Agenda - Council - 06/11/2024 - Laserfiche WebLink

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BMP Descriptions Wet retention ponds, although very effective in treating stormwater for suspended sediment and nutrients bound to sediment, have shown a limited ability at retaining dissolved species of nutrients. This is most notable for phosphorus, which easily adsorbs to sediment when in particulate form. Median values for pollutant removal percentage by wet retention ponds are 84% for TSS and 50% for TP (MN Stormwater Manual). For the case of phosphorus, dissolved species typically constitute 40-50% of TP in urban stream systems, but only 34% (median efficiency; Weiss et al., 2005) of dissolved phosphorus is treated by the pond. Thus, a majority of the phosphorus escaping wet retention ponds is in dissolved form. This has important effects downstream as dissolved phosphorus is a readily available nutrient for algal uptake in waterbodies and can be a main cause for nutrient eutrophication. To address this deficiency, researchers at the University of Minnesota developed a method to augment phosphorus retention within a sand filter. They've named this technology the "Iron Enhanced Sand Filter (IESF; Figure 8)". Locally, this practice has also gone by the name "Minnesota Filter." IESFs rely on the properties of iron to bind dissolved phosphorus as it passes through an iron rich medium. Depending on topographic characteristics of the installation sites, IESFs can rely on gravitational flow and natural water level fluctuation, or water pumping to hydrate the IESF. IESFs must be designed to prevent anoxic conditions in the filter medium because such conditions will release the bound phosphorus. Because IESFs are intended to remove dissolved phosphorus and not organic phosphorus, they are typically constructed just downstream of stormwater ponds, minimizing the amount of suspended solids that could compromise their efficacy and drastically increase maintenance. As an alternative to an IESF, a ferric -chloride injection system could be installed to bind dissolved phosphorus into a flocculent, which would settle in the bottom of the new pond. Figure 8 shows an IESF that is I��,,��s� T� '.La�:l installed at an elevation slightly Trench �� , ter aurae)Grate� above the normal water level � of the and so that following a p 8 I��'�I �t�r � � ��f �-v�l ° • storm event the increase in �_. 6u r I &U& El"e ation ontml Weir depth of the pond would be _. first diverted to the IESF. The \1 I filter would have drain the installed along the base of the — 4 Natural trench and would outletDrain P� tile downstream of the current Iron Enhanced pond outlet. Large storm Sand Filter Drain W events that overwhelm the Figure 8: Iron Enhanced Sand Filter Concept (Erickson & Gulliver, 2010) IESF's capacity would exit the pond via the existing outlet. Benefits for stormwater ponds were modeled utilizing WinSLAMM. After selecting an optimal pond configuration in terms of cost -benefit, or by using the existing pond configuration if no updates are needed, modeling for an IESF was also completed in WinSLAMM. WinSLAMM is able to calculate flow through constructed features such as rain gardens with underdrains, soil amendments, and controlled overflow elevations. An IESF works much the same way. Storm event based discharge volumes and phosphorus concentrations estimated by WinSLAMM at the pond outlet were entered into WinSLAMM City of Ramsey Stormwater Retrofit Analysis