Analysis of the method of calculating high resistance drainage of rapid filters

Summary

The methods of calculating high resistance drainage distribution systems for rapid filters are considered. In the practice of designing filtration plants in Russia several approaches to the structural concept of drainages that provide for equilibrium distribution of wash water in area have been worked out. It is shown that the method proposed by VNII VODGEO is the most substantiated; therefore it seems advisable to restore the recommendations on the use of this method in the practice of designing filtration plants in the regulatory documents. The analysis of the practical use of different approaches to the most useful method of calculating high resistance drainages for rapid filters shows the need to take into account a whole complex of indicators in these calculations. The most important among them is the indicator of drainage open ratio as well as assumed values of original flow rate of wash water in collectors (channels) and branches that determine the value of recovered head.

Key words

rapid filter , drainage-distribution system , wash uniformity , drainage open ratio , backwash loss

On some specific features of calculating drainage-distribution systems of large area rapid filters

Summary

Two main methods of calculating drainage-distribution systems for uniform washing of rapid filters are known: as regards the relation of the total surface area of drainage slots to the filter area and as regards the so-called relation of the total surface area of drainage slots to the cross-sectional area of the collecting manifold. The second method seems to be more consistent since it provides for the direct link between the values of the recoverable dynamic pressure in the pipes and the total head loss in the drainage required for the uniform filter wash. When using this method for calculating large area filters it is suggested to take into consideration the head loss at the sudden expansion of the cross section of the wash water flow at the head of the central (side) canal.

Key words

rapid filter , drainage-distribution system , pressure losses , wash uniformity , central canal

Method of designing tubular porous rapid filter drainage

Summary

In recent years the specialists of water treatment plants pay much attention to rapid filter drainage systems of porous type. This design excludes the drawbacks typical for drainage systems with supporting gravel layers, major of them being gravel layer slip that causes frequent filter reloading (typically every 3–5 years). The method and basic formula for calculating tubular porous drainage are presented. In view of specific design tubular porous drainage calculations differ significantly from calculating widely used tubular perforated drainage systems of high resistance with supporting gravel layers. In the latter the pressure loss required for uniform filter wash occurs in one stage – in drainage openings. In tubular porous drainage design the pressure is successively lost in two stages – in the openings and outside porous layer, or in three stages – in the upper drain openings, inner porous layer, bottom drain openings. The summarized pressure loss in the mentioned two stages must provide for 95% uniformity of filter wash.

Key words

water supply plant , rapid filter , pressure losses , tubular porous drainage , wash uniformity