High-rate biofiltration differs from traditional bioretention in that it has been optimized for high volume/flow treatment and high pollutant removal. Traditional bioretention footprints with typical design flow rates of 1-12 inches per hour can occupy up to 10% of the contributing impervious drainage area. In addition, the individual components of traditional bioretention (mulch, soil, underdrain and plants) are designed by the engineer and sourced and installed by the contractor.

In contrast, high-rate biofiltration systems contain an engineered high-flow biofiltration media with flow rates up to 175” per hour (depending on the jurisdiction). In addition, these systems are packaged and delivered with all the components necessary for system performance: mulch, media, vegetation and underdrain.

While high-rate biofiltration offers benefits such as simple and easy maintenance, regulatory compliance, and aesthetic appeal, the two most significant benefits are a reduced footprint and consistent, reliable performance.

#1 Reduced Footprint

Due to the high media flow rates (100-175 inches per hour), high-rate biofiltration has a much smaller footprint than traditional biofiltration, typically less than 0.5% of the impervious drainage area (see Table 1). The smaller footprint results in more usable land, which can be advantageous on sites where the cost of land is expensive or on tight, highly developed sites such as urban development projects, commercial parking lots, residential streets and streetscapes. The small footprint also reduces installation and lifecycle costs vs. traditional bioretention, as there’s less square footage to maintain. However, it’s important to note that a smaller bioretention system footprint doesn’t necessarily mean the total amount of green space on a site will be reduced. For example, the land area saved by specifying high-rate biofiltration can be used for conventional landscaping or for dedicated recreation or habitat areas where stormwater pollutant accumulation and prolonged inundation during storm events could be problematic.

Table 1. Benefits of High-Rate Biofiltration

#2 Consistent, Reliable Performance

The second most important benefit of high-rate biofiltration systems is that they are manufactured in a controlled environment, with all the components sourced by the manufacturer to ensure the final media product performs per design specifications. This differs from traditional bioretention, where the contractor sources and installs each component separately, and the media is blended onsite. Media blending methods vary, most often depending on the amount of media to be mixed and the equipment available. Like two different cooks can use the same ingredients to create two different meals, different blending methods can have different final results, even with the same components. To reliably achieve consistent media properties, specific standards of practice for media production must be followed in a controlled environment.