Factors Affecting Capillary Suction Time of Sludge Using Wet Front Distance and Filtrate Flow Rate Methods

Sludge generated during wastewater treatment is difficult to handle due to their high water content, hence a need for dewatering. The factors that affect the outcome of capillary suction time (CST) were studied using piggery and domestic sludge samples conditioned with ferric chloride. Effects of conditioner concentration, hydrostatic pressure and mixing time were investigated. Filtrate flow rate studies were also carried out as an alternative to wet front progression to determine CST. Ferric chloride was varied from 0.002 to 0.008 g/ml, hydrostatic height was varied from 20 to 40 mm while mixing time ranged from 0 to 120 sec. The optimum conditioner concentration was observed to be 0.0055 and 0.0035 g/ml for piggery sludge and domestic sludge respectively. At 5 % level of significance, hydrostatic height was observed to have no significant effect on the CST for the range of hydrostatic heights chosen. The optimum mixing time observed for 90 g/l and 30 g/l piggery sludge was 50 and 25 sec respectively. While the optimum mixing time for 28 g/l and 14 g/l domestic sludge was 10 and 5 sec respectively. Flow rate was discovered to have an inverse relationship with CST, with the optimum conditioner concentration corresponding to the peak flow rate and minimum CST for both test samples. Hence, in other to eliminate the difficulty presented by the anisotropic property of the capillary suction apparatus (CSA) filter paper, filtrate flow method can be used instead of wet front progression method.

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.