Pretreatment
Overland sheet flow[edit]
The following pretreatements are recommended where sheet flow is being directed to a BMP (typically a bioretention cell) 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 two 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:
- Forebays (well suited for bioretention cells)
- Sand filters (well suited for underground infiltration)
- Screens and racks
Concentrated underground flow[edit]
Oil and grit separators (OGS) are devices which use physical principles to improve water quality. Typically these are underground chambers installed in catchbasins or manholes. Many commercial and custom configurations exist for both new installations and retrofit scenarios.
- One example might employ baffles within a tank or chamber:
- The first baffle slows the incoming water flow, encouraging any larger, heavier particles to drop out of the water,
- A second baffle skims the surface of the water, preventing floating debris, oils and grease from passing,
- Another option is a hydrodynamic device, which forces the incoming water flow into a circular vortex:
- This motion 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.
External Links[edit]
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