Horizontal-Flow Biofiltration: Expanding the Engineer’s Toolbox for Stormwater Treatment
As urban development intensifies and stormwater regulations grow more complex, engineers are challenged to design treatment systems that perform effectively within constrained site layouts. Engineered biofiltration has long been a proven solution, with vertical-flow systems like Filterra® delivering reliable, high-rate performance in compact sites. Building on that foundation, Contech’s Modular Wetlands® Linear (MWL) uses horizontal-flow technology to address hydraulic and spatial challenges, giving engineers greater flexibility in how they design for water quality.
When Site Conditions Call for a Different Approach
Vertical-flow biofiltration systems excel in applications where runoff is concentrated and a sufficient vertical drop exists to drive flow through the media profile. However, urban sites may present constrained conditions, including but not limited to depth restrictions due to utilities, shallow drainage networks, or minimal allowable drop between inlet and outlet piping.
In these cases, a horizontal-flow configuration can be an ideal solution. Instead of water filtering vertically through a media bed, water flows radially through the media profile to a vertical drain pipe, allowing effective pollutant removal even where space or elevation is limited. The result is expanded design flexibility without sacrificing treatment performance.
How Horizontal Flow Works
The Modular Wetlands Linear uses a multi-stage process that combines physical separation and biological treatment in one unit.
- Pretreatment chamber: incoming stormwater first enters the inlet chamber, where trash, sediment, and debris are captured via hydrodynamic separation and media filtration in prefilter cartridges.
- Biofiltration cell: water then fills the void space along the perimeter of the biofiltration chamber and moves radially inward through the engineered high-rate media to a vertical underdrain pipe. The media bed incorporates microbial activity to remove advanced pollutants before being discharged through an underdrain system.
This configuration maximizes the stormwater-media contact area by increasing the vertical head within the system.
Engineering Benefits of Horizontal-Flow Biofiltration
Space Efficiency
The horizontal-flow technology of the Modular Wetlands Linear is purpose-built for constrained sites such as streetscapes, medians, and parking lot perimeters. Modular Wetlands Linear systems can be installed in series or in parallel to provide distributed treatment, reducing the need for large, centralized facilities.
Hydraulic Performance
The even lateral distribution of flow helps maintain infiltration capacity and prevent localized clogging. The shallow flow path enables Modular Wetlands Linear units to perform well in areas with limited vertical elevation change. The required drop between inlet and outlet pipes is reduced compared to vertical-flow systems, as the media bed depth does not dictate the flow path.
Simplified Maintenance
The Modular Wetlands Linear is a self-contained treatment train. Maintenance requirements for the unit consist of simple steps that include trash and sediment removal, replacing the prefilter cartridge filter media, and trimming of vegetation. Because sediment and debris are captured in the pretreatment chamber, the life of the high-rate biofiltration media is extended.
Proven Water-Quality Results
Multiple independent stormwater technology evaluation organizations, including the Washington Department of Ecology and the New Jersey Department of Environmental Protection, have evaluated the Modular Wetlands Linear. These programs test and confirm the system’s ability to effectively remove total suspended solids, nutrients, hydrocarbons, and metals. Choosing Between Vertical and Horizontal Systems
The decision to use vertical or horizontal flow high-rate biofiltration systems depends on the site constraints for a given project. Vertical-flow systems like Filterra are ideal for small drainage areas or retrofit projects, while the Modular Wetlands Linear provides an optimized solution for deeper, end-of-pipe installations or projects with minimal allowable drop through the system.
Whether configured vertically or horizontally, engineered high-rate biofiltration systems give civil engineers the flexibility to design with confidence, delivering reliable, high-performance stormwater treatment in any setting.