Slide 21 of 52
Notes:
The underground sand filter, pioneered in the District of Columbia, was adapted for sites where space is at a premium. In this design, the sand filter is placed in a three chamber underground vault accessible by manholes or grate openings. The vault can be designed either on-line or off-line in the storm drain system. The first chamber is used for pretreatment and relies on a wet pool as well as temporary runoff storage. It is connected to the second sand filter chamber by an inverted weir, which keeps the filter surface free of trash and oil. The filter bed is 18 inches deep and may have a protective screen of gravel or permeable geotextile to limit clogging. During a storm, the water quality volume is temporarily stored in both the first and second chambers. Flows in excess of the filter's capacity are diverted through an overflow weir. Filtered runoff is always collected, using perforated underdrains that extend into the third �overflow� chamber. One concern associated with this design is that OSHA confined space regulations for individuals conducting system maintenance.