Improving the mixing performance of anaerobic digesters in wastewater treatment
Creators: name: Kennedy, Stephen
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400410
subject: Wastewater treatment processes
classificationCode: 401212
subject: Non-Newtonian fluid flows (incl. rheology)
classificationCode: 401299
subject: Fluid mechanics and thermal engineering not elsewhere classified

description: Effective treatment of municipal wastewater is becoming increasingly important to good water governance. It is vital that every operation in the wastewater treatment process be optimised to achieve maximum efficiency and sustainability. One such operation is anaerobic digestion, in which microbes act in the absence of oxygen to turn sludge into odour free bio-solids and methane-rich biogas, from which energy can be recovered. Tenney &amp; Budzin (1972) argued that around 50% of the geometric volume of an anaerobic digester is stagnant and thus oversizing is a necessity. It is the contention of this project that an increase in capital cost of such magnitude is unnecessary and can be avoided if the all parameters including mixing requirements are carefully considered. Mixing is important to the digestion process as it provides the necessary contact between the feed sludge and the active biomass, which in turn gives uniform temperature and substrate concentration throughout the digester. In the interest of sustainability, there is a push in the wastewater industry to move to more concentrated feed stocks to conserve water. While an increase in solids concentration provides a richer feed stock for digestion, it also presents a challenge regarding the hydrodynamics of mixing in anaerobic digesters.<br><br>There are three mixing techniques usually employed for anaerobic digesters, mechanical mixing, gas sparging and liquid jet recirculation mixing. This project focuses on the latter, which involves pumping sludge out of the digester, passing it through a heat exchanger and returning it to the digester through a nozzle submerged in the liquid body of the digester. As sludge is an opaque fluid and therefore difficult to study on a lab-scale, this work employed a model digester using xanthan gum Keltrol T (XGKT) solution, which has been shown to mimic sludge across the range of shear rates found in anaerobic digesters.<br><br>A flow visualisation technique was employed to study the effect of a number of factors on the creation of active volume over time. In the tank with a 1:1 aspect ratio, three distinct mixing regimes were identified using a downward-facing nozzle placed at half the liquid height, each associated with different hydrodynamics at play. The effects of jet velocity and fluid rheology were investigated, and it was found that fluid rheology has a much greater impact on mixing performance than jet velocity. Increased jet velocity was even shown to have a detrimental effect on mixing performance over the observed timescale.<br><br>Through first-principles analysis of advection-diffusion, solutions were developed to describe the expansion of active volume in terms of advection-diffusion in bounded and unbounded domains. It was found that the solution for the unbounded domain can be fit to the creation of active volume in the early stages of mixing and the solution for the bounded domain can be fit to later stages of active volume creation. However, the majority of active volume creation occurs in transition between the two solutions. <br><br>To understand why the majority of active volume creation occurred between the two ideal advection-diffusion equations, a state-of-the-art electrical resistance tomography (ERT) technique was employed to fully investigate the 3D mixing environment. It was found that the creation of active volume begins to deviate from the unbounded advection-diffusion curve as the suction flow field begins to interact with the jet flow field and ‘short-circuiting’ occurs. This short-circuiting effect becomes more pronounced in fluids with more complex rheology such that steady-state cavern formation occurs in fluids with a yield stress. This has not been observed in jet recirculation mixing before.<br><br>To overcome the steady-state carven formation in rheologically complex fluids, a simple nozzle inversion was employed such that the nozzle is pointing upwards towards the flexible surface. In all cases, an upward-facing nozzle performed better than a downward-facing nozzle because a surface velocity induced by the jet aided in the mixing process, which was not found when the jet was pumping against the rigid floor of the tank. Furthermore, rather than generating a short-circuiting effect that results from having two competing flow fields, an upward-facing ‘jet’ nozzle and a downward-facing ‘suction’ nozzle lead to two complimentary flow fields that greatly enhances the mixing performance.<br><br>Based on the interaction between jet and suction flow fields, a design equation was formulated for jet mixing based on the separation of the jet and suction nozzles. The equation was based on the observed trend that mixing time decreases with increased separation of the two nozzles up to a certain point. Beyond this point, the further separation of nozzles has no effect on mixing time. This has beneficial implications in designing jet recirculation systems with multiple inlet and outlets, although, further tests need to be conducted with different configurations on the pilot and industrial scales to verify this.