Rheological measurements on different types of sewage sludge for pumping design

• The rheology of recycle, anaerobically digested and raw mixed sludge was examined.
• Bingham parameters at various concentrations and temperatures were evaluated.
• Correlations of such parameters with concentration or temperature were determined.
• Correlation of thixotropy with concentration or temperature was established.
• These correlations can be used for sludge pipeline design.

Sewage sludge pumping could represent an optimal solution to assure adequate treatment of sludge in centralized plants, with a consequent reduction of the environmental impact of sludge disposal (volume, odour, putrescence), because small wastewater treatment plants usually do not provide an adequate treatment due to high costs. An accurate knowledge of rheological parameters is required to compute head loss for pipeline design, but only few data are available. In order to circumvent the problem of the scarcity of sludge rheological data, we have performed tests on different types of sludge, with solids concentration and temperature in the typical range of a conventional wastewater treatment plant. Bingham rheological parameters and sludge thixotropy values have been processed by regression analysis to identify their dependence on solids concentration or temperature. The results of this study allow the definition of guidelines and optimal strategies for designers in order to reduce pumping costs.