The self cleaning autonomous valveless Gravity Sand Filter consists of a 3-compartment vertical steel tank divided into a backwash storage compartment, a filter bed compartment and a filtration collection compartment. It operates entirely without any outside energy supply, whereby water is clarified solely throughout the process of sand filtration. Backwashing occurs on the loss of head principle as described below. The design eliminates the need for backwash pumps and valves.
The backwash storage compartment is designed to hold
an adequate amount of backwash water. It always fills to
the same height, thus providing a constant backwash volume.
Also, as the level of water in the backwash compartment drops,
the backwash rate diminishes so that the backwash rate
actually starts at about 44m/h and gradually slows down to
30m/hr at the end. Extensive experience with this diminishing
backwash rate has indicated it to be completely satisfactory for
cleaning the filter bed. The high initial flow rate provides greater
initial turbulence to wash the sand. The lower flow at the end of
backwashing permits the bed to settle evenly and smoothly.
The filter-bed compartment contains fine filter sand. Unique
strainers uniformly collect the filtered water and distribute the
backwash water without the need for any gravel layer.
The unit operates to a mere 1,2 to 1,5 metres head loss.
Experience has shown that under these conditions a surface
wash and/or an air backwash is not required to break the
surface mat to achieve effective regeneration of the filter bed.
Units are available for flows up to 1,130m3/hr through a
single vessel uo to 12m diameter and a double bed version is
also available up to a maximum flow of 2,260m3/hr. Using
double bed units has advantages in certain installations relating
to space saving, transport and instantaneous discharge while backwashing.