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The levels of phosphorus in media can also be evaluated and discussed as Phosphorus Saturation Index (PSI). PSI is the proportion of extractable phosphorus to extractable aluminum and iron in the soil sample.
The levels of phosphorus in media can also be evaluated and discussed as Phosphorus Saturation Index (PSI). PSI is the proportion of extractable phosphorus to extractable aluminum and iron in the soil sample.
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==Design and Maintenance considerations==
The mechanisms for phosphorus removal are sedimentation, [[filtration]], adsorption, [[precipitation]], and [[Plants#Plant Characteristics|plant uptake]]. Particulate phosphorus can be removed through sedimentation and filtration and is often trapped among other solids within a shallow depth at the media surface (Hsieh et al. 2007<ref name="example3">Hsieh, C.-H., Davis, A. P., and Needelman, B. A. (2007).“Bioretentioncolumn studies of phosphorus removal from urban stormwater runoff.” Water Environ. Res., 79(2), 177–184.</ref>). Dissolved phosphorus is removed deeper in the media as it requires higher retention time (Hsieh et al. 2007<ref name="example3" />). Hunt et al. (2006)<ref name="example4">Hunt, W. F., Jarrett, A. R., Smith, J. T., and Sharkey, L. J. (2006).“Evalu-ating bioretention hydrology and nutrient removal at three field sites in North Carolina.” J. Irrig. Drain. Eng., 132(6), 600–608.</ref>, suggested a minimum depth of 0.6m and recommends 0.9m and infiltration rate of 0.007- 0.028 mm/s (1-4 in/h) for targeted removal of dissolved phosphorus. Removal of dissolved phosphorus relies heavily on the specifications of the LID media, it’s phosphorus content, type and percentage of organic matter, its potential hydrogen (pH), and temperature.
Media with high phosphorus content provides additional support for plant growth, however it will harm the phosphorus removal capability of the feature (Hunt et al. 2006)<ref name="example4" />). Similarly high percentage of organic matter in the media can increase the phosphorus content after degradation and lead to leaching of phosphorus (Clark and Pitt 2009<ref>Clark, S. E., and Pitt, R. (2009).“Storm-water filter media pollutantretention under aerobic versus anaerobic conditions.” J. Environ.Eng., 135(5), 367–371.</ref>). Different types of organic matter have various degrees of phosphorus leaching. To ensure that an LID can provide phosphorus removal, the phosphorus content and percentage of organic matter must be carefully selected and implemented during construction. For proper ranges of these values refer to the [[Bioretention: Filter media|bioretention media page]]. The suggested ranges should be met during the design phase and inspected before assumption of the feature.
==Enhanced Phosphorous Removal==
Particulate phosphorus is removed to a good extent in LIDs due to the sedimentation and filtration mechanisms offered by these features. To further improve the removal of total phosphorus, the removal of dissolved phosphorus is targeted. As explained in the previous section, adsorption is the main removal mechanism for dissolved phosphorus and aluminum and iron are the main sorptive elements. Therefore, additions of [[Bold & Gold
|aluminum and iron to the media]] can enhance phosphorus removal. Examples of such additives are [[Iron filings (ZVI)|iron filings]] or zero valent iron, iron-enriched or [[red sand|“red” sand]], and [[water treatment residuals]]. Other types of additives that can also provide sorption capacity are [[biochar]], [[bold & gold]], [[smart sponge]], and [[sorbtive media]].
==References==
[[Category:Phosphorus]]
[[Category:Phosphorus]]
[[Category: Water quality]]
[[Category: Water quality]]