Stormwater treatment regulations are generally focused on total suspended solids and particulate-bound pollutants. With widespread research demonstrating that dissolved pollutants contribute to water body impairments, many stormwater filtration medias incorporate active amendments providing various sorptive mechanisms to remove dissolved pollutants. Commonly used active amendments in bioretention and cartridge filters include iron, alumina, biochar, GAC and water treatment residuals. While many active medias enhance dissolved pollutant removal over inert medias, the reduction in sorption capacity of the active component and media replacement frequency is not well understood. This research examined new and spent media from a phosphorus filtration project in the Capitol Region Watershed District in Minnesota, USA. Spent media comprised of perlite with activated alumina was collected after 23 months of operation from a 53-cartridge underground filtration system. New media was collected and preserved from the same initial batch. Physical characteristics were evaluated including particle size distribution, porosity, bulk density, moisture, pH, and surface area. Elemental and adsorption characteristics were evaluated via scanning electron microscopy, x-ray fluorescence (XRF), fertility, and isotherm tests. Results from comparative analysis between the new and spent media will be discussed for determining when replacement is required. Additional discussion will include methods for active media evaluation, the practicality of site owners and municipalities to conduct these evaluations, and insight into the inspection and maintenance implications as it relates to design. As maintenance enforcement becomes more stringent, a better understanding of maintenance and replacement of active medias is needed to protect impaired water bodies.
.jpg "Craig Fairbaugh, A.M. ASCE (he/him/his) photo")