The hydraulic transportation of thickened sludges

Industries which pump sludges are under continuous pressure to decrease water content, and increase concentration. Environmentally superior disposal techniques are demanding that such sludges have high mechanical strength properties. This results in a sludge with an increasing viscous character. At high concentration, the viscous forces - which are usually highly non-Newtonian and yield stress in nature - become dominant, and flows inevitably become laminar. The objective of this paper is to demonstrate the effect and evaluate the impact that increasing non-Newtonian viscous stresses - particularly yield stress - have on the pipelining problem. An industrially relevant sludge pipe flow study is presented, demonstrating and quantifying the relationship between sludge rheology and flow regime. It is argued that laminar flow will result in settlable solids accumulating on the pipe invert, leading to pipe blockage. Although some practical remedies have been proposed, this problem requires urgent and focussed research.