Improvements of sewage sludge characterization | Optimiser la charactérisation des boues résiduaires

According to the current social economical and environmental context, sewage sludge production is increasing and their management is of major concern. On a practical point of view, to limit the volumes and to assure their valorisation success (for example agricultural spreading), it is necessary to optimize sludge secondary treatment and to characterize their consistency. With this purpose, taking into account the difficulties the professionals are confronted with, we focused on the combined contribution between rheological and electrical measurements, the latter aiming at overcoming the lack of knowledge about sludge composition. Indeed, though rheology is a performing tool, it needs a real expertise and often expensive equipments, whereas electrical measurements are cheaper and easier to perform. The similarities existing between rheological and electrical domains just as the use of electrical techniques to analyse materials such as concrete (which has a behaviour close to sludge) leads us to choose conductivity measurements and electrical impedance spectroscopy as new ways of analysis. Thus, in order to substitute electrical for rheological measurements during sludge treatment process and also to develop our knowledge of sludge behaviour we studied both sludge and other materials. First of all, experiments were realised on different materials such as Newtonian fluids, Carbopol® gels and bentonite suspensions, chosen for their specific structure and rheological behaviour. Through the study of many parameters (concentration, pH, aging, temperature), we demonstrated the importance of the nature of the charges, bound or free, on electrical and rheological characteristics of the material. Moreover, similarly to liquid electrolytes or solids as rocks, we established the existence of a relationship between the conductivity and the viscosity of pasty materials. Secondly, this analytical method was applied to sewage sludge. The impact of many parameters on the electrical and rheological behaviour of sewage sludge was determined: dry content, temperature, fermentation degree, salts. That allowed us to improve sludge's characterization and to determine an indicator for their dewaterability which would lead to a better control of sewage sludge treatment processes. The purpose of this communication is to show the potential of this double analytical method and especially how the combination of rheological and electrical measurements on sewage sludge leads to the improvement of their treatment and valorisation.