Media Surface Area
Filtration flow rates are typically expressed as a surface area specific operating rate such as gallons per minute per square foot (gpm/ft2) of surface area. Lower specific operating rates translate to better performance and longer maintenance cycles. Specific operating rates higher than 2 gpm/ft2 of media surface area negatively impact performance and longevity.
Surface vs. Bed Filtration
When assessing filtration systems, it is important to consider whether filtration occurs primarily at the media surface or throughout a bed of media. All else equal, bed filters are longer lasting, since pollutants are captured and stored throughout the bed, as opposed to predominantly on the media surface. Bed filters capture more mass of pollutants per unit area of filter surface. Surface filters, such as membranes, are prone to rapid failure as pollutants occlude the media surface.
Media Hydraulic Conductivity and Flow Control
Filtration media is able to pass more flow per unit of media when it is new versus when it has been in operation for a while. With time, pollutants accumulate in the media bed and reduce its hydraulic capacity. It is critical that filtration devices are designed with excess hydraulic capacity to account for this loss. Also, finer media gradations remove finer particles, but have lower hydraulic capacity and occlude more rapidly. High performance and superior longevity can be achieved by controlling the flow through a more coarse media bed.
Laboratory testing provides a means to generate hydraulic and basic performance data, but should be complimented with long-term field data. Laboratory performance trials should be executed with a fine sediment gradation such as Sil-Co-Sil 106 which has a median particle size of 22 microns. Testing with coarser gradations is not likely to be representative of field conditions.
Long-term field evaluations should be conducted on all filtration devices. Field studies should comply with the Technology Acceptance Reciprocity Partnership (TARP), Environmental Technology Verification (ETV) or the Technology Assessment Protocol – Ecology (TAPE) protocols. Testing should be overseen by a reputable third party to be considered valid.
It is essential that loading trials be conducted to evaluate the longevity of a media filter. These trials must be executed with “real” stormwater solids and not silica particles. Reliance on silica particles to assess longevity grossly overstates the loading capacity of the media and the results of such trials should not be relied on. Knowing how much mass a media filter can capture before failure allows it to be sized for a desired maintenance interval by estimating the pollutant load that will be delivered to the filter.