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 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). 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[edit]

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
  • Rain Guardian
Catch basin inserts Great for improving water quality in retrofit situations
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
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.
Isolated chamber row filters 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.

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