It's hard to believe, but it has been almost twenty years since Filterra bioretention was introduced as a high-flow stormwater treatment system. For those that don't know, Filterra is an engineered high-flow biofiltration/bioretention system that operates similar to traditional bioretention but with high flow rates in a much smaller footprint.
Despite being introduced over two decades ago, engineers still have questions about Filterra, and the question we get asked the most is how we prevent the Filterra media from clogging. It's not surprising this is the most common question, as traditional bioretention systems have been known to clog. The three ways we prevent Filterra from clogging include a robust media QA/QC program, proper use of mulch, and proper system maintenance.
Robust media QA/QC program
Engineered media is the heart of bioretention system performance. The media must provide the desired infiltration rate, support vegetation, and provide water quality treatment. Therefore, preserving media integrity is of the utmost importance. A successful bioretention installation involves oversight not just onsite but also having a framework for transferring raw materials to a blended, commercially installed product. We employ rigorous standards of practice (SOPs) utilizing verification testing for qualifying, sourcing, verifying, producing, storing, and handling media. Unfortunately, many traditional bioretention facilities are built using media that has not been correctly sourced, mixed, and stored, which can impact hydraulic capacity and contribute to clogging.
Using the Proper Mulch
Often seen as just an aesthetic feature, mulch plays an essential role within the system. It provides physical filtration and acts as a pretreatment and anti-scour layer for the media. In addition, the mulch layer protects bioretention media from occlusion by capturing sediment loads and debris. Maintaining the mulch layer will indefinitely extend the bioretention soil media's service life in both conventional and high-rate systems. In addition, mulch provides benefits beyond pretreatment. Since the mulch is made from organic material, as it biodegrades, it releases new organic material into the media, which helps regenerate the media and sustain the biological processes taking place within the media. The mulch also provides water retention for the system's vegetation. We properly qualify mulch to protect the underlying treatment media, which reduces the maintenance burden and ensures sustained hydraulic and pollutant adsorption function.
Designed with Maintenance in Mind
Traditional systems are often not designed with maintenance in mind. In instances where maintenance does occur, for example, mulch may be replenished without removing degraded mulch and captured sediment. Long-term, this can impact hydraulic capacity, with greater impacts on total annual volume treatment since traditional systems are low flow. With Filterra, the mulch is the only component of the system that needs to be replaced regularly, usually one to two times per year. We also provide the first year of maintenance for every Filterra system to ensure proper system functionality.