Slide 8 of 52
Notes:
The earliest filter design was the surface sand filter (or Austin sand filter, as it has often been called). A flow splitter is used to divert the first flush of runoff into an off-line sedimentation chamber. The chamber may be either wet or dry, and is used for pretreatment. Coarse sediments drop out as the runoff velocities are reduced. Runoff is then distributed into the second chamber, which consists of an 18 inch deep sand filter bed and temporary runoff storage above the bed. Pollutants are trapped or strained out at the surface of the filter bed. The filter bed surface may have a sand or grass cover. A series of perforated pipes located in a gravel bed collect the runoff passing through the filter bed, and return it into the stream or channel at a downstream point. If underlying soils are permeable and groundwater contamination unlikely, the bottom of the filter bed may be designed without a lining, and the filtered runoff allowed to exfiltrate. The surface sand filter is usually surrounded by a concrete shell, although earthen embankments are equally acceptable. The surface sand filter can be designed to receive runoff from larger drainage areas (i.e., up to 10 acres), compared to the other filter types