Nonlinear filtration behavior of soft particles: Effect of dynamic cake compression

For activated sludge, manure, dextran-MnO2 particle suspensions, and other organic slurries, the filtrate volume increases nonlinearly with the square root of time (nonlinear filtration behavior). This study uses dextran-MnO2 particles as a general model compound, and the observed behavior is discussed in relation to sedimentation, blinding, medium resistance, and dynamic cake compressibility. Only dynamic cake compressibility is able to explain the experimental data. The soft solvent-swollen dextran materials deformed at high pressure, filling the voids in the cake. This was a dynamic process: specific cake resistance increased with time, resulting in nonlinear filtration behavior. The dynamic cake compressibility implies that the equilibrium porosity in the cake is not reached instantly during filtration. Thus, the effect of time on porosity and resistance must be considered when dewatering organic materials.