With stormwater pollutant removal requirements increasing nationwide, the demand for robust filtration has also increased.  Designing and specifying a properly functioning filter extends beyond specifying the correct product (media or membrane filter) and sizing correctly (flow or volume based).  These additional considerations are classified as design red flags.  Red flags include elements that can be overlooked and can significantly impact the effectiveness and longevity of a stormwater filtration system.

1. Hydraulic Impacts
   1. All filter products will require a driving or activation head.  This is a built-up of pressure that will force the water through the filter or provide a water level that will activate a filter cartridge.  Reviewing this head level is essential to reviewing the hydraulic grade line impacts of using a filter on site.
   2. In addition to the treatment flow and head build up, the peak storm events conveyed to the unit should be considered.  A ten-year storm, for example, will likely send more flow to the unit than a water quality event.  Considering whether this flow, and ones generated from other large events, will sufficiently bypass within the unit or if a separate bypass structure is required is essential to system design.

1. Atypical Treatment Requirements
   1. While filters may remove high levels of typical stormwater pollutants (TP, TSS, trash, etc.), non-standard pollutants (bacteria, metals, etc.) may require special filtration practices.  Some pollutants may be removed with a slight adjustment to standard stormwater filters (i.e., a custom media blend) while other pollutants are best suited with a more active filter – like a wastewater treatment plant.  Correct application of the filter design intent is an important consideration for an efficient filter design.

1. Limited Maintenance Access
   1. There are two main components to maintenance access.  The first involves access from the surface to the vault.  Placing a structure far away from access road ways will make future maintenance access difficult.  A larger filter structure should also consider multiple access points (covers, grates, hatches) to ease maintenance activity.
   2. The second component of maintenance access is the physical space within a filter structure.  Structures that offer restricted access will make maintenance difficult for a person to properly operate and replace any required media or cartridges.
2. Extraordinary Loading
   1. Most structures are suitable for loading from pedestrians and traffic commonly seen within parking area and drive areas.  A site with uncommon loading such as frequent firetruck or airplane loading or a roadway with high speeds, may provide concerns for the structural integrity of the filter.  A structural engineer should always be consulted to confirm the safety of the design.
   2. Buoyant forces may also be a concern.  In tidal areas or locations with high groundwater, there may be enough buoyant force to potentially float a standard filter.  In these locations a buoyancy check should be run and accommodations made to the structure’s design to prevent any potential buoyancy.
   3. Structure located beneath significant surface features such are light poles and retaining walls should also be examined for the impact of these unusual loads.

When designing you may encounter any of these or other challenges.  Contech’s team of stormwater design engineers have successfully engineered thousands of filter systems and can provide design assistance for your specific project needs.