CFD simulation of anaerobic digester with variable sewage sludge rheology

Abstract

A computational fluid dynamics (CFD) model that evaluates mechanical mixing in a full-scale anaerobic digester was developed to investigate the influence of sewage sludge rheology on the steady-state digester performance. Mechanical mixing is provided through an impeller located in a draft tube. Use is made of the Multiple Reference Frame model to incorporate the rotating impeller.The non-Newtonian sludge is modeled using the Hershel-Bulkley law because of the yield stress present in the fluid. Water is also used as modeling fluid to illustrate the significant non-Newtonian effects of sewage sludge on mixing patterns. The variation of the sewage sludge rheology as a result of the digestion process is considered to determine its influence on both the required impeller torque and digester mixing patterns. It was found that when modeling the fluid with the HershelBulkley law, the high slope of the sewage stress-strain curve at high shear rates causes significant viscous torque on the impeller surface. Although the overall fluid shear stress property is reduced during digestion, this slope is increased with sludge age, causing an increase in impeller torque for digested sludge due to the high strain rates caused by the pumping impeller. Consideration should be given to using the Bingham law to deal with high strain rates. The overall mixing flow patterns of the digested sludge do however improve slightly.

CFD Analysis of Slurry Flow in an Anaerobic Digester

“…Anaerobic digestion is a process that uses micro-organisms to break down biodegradable materials into other organic and inorganic compounds without using oxygen (Craig et al, 2013;Mendoza et al, 2011). In anaerobic digestion, as a result of these anaerobic bacterial activities, the sludge will be transformed into a stable product and a gas, called biogas, which can be used as an energy source alternative to fossil fuel (Coughtrie et al, 2013;Lopez-Jimenez et al, 2015).…”

“…Some experiments have shown laminar flow while others, at higher velocities, have shown transitional or turbulent flows. Wu, as quoted by Craig (Craig et al, 2013), stressed that most digester flows are turbulent and used a turbulence model to analyze the flow phenomena using CFD. The most appropriate model is the Large Eddy Simulation (LES), but it is expensive, whereas the Reynolds Averaged Navier-Stokes (RANS) models are low-cost but less precise.…”

“…The most appropriate model is the Large Eddy Simulation (LES), but it is expensive, whereas the Reynolds Averaged Navier-Stokes (RANS) models are low-cost but less precise. The Shear Stress Transport (SST)k-ω of RANS models provides the best results in a digester regarding gas mixing and agitating non-Newtonian slurry (Craig et al, 2013) This study evaluates a laboratory-scale anaerobic digester producing biogas used to drive a micro-gas turbine (MGT). Such flow simulation is necessary before constructing the actual digester to reduce the costs of design and construction.…”

Optimización de la agitación de un digestor anaerobio mediante mecánica de fluidos computacional

“…El segundo es la imposibilidad de realizar ensayos de laboratorio para validar los resultados obtenidos. Sin embargo, gracias a un profundo estudio la bibliografía existente, se han tomado los parámetros validados por otros autores mediante diferentes medios, tal y como hicieron anteriormente otros investigadores (Karim et al, 2007;Wu 2010cWu , 2011Wu , 2014 Yu et al, 2012;Coughtrie et al, 2013;Craig et al, 2013).…”

“…El segundo es la imposibilidad de realizar ensayos de laboratorio para validar los resultados obtenidos. Sin embargo, gracias a un profundo estudio la bibliografía existente, se han tomado los parámetros validados por otros autores mediante diferentes medios, tal y como hicieron anteriormente otros investigadores (Karim et al, 2007;Wu 2010cWu , 2011Wu , 2014 Yu et al, 2012;Coughtrie et al, 2013;Craig et al, 2013).En cuanto al sistema de agitación, se ha elegido la recirculación de fango mediante bomba ubicada en el exterior del digestor debido a los buenos resultados que este sistema ha dado en instalaciones existentes (EDAR Valdebebas) y a que el tiempo de cálculo es menor que para otros sistemas de agitación. La bomba toma el fango de la parte inferior y lo eleva hasta la parte superior del tanque.…”