for these chemicals, there is insufficient data in the literature. Sources. TSS=total suspended solids; TP=total phosphorus; PP=particulate phosphorus; DP=dissolved phosphorus; TN=total nitrogen | |||||||
TSS | TP | PP | DP | TN | Metals | Bacteria | Hydrocarbons |
85 | 77 | 91 | 60 | 35 | 50 | 80 | 80 |
Credit refers to the quantity of stormwater or pollutant reduction achieved either by an individual Best Management Practice (BMP) or cumulatively with multiple BMPs. Stormwater credits are a tool for local stormwater authorities who are interested in
This page provides a discussion of how enhanced sand filter practices can achieve stormwater credits.
For a detailed overview of iron enhanced sand filters, see the Overview section for iron enhanced sand filters.
Volume credits are not applicable to iron enhanced sand filters.
TSS credits for enhanced sand filters are the same as those for sand filters and are described on the page discussing credits for sand filters.
The primary advantage of iron-enhanced filtration is that it removes dissolved constituents including phosphate, color and some metals by chemical binding.
Iron-enhanced sand filters can be used as a retrofit to existing BMPs or in new construction. If the iron-filtration bed remains oxygenated, iron will be retained in the bed. Iron-filtration beds that are persistently deoxygenated risk iron loss or migration and clogging.
Annual total phosphorus (TP) reductions were divided into two components: particulate phosphorus (PP) and dissolved phosphorus (DP). Filtering systems were assumed to provide zero DP reduction without the incorporation of iron in the filter media. It was also assumed that of TP, 55 percent is PP and 45 percent is DP. Using these assumptions the TP removal can be described by
\(R_{TP} = 0.55R_{PP} + 0.45R_{DP}\)
where
The removal efficiency for R_{PP} is based on the annual TP reductions provided by each of the filtering BMPs without the inclusion of iron in the filter media. It was assumed that all removal of phosphorus in these systems is provided through the removal of particulate phosphorus. Therefore, the R_{TP} reductions can be converted to R_{PP} using the above equation by setting RDP to 0.
Total P and particulate P removal from BMPs without iron in the filter media.
Link to this table
BMPs without iron | R_{TP} (%) | R_{PP} (%) |
---|---|---|
Stormwater pond | 50^{1} | 65 |
Sand filter | 45^{1} | 85 |
^{1} Source (CWP and CSN, 2008)
Once iron is added to the media, R_{DP}, can be broken down into the product of the DP removal effectiveness of iron-enhanced sand and the fraction of the annual runoff that passes through the sand filter. This assumes that the volume of annual runoff that bypasses the sand filter and the BMP through an overflow structure receives no treatment of the dissolved portion of the phosphorus loading. Thus, R_{DP} can be represented by
\(R_{DP} = R_{FE}V_F / V_T\)
where
The ratio of V_{F} to V_{T} will be called F, which is the fraction of the total annual runoff volume that is filtered by the sand filter. Therefore, the equation for total phosphorus removal can be rewritten as
\(R_{TP} = 0.55R_{PP} + 0.27F\)
To calculate the total phosphorus removal efficiency for each BMP, F, or the fraction of annual runoff that passes through the sand filter, must be calculated. This calculation is made in a two-step process.
The treatment volumes of the various BMPs are defined as the amount of water that can be stored by the BMP above the filter media at any given time. All of this water is able to pass through and be treated by the filter media. Iron-enhanced sand filters should be designed to drain in 48 hours, so the filter media should be designed to discharge the entire treatment volume below the outflow in no more than 48 hours (24 hours if the BMP drains into a trout stream). The limiting factor in the discharge rate of the sand filter can be the saturated hydraulic conductivity of the sand media, the surface area of the sand filter, or the capacity of the underlying underdrain. The designer must consider these factors when determining the volume of potential treatment. The treatment volume calculations for each of the three filter BMPs are described below.
The treatment volume capacity of the iron enhanced sand filter basin in given by
\(V_T = D(A_S+A_M) / 2\)
where
The treatment volume capacity of the iron enhanced sand filter bench in given by
\(V_T = D_O(A_N+A_O) / 2\)
where
The designer must make sure that the final treatment volume is able to drain through the filter media and out the underdrain within the required drawdown time. If this criterion is not met, the designer should redesign the system to meet the requirement.
Once the treatment volume is calculated for the BMP, this value is converted to the fraction of annual runoff volume treated (F) using the treatment volume capacity (V_{T}) of the iron-enhanced sand filter, tributary watershed size, soils and imperviousness of the watershed. Below is a description of the methods employed to determine the fraction of runoff treated.
The P8 model was used to calculate runoff from several hypothetical 10‑acre development scenarios with varying levels of imperviousness and different soil types. Twenty hypothetical watersheds were included in the P8 modeling analysis, including type A, B, C and D soils with 10, 20, 50, 70, or 90 percent imperviousness. Through the use of look up tables created from P8 modeling results, treatment volumes were converted into a percent annual volume filtered.
Watershed runoff volumes from pervious areas were computed in P8 using the SCS Curve Number method. Pervious curve numbers were selected for each hypothetical watershed based on soil type and an assumption that the pervious areas within the hypothetical development would be open space areas in fair to good condition. References on SCS curve numbers provide a range of curve numbers that would apply to pervious areas in fair to good condition. Pervious curve numbers of 39, 65, 74, and 80 were used for hydrologic soil groups A, B, C, and D, respectively.
Depression storage represents the initial loss caused by such things as surface ponding, surface wetting, and interception. As previously discussed, the P8 model utilizes the SCS Curve Number method to estimate runoff from pervious areas. For impervious areas, runoff begins once the cumulative storm rainfall exceeds the specified impervious depression storage, with the runoff rate equal to the rainfall intensity. An impervious depression storage value of 0.06 inches was used for the P8 simulation.
The P8 model requires hourly precipitation and daily temperature data; long-term data was used so that watersheds and BMPs can be evaluated for varying hydrologic conditions. The hourly precipitation and average daily temperature data were obtained from the National Weather Service site at the Minneapolis-St. Paul International Airport. The simulation period used for the P8 analysis was January 1, 1955 through December 31, 2004 (50 years).
For the P8 analysis, the 50-year hourly dataset was modified to exclude the July 23-24, 1987 “super storm” event, in which 10 inches of rainfall fell in 6 hours. This storm event was excluded because of its extreme nature and the resulting skew on the pollutant loading and removal predictions. Excluding the July 23-24, 1987 “super storm”, the average annual precipitation throughout the 50-year period used for the P8 modeling was 27.7 inches.
Infiltration basins were used to simulate the amount of water treated by each BMP. Infiltration basins were sized for a range of treatment volumes with depths of 1.5 feet and infiltration rates of 5.3 inches per hour to simulate flow through a sand media as reported by Rawls et al. 1998. Infiltration basins were modeled using each of the drainage characteristic combinations of soils type and impervious surfaces. The final results from the modeling were total runoff volume reductions based on the size of the basin and infiltration rate. This runoff reduction was converted to percent annual reduction based on the total inflow into the system and the total overflow. The percent annual reduction represents the annual percent runoff volume that passes through the BMP and is treated. Modeled results are shown in the figures to the right. The figures can be used to determine the annual runoff volume filters (F) using the BMPs treatment volume (V_{T}):total drainage area ratio. The calculated runoff volume filtered (F) can then be plugged into the equation to calculate RTP. Results are divided by watershed soil type and percent impervious surfaces.
A 2 acre site with 50 percent impervious and hydrologic soil group B soils is routed to an iron-enhanced sand filter basin. The depth between the overflow structure and the sand filter bed (D) is 1.5 feet. The area of the sand filter at the overflow structure (A_{S}) is 1,000 square feet and the surface area of the sand filter (A_{M}) is 800 square feet. V_{T} is calculated to be 1350 cubic feet or 0.031 acre-feet.
\( V_T = 1.5(1000 + 800) / 2 = 1350\)
The ratio between the BMP treatment volume (V_{T}) and the drainage area then becomes 0.0155 feet (1350 ft^{3}/2 acres * 43560 ft^{2}/1 acre). This value can be used for a watershed with HSG B soils and 50 percent impervious to get a percent of annual runoff volume filtered (F) of 88 percent. Using this value in the following equation
\(R_{TP} = 0.55R_{PP} + 0.27F = (0.55)(0.85) + (0.27)(0.88) = 0.71\)
results in an annual TP removal rate (R_{TP}) of 71 percent.
This page was last modified on 13 August 2019, at 14:02.
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