Hydrodynamics Study of Sludge in Anaerobic Digesters

Inadequate design and improper mixing operation in anaerobic digesters often lead to poor mixing and digester failures due to the presence of dead zones formed in the digester. More research is required to understand the mixing phenomena in anaerobic digesters and thus improve their efficiency. This paper investigates the efficiency of mixing of digester sludge in a mechanically agitated vessel. A transparent model fluid (Xanthan Gum Keltrol T – XGKT) was used as a model fluid. Four different concentrations of XGKT solutions (0.15, 0.225, 0.3 and 0.4 wt%) were studied as they exhibits similar rheological characteristics of digested sludge with different solids concentration. Experiments were conducted using a standard six-bladed Rushton turbine in an unbaffled mixing vessel. Direct visualization of an acid-base neutralization reaction with fluorescent green dye was used to measure the mixing time. Changes in volume of unmixed flow elements i.e. ‘isolated mixing regions’ (IMRs) and well mixed flow elements i.e. caverns were studied by carrying out image analysis. Results showed that active volume decreases with increasing liquid viscosity at higher Xanthan Gum concentrations. In 0.15 wt% solution, IMRs are found above and below the impeller. With increase in mixing time, the IMRs destroyed continuously until disappeared. For 0.225, 0.3 and 0.4 wt% solutions, caverns were found around the impeller. With increasing mixing time, cavern grows and reaches a constant value. Homogenous mixing cannot be achieved with sludge, whose rheology is similar to that of 0.225, 0.3 or 0.4 wt% solutions, even after a long mixing period.