Transparent fluids for the rheological behaviour of thickened municipal sludge

Optimizing flow processes in wastewater treatment plants requires that designers and operators take into account flow properties of the sludge. Moreover, due to increasingly more stringent conditions on final disposal avenues such as landfill, composting, incineration etc., practitioners need to produce safer sludge in smaller quantities. Anaerobic digestion is a key treatment process for solids treatment and pathogen reduction. Due to the inherent opacity of sludge, it is impossible to visualize the mixing and flow patterns inside an anaerobic digester. Therefore, choosing an appropriate transparent model fluid which can mimic the rheological behaviour of sludge is imperative for visualization of the hydrodynamic functioning of an anaerobic digester. Digested sludge is a complex material with time dependent non-Newtonian thixotropic characteristics. In steady state, it can be modelled by a basic (but truncated) power-law. However, for short-time processes the Herschel-Bulkley model can be used to model liquid-like properties. The objective of this study was to identify model fluids which will mimic the behaviour of real sludge. A comparison of three model fluids, Carboxymethyl Cellose (CMC), Carbopol gel and laponite clay revealed that these fluids could each model certain aspects of sludge behaviour. It is concluded that the rheological behaviour of sludge can be modelled using CMC in steady state at high shear rates, Carbopol gel for short-time processes and laponite clay suspension for thixotropic.