Filtration of submicron particles: effect of ionic strength and organic substances

Hematite (α-Fe₂O₃) suspension was used in this study to investigate the effects of aggregate size and solution chemical characteristics on particle removal and headloss development in a deep bed filter. When electrolyte (KCl) concentration in the suspension was increased from 0-80 mM, the size of hematite aggregates in suspension increased from 85 to 990 nM but had no significant effect on zeta potential. It was found that at low KCl concentrations (0-10 mM), the initial removal is governed by surface chemical properties while at higher concentrations (40-80 mM), the effect of particle size (steric effect) predominates. The transient stage filter performance was found to be controlled by the favourable chemical conditions prevailing in the filtration system rather than the size of the particles.

Studies of the effect of a natural organic material (fulvic acid) on particle removal indicated that fulvic acid at low concentrations (less than 0.75 mg/L) enhanced particle aggregation and improved filtration efficiency. At concentrations higher than 2 mg/L, it was found to increase the stability of particles thus affecting filtration efficiency.