Studied sludge samples are composed of major geothite and quartz; less clay minerals; and minor magnetite, hematite, clinochlore and todorikite. They have quite similar qualitative, but different semi- quantitative compositions. There are similar particle size distributions between the samples, and the higest contents of ~50% belongs to the finest classes of <6_m. Among size classes within the samples, almost identical iron contents are present; indicating their similar mineral compositions, which make these systems very complex for further separation processes. Sludge II has a higer natural settling rate, due to its higer density and mineral composition. With addition of the flocculant, settling rates increase significantly with increase of liquid component in both of the samples. The effect of flocculant on the settling rates is different between samples, and depends on their mineral composition.
