Correlation between impedance and rheology measurements on sewage sludge: a preliminary study

Sewage sludges treatment and valorization depend upon their consistency: liquid, pasty or solid. Evident at first sight, the distinction between these states turns out to be quite complex to practically determine. The question is then: how define clearly the transition between these different states of sludge consistency and explain it? Traditionally used to characterize them, siccity is not the only criteria determining rheological sludge behavior (Baudez, 2001). Similarly, composition can not be considered as the main parameter, since there are strong similarities between different ages and origins sludges (Baudez et al., 2004). Since water is the common element to all types of sludge, we could wonder about its qualitative but no quantitative impact on the consistency. Performing technique to study a media flow properties, rheometry allows to obtain sludge consistency descriptors. In the same way electrical impedance spectroscopy gives data on a material humidity or its states of water (free or bound water). Thus, we aimed at finding a relationship between these both information. Our experiments consist in doing on many materials a sweep over a range of frequencies from 42 Hz to 5 MHz. The electrical parameters evolution (instance, magnitude of complex impedance) is then related to the torque measured by submitting substrates to shear rate levels in a Couette geometry. The results show there is a correlation between rheology and impedance: at a defined frequency, module decreases as sludge flow threshold increases. Such a relationship opens new perspectives to the comprehension of sludge structure evolution under a mechanical solicitation.