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

National audienceIn 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

Using water activity measurements to evaluate rheological consistency and structure strength of sludge

In this study, an original method of characterization of water/solid matter interactions in sewage sludge has been developed, based on both rheological characteristics and thermodynamic water activity determinations, in order to check whether a link could be made with water activity and its mechanical properties. The effect of solid matter content, flocculation and ageing time on water activity and rheological parameters has been investigated. Through this study, we showed that rheological parameters (G’ and G’’) of both raw and flocculated sewage sludge at optimal dose of polymer increase with solids concentration following a power-law, whereas in the same way water activity decreases following an exponential relationship (Arrhenius dependence). A slight increase of water activity values and those of G’ and G’’ moduli with added polymer was also highlighted. On the other hand, we have shown that during ageing, the rheological parameters G’ and G’’ decreased upon increasing the ageing time whereas in the same way, water activity increases. Rheological parameters clearly evidenced thermodynamic water activity dependence, regarding solid concentration and ageing time, with a decrease of G’ and G’’ upon increasing water activity. Thus, water/solid matter interactions were supposed to intervene importantly in the rheology of sewage sludge. From all these results, it appears clearly that water activity measurements can be regarded as an effective and easy tool for evaluating the state of structuration of sewage sludge and for predicting its structural, textural and mechanical properties during dewatering and ageing

Near-infrared spectroscopy and multivariate curve resolution-alternating least squares for the study of time-dependent structural changes of laponite aqueous colloidal gels

International audienceLaponite aqueous colloidal suspensions undergo structural evolution with time. The time-dependent changes in chemical or physical structure are usually termed as ageing process. In this work, we study the ageing behavior of Laponite aqueous colloidal gels having 2 and 3 % (w/w) of Laponite using near infrared spectroscopy (NIRS). The aim of this study is to investigate the ability of near-infrared spectroscopy combined with Multivariate Curve Resolution-Alternating Least Squares (MCR-ALS) analysis to monitor and detect changes in structural properties of Laponite aqueous colloidal suspensions during the ageing process. To enable resolution and provide meaningful results, difference spectra were used. This strategy enhances the fine spectroscopic changes undergone during the process and allows characterizing the effects of the perturbations on the NIR spectra in terms of changes in hydrogen bonding and scattering phenomena

Study of time-dependent structural changes of laponite colloidal system by means of near-infrared spectroscopy and hybrid hard- and soft-modelling multivariate curve resolution–alternating least squares

International audienceIn this work, we study the gel formation dynamics after shear rejuvenation of a laponite colloidal model system using near-infrared (NIR) spectroscopy. The aim of this study is to investigate the ability of NIR spectroscopy combined with hybrid hard- and soft-modelling based on multivariate curve resolution–alternating least squares (MCR-ALS) analysis to monitor and detect changes in structural properties of colloidal systems during dynamic processes. To enhance the f ine spectroscopic changes occurring during the process and to characterize the effects of slight perturbations on the NIR spectra, difference spectra were used. The hybrid hard- and soft-modelling ver- sion of MCR-ALS (HS-MCR), which allows the introduction of kinetic models to describe the process behavior, is used in this case to separate the contributions found to be present during laponite ageing ("gel formation") dynamics: one related to a chemical effect (loss of solvated water in the network structure of the laponite system, following a kinetic model, i.e. hard-modelled) and one related to a physical effect (changes in scattering phenom- enon due to the presence of laponite particles, i.e. soft-modelled). The presented methodology is proposed as a general modus operandi applicable to time-dependent kinetic processes in colloidal systems, where the nature of the colloid can produce chemical and physical contributions to the NIR measured signal and transformations may have to be followed through the subtle changes of the water environment surrounding the colloid particles