Hidrológiai Közlöny 1971 (51. évfolyam)

1. szám - Varró István: Felszíni vizek tisztításának korszerű módszerei és anyagai

42 Hidrológiai Közlöny 1971. 1. sz. Special Conference Number, Szebellédy, L. Filtration The important step in the teehnologieal process after coagulation, flocculation and clarification is filtration. The problems associated with filtration are considered by Professor íves in his paper on the role of theory in the advancement of rapid filtration technology. He established that during the last 10 years the technology of rapid filtration underwent considerable changes, thanks to the increased attention devoted thereto. This development can be attributed to the research work by Professor Mints, 1966, Soviet Union and Professor íves (1969) in the United Kingdom. Several different mathematical models have been conceived for describing the process of filtration, but all of them are based on a fundamental assumption, namely that the volume of suspension retained over unit depth of the filter is proportionate to the particular local concentration, where X is the filter coefficient. As will be perceived from the expression, the decreaseof concentration C depends on the distance L from the surface of the filter layer. Consequently, less of the material suspended in the water is carried further as the value of L increases. Considering the fundamental formuláé relating to the mechanism of filtration the following conclusions are arrived at: Owing to the wide variety of particles, the flow of fluid in the pores of the filter and to the great differences in the quality of natural waters, no theory can be developed by which the operation of filters could be accurately described. Nevertheless, a few useful hints can be derived from the theories forewarded so far on the fundamental phenoniena. Interception: where the contact between particles and filter grains results in adhesion. Diffusion: minute particles perform, owing to the thermal energy of surrounding water molecules, irregular, random movement and come thereby into contact with the partiele surface. Inertia: in the flowing suspension particles move at a certain velocity and tend to proceed in a straight line, even when surrounded by streamlines. Settling: the fundamental concept is that particles move in the direction of gravitational acceleration at a certain fali velocity (Stokes), as a consequence of which they are forced to cross the streamlines. Thus they come into the vicinity of the surface of filter particles, especially those facing the direction of flow. Hydrodynamics: where particles are carried through the streamlines in filter runs. The transport mechanism of particles are by no means exclusive, and particles may be exposed to the combined effect of several mechanisms. The adherence of particles to the surfaces of filter grains can be attributed to physical-chemical forces. However, the relevant theories are in need of further refinement. The useful hints derived from theory are as follows: 1. It is indicated by theory that certain measurements may be advantageous (e. g. pressure tests across the filter wall). 2. Observation data may be extended on the strength of theory (e. g. the linear variation of head loss with time if there are deposits förmed in the pores). 3. The development of experimentál models is made possible by a theoretical foundation (e. g. laboratory filtration equipment — íves, 1970). 4. Theoretical considerations help in deciding what new, or alternative designs are advantageous (e. g. linear, or radial filtration — Homer, 1969). It appears that theoretical and practical research conducted and completed so far has already established the foundations for further experimentál work aimed at the refinement ofexisting theories and the clearing of details concerning the individual processes. It would be appropriate to enlarge more on this valuable, paper, but the above remarks should already suffice to indicate that practical filtration technology is promoted and improved by correct theoretical research. I wish to add that the physico-chemical, and especially the electro-chemical processes involved in filtration are still far from being fully understood, so that more detailed research would be desirable in this field. Besides the thickness of the filter layer, the filter effect is affected alsó by the height of the standpipe, i. e., the height of the water column contained therein. Different results are obtained if the filtered water is released directly from the filter than when raising the standpipe to different heights above the surface of the sand filter layer. It is the resistance to flow which becomes effective here. Preliminary teehnologieal researeh In special cases the designing of water works drawing on surface water must be prececed by pilot plánt experiments in different seasons. The actual cüfficulties encountered in the development of a treatment technology are described in a highly visual manner in the paper by A. Pálhidy (MÉLYÉPTERV). The problem consisted in producing 20,000 cu.m drinking water per day from the water of the Eastern Main Canal. The flow in the canal being intermittent, the quality of water in the canal differs appreciably from that commonly encountered in rivers, or reservoirs. Flow and temperature conditions varv between very wide limits and give rise to extrernely variable biological life in the water. Drainage of excess waters may carry from agricultural areas a wide variety of dangerous herbicides into the canal. As a first step towards the solution of the problem the operation of the canal was changed.