Experimental And Numerical Modeling Of High-Solids Anaerobic Digestion Of Dairy Manure In A Pilot-Scale Stirred Tank Digester

Experimental and computational fluid dynamics(CFD)simulations were conducted for highsolids anaerobic digestion of dairy manure in a mesophilic pilot-scale stirred tank digester operated at a 30 day hydraulic retention time.Mixing of the digester contents were achieved using an optimized dual six pitched blade disc turbine impellers,pitched at a 45° angle.Three mixing intensities: 50,100,and 150 rpm were investigated to determine the optimum range of mixing intensity at optimal power consumption to improve the anaerobic digestion energy efficiency.A Non-mixed experiment was conducted to serve as a baseline.The results of the methane yield and the specific methane production rate show that the 100 rpm performed better than the 50 rpm which also performed better than the 150 rpm mixing intensity.The reduction in the methane yield of 150 rpm compared with 50 and 100 rpm were 18% and 21% respectively.The results of the net energy production in kilowatt hours confirms that the 100 rpm is the economical speed,followed by 50 and 150 rpm.Dairy fresh manure slurry and digestate exhibits non-Newtonian fluids behavior with can be modeled using the power law model.The power consumptions predicted by the CFD non-Newtonian power law model were in good agreement with the experiment measurements of the power consumption.Further CFD simulations conducted to optimize the mixing intensity from 50 to 100 rpm at an increment of 10 rpm identified 70 to 80 rpm as the optimum threshold range for optimum methane production with 70 rpm as the optimal mixing intensity based on the average velocity and average velocity gradient.The net energy production from both the experimental and the CFD calculations indicates that about 99% of the energy produced from the methane output could be saved with minimal mixing once a day.The CFD analyses of the hydrodynamics parameters support the claim that increasing the mixing intensity above a certain threshold is not beneficial to microbial activities.Cautious applications of rheological properties presented in literature could therefore provide useful data for modeling purposes.The simulations results proved the applicability of CFD as auseful tool for modeling mixing in digesters.This research confirms that there exists a mixing intensity threshold for every anaerobic digestion setup and that above which increasing the mixing intensity is a waste of energy and does not increase methane production but rather may reduce it.Mixing intensity within the ranges of 50 to 100 rpm is therefore ideal for optimum methane production under the study conditions.Using the net energy production is the best criteria for determining the mixing mode,mixing intensity,mixing time and mixing interval for every anaerobic digestion operational plan.When the initial 30 day hydraulic retention time was reduced to 20 day hydraulic retention time,digestion process instability was experienced causing severe foaming which could not be controlled,the experiment was discontinued.At a steady state operating condition,a simple mathematical model could be successfully used to model the methane yield and the specific methane production rate.Including the biodegradability factor into the stoichiometric method is a simple but accurate method to model the influence of operational conditions on the methane specific methane production rate.