Calculating credits for iron enhanced sand filter
Contents
Volume credits
Volume credits are not applicable to iron enhanced sand filters. Some volume credit may be given for evapotranspiration and evapotranspiration below underdrains for certain BMPs.
TSS credits
TSS credits are provided for filtration practices.
Phosphorus
The primary advantage of iron-enhanced filtration is that it removes dissolved constituents including phosphate, color and some metals by chemically binding. {{alert|Dry or wet pre-treatment is Required prior to media filter treatment for all filters, unless influent is relatively free of solids (pre-treatment volume equivalent to at least 25% of the computed Vwq is Recommended).|alert-danger]] 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.
Total phosphorus reduction
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
- R is the removal efficiency for each of the phosphorus constituents.
The removal efficiency for RPP 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 RTP reductions can be converted to RPP 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 | RTP (%) | RPP (%) |
|---|---|---|
| Stormwater pond | 501 | 65 |
| Sand filter | 451 | 85 |
1 Source (CWP and CSN, 2008)
Once iron is added to the media, RDP, 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, RDP can be represented by
\(R_{DP} = R_{FE}V_F / V_T\)
where
- RFe is the removal efficiency of an iron-enhanced sand filter (determined by experiments and monitoring to be approximately 60%);
- VF is the annual volume of runoff filtered; and
- VT is the total annual volume of runoff.
The ratio of VF to VT 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.
- Step 1: Calculate the treatment volume of the BMP based on design parameters. The treatment volume represents the amount of water that the BMP is capable of holding at any one time.
- Step 2: Use modeling results (i.e., from P8 modeling) to convert the treatment volume into a percent annual runoff volume filtered.
Treatment volume calculation
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.
Iron enhanced sand filter basin
The treatment volume capacity of the iron enhanced sand filter basin in given by
\(V_T = D(A_S+A_M) / 2\)
where
- VT is the treatment volume capacity of the sand filter (cubic feet);
- AS is the surface area of the sand filter at the basin overflow (square feet);
- AM is the surface area of filter media (square feet); and
- D is the depth of water between overflow outlet structure and the sand filter media (feet).
Iron enhanced sand filter bench in wet ponds
The treatment volume capacity of the iron enhanced sand filter bench in given by
\(V_T = D_O(A_N+A_O) / 2\)
where
- VT is the treatment volume capacity of the filter bench (cubic feet);
- AN is the surface area of the pond at the normal water level (square feet);
- AO is the surface area of the pond at overflow outlet structure elevation (square feet); and
- DO is the depth of water between the outflow structure elevation and the normal water level (feet).
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.