There are several factors to consider when selecting the most appropriate stormwater filtration solution for a specific site.  Several of these factors easily come to mind, such as: state or jurisdiction regulations, hydraulic grade line limitations, footprint constraints, and other site restrictions.  One variable that is often overlooked when reviewing a site is the methodology used to determine the specific quantity of filter cartridges or filter media.  Filter designs fall under one of three methods: flow-based design, volume-based design, or mass-load design.  All three options offer different benefits and limitations.

Flow-Based Sizing

To size a filter using the flow-based sizing method, a treatment flow rate needs to be determined for the contributing drainage area.  A treatment flow rate can be determined using the modified rational method, NRCS TR-55, hydrographs, etc. that meet the local regulations.  Once a treatment flow rate has been established, the size of the filter device will be determined by its hydraulic capacity while removing targeted pollutants.  The resulting stormwater treatment system will be a stand-alone filter system.  For this reason, the flow-based method provides a solution with the smallest footprint.  The downside to this approach relates to a required treatment flow rate.  For geographies with high water quality storm events, the treatment flow rate and runoff will be high. This will require increasing the filter surface area to adequately treat the storm event.  For locations with smaller water quality events, the required filter surface area corresponding to the treatment flow rate may be too small to ensure a reasonable long-term maintenance cycle.  Depending upon the flow-based sizing results, it may be beneficial to explore additional other sizing methods.

Volume-Based Sizing

The second sizing method listed above is a volume-based approach.  The volume-based approach incorporates not only a filter device but also an upstream detention component.  Again, based upon local regulations, a runoff treatment volume can be ascertained from the treatment storm event.  The volume-based design detains the treatment volume within upstream detention.  For this design, the detention system could be a pond, an underground metal pipe, a concrete solution, or other design.  Using a known treatment volume and target drain down time, a filter system is sized to treat the full treatment volume at an attenuated flow rate.  Due to the smaller treatment flow rate, the filter component of the system will be smaller, resulting in reduced filter capital costs.  Drawbacks to the volume-based design include an overall larger footprint to accommodate the detention system and a potentially shorter maintenance cycle as pollutants are routed through the smaller filter device. 

Mass-Based Sizing

The final sizing method, mass-based, is a combination of the two methods described above.  This means the design will require detention of the treatment volume as with the volume design.  The mass-based design method compares the detention release rate with an anticipated mass load from the contributing drainage area and is dependent upon criteria such as annual rainfall, site characteristics, and event mean concentration.  Based upon laboratory and field testing, filters have an associated pollutant capture capacity.  Therefore, a filter size can be determined to meet the mass load criteria for a desired maintenance interval.  The mass-based design method compares the filter size required to meet the mass load requirement with the filter size to treat the release rate from detention.  The larger of these two values will be selected to properly treat the drainage area.  The mass-based design ensures the most efficient filter design is provided when considering performance and longevity. 

As with all facets of land development there is no one single solution that will work best on all projects.  Based upon site conditions, maintenance requirements and design limitations, certain sizing methods provide more value than others.  Careful consideration and proper application of all three methods ensures the optimal design.