Pretreatment

Example oil-grit separator USGS 2002 (Wikimedia commons)

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

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 three 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);
• 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).

Concentrated underground flow

This type of pretreatement is dominated by proprietary systems, some of which have undergone verification under the Environmental Technology Verification (ETV) program.

Pretreatment options

Type	Description	ETV products	Other products (not verified)
OGS 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
Hydrodynamic OGS	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 SDD Stormceptor EF
Cartridge filters	Uses advanced membrane technology adopted from water treatment plants.	Jellyfish filter
Catchbasin inserts	Great for improving water quality in retrofit situations	CB Shield	Littatrap Snout

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

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.

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.

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.

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

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.

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.

Systems designed to remove trash, debris and some amount of sediment, oil and grease from stormwater runoff based on the principles of sedimentation for the grit and phase separation for the oil.

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