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===Total phosphorus reduction=== | ===Total phosphorus reduction=== | ||
− | Annual total phosphorus (TP) reductions were divided into two components: particulate phosphorus (PP) and dissolved phosphorus (DP). | + | 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 |
<math>R_{TP} = 0.55R_{PP} + 0.45R_{DP}</math> | <math>R_{TP} = 0.55R_{PP} + 0.45R_{DP}</math> |
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 are provided for filtration practices.
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.
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 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
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.