SOLIDS ACCUMULATION DURING DEEP BED FILTRATION

A series of pilot plant experiments provide data on particle accumulation and increasing hydraulic gradients within packed beds. For two medium sizes and three filtration velocities, increased hydraulic gradients normalized by clean-bed gradients all follow the same dependence on sigma, the mass of solids deposited per filter volume. For sigma less than 4 g/L, the normalized gradient goes as sigma2/3. Particle accumulation ceases when solids have accumulated to only 15 g/L, which corresponds to less than 2% of the pore space occupied by deposited solids. After the mass of retained solids reaches a steady-state value, the hydraulic gradients continue to increase. Quantitative models can be fitted to these observations, but only if the accumulated particles form a porous deposit that decreases in porosity as particles accumulate. The processes causing this porous deposit to decrease in porosity could be internal filtration caused by flow through the deposit or consolidation caused by hydrodynamic forces.