Clear model fluid with similar rheological behavior of municipal sludge

Anaerobic digestion is a key treatment process for solids treatment and energy generation in sewage treatment plant. Currently, research is focused on solving the issues associated with efficiency of mixing in anaerobic digesters. This includes experimental investigations on real sludge followed by Computational Fluid Dynamics (CFD) modeling. Using opaque liquids such as sludge for experimental work limits visualization of mixing flow and subsequently the digester is treated as a "black box". Digested sludge is a complex material with both non-Newtonian and thixotropic characteristics. In steady state, it can be modeled by a basic (but truncated) power-law, however, for short-time processes the Herschel-Bulkley model can be sufficiently powerful to model liquid-like properties. A comparison of three model fluids, Carboxymethyl Cellulose (CMC), Carbopol gel and Laponite clay revealed that these fluids could only model certain aspects of sludge behaviour. The rheological behaviour of sludge was therefore modeled using CMC in steady state, Carbopol gel for short-time flows and clay suspension for thixotropic modeling.