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Water clarification in bed filters

The water clarification process by means of filtration basically is performed by adhesive volumetric filtration in the vertical clarifying bed filters. The filter consists of a cylindrical vessel with spherical bottoms welded to it. The vertical one-chamber filters with diameters of 1.0, 1.4, 2.0, 2.6, 3.0 and 3.4 m are commercially available. Inside the filter there are filtering bed layer and drainage and distribution assemblies intended for uniform distribution and collection of water per filter's cross section area. The upper drainage and distribution assembly is made in the form of the cover plate that absorbs incoming water energy; bottom drainage and distribution assembly consists of a collector with the side bends equipped with specially designed slotted caps or slotted openings with a width of 0.4 mm to discharge water and prevent loses of the filtering material.

The filtering material of bed filters shall have required grain-size distribution, sufficient mechanical strength and chemical stability of grains. Granulated anthracite and silica send meet these requirements, but the last one dissolving in water enriches it with silicon compounds. Anthracite' grain size shall be within the limit of 0.6-1.4 mm for one-layer filtration. In compliance with mechanical strength requirements (filtering material attrition during the loosening) annual attrition of the filtering material shall not exceed 2.5%.

The height of filtering material in clarifying filters amounts to 1 m. The operation cycle of clarifying filters is divided into three stages:

  • useful work of the clarifying filter;
  • filtering material backwash;
  • discharge of the primary permeate.

The useful work of the bed filter is performed at a filtration rate up to 10 m/h with preliminary treatment of water in the clarifying filters and at a rate of 4-5 m/h without such treatment.

During the operation of clarifying filters it is essential to keep the constant filtration rate, control the pressure difference at the filtering material layer and water consumption, take samples of incoming water and permeate to determine water clarity.

The filter is stopped for backwash to remove retained suspended solids as scheduled or in case the pressure difference increases up to 0.1 mg/dm3, suspended solids' content - up to 1 mg/dm3.

The main stage of the backwash is performed by an upward wash water flow. An intensive wash water flow loosens and transfers entire filtering layer into suspended state with expansion by 40-50%. Thus filtering material grains move at a random way in the water flow and get free from suspended particles stuck to them.

The backwash intensity shall ensure sludge removal, but shall not cause the removal of effective size fraction of the filtering material.

The backwash intensity depends on material type, its grain size and usually amounts to 10-12 dm3/(m2*s).

Upon completion of the backwash process the first turbid permeate is being discharged to the tank within 5 minutes, where the loosening waters are collected as well. To save resources wash waters along with the sludge and the first permeate are uniformly pumped out from the tank within 24 hours and supplied to the clarifier.

Prior the backwash the filtering material may be treated by the compressed air after the water is discharged or by air-and-water washing with simultaneous supply of compressed air and wash water to the bottom drainage and distribution assembly. The washing modes of clarifying filters are specified during their set-up stage.

Sulphated coal is used as cation-exchange resin in the mechanical bed filters intended for purification of the turbine condensate and return industrial condensates, if the condensate temperature does not exceed 50 °C, and cation-exchange resin KU-2-8 is used, if the condensate temperature does not exceed 100 °C. The condensate filtration rate is deemed to be equal to 30 m/h for sulphated coal and 50 m/h for KU-2-8 cation-exchange resin.

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* The text is taken from:
A.S. Kopylov, V.M. Lavygin, V.F. Ochkov, "Water-treatment in power energetics"
(M. MEI Publishing House, 2003)