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. (Source: Green Synergy Engineering)

Filtration is the process by which stormwater runoff is conveyed through a filter medium to remove solid particles and associated pollutants. Several stormwater practices use filtration through sand or gravel media as the primary mechanism for treatment of urban runoff. Stormwater passes through a filter based on the pressure difference on either side of the filter. In most stormwater treatment applications, the pressure difference is created by gravity (or hydraulic head). As the filter becomes clogged, higher pressure is needed to drive stormwater through the filter, until 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).

Types of filtration

Two common types of filter media used in stormwater treatment are granular media, such as sand, and membranes:

Granular media is a multilayer lattice or matrix that traps solids as water flows through the tortuous channels in the media. Clogging may occur when solid particles form a film or cake on top of the filter or accumulate within the media channels to the point where the channels become occluded.

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.

Key system design parameters

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.

Permeable pavements can also be considered to be a type of filter. In this case, runoff and rainwater is filtered through the surface pores or granular media between the 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. Since permeable pavements need to maintain a high rate of infiltration to avoid ponding, the area of impervious surface draining on to them should be no larger than the area of the permeable pavement itself.

See also

• Filtration Manufactured Treatment Devices
• Media filters
• Pretreatment
• Oil and grit separators

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

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

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.

Mineral particles which are smaller than 2 mm, and which are free of appreciable quantities of clay and silt. Coarse sand usually designates sand grains with particle size between 0.2 and 0.02 mm.

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.

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.

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

Penetration of water through the ground surface.

An alternative practice to traditional impervious pavement, prevents the generation of runoff by allowing precipitation falling on the surface to infiltrate through the surface course into an underlying stone reservoir and, where suitable conditions exist, into the native soil.