Extreme Solid Compressibility in Biological Sludge Dewatering

The degree of compressibility in the filter cake structure associated with mechanical dewatering of a polyelectrolyte conditioned biological sludge is investigated. Filtration/expression tests are carried out using both an A1(OH)3 and a biological sludge. A numerical model for filtration/expression is established and used to simulate liquid pressure development A comparison between measurements and calculations shows that the compressibility of the biological sludge is extremely high. Using filtration theories, the degree of compressibility is shown to be so high that a thin skin which accounts for nearly all the hydraulic pressure drop across the filter cake is likely to be formed just above the medium. This extreme compressibility behaviour suggests a reconsideration of the usual power law description of the relations between solid contact pressure, porosity and specific flow resistance. An equation showing the effect of the compressibility on the relation between filtrate flow and applied pressure during filtration is derived, and it is concluded that for extremely compressible solids, the filtrate flow becomes independent of the pressure applied.