Sludge porosity

Filter aids may be applied in one of two ways. The first method involves the use of a precoat filter aid, which can be applied as a thin layer over the filter before the suspension is pumped to the apparatus. A precoat prevents fine suspension particles from becoming so entangled in the filter medium that its resistance becomes exces-sive. In addition it facilitates the removal of filter cake at the end of the filtration cycle. The second application method involves incorporation of a certain amount of the material with the suspension before introducing it to the filter. The addition of filter aids increases the porosity of the sludge, decreases its compressibility, and reduces the resistance of the cake. In some cases the filter aid displays an adsorption action, which results in particle separation of sizes down to 0.1 /i. The adsorption ability of certain filter aids, such as bleached earth and activated charcoals, is manifest by a decoloring of the suspension s liquid phase. This practice is widely used for treating fats and oils. The properties of these additives are determined by the characteristics... 

The characteristics of the pump relate the applied pressure on the cake to the flowrate at the exit face of the filter medium. The cake resistance determines the pressure drop. During filtration, liquid flows through the porous filter cake in the direction of decreasing hydraulic pressure gradient. The porosity (e) is at a minimum at the point of contact between the cake and filter plate (i.e., where x = 0) and at a maximum at the cake surface (x = L) where sludge enters. A schematic definition of this system is illustrated in Figure 2. 

The fractions of iron and arsenic in the sediments that would have to be released to support the observed concentrations of dissolved iron and arsenic in the porewaters can be calculated using Eq. (1) with appropriate values of specific gravity, D = 2.5 g/cm for coagulant sludge (126), and porosity (([) = 0.9). Figure 5 illustrates that the calculated fractional solubilization is quite low, less than 2.5% for all samples, and is similar for iron and arsenic for most samples. The... 

Li D. and Ganezarezyk, J. (1987). Stroboscopic determination of settling velocity, size and porosity of activated sludge floes. Water Res., 21, 257-262. 

The second application method involves incorporation of a certain amount of the material with the suspension before introducing it to the filter. The addition of filter aids increases the porosity of the sludge, decreases its conpressibility, and reduces the resistance of the cake. 

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