Pretreatment

This non-proprietary pre-treatment device, known as a "Sediment Pad" that is used by the City of Toronto to capture coarse sediment, trash and debris in runoff before water drains into bioretention or other similar green stormwater infrastructure practices. Centralizing sediment and debris capture in a small area near the inlet to the bioretention feature helps simplify maintenance (City of Toronto, 2021.

Pretreatment devices may be constructed in close association association with one or more inlets. Different configurations of each exist to accommodate any design.

Another example of the City of Toronto's non-proprietary pre-treatment device, known as a "Sediment Pad" shown being used at the curb inlet of a bioretention cell on Danforth Av. to capture coarse sediment, trash and debris in influent entering the BMP.

Overland sheet flow

The following pretreatement features are recommended where surface sheet flow is being directed to a BMP (e.g. a bioretention cell) receiving flow from the edge of an impermeable surface such as an asphalt parking lot or a paved plaza:

• Level spreaders
• Gravel diaphragms
• Vegetated filter strips

A gravel diaphragm typically requires less setback between the catchment and the BMP. Where space permits, these designs can be used in combination to even greater effect!

Concentrated overland flow

Where a curb cut, a disconnected roof downspout pipe, swale, trench drain, side inlet catch basin or other pre-fabricated inlet structure collects and conveys runoff as concentrated overland flow, which increases the flow rate, one or more of the following pretreatment features should be considered:

• Catch basin or other pre-fabricated inlet structure sump;
• Level spreader;
• Forebays (well suited for bioretention cells). See Pretreatment features for guidance on sizing.;
• Stone and geotextile filter: A shallow excavation on the filter media bed surface at the inlet, lined with geotextile and rip rap or decorative stone cover; and,
• Sand filters (well suited for underground infiltration);
• Concrete sump pretreatment device. Rain Guardian is one example of a proprietary sump device.

Concentrated underground flow

This type of pretreatment is dominated by proprietary systems, some of which have undergone verification under the Environmental Technology Verification (ETV) Canada program. Pretreatment devices suited to concentrated underground flow, or pipe flow include the following:

• Oil and grit (hydrodynamic) separators
• Manhole baffles
• Isolated chamber row filters
• Membrane filters
• Media filters

Membrane and media filters are more commonly used as "stand alone" water quality treatment practices when space for surface practices is not available, but may also be used downstream of LID facilities or treatment trains to enhance removal of sediment and other targeted pollutants (e.g., nutrients, metals) where warranted by receiving water sensitivity.

Pretreatment options

Type	Description	ETV Canada verified products	Other products (not verified)
Overland flow sumps	Collects and settles debris as overland flow passes through sump to treatment practice.		Catch basin or other non-proprietary pre-fabricated inlet structure sump. Small forebays may also be used for treatments systems receiving concentrated overland flow’ Rain Guardian
Catch basin inserts	Great for improving water quality in retrofit situations	CB Shield Littatrap (Now, EnviroBasin™)	Snout
Oil and grit (hydrodynamic) separators	The shape causes rapidly flowing water to form a vortex which forces the larger, heavier particles are outwards and downwards into a lower chamber. Floating debris, oils and grease are trapped behind a baffle or in small tank area, towards the top of the main vortex chamber	CDS Downstream Defender First Defense Hydrostorm SCICLONE HDS SDD Stormceptor EF StormTrap Storm Settler	SAFL Baffle
Manhole baffles	Baffles slow flow, encouraging larger, heavier particles to drop out of the water column. A skimmer may also trap floating debris, oils and grease.		SAFL Baffle
Isolated chamber row	Separate rows at inlets, not connected by pipes to the other chambers, that isolate the bulk of sediment and associated pollutants and designed for ease of access by jet flushing cleaning equipment.	Cultec Separator Row StormTech Isolator Row Plus®	ChamberMaxx®
Membrane filters	Uses advanced membrane technology adapted from water treatment plants.	Jellyfish Filter (TARP) Jellyfish Filter (TAPE) Kraken Membrane Filtration (TAPE)
Media filters	A variety of proprietary designs and media mixes are available.	Filterra BaySaver BayFilter The Stormwater Management StormFilter (TAPE) Up-Flo Filter	Sorbtive Media Bioretention Filter Media Sand Filters[2] Gutter Guards/Filters Downspout Filters Media filters High Performance Soil Media Mixes[3]

This table and page are particularly heavy on proprietary systems and information. Please inform us of any omissions or broken links using the box below:

Gallery

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  This pretreatment device, known as a baffle helps to remove trash, oil, TSS, larger sediments (silts, sands, etc.) and other floatables in roadway catch basins. This picture was taken of a SNOUT BAFL in Myrtle Beach, S.C. (Mullen, 2022)

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  Example of an overland flow sump, capturing stormwater runoff from the adjacent roadway. This example showcases a Rain Guardian^(TM) Turret structure with a metal grate overtop to capture larger floatables, trash and detritus from entering a stormwater facility or in this case a bioretention LID feature.

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  This inlet channels flow from an underground pretreatment unit onto the surface of a large bioswale, Mississauga Road, ON

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  A stormwater tree trench, including a sump inlet design used as part of pretreatment. The feature is located in the City of Cambridge, Massachusetts, US. The design was developed and lead by HDR Inc., and Halvorson, Tighe & Bond Studio (Halvorson, Tighe & Bond Studio, n.d.)

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  Example of a sump inlet being used in conjunction with a large bioretention basin feature, located in park space receiving stormwater off of the adjacent roadway (Source: Philadelphia Water Department, n.d.)

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  An inlet sump leading into a bioretention bump out for traffic calming. Inlet sumps help both settle and separate sediment from stormwater road runoff before depositing excess sediment and silt into an infiltration BMP. These sumps allow stormwater to flow into the feature through an underdrain pipe (Source: NACTO, 2017)

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  Example of a sump inlet allowing sediment to settle out of influent stormwater before entering a large infiltration chamber housed under the parking lot (Source: Philadelphia Water Department. 2020)

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  Example of a proprietary inlet device known as a Rain Guardian Bunker, that was installed at the Morningside Extension Project in Scarborough, ON.

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.

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.

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

Initial capturing and removal of unwanted contaminants, such as debris, sediment, leaves and pollutants, from stormwater before reaching a best management practice; Examples include, settling forebays, vegetated filter strips and gravel diaphragms.

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.

A mixture of mineral aggregates bound with bituminous materials, used in the construction and maintenance of paved surfaces.

A level spreading device placed at a runoff discharge location, perpendicular to flow, to maintain sheet flow and distribute runoff as evenly as possible across a pervious area or stormwater infiltration practice. A gravel diaphragm acts as a pretreatment device, settling out suspended sediments before they reach the practice.

The land draining to a single reference point (usually a structural BMP); similar to a subwatershed, but on a smaller scale.

Open space floodway channels, road reserves, pavement expanses and other flow paths that convey flows typically in excess of the capacity of the Minor Drainage System.

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

Initial capturing and removal of unwanted contaminants, such as debris, sediment, leaves and pollutants, from stormwater before reaching a best management practice; Examples include, settling forebays, vegetated filter strips and gravel diaphragms.

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.

Filter fabric that is installed to separate dissimilar soils and provide runoff filtration and contaminant removal benefits while maintaining a suitable rate of flow; may be used to prevent fine-textured soil from entering a coarse granular bed, or to prevent coarse granular from being compressed into underlying finer-textured soils.

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.

Total suspended solids

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.

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

Soil or media particles smaller than sand and larger than clay (3 to 60 m)

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.

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