Mixing characteristics of a model anaerobic digester
Creators: name: Low, Siew Cheng
Publisher: name: RMIT University
Year: 2024

description: Anaerobic digestion is used to convert unusable waste such as livestock manure, wastewater, solid waste and food waste into valuable end products such as biogas. Wastewater treatment plants are required to intensify wastewater digestion process using existing digester capacity by processing concentrated sludge, which affects digester mixing performance. There have been many studies on digester hydrodynamics, but digester mixing is still carried out based on empirical evidence rather than fluid mechanical studies. The main objective of this work is to determine the optimum mixing method/s for digesters using a hydrodynamic study. This involved a model liquid to represent sludge. Flow visualisation and electrical resistance tomography (ERT) are employed in this work. Also, computational fluid dynamics (CFD) models are developed and validated using flow visualisation experimental data. <br><br>All experiments were conducted in a 0.19 m diameter cylindrical tank. Three mixing systems, namely mechanical mixing, liquid recirculation and gas sparging, were implemented. Xanthan gum Keltrol-T (XGKT) solution was selected as the model liquid because its rheological characteristics are similar to digester sludge. Specific power inputs ranging from 3.5 to 10.5 W/m3 were employed. Dye tracer and brine solution were used as tracers in flow visualisation and electrical resistance tomography (ERT) techniques respectively. Inactive mixing volume was determined by measuring the volume of the coloured and low conductivity regions in flow visualisation and ERT experiments, respectively.<br>Experimental results show that the total inactive volume in the model digester decreases with increasing mixing time and reaches a plateau. Larger inactive volume was found in solutions with higher apparent viscosity regardless of the mixing mode used. Increasing power input beyond a certain value was found to have no influence in the reduction of inactive volume in liquid recirculation and gas sparging. In case of mechanical mixing, a threshold power input is required to achieve an acceptable value of the active volume.<br><br>Experiments with liquid recirculation and gas sparging systems were conducted with different nozzle orientations to study their effect on inactive volume reduction. Downward jet in liquid recirculation system produces the largest inactive volume, and upward and angled jets lead to smaller inactive volumes. In gas sparging system, the downward sparger with a low tank bottom clearance leads to the smallest inactive volume. Therefore, the position and orientation of fluid inlet nozzles are very important in the design and operation of digesters processing thicker sludge.<br><br>ERT can be used to estimate the shape and size of the active mixing region. It provides information on the inactive volume reduction rate and the final dead volume up to a certain level of accuracy depending on the mixing system. However, the estimations require further investigations and improvements. Overall, ERT has the potential of being used in the study of inactive volume in real sludge.<br><br>CFD models are developed only for mechanical mixing and liquid recirculation systems. The models estimate the flow patterns and the inactive volume fairly well. However, the CFD model for the mechanical mixing system could not predict the presence of a threshold power input).<br>