Filtration by falling particles

In one variation of deep-bed filtration the liquid is passed upwards throug h a fluidized bed of particles. In this variation, filtration efficiency is very much dependent on some type of attraction between the bed particles a nd the suspended particles, otherwise the suspended particles flow easily t hrough the much larger voids in the fluidized bed, and filtration efficienc y is low. When this inter-particle attraction exists, another variation in filtration procedure is possible. This is to pass the fluidized bed downwar ds, by gravity, through the liquid to be filtered. This variation is the su bject of this paper. The ability of falling particles to induce a flow patt ern in the body of the liquid makes it possible to devise a very large scal e continuous filtration operation, to clarify volumes too large for other f iltration techniques, especially when the suspended solids are at a very lo w concentration. In all of the above, the word gas can be substituted for t he word liquid, and the whole spectra of gas cleaning by falling liquid or solid particles opens up. One of these - the cleaning of dust from the atmo sphere by falling raindrops - is a familiar and interesting example of simu ltaneous filtration and flow inducement. (C) 2000 Elsevier Science B.V. All rights reserved.