The Value of High-Rate Biofilters for Urban Stormwater Retrofits

Introduction

In the nearly 35 years since the National Pollutant Discharge Elimination System (NPDES) Phase I Stormwater Rule was implemented, there has been an incredible amount of progress in the field of stormwater management. Advances in regulations and stormwater management solutions have enhanced the effectiveness of stormwater control measures (SCMs). These developments have established robust standards and best practices for managing stormwater runoff from new and redevelopment sites, ultimately reducing the quantity of pollutants and excess runoff entering receiving waters.

However, the new and redevelopment centric approach to stormwater management has left a major source of stormwater pollution largely unaddressed. Prior to the adoption of modern stormwater standards, millions of acres of impervious surfaces were created that, for the most part, still lack effective SCMs. It’s difficult to know exactly how much existing impervious area lacks modern stormwater management, but research published in 2009 by Theobold et al. estimated the amount of existing impervious surface in the United States in the year 2000 was greater than 20 million acres (83,749 km2)1.

Prioritizing Stormwater Retrofits

Not surprisingly, large amounts of impervious surfaces exist in older cities and surrounding urban areas that were developed prior to the adoption of modern stormwater management standards. Many of these older urban areas also have impaired watersheds that fall short of meeting their designated uses and are subject to a variety of Total Maximum Daily Loads (TMDLs) intended to improve water quality. While redevelopment has helped improve stormwater management on many sites, the pace of redevelopment has not been sufficient to address the problem outright. To restore impaired urban watersheds, stormwater retrofits will need to be prioritized.

Stormwater retrofitting in urban environments refers to the process of upgrading or modifying existing stormwater management systems to improve their performance and adapt to current environmental standards and regulations. This involves integrating new technologies and infrastructure into the already developed urban landscape to address issues such as flooding, water quality and inadequate drainage.

The goal of stormwater retrofitting is to enhance the capacity, efficiency and sustainability of stormwater systems while minimizing disruptions to the existing urban infrastructure and daily life. Unlike stormwater redevelopment standards that are triggered when a property is going through a major repurposing, retrofits occur on existing sites to incorporate SCMs to the extent feasible, even if the property is otherwise to remain unchanged. For many urban watersheds, it’s the only viable way to meaningfully reduce the impact from existing sites that are contributing to impairments.

The implementation of TMDLs in older urban watersheds is beginning to bring stormwater retrofits to the forefront. For example, in Maryland’s watershed implementation plan (WIP) to comply with the Chesapeake Bay TMDL, the state set a goal to retrofit 2% of existing impervious surfaces each permit year, aiming for a total of 10% over the five-year permit cycle. Additionally, Maryland plans to offset the impact from another 10% of existing impervious surfaces through the generation and purchase of nutrient credits2.

Similarly, Vermont recently passed what’s known as the “3-acre rule,” which is aimed at restoring and protecting Lake Champlain and surrounding watersheds from existing impairments. This rule requires all existing properties with greater than 3 acres of impervious surfaces that were built prior to the adoption of modern stormwater rules (2002) to obtain permit coverage and ultimately retrofit those properties with SCMs3. To give a sense of the scale of the problem, the 3-acre rule is expected to impact more than 1,000 sites in Vermont, one of the least populated and least densely developed states.

Perhaps even more impactful, EPA Region 1 plans to use Residual Designation Authority (RDA) to require existing properties within the Charles, Mystic and Neponset watersheds in the greater Boston area to retrofit properties to include SCMs4. RDA is a tool within the Clean Water Act that allows the EPA to require permits for existing properties contributing to impairments, and it’s likely that it will be applied in additional urban watersheds moving forward.

Challenges of Stormwater Retrofits

Retrofitting stormwater systems in urban environments presents a complex array of challenges that civil engineers must navigate to ensure successful implementation. These challenges stem from the intricacies of existing infrastructure, the constraints of urban spaces and the necessity of maintaining urban functionality during construction. The following are some of the primary challenges faced by engineers:

Limited Land Space

The scarcity of available land in urban areas complicates stormwater retrofit projects. Space constraints limit the options for implementing traditional stormwater management practices such as large detention basins. Engineers must therefore innovate with space-efficient solutions. These solutions require careful integration into the existing urban fabric without compromising other land uses or aesthetic values.

Poor/Contaminated Soils

Existing urban areas often have soils that have been heavily compacted and/or contaminated with potentially harmful substances, both of which create unique stormwater management challenges. In general, soils that are compacted and/or contaminated are not suitable for infiltrating runoff onsite. Managing stormwater on these sites often requires utilization of some form of flow-through treatment or lined SCM to avoid contamination risks.

Integration with Existing Utilities

Navigating the dense network of existing utilities is a significant challenge. At grade, utilities such as water and gas lines must be carefully considered to avoid disruptions. Below grade, sewer lines and stormwater pipes add layers of complexity, often lying at varying depths and orientations. Overhead, electrical and fiber optic lines further complicate the landscape, requiring careful planning to avoid interference during the retrofit process.

Additionally, accurate and up-to-date maps are essential to avoid damaging these utilities during construction. However, discrepancies often exist between documented maps and actual conditions, necessitating thorough site investigations and advanced technologies such as ground-penetrating radar to accurately locate and map these utilities. Ultimately, engineers must devise strategies that either work around these utilities or include provisions for temporarily relocating or protecting them during construction.

When tying SCMs into existing storm sewers, engineers must carefully assess elevation and hydraulic properties of existing stormwater pipes to ensure new systems will integrate seamlessly with old ones, maintain proper flow and prevent issues such as backflow or insufficient drainage. This often requires detailed hydraulic modeling and precise engineering to accommodate the existing topography and infrastructure.

Resident Access and Traffic Management

Urban construction projects must coexist with the daily lives of residents and the functionality of the city. Ensuring driveway access and minimizing disruptions to flow-through traffic are essential to maintain public support and minimize inconvenience. Additionally, noise restrictions often limit construction hours, further complicating scheduling and extending project timelines. Engineers must design detailed traffic-management plans and communicate effectively with residents to minimize disruptions and ensure safety.

Narrow Roads and Construction Equipment Access

Urban environments typically feature narrow roads, which pose a significant challenge for construction equipment access and maneuverability. Heavy machinery required for stormwater system retrofits, such as excavators and cranes, often struggle to navigate tight urban spaces. Engineers must plan for the logistics of equipment staging, possibly requiring road closures or detours, which further complicates traffic management.

Advantages of High-Rate Biofiltration (HRBF) in Urban Retrofits

HRBF is an advanced stormwater management technique that enhances features of traditional biofiltration systems to handle higher volumes and flow rates of stormwater runoff by optimizing the hydraulic capacity and performance of the media gradation. The following are some of the key benefits of utilizing HRBF:

Increased Flow Rates

The most significant advantage of HRBF systems is the increased flow rate through the media. Engineered media can achieve flow rates exceeding 3 gallons per minute per square foot (gpm/sf) of media surface area, which is an order of magnitude greater than conventional biofiltration systems. This results in a reduced treatment system footprint, which is critical in urban retrofits that often have space constraints that prohibit the use of traditional biofiltration. The small footprint also reduces installation and lifecycle costs as there’s less square footage to install and maintain.

Superior Pollutant Removal

In addition to higher flow rates, HRBF systems deliver superior pollutant removal efficiencies relative to many types of SCMs.

Data from the 2020 International BMP Database summary statistics5 show that HRBFs are among the most effective SCMs for many priority stormwater pollutants. Table 1 compares the median effluent concentrations achieved by traditional biofiltration (labeled as bioretention) systems and HRBF studies included in the database. The HRBF median effluent concentrations are significantly lower for total suspended solids (TSS), total phosphorus (TP), total copper (TCu) and dissolved copper. TP performance is particularly noteworthy, with traditional biofiltration showing a significant increase from influent to effluent and HRBF showing a significant reduction. Traditional biofiltration median total zinc effluent is significantly lower than HRBFs; however, both provide statistically significant reductions.

Quality Control

Another important benefit of HRBF systems is that they’re manufactured in a controlled environment. All media components are provided by the manufacturer to ensure the final product performs according to design specifications.

The engineered biofiltration media is blended under strict QA/QC guidelines, unlike traditional biofiltration systems where the contractor sources and installs each component separately and the media is blended onsite. Traditional media sourcing and blending methods can vary significantly, often depending on the source and amount of media to be mixed as well as the equipment available. Just as two different cooks can use the same ingredients to create different meals, varying sources and blending methods can result in meaningfully different media. To reliably achieve consistent media properties, specific standards of practice for media production must be followed in a controlled environment.

Installation Efficiency and Adaptability to Different Site Layouts

HRBF systems are offered in various configurations to accommodate different retrofit project types. In areas with scattered existing inlets, smaller systems can be decentralized and placed adjacent to existing inlets. Their shallow depths and ability to be paired with existing inlets (as a bypass) allows for quick and easy installation without modifying existing underground infrastructure.

On sites where existing flows drain to a single outfall point, larger HRBF systems can be placed in an end-of-pipe configuration, collecting all flows at a single location. In some cases, HRBF systems can be installed downstream of the site’s existing stormwater detention infrastructure. This centralizes maintenance to a single location, benefiting municipal maintenance crews and, most importantly, budgets. Some HRBF systems can also be used outside of precast vaults, in basin orientations or custom structures. The reduced footprint, compared to conventional practices, is maintained due to the higher media flow rates, allowing the system to treat a larger drainage area.

Example Stormwater HRBF Retrofit Projects

Market Street Retrofit, Spokane, Wash.

The Market Street – Francis to Lincoln Stormwater Retrofit project involved removing and replacing existing stormwater structures on Market Street between Francis Avenue and Lincoln Road in Spokane County, Wash. The dry wells along this transportation corridor didn’t meet current regulations as outlined in the Stormwater Management Manual for Eastern Washington—injection wells have the potential to place contaminants below the root zone so they may reach underground sources of drinking water.

To address this issue, the county replaced 15 catch basins with Filterra HRBF systems planted with dwarf lilacs. The county selected Filterra based on Washington Ecology General Use Level Designation approval as well as ease of inspection and maintenance.

Two of the Filterra systems were the Internal Bypass – Curb configuration that incorporates a curb inlet treatment chamber and an internal high-flow bypass in a single structure. Incorporating a high-flow bypass eliminates the need for a separate bypass structure and enables placement on grade or in a “sag” or “sump” condition. The remaining systems were the Filterra Offline configuration, the most economical and simplest Filterra configuration. It utilizes a downstream catch basin or curb inlet for bypass flows, allowing for the shallowest profile of any Filterra configuration.

Costco Upgrades, Frederick, Md.

The Costco location in Frederick, Md., was making upgrades for ADA compliance and was required by the state to also provide treatment for some existing areas near the building’s receiving dock and tire-installation areas. The Engineer of Record sized multiple drainage areas that could be retrofitted with Filterra HRBF systems placed within existing parking islands. The engineer targeted four drainage areas for treatment with Filterra units. The approach was minimally invasive, using the existing catch basin as an overflow where possible and allowing the business to stay open during the entire process.

Glenmont Forest Green Streets, Wheaton-Glenmont, Md.

The Glenmont Forest subdivision was built in the 1940s-1950s, long before modern SCMs were in place. The project involved installing 53 small-scale stormwater practices within the county-owned right-of-way to capture water from roadways and sidewalks.

Four types of SCMs were constructed in existing roadside facilities to collect as much first-flush runoff as possible. All the SCMs used needed to be approved to treat the 1-inch water-quality storm by the state of Maryland to be considered Green Infrastructure. Three of the SCMs selected were traditional systems, including bioretention, rain gardens and downspout rain barrels. The fourth SCM was the Filterra system, which has been approved as a Green Infrastructure Practice in Maryland since 2013.

Twenty-two Filterra offline systems in sizes from 6 feet by 6 feet through 12 feet by 6 feet were specified. All units were installed offline and upstream of existing storm drain inlets installed more than 60 years ago.

The existing neighborhood featured crowded lots and narrow roads, sidewalks and infrastructure for subsurface utilities. As a result, the space available to retrofit surface stormwater SCMs was extremely limited. HRBFs small surface footprint was ideal for this type of application.

The primary challenge with installation was delivery vehicle access, given narrow roads and street parking. Cranes couldn’t be used due to clearance issues from overhead utilities. The solution was to use a large mobile forklift to offload each unit from the truck, drive it to its final location and drop it in place with a forklift.

Conclusion

In conclusion, HRBF systems represent a significant advancement in urban stormwater management by consistently achieving a high level of water quality protection in a smaller footprint, making them an ideal tool for addressing the challenges posed by existing impervious surfaces in older urban areas. The increased flow rates, superior pollutant removal and adaptable installation methods make these systems particularly well-suited for space-constrained environments.

By utilizing engineered biofiltration media, HRBF systems can effectively manage stormwater runoff in smaller footprints, offering a cost-effective solution for retrofitting existing sites. As regulatory requirements continue to emphasize the importance of stormwater retrofits, HRBF systems will play a crucial role in enhancing water quality and restoring impaired urban watersheds. Through innovative design and strategic planning, civil engineers can overcome the complexities of urban retrofits, ensuring sustainable and efficient stormwater management for the future.

References

1. Theobold, D.M.; Goetz, S.J.; Norman, J.B.; Jantz, P. (2009). “Watersheds at risk to increased impervious surface cover in the conterminous United States,” Journal of Hydrologic Engineering, 14, 362-368.

2. Maryland Department of Environmental Protection. (2019). “Maryland Phase III Watershed Implementation Plan,” Online: https://mde.maryland.gov/programs/water/TMDL/TMDLImplementation/Pages/Phase3WIP.aspx, Accessed: 6/28/24.

3. Vermont Department of Environmental Conservation. (2020). “Vermont General Permit 3-9050 (3- Acre Rule),” Online: https://dec.vermont.gov/watershed/stormwater/permit-information-applications-fees/operational-stormwater-discharge-1, Accessed: 6/28/2024.

4. U.S. Environmental Protection Agency Region 1. (2022). “EPA’s RDA Designation for the Charles River, Mystic River and Neponset River Watersheds,” Online: https://www.epa.gov/npdes-permits/watershed-based-residual-designation-actions-new-england#EPAsRDADetermination, Accessed 6/28/2024.

5. International BMP Database. (2020). “2020 BMP Summary Statistics,” Online: https://www.waterrf.org/system/files/resource/2020-11/DRPT-4968_0.pdf. Accessed: 6/25/2024.