Media-based stormwater filtration systems are much more flexible in their design than their rigid, precast concrete structure would suggest. A brief look inside that durable housing identifies several design options these systems offer to further reinforce their flexibility.
Flexibility to treat site-specific pollutants
Just as each site differs in the way stormwater filtration practices are designed and deployed, state and local jurisdictions enforce a variety of regulations for pollutant removal. To address this, media filtration systems can be customized using various filter media to target site-specific pollutants. Specific media targeting sediment (TSS or total suspended solids), nutrients such as total phosphorus, soluble metals, oil and grease, or organics such as herbicides and pesticides are common in the industry. A combination of media is often used to further maximize pollutant removal. Media type and gradation differ by manufacturer, and each has unique characteristics with respect to removal performance and capacity. In other words, all media is not the same. Additionally, peer-reviewed, long-term field performance data collected in accordance with a nationally recognized protocol such as Washington’s TAPE protocol should be considered to help make an informed decision as to the best option for the site in question.
Flexibility to reduce system size and ownership costs
Some manufacturers offer modules with differing media volumes, giving engineers design flexibility, which can reduce long-term ownership costs of the system. For example, larger modules correlate to sites with access to more available driving head. This increases the treatment rate per module and creates a reduction in the number of modules and the required footprint of the device. Both reductions mean lower installation and per event maintenance costs. For sites with hydraulic constraints, smaller modules can be used to accommodate the lower available driving head.
Flexibility to accommodate flows, project footprints, and hydraulics
The structure that houses the media can be constructed in a variety of configurations to accommodate a wide range of treatment goals, peak flows, influent delivery methods, project footprints, and hydraulic conditions. Standard structural configurations include catch basin, manhole, vault, curb inlet, and cast-in-place; the latter of which allows the highest degree of flexibility for installations within buildings or other areas where precast cannot be accommodated. Systems can be combined with upstream storage and designed to meet volume-based regulations, treating and releasing within a required drain downtime. Some manufacturers make systems that can incorporate a peak event bypass, eliminating the need for external bypass or junction structures.
Flexible maintenance operations
Some manufacturer’s media filter systems deploy filter modules that are of reasonable weight and size to allow maintenance operators to maneuver without specialized lifting equipment or an apparatus cluttering the structure’s interior space. Similarly, when designed with typical manhole covers or hatches, entry can be safely accomplished using typical access devices. A media filter that dewaters between events and contains modules that are reusable, where only the spent media is discarded, reduces disposal volumes and allows for a greater variety of maintenance equipment to be used.
In summary, media filters can be designed and customized to meet the unique needs of the site, the regulators, and the end-user. Specifying a device with these capabilities will ensure maximum flexibility afforded to the remaining aspects of the site design.