(181ah) Selecting a Clear Model Fluid with Similar Rheological Behavior to Municipal Sludge

Anaerobic digestion is a key treatment process for solids treatment and energy generation in sewage treatment plants. 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 excludes visualization of mixing flow patterns, 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 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. The rheological behaviour of sludge was therefore modeled using CMC in steady state, Carbopol gel for short-time processes and laponite clay for thixotropic modeling.