Towards a rheological parameter for activated sludge bulking characterisation

An examination of the rheological behaviour of activated sludge presenting filamentous bulking was carried out. It was shown that rheograms from filamentous sludge exhibit a particular behaviour due to its thixotropic characteristic, contrary to non-filamentous ones. A rheological protocol of measurement of thixotropy consisting of submitting sludge to increasing–decreasing shear rates was proposed and optimised to obtain the greatest sensitivity as possible on the determination of the proliferation of filamentous bacteria in activated sludge. A relevant parameter, consisting in the determination of the reduced hysteresis area developed (rHa) from rheograms, for indicating the presence and the amount of filamentous bulking was deduced. An application to the study of several bulking developments was carried out. The thixotropic characteristic of filamentous sludge was shown to originate from an irreversible breakdown of the filamentous matrix. The total suspended solid (TSS) content was shown to have a great influence on the value rHa. The morphological characteristics of filaments were supposed to have an influence on the value rHa too. The carbohydrate content of activated sludge was shown to be linearly linked to the rheological parameter rHa/TSS during the filamentous growth phase and an increase in rHa/TSS was followed by an increase in SVI.