Water/solid matter interactions in sewage sludge: Linking rheology and water activity

In the current economical and environmental context, biosolids management is of major concern. Specific attention is particularly paid to rheological measurements which are presented as of special interest for mixing, chemical conditioning or dewatering [1]. However, sewage sludge is often seen as a complex mixture and its rheological behaviour is considered to be highly dependent on the implemented treatments [2-3] and on its composition [4]. Thus, engineers need reliable data to model their process and research is focusing on parameters to evaluate physical consistency and rheological characteristics. On a practical point of view, because sewage sludge can be regarded as soft matter, improving knowledge on water/solid matter interactions is one of the clues to improve sludge rheology. In that purpose, this work focuses on the characterization of the water/solid matter interactions, based on water activity determination. These results are then related to sludge rheological properties, in order to check whether water activity can be representative of the evolution of sludge structure. Water activity is determined thanks to relative humidity measurements whereas oscillatory tests are performed to define sludge rheological behaviour. The impact of flocculation, solid content and ageing time on water activity and rheological parameters is studied. In accordance with the literature, in the range of considered solid contents, sewage sludge shows viscoelastic behaviour and the impact of flocculation is clearly highlighted. For raw sludge, both elastic and viscous moduli increase with the solid content whereas water activity decreases: the link between rheological characteristics can be modelled with two linear relationships, underlying a change between diluted and concentrated pastes. Similar tendencies are obtained with flocculated sludge. Polymer addition basically increases the values of elastic and viscous moduli and tends to slightly increase water activity values (for a given solid content), indicating water is becoming more accessible. The evolution of water activity and rheological properties with the ageing time is also shown, indicating that the change of sludge composition impacts water mobility.