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APPLICATION AND PROPERTIES OF FILTERS WITH A DRAINAGE TUBING SYSTEM MADE FROM PLASTIC MATERIALS


A drainage tubing system can be utilised for both open and pressure single-bottom filters intended for filtering drinking, utility or waste water (third stage of purification). The drainage system allows washing of the filtering material with air and water. It is applicable for simple, coagulation, and follow-up filtration (behind the decantation tanks or the sludge contact clarifiers). It has been applied in filters with single-layer as well as multi-layer filtering materials. It can be placed in filters with rectangular as well as circular plans.

At the bottom of the filter, washing air and washing water are distributed separately in individual tubing grillages, assembled and anchored to the filter bottom in a horizontal position. The grillage for washing air is composed of PVC Ø50 or Ø60 mm tube bodies (so-called "washing air bodies") situated on the filter bottom parallel to each other. The grillage for washing water distribution and filtered water disposal is assembled using PVC Ø160 mm tube bodies (so-called "washing water bodies"). There is always a washing water body embedded between two washing air bodies. The washing water distribution grillage can be used for individual distribution of washing air. This capability may be employed for more intensive washing with air or for occasionally verifying the condition of washing water bodies under the filtering material.

The drainage system can be used for separating, at the filter bottom, the washing air at an intensity from 3 to 30 l/s and the washing water at an intensity from 3 to 18 l/s per 1m2 of filtering area into the filtering material. The applicable rates of filtration depend on the filtering material, the height of the water column or overpressure of the water above the filtering material, on the requirements for a filter liquor, and so on. In water-plant gravity rapid sand filters where this drainage system is applied the rates of filtration ranging from 0.8 to 9.6 m/h have been so far employed. The practically utilisable limit for this type of filters is approximately 15 m/h.

In the washing air bodies as well as washing water bodies there are fitted special nozzles made of PP that are used for transferring washing air or washing water from the inside of tube bodies into filtering material and, in the opposite direction, the filtered water. Approximately 105 water and air nozzles with removable filter nozzles fitted with slots of a width of 0.4 mm cover 1m2 of filtering area. Filtering material with a minimum grain size of 0.6 mm can be placed on the drainage system directly, that is, without a transition layer of coarser material. 

The structural and hydraulic solution to the drainage system allows for uniform distribution of washing air and washing water over the entire filtering area, with deviations of up to ±2.5% (these differences in interbottoms with the nozzles are substantially greater even at the nozzle fitting accuracy of ±1mm). Due to this solution a very good washing efficiency is obtained as well (verified by a series of measurements performed by a water development agency) and, when compared to nozzle interbottoms, the occurrence of unwashed spots in filtering material is substantially reduced. When assembling the drainage system, tolerances +15 and -10 mm from a specific height level of the nozzles must be adhered to (as for the nozzles in interbottoms, adherence to tolerances of ±1mm is required).

The applied spigot and socket joints and anchorage of the drainage system at the filter bottom have a minimum of 300 % resistances to overpressures and forces acting on the drainage system on the filter bottom during the filtering process or during washing of the filtering material.

A simple provision was put into practice to eliminate leakages of filtering material via the drainage system into the follow-up piping or ducts for washing air or washing water intake and for filtered water disposal (for example, during possible damage to any of the nozzles).

It is possible to walk on the drainage system without any risk of damaging it. Individual burdens up to a weight of 120 kg can be placed on the drainage system. Both capabilities can be made use of when assembling the drainage system or during possible repair works.

Filters with a specific tube drainage system are of approximately 0.7m lesser structural height than the height of the filter interbottoms. This fact can be used advantageously during a reconstruction of the filters with interbottoms. With the reconstruction in progress, the interbottom is removed, the tube drainage system is laid upon the adapted filter bottom and the substantial remaining height under the removed bottom is utilised actively for filtering (increase in the filtering material thickness or height of the water column above the filtering material). A reconstruction performed in this way does not require any change in the existing washing pump (if it is in a satisfactory technical condition). However, an assessment must be made as to the sufficiency of the pressure head of the blast machine in the case the washing air outlet into filtering lands is lower by more than 0.2m of the water column after reconstruction.