A common design issue for sand filters, multi-media filters, greensand filters, arsenic removal units, etc. is the large difference between the backwash flow rate and the flow rate during normal operation. Depending upon the media used, the backwash flow rate can be 2X to 10X the service rate. Due to restrictions in the piping coming to the unit or the well feeding the plant, the unit cannot get an adequate backwash flow rate. Most water supplies are not designed to handle such a wide swing in flow rate and the drain line will have to be appropriately up-sized. In particular, a plant well water source can not be cost effectively sized for such a large flow difference.
To address this predicament, Res-Kem has been selling triplex sand filters, multi-media filters, greensand filters, arsenic removal units, etc. to reduce the backwash flow rate. The tradeoff is the cost of the extra valves and tanks with a backwash flow rate that is reduced by 50% for a continuous system and 67% for a non-continuous system.
Depending upon the size of the units, all of them can be skid mounted. This makes for a very simple field installation with a single electrical connection. Also, the skid will have a single feed, product, and drain connection.
An optional feature of a triplex skid is that a separate line for an alternate backwash source can be added. This allows for the use of a "clean" water source to backwash the units. If water storage is not available, two units can be used to backwash the third.