Water Works: Making Large-Scale Green Stormwater Infrastructure Possible in Ultra-Urban Areas

Stormwater treatment in the United States faces significant challenges, most notably increased development, rising pollution and a lack of funding to address the issues. This was demonstrated in the American Sociey for Civil Engineers (ASCE) 2021 “Infrastructure Report Card” in which the Stormwater Sector received a “D” rating with more than 600,000 miles of impaired rivers and streams as well as more than 13 million acres of impaired lakes and ponds. In addition, a 2022 Water Environment Federation survey estimated a $6.2 billion annual funding gap for stormwater infrastructure. To throw another wrinkle into the equation, most jurisdictions and permits prioritize green stormwater infrastructure (GSI) due to the water-quality and ancillary benefits such as heat-island mitigation and the creation of pollinator habitat.

Cities, site owners and engineers clearly have a lot of work to do to protect our streams, rivers, lakes and oceans from pollution, and we need to do it efficiently with limited resources. Engineers have increasingly been turning to regional high-rate biofiltration (HRBF) systems to achieve this feat. This colum will briefly discuss the benefits of regional systems and HRBF as well as how to harness both to make large-scale GSI possible.

Efficiencies of Scale

Regional stormwater treatment systems utilize economy-of-scale benefits: the larger the systems get, the more efficient they become. Instead of hundreds of decentralized treatment systems spread throughout a large basin, a single, larger, regional treatment system can optimize inspection and maintenance requirements as well as provide more funding sources.

Inspection and maintenance costs are reduced when crews don’t have to visit dozens or hundreds of individual systems, with regional systems eliminating costly travel time and setting up/breaking down maintenance and safety equipment. Funding sources for capital and long-term maintenance costs can be spread across multiple stakeholders as larger drainage basins provide more partnership opportunities.

For example, some departments of transportation (DOTs) will partner with local cities on large projects in which the DOT will cover the capital cost of the treatment system, and the adjacent city will provide land space and long-term maintenance. Runoff from DOT and city drainage basins feeds into the regional system and creates a shared resource.

Regional systems also can be intentionally oversized to accommodate connections from future development. For example, a regional project in Minnesota built an oversized stormwater reuse system in which future development can tie into the existing system and have “built-in” stormwater compliance. Since the new development doesn’t have to pay capital costs for additional stormwater treatment infrastructure, those funds are redirected into a “connection fee” used as a dedicated long-term funding source to ensure operation and maintenance needs are met. These are but a few of the benefits of regional treatment systems.

How Does HRBF Fit?

HRBF is a GSI design approach that utilizes highly engineered media with vegetation to shrink the footprint and space needed for treatment. Conventional bioretention and biofiltration (BF) operate at hydraulic loading rates of 1 to 12 inches per hour. In contrast, HRBFs can operate at more than 100 inches per hour, often making HRBF up to 10 times smaller than conventional BF. This is a major benefit for regional systems in which there’s often a preference for GSI, but engineers pivot to gray infrastructure due to lack of available space.

HRBF makes green infrastructure solutions possible for even the largest of regional systems, especially in urban areas that have the highest levels of pollutants and the least amount of space available for treatment. The small footprint of HRBF systems has facilitated regional installations in the United States, Australia and New Zealand, with some regional HRBFs treating stormwater runoff and pollutants from more than 700 acres.

In addition to the high treatment rates and smaller footprint, regional HRBF typically has third-party-verified pollutant removal performance and strict media quality assurance and quality control (QAQC) procedures. The Washington State Department of Ecology TAPE (Technology Assessment Protocol – Ecology) program has verified several HRBF systems for total suspended solids, total phosphorus and dissolved metals treatment, providing proven pollutant-removal performance through the most-difficult field-monitoring test protocols.

Most HRBF systems also are provided by manufacturers, which allows for inhouse and robust QAQC procedures to ensure the HRBF media is the same as the media that was tested for pollutant-removal performance. This critical QAQC aspect sets HRBF apart from its conventional BF counterpart—conventional bioretention and biofiltration media notoriously lack QAQC oversight and, as a result, have well-documented and researched pollutant-export issues. HRBF systems help eliminate many of the failure modes that plague conventional BF by focusing on strict media procurement, storage, blending, testing, delivery and installation procedures. With many of the aforementioned challenges that stormwater treatment faces, ensuring our treatment media are of the highest quality possible is critical.

Last, but not least, one of the biggest economies-of-scale advantages of regional HRBF is maintenance: when the HRBF system is 1/10th the size of conventional BF, it can directly translate to having 1/10th the maintenance requirements. Both HRBF and conventional BF maintenance typically require the removal and replacement of the top layer of mulch, and the annual maintenance savings can significantly add up to make regional HRBF the most-efficient system from a lifecycle cost perspective.

Large-scale GSI often is challenging to implement; however, regional HRBF systems have been operating for more than a decade and are quickly becoming a preferred design approach to address the persistent challenges of treating large volumes of stormwater in highly developed urban areas. The small footprint of regional HRBF allows the use of green infrastructure in large urban areas with reduced lifecycle costs and proven, reliable water-quality performance.