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Continuously washing sand filter

A common problem for pressurized or non-pressurized fixed-bed filters, typically sand filters, is a constant plugging tendency, especially when treating water with medium to high ratios of suspended solids. The problem can sometimes be resolved by using multi-layer filters in which the capacity for holding suspended solids can easily be doubled. However, both types of filters must be backwashed regularly to remove the foulants and to attain the desired head loss. When using feed water as wash water, the feed water contaminants can be collected by the lower parts of the filter media or the lower collectors. This undesirable phenomena can cause a fluctuation in the quality of the treated water.

Continuously washing WatMan sand filters will never get plugged by the suspended solids or need to be taken out of service for the backwash. The amount of constant wash water needed is 5-10% of the feed; there are no high peak flows to balance. There are no channels causing dead zones and the whole filtering area can be utilized efficiently. Low feed pressure is needed.

The extremely tough and simple structure will minimize the need for service and down time, and all the vital parts including the filter chassis are manufactured using chemically pickled acid proof steel. The basic filter has only one moving part, which is the sand washing screw. The main task is carried out by an air lift pump, which transfers the dirty sand into the washing screw chamber. As an option a sand balancing scraper and a filtered water wash pump can be delivered.

The feed water meets the distributor chamber in the lower section of the unit and flows upwards through the sand bed. As the water rises towards the surface, the debris remains in the sand bed. Water flows through the high sand bed and leaves the filter unit in the upper part outlet. In the opposite direction to the water, the dirty sand and debris first sink into the bottom of the unit. From the bottom the air lift pump draws and transfers the sand up into the washing screw chamber. The purified sand drops into the filter chamber forming the top filtering layer. The dirty wash water is led into the sewer or onwards for further treatment.

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