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Line 27: − ==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. Line 27:
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(Source: Green Synergy Engineering)]]
(Source: Green Synergy Engineering)]]
==Overview==
==Overview==
Filtration is the process by which stormwater [[Runoff volume control targets|runoff]] is conveyed through a filter medium to remove solid particles and associated pollutants.
Stormwater filtration is an essential component of urban runoff management. Filtration can improve water quality by removing suspended solids and pollutants, helping to meet TSS removal targets and maintain watershed health. Filtration is the process by which stormwater [[Runoff volume control targets|runoff]] is conveyed through a medium to remove solid particles and associated pollutants. Filter media can include materials such as gravel, native or engineered soils, and [[geotextiles]]. 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).
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 [[clogging|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 [[overflow|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. Care should be taken when selecting the type of [[Geotextiles|geotextile]] used in an LID system.
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.
==Maintenance Considerations==
As the filter becomes [[clogging|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 [[overflow|bypass]] within or upstream of the system).
==Applications==
==Applications==
Stormwater practices use filtration through as the primary mechanism for treatment of runoff include:
*[[Bioretention]] systems such as <ref>https://sustainabletechnologies.ca/app/uploads/2020/06/High-rate-treatment-bioretention-filter-tech-brief.pdf<ref>
*[[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]]
*[[Filtration Manufactured Treatment Devices]]
*[[Media filters]]
*[[Media filters]]
*[[Oil and grit separators]]
*[[Oil and grit separators]]
==Key system design parameters==
==Key system design parameters==
Non-proprietary [[bioretention]] systems are sized as filters much more conservatively based on design storms, surface ponding depth and [[Flow through media|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.
Non-proprietary [[bioretention]] systems are sized as filters much more conservatively based on design storms, surface ponding depth and [[Flow through media|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.
[[Permeable pavements]] can also be considered to be a type of filter. In this case, runoff and