Improved design and performance of continuous thickeners should be achieved through a better understanding of the sludge transport processes involved and the variability of sludge thickenability. A software package called PHOENICS was used to model the three-dimensional flow of sludge in the transport zone of a 20m diameter thickener. A mass transport efficiency was evaluated to test the efficacy of the ploughing system, and tracer simulations were performed to study the degree of back-mixing induced. Interpretation of the results suggests that:
Optimum orientation of the ploughs to the radial arm is 60° to 70°.
Transport of sludge to a central outlet occurs principally in the wake of the advancing ploughs, and that ploughs are less efficient than conventional theories dictate.
Sludge in the ploughing zone is mixed, thus potentially impairing thickener performance.
The thickenability of mixed raw sludge arising at a large activated sludge works was monitored. Pronounced seasonal variations were observed, with a marked deterioration in thickenability in late summer and autumn. These were confirmed in trials of a 20m diameter continuous thickener. Consequently the proposed thickening strategy for this works has been revised.