automatically washed filters without valves

The operation of these filters is completely independent and automatic, in filtration and in wash modes. The water to be filtered comes from a header tank and, after filtration through a low granulometry bed, this water returned to the superimposed filtered water tank. When the tank is full, the water is released for use by overflow.

When a filter bed becomes fouled, the level rises in the header tank and in the upstream arm of a siphon. When maximum head loss is reached, the compressed air in the siphon escapes and the siphon is primed. The reserved filtered water backflows through the filter, washing it.

With this type of filter, we can be certain that the filter bed will never undergo abnormal levels of fouling because the wash is carried out automatically at a fixed and preset head loss figure.

These filters are especially valuable when no compressed air or electricity is available. They are appropriate for use with water that has a low to medium suspended solids loading and when the distribution system tolerates a shutdown so that the wash water capacity can be reinstated.

Given the fine nature of the sand (NES 0.55 or 0.65 mm) and the reduced of media layer, its retention capability is quite low and velocities, in practice, in the region of 5 to 7.5 m · h^–1, cannot exceed 10 m · h^–1.

These filters can be used :

• in direct filtration applications with neither coagulant nor flocculent (except in special cases) and on water that does not cause much fouling (open recirculating systems used for cooling);
• in settled water filtration.

They have been designed for 1.6 to 4 m diameters.