Filtration

Media based systems are depth filters that capture particles both on the surface and within the media matrix. Membrane filters are surface filters that remove all particles greater than a specified size at the surface of the filter. Membranes are typically much thinner than shown in the diagram above. Both filtration systems can be oriented vertically or horizontally (Green Synergy Engineering, 2024).

Overview

As stormwater flows over impervious surfaces such as pavement and rooftops, it collects sediment and other pollutants before entering nearby waterways. Stormwater filtration is a key element of urban runoff management, helping to improve water quality by removing suspended solids and associated pollutants. Filtration can help achieve total suspended solids (TSS) removal targets and contributes to overall watershed health (Purvis et al., 2018). Filtration works by conveying stormwater through a filter medium that captures solid particles. In most stormwater treatment systems, water is driven through the filter by a pressure difference created by gravity or hydraulic head.

Applications

Stormwater practices that use filtration as a mechanism for treatment of runoff include:

• Bioretention technologies have been shown to effectively filter sediment from parking lot, road and residential runoff (STEP, 2019).
• Permeable pavements can filter runoff through the surface pores or media between pavers, as well as through the bedding and base layers. Most of the sediment directed onto the pavements is trapped near the surface where it can be more easily accessed for cleaning.
• Filtration Manufactured Treatment Devices such as the Filterra® high flow curb inlet filtration MTD have achieved 80% TSS removal during runoff events.
• Stormwater planters filter runoff through a planting bed of filter media and can be installed in tight urban spaces
• Pretreatment options, such as vegetated filter strips and catch basin inserts

• 

  Bioretention cell with a pre-treatment grass filter strip. Stormwater filters through mulch, filter media, and two sizes of clear stone before infiltrating into native soil.

• 

  LittaTrap catchbasin insert filters trash, debris, and sediment from runoff before it enters the catchbasin (Enviropod, 2022).

• 

  Stormwater planter allows for storage, filtration, and evapotranspiration of runoff (Philadelphia Water Department, 2025).

Key system design parameters

Types of filter media

Careful consideration should be given to the type of filter media used in an LID system. Two common types of filter media used in stormwater treatment are granular media and membranes:

• Granular media is a multilayer lattice or matrix that traps solids as water flows through the tortuous channels in the media. Examples of granular media include sand and gravel.

• Membranes have very thin separation lattices with openings or pores that remove all particles larger than the nominal pore size of the filter at the surface, thereby acting much like a sieve. Membranes are often pleated to increase the surface area, allowing for more particles to be trapped prior to clogging.

Some filters include additives such as zeolite or iron to enhance the contaminant removal process for target water quality variables by increasing chemical adsorption and/or precipitation.

Sizing

When sizing filtration systems for specific sites using local historical rainfall records, several key system design parameters need to be considered. These are defined in the sizing section of the Filtration MTD page. Non-proprietary bioretention systems are sized as filters much more conservatively based on design storms, surface ponding depth and minimum media infiltration rate. The ratio of drainage area imperviousness to the surface area of bioretention facilities (referred to as the impervious-to-pervious ratio) is often many times lower than that for filtration MTDs. This ensures longer intervals for sediment maintenance and significant reductions in stormwater volumes through infiltration.

Filter media replacement in bioretention system (Filterra®)

Maintenance considerations

Vehicle used for vacuum maintenance of permeable pavements.

Practice-specific maintenance guides are available for each LID BMP. General maintenance considerations for filter-based LIDs include:

• Inspection and maintenance frequency: Routine inspections and maintenance are needed to assess and resolve issues, such as sediment buildup. Frequency of visits depends on a variety of factors. LID BMP designs should include safe access for maintenance and inspection crews and confined space entry procedures should be considered for subsurface systems. Monitoring should also be undertaken to assess function.
• Sediment removal: Filters can become clogged with sediment, organic material, and trash, reducing flow capacity and treatment performance. When solid particles form a film on the filter's surface or accumulate within the media channels, higher pressure is needed to drive stormwater through the filter. Eventually, a point is reached at which flow through the treatment system must be redirected to another flow path (usually a bypass within or upstream of the system). Regular vacuuming or manual removal of accumulated materials is commonly required.
• Media replacement: Filter bed media can lose effectiveness over time due to clogging, chemical saturation, erosion, animal burrows, and compaction. Media may need to be replaced, regraded, or core aerated.
• Overflow bypass functionality: Ensure bypass structures are functioning to prevent flooding or erosion when filters are clogged or overwhelmed by large runoff events.

The technique of removing pollutants from runoff as it infiltrates through the soil.

A hard surface area (e.g., road, parking area or rooftop) that prevents or retards the infiltration of water into the soil.

Soil, sand and minerals washed from land into water, usually after rain. They pile up in reservoirs, rivers and harbors, destroying fish-nesting areas and holes of water animals and cloud the water so that needed sunlight might not reach aquatic plans. Careless farming, mining and building activities will expose sediment materials, allowing them to be washed off the land after rainfalls.

The portion of water precipitated onto a catchment area, which then flows as surface discharge from the catchment area past a specified point.

Water from rain, snow melt, or irrigation that flows over the land surface.

The technique of removing pollutants from runoff as it infiltrates through the soil.

Total suspended solids

Sustainable Technologies Evaluation Program

The engineered soil component of bioretention cell or dry swale designs, typically with a high rate of infiltration and designed to retain contaminants through filtration and adsorption to particles.

Ground depression acting as a flow control and water treatment structure, that is normally dry.

A shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation.

a gently sloping, densely vegetated areas that treat runoff as sheet flow from adjacent impervious areas. They function by slowing runoff velocity and filtering out suspended sediment and associated pollutants, and by providing some infiltration into underlying soils.

a top dressing over vegetation beds that provides suppresses weeds and helps retain soil moisture in bioretention cells, stormwater planters and dry swales.

The natural ground material characteristic of or existing by virtue of geographic origin.

Box like underground concrete structure with openings in curb and gutter designed to collect runoff from streets and pavement.

A vegetated practice that collects and treats stormwater through sedimentation and filtration. Contributions to water cycle/water balance are through evapotranspiration only; no infiltration.

The quantity of water transpired (given off). Retained in plant tissues, and evaporated from plant tissues and surrounding soil surfaces. Quantitatively it is usually expressed in terms of depth of water per unit area during a specified period. e.g. mm/day

The combined loss of water to the atmosphere from land and water surfaces by evaporation and from plants by transpiration.

Low Impact Development. A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting.

Gravel, or crushed stone of various size gradations (i.e., diameter), used in construction; void forming material used as bedding and runoff storage reservoirs and underdrains in stormwater infiltration practices.

Gravel, or crushed stone of various size gradations (i.e., diameter), used in construction; void forming material used as bedding and runoff storage reservoirs and underdrains in stormwater infiltration practices.

The attachment of gas, vapour or dissolved matter onto the surface of solid materials.

Any form of rain or snow.

The total surface area upstream of a point on a stream that drains toward that point. Not to be confused with watershed. The drainage area may include one or more watersheds.

A shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation.

The slow movement of water into or through a soil or drainage system.

Penetration of water through the ground surface.

The engineered soil component of bioretention cell or dry swale designs, typically with a high rate of infiltration and designed to retain contaminants through filtration and adsorption to particles.

Best management practice. State of the art methods or techniques used to manage the quantity and improve the quality of wet weather flow. BMPs include: source, conveyance and end-of-pipe controls.

Soil, sand and minerals washed from land into water, usually after rain. They pile up in reservoirs, rivers and harbors, destroying fish-nesting areas and holes of water animals and cloud the water so that needed sunlight might not reach aquatic plans. Careless farming, mining and building activities will expose sediment materials, allowing them to be washed off the land after rainfalls.

(1) The wearing away of the land surface by moving water, wind, ice or other geological agents, including such processes as gravitation creep; (2) Detachment and movement of soil or rock fragments by water, wind, ice or gravity (i.e. Accelerated, geological, gully, natural, rill, sheet, splash, or impact, etc).

Linear bioretention cell designed to convey, treat and attenuate stormwater runoff. The engineered filter media soil mixture and vegetation slows the runoff water to allow sedimentation, filtration through the root zone, evapotranspiration, and infiltration into the underlying native soil.