Pretreatment

From LID SWM Planning and Design Guide
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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[edit]

The following pretreatements are recommended where 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:

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[edit]

Where a curb cut, a disconnected downspout or a swale directs surface water to a point, increasing the flow rate, one or more of the following pretreatments should be considered:

- Also see stone for erosion control at inlets.

Concentrated underground flow[edit]

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.
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
Cartridge filters Uses advanced membrane technology adopted from water treatment plants.
Catchbasin inserts Great for improving water quality in retrofit situations


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