The primary interest of this work is to determine the effect of floc concentration on ferric floc particle interactions and hence, theology. In accordance with established theory, ferric floc viscosity exhibits a non-linear increase with solids content, with the viscosity increasing sharply above a concentration of 12 000 ppm. Shear effectively breaks down floc structure, and thus reduces viscosity, the extent of which is independent of solids content. The effect of shear is also observed to diminish over time and can be adequately characterised by a power law model. Addition of aluminium nitrate to ferric floc increases the quantity of water bound within the floc, and thus produces suspensions which are more viscous in nature. (C) 2004 Society of Chemical Industry.