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.

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