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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 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:

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[edit]

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);
  • Concrete sump pretreatment device. Rain Guardian is one example of a proprietary sump device.

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)
Catch basin inserts Great for improving water quality in retrofit situations
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.
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
Isolated chamber row filter 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.
Membrane filters Uses advanced membrane technology adapted from water treatment plants.
Media filters A variety of proprietary designs and media mixes are available.
Overland flow sumps Collects and settles debris as overland flow passes through sump to treatment device.
  • Rain Guardian
  • Catch basin or other pre-fabricated inlet structure sump
  • Nonproprietary concrete inlet sump

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