The reported literature on c. This review finds that protocols for proper rheological measurements are absent, and that good practice is often violated when modelling the rheological behaviour of activated sludge.
Rheological behaviour is an important fluid property that severely impacts its flow behaviour and many related aspects.
In the case of activated sludge, the apparent viscosity has an influence on e.g. pumping, hydrodynamics, mass transfer rates, and sludge–water separation (settling and filtration). It therefore is an important property related to process performance, including process economics.
To account for this, rheological behaviour is included in process design, which necessitates its measurement. However, measurements and corresponding protocols in the literature are quite diverse, leading to varying results and conclusions.
Here a multinational team critically reviews a large number of papers, and highlights important flaws with respect to rheometer choice, rheometer settings, and measurement protocol.
The rheograms obtained from experimental efforts have frequently been used to build viscosity models. But it is not that straightforward, and many errors can be detected with respect to good modelling practice, including fair model selection criteria, qualitative parameter estimations, and proper model validation.
However, these important steps are repeatedly violated, severely affecting the model reliability and predictive power. This is illustrated here with several examples.
The researchers conclude that dedicated research is required to improve rheological measurements and the models derived from them.
But at this moment, there is no guidance with respect to 'proper' rheological measurements. Moreover, the rheological models are not very trustworthy, and remain very much a ‘black box’.
Thus more insight needs to be gained into the physical background. A model-based approach with dedicated experimental data collection is the key to addressing this.
