"2.2.2 How to solve Sub-problem 2: Tracer Concentration and Mixing UniformityRelationshipThe jet mixing system’s energy input is required to be calculated by performing a hydraulicsystem calculation to calculate the pump performance required. The mechanical mixingsystem’s energy input is also required by calculating the power and torque required tocreate the flow in the simulation.The gas mixing system’s energy input is needed bycalculating the energy required to produce the gas pressure used to achieve the mixingbeing simulated. Typical PS and WAS viscosities are required to build the digester fluidmodel.The fluid viscosity will be used to represent the fluid’s sludge constituency of PS and WAS.Modifying the sludge viscosity will be the method of modifying the fluid make up withineach CFD model. The mixing will be modelled using the energy input calculations toreplicate the mixing intensities used for the gas production data collection. The flow fieldwill be interrogated to determine the shear rate within the modelled fluid. Interpretation ofthe shear rate will indicate the quality of mixing being performed. This will be demonstratedby differing shear rates within the fluid indicating a non-uniform mixing regime while auniform shear rate will indicate uniform mixing. Particle trajectory and velocity will also beinterrogated to validate the shear rate results. Particles moving in similar directions withsimilar velocities will indicate the fluid is moving in a homogenous state were differingtrajectories and velocities will indicate greater levels of shear. The mixing uniformity will becompared to the tracer studies’ concentration dispersion and variance to develop arelationship between the empirical data and the calculated results.2.2.3 How to solve Sub-problem 3:Mixing Uniformity and Gas ProductionRelationshipData from the results of the modelled mixing regimes and the data of gas production withrespect to energy input from the previous two sub-problems will be needed.The mixing uniformity data for the CFD model will be analysed in relation to the observedgas production data with respect to the mixing energy input required. A mathematicalmodel will be developed calculating the minimum mixing energy required to mix a varyingsludge concentrations for the maximum gas production possible for each techniquemodelled.11 | P a g e 2.2.4 Ethical ImplicationsThere are no ethical implications of this research.3 An Outline of the Proposed StudyThe research will be conducted using Ansys Fluent CFD software. Empirical data will besourced from existing SA Water assets by liaising with their research and developmentteam. Figure 1 represents the proposed project lifecycle. The result of this research is theidentification of the most efficient method of mixing in terms of energy produced relative toenergy consumed for a range of sludge concentrations. A mathematical model of thedifferent mixing techniques over various sludge concentrations will calculate the expectedgas production from the energy consumed in mixing. A graphical representation of themodel will provide direct comparison between the different mixing techniques enablingselection of the most efficienttechnique. 12 | P a g e Figure 1 4 ReferencesBridgeman, J. (2012) 'Computational fluid dynamics modelling of sewage sludge mixing inan anaerobic digester', Advances in Engineering Software, 44(1), 54-62.Ding, J., Wang, X., Zhou, X.-F., Ren, N.-Q. and Guo, W.-Q. (2010) 'CFD optimization ofcontinuous stirred-tank (CSTR) reactor for biohydrogen production', BioresourceTechnology, 101(18), 7005-7013.Karim, K., Thoma, G. J. and Al-Dahhan, M. H. (2007) 'Gas-lift digester configuration effectson mixing effectiveness', Water Research, 41(14), 3051-3060.Karim, K., Thomas Klasson, K., Hoffmann, R., Drescher, S. R., DePaoli, D. W. and Al- Dahhan, M. H. (2005) 'Anaerobic digestion of animal waste: Effect of mixing',Bioresource Technology, 96(14), 1607-1612.Meroney, R. N. and Colorado, P. E. (2009) 'CFD simulation of mechanical draft tube mixingin anaerobic digester tanks', Water Research, 43(4), 1040-1050.Rahimi, M. and Parvareh, A. (2005) 'Experimental and CFD investigation on mixing by a jetin a semi-industrial stirred tank', Chemical Engineering Journal, 115(1 –2), 85-92.Terashima, M., Goel, R., Komatsu, K., Yasui, H., Takahashi, H., Li, Y. Y. and Noike, T.(2009) 'CFD simulation of mixing in anaerobic digesters', Bioresource Technology,100(7), 2228-2233.Wu, B. (2010) 'CFD simulation of gas and non-Newtonian fluid two-phase flow in anaerobicdigesters', Water Research, 44(13), 3861-3874.Wu, B. (2011) 'CFD investigation of turbulence models for mechanical agitation of non- Newtonian fluids in anaerobic digesters', Water Research, 45(5), 2082-2094.Wu, B. (2012) 'Advances in the use of CFD to characterize, design and optimize bioenergysystems', Computers and Electronics in Agriculture, (0). "