Changes

[[File:Filtration.png|right|frame|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.

[[File:Filtration.png|500px|right|frame|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).<ref>Green Synergy Engineering. 2024. Depth vs Membrane Filter. https://www.greensynergy.my/post/depth-vs-membrane-filter</ref>]]

 

(Source: Green Synergy Engineering)]]

==Overview==

==Overview==

Stormwater filtration is an essential component of urban runoff management. Filtration can improve water quality by removing suspended solids and pollutants which helps 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).

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|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).<ref>Purvis, R. A., Winston, R. J., Hunt, W. F., Lipscomb, B., Narayanaswamy, K., McDaniel, A., Lauffer, M. S., & Libes, S. (2018). Evaluating the Water Quality Benefits of a Bioswale in Brunswick County, North Carolina (NC), USA. Water, 10(2), 134. https://doi.org/10.3390/w10020134</ref> Filtration works by conveying stormwater through a [[Bioretention: Filter media|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.

==Types of filtration==

==Applications==

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

*[[Bioretention]] technologies have been shown to effectively filter sediment from parking lot, road and residential runoff (STEP, 2019).<ref>Sustainable Technologies Evaluation Program. 2019. Comparative Performance Assessment of Bioretention in Ontario. https://sustainabletechnologies.ca/app/uploads/2019/10/STEP_Bioretention-Synthesis_Tech-Brief-New-Template-2019-Oct-10.-2019.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]] such as [https://sustainabletechnologies.ca/app/uploads/2020/06/High-rate-treatment-bioretention-filter-tech-brief.pdf 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|vegetated filter strips]] and catch basin inserts

'''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.

===Types of filter media===

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

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.

*'''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 [[Aggregates|gravel]].

*'''[[Geotextiles|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.

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==

===Sizing===

Stormwater practices use filtration through  as the primary mechanism for treatment of runoff include:

When sizing filtration systems for specific sites using local historical [[Understanding rainfall statistics|rainfall]] records, several key system design parameters need to be considered. These are defined in the sizing section of the [[Filtration MTD]] page.  

*[[Bioretention]] systems such as <ref>https://sustainabletechnologies.ca/app/uploads/2020/06/High-rate-treatment-bioretention-filter-tech-brief.pdf<ref>

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 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]]

*[[Media filters]]

*[[Pretreatment]]

*[[Oil and grit separators]]

==Key system design parameters==

==References==