Sewage sludge ash (SSA) arises as a by-product of the treatment of wastewater by combustion and has been identified for potential use in various construction products, such as brick manufacture, artificial aggregate and cement replacement in concrete. This paper reports on research that looked at replacing the cement or fine aggregate fraction in concretes with SSA from three UK wastewater plants. The first phase of the study established whether the physical and chemical properties of SSA were suitable for use in concrete. The potential reactivity of the material, when used as a cement replacement, was assessed for mortar specimens. In the second phase, SSA was used as a cement replacement in structural grade concrete and as a fine aggregate in foamed concrete. The overall findings of the study were that although the chemical properties of SSA indicated that it had limited reactive abilities, the porous and friable nature of SSA was such that it masked any strength development in conventional concrete and may require further processing to be used effectively. However, this characteristic was a benefit when used as a fine aggregate in foamed concrete, as it enhanced the thermal properties of the concrete by more than a third.

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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.