Viscosity evolution of anaerobic granular sludge

The evolution of the apparent viscosity at steady shear rate of sieved anaerobic granular sludge (20–315 μm diameter) sampled from different full-scale anaerobic reactors was recorded using rotation tests. The “limit viscosity” of sieved anaerobic granular sludge was determined from the apparent viscosity. The limit viscosity values depended on the applied shear rate, indicating a non-Newtonian behaviour of anaerobic granular sludge. The tests to determine the apparent viscosity values were carried out at a constant shear rate of 500 s−1, which was found to be the optimal value to obtain the limit viscosity. The total suspended solids (TSS) content of the sieved granular sludge was shown to strongly influence the limit viscosity value and an exponential relationship was found between the TSS content and this rheological parameter. The limit viscosity value increases with decreasing size and surface charge of the granules. This underlines the importance of the basic physical sludge characteristics (TSS content, size of granule, surface charge) on the quantitative interactions between granules (i.e. limit viscosity value). Significant differences in limit viscosity values were found for granular sludges of different origin. This indicates the ability of the rheological parameter “limit viscosity” to describe different overall characteristics (TSS, granulometry, origin, charge of surface…) of anaerobic granular sludge within the size class investigated.