Patent GB-578811-A (GB)
Title: Improvements in or relating to filter plants with automatic wash-out
IPC: B01D33/00
Filed: 1944-02-29
Abstract: 578,811. Filters. BENDEL, H. Feb. 29, 1944, No. 3763. Convention date, July 14, 1943. [Class 46] A sand bed filtration plant with automatic cleaning by periodic circulation of the sand with compressed air and filtered liquid forced into the vessel containing the sand has a siphon for discharging supernatant soiled liquid from the top of the sand bed and such siphon is continuously primed during the whole of the cleaning period; during normal filtration, priming not being then operative, such siphon acts as a safety overflow to prevent excessive rise in level should there be a failure of the means for initiating the cleaning. The plant shown, comprising a body of sand 2 in a reservoir tank 3 is wholly automatically controlled to effect as necessary and without the operation of any sluice valves a washing and aeration of the sand by an air-lift pump-action causing a circulation of sand and water up the air-lift shaft or tube 8, the sand being guided thereto by the bottom sloping walls 4 of the reservoir tank 3 whereby the whole body of sand is circulated ; the bottom of the air-lift shaft or tube 8 has a bell enlargement 14 which serves as a gathering space for filtered water, this water then passing to a filtered-water draw-off tank 20 by a pipe 17 having a downwardly extending U-loop 23a and pipe 17 serving also for the supply of compressed air (for the air-lift action) from a nozzle 24 when a compressor 26 is switched into operation by a float contact 33 consequent on the rise of the raw water level above the sand level due to the clogging of the body of sand after a period of use. The sand is circulated a number of times during the washing phase since the liquid level in the reservoir tank 3 is allowed to descend only comparatively slowly by a discharge siphon 28 being throttled at 27a. The sludge water arising from the cleaning of the sand passes away by this siphon 28 which has a flared inlet 29a whereby the entrance velocity is kept below a value that would maintain the sand particles in suspension and thus only the washed-out impurities are carried away in the sludge water. At the beginning of the washing phase the raw water is at the level 7 so that the compressed air pressure at the pipe 17 is such as to force filtered water lying in the U-loop 23a downwardly in the near limb ; the air-lift circulation is at first therefore effected with raw water but when the liquid level has been lowered by the siphon 28 the air pressure in pipe 17 is such as to allow induction of filtered water by this latter having risen in the near limb of the U-loop 23a and the final washing therefore is by filtered water. The duration of the washing phase is determined by a clock contactor 34. The air-lift shaft 8 is normally closed by a weighted valve plate 10 but this is lifted by the compressed-air pressure. The siphon 28 is primed by a connection 30 to the suction side of the compressor. The discharge side of the siphon is sufficiently drowned in a chamber 35 to prevent uncovering during the priming period. A plurality of air-lift shafts 8 and discharge siphons 28 may be provided. The Specification as open to inspection under Sect. 91 comprises also an arrangement in which the admission of raw water to be filtered and the discharge of sludge water are controlled by a slide valve operated by the compressed air from the compressor which is switched into operation by a manometer device. This subject matter does not appear in the Specification as accepted.