Pressure Sand Filters

The inlet consists of a method of maintaining a set water level during filtration and removing waste during the backwash cycle.
Weirs are the most common method used to maintain the water level.

Wash trough photos courtesy of Dr. Daniel L. Gallagher
A series of wash troughs are used to collect and remove backwash water. Wash troughs are usually located 3.5 - 4 feet above the bed, 6 - 10 feet apart. Correct spacing and height of troughs ensures the dirty wash water is efficiently skimmed off, without washing out any of the filter media.

Filter media selection (both size and type) may be the most important decision in a rapid sand filter design. Its selection is specific to each treatment plant and depends on the type of influent being filtered, media available and expected quality of effluent.

The higher specific gravity of garnet allows a finer pore size without the danger of mixing layers during backwashing. After the larger particles have been trapped by the anthracite, the sand media traps the remaining particulate matter by a combination of adhesion and straining.
16-24 inches of 3-6 carefully graded layers of successively coarser gravel. Preferred gravel size is 5 - 60 mm. The gravel maintains a consistent and diffused flow during filtration and prevents disruption of the sand layer during backwash.

The most effective is a coarse to fine pore size (the hydraulic radius of the stream line between particles). This allows for a more complete use of the bed area and reduced backwash frequency. Mixed media filter beds are the most common. An ideal mixed media bed may be composed of approximately 30-60 inches of granular media with the following makeup, from inlet to outlet:
12 - 18 inches of anthracite coal with a specific gravity of 1.5 and grain size of 0.7 - 2 mm.. The anthracite has a larger, more angular particle size to trap particulate matter before it gets to the sand layer and clogs it. Anthracite's lower specific gravity allows it to remain on top of the sand layer after backwashing. The anthracite layer also allows a lower backwash velocity to give the same bed expansion than if sand alone were used.
9 - 16 inches of sand - this can be composed of two types.

Pressure  sand filtration is contrasted to Slow Sand Filteration by increased flow rate, method of cleaning the filter bed. A rapid sand filter can operate up to 40 times faster than a slow sand filter. Rapid sand filters are cleaned often, usually daily, by reversing the flow of water through the entire filter bed, referred to as backwashing Slow sand filters are cleaned less frequently by removal of the top layer of media.