"Literature Review1.5 IntroductionAnaerobic digesters in municipal WWTP’s are usually large tanks filled with both raw andactivated sludge, in ratios dependent on local conditions. Wastewater sludge is a shearthinning non-Newtonian fluid where its viscosity reduces as the fluid is sheared. The studyof this phenomenon is known as rheology and influences the way in which a fluid is mixed.The primary reason for mixing in an anaerobic digester is to achieve a uniform distributionof solids present within the liquid. When this is achieved, maximum contact between theraw sludge and biological mass is attained resulting in the maximum production of biogasfrom the breakdown of raw sludge. Mixing also prevents scum build up on internal tankswalls which in turn can prevent the tank volume from decreasing with time.In additionmixing reduces thermal stratification of the sludge which can also cause poor digestion ofthe organic material within the sludge, yielding the process ineffective.However, if sludge isovermixed the mesophilic bacteria may be disrupted from breaking down the organicmatter. Depending on the type of sludge and the amount of solids in the fluid beingdigested, there are different methods to mix the sludge. The three main mixing methodsare;gas circulation, pumped circulation and mechanical agitation (Wu 2010). CFD haspreviously been used in various degrees to help understand the effects mixing plays inanaerobic digestion and the different methods of mixing. This literature review provides asummary of current practices and methodologies with respect to anaerobic digestion.1.6 Gas CirculationGas circulation utilisesbiogas produced through the digestion process by drawing gas fromthe top of the tank and reintroducing the gas near the bottom of the tank. The gas is lessdense than the fluid in the tank and therefore rises to the top surface again, mixing throughthe fluid. Mixing can also be achieved via thermal stratification, however this still requiresmechanical recirculation through heat exchangers to achieve the optimal mesophilictemperatures (Karim et al. 2007). Different factors affect gas mixing, such as the gas volumethat is recycled through the mixing system, the tank geometry, the location of the gasinjection points and the sludge content (Karim et al. 2005).6 | P a g e When incorporated with a draft tube, where the gas in introduced in the centre of a largevertical venturi, fluid can begin to flow via the uplift of the gas rising to the surface. Thewastewater industry has previously used gas lift mixing to undertake anaerobic digestion ofsewage sludge (Wu 2010).The literature suggests that CFD utilisation to simulate gas mixing in anaerobic digestersisstill in its infancy (Wu 2012). The literature indicates that CFD is not widely used becauseconclusive results are not consistent or easily obtained. This may be a result of thesimplification required to create easily computablemeshes which results in loss of definitioncombined with the necessary inter- phase fluid behaviour simplifications adopted to runCFD models. Despite these identified limitations, methodologies have been developed toovercome them. This includes work completed by Wu (2010) who has developed a singlephase liquid model that behaves like a 2 phase model. This model wasverified usingexperimental data in a small digester. Results from the CFD and experimental datawerecomparable and therefore it could be concluded that CFD is accurate, although thiswas conducted at lab scale (Wu 2010). In support of this, some research indicates accuraterepresentation in flow patterns in gas lift digesters through the use of CFD (Karim et al.2007).The definition of good and poor mixing is important to understand. Within gas lift mixing, ifthe velocity of the rising gas and fluid is lower than the velocity of the settling floc, themixing would be considered poor. This suggests that good mixing as a bare minimum couldbe described by any mixing system that keeps solids in suspension with no settlement(Karim et al. 2007).1.7 Mechanical AgitationMechanical agitation involves the creation of a flow field within the tank via the use ofmechanical device such as an impeller(Meroney and Colorado 2009).Studies in mixing using impellers in draft tubes have identified other issues with regards tothe use of CFD to accurately predict the mixing regime of sludge in a tank. It has beenidentified that the concentration of solids within the sludge affects the mechanical input7 | P a g e required. Sludges with low solids concentration appear to behave similar to simple tapwater, with a transition from laminar to turbulent flow as the amount of mechanical effort isapplied to the fluid. However sludges with a higher concentration of solids appeared toremain in a laminar flow regime even as the mechanical effort input into the fluidincreased(Terashima et al. 2009).The literature identified that CFD has been used to both calculate and evaluate impellergeometries or used as a verification method to confirm results from laboratory scaleexperiments (Ding et al. 2010).The use of CFD in this manner has been widely reported as atool for assessing the design and mixing efficiency in existing digester tanks and therelationship that digester design and mixing has on the biogas production. It has beenrecognized that the use of CFD has increased over the last few years from a water industryperspective, but using CFD to study multiphase fluids such as sludge in an industrial scalehas been less advanced (Bridgeman 2012). As such CFD has not been commonly used todesign the entire mixing system.1.8 Pumped CirculationPumped Circulation removes a portion of fluid within the tank and reinjects it back into thetank in specific locationsto create a mixing flow. There have been studies for jet mixed tankswhere CFD was used to adjust the geometry of the mixing system in an attempt to reducedead zones within the tank (Rahimi and Parvareh 2005).Much of the literature discusses mixing efficiency with regards to the velocity of the sludgewithin the tank. Careful consideration needs to be taken when using particle velocity tomeasure mixing efficiency as the sludge may be moving homogenously as a large mass.Thiscould be concluded when reviewing particle velocity in isolation.Therefore it is importantto consider the velocity of particles relative to each other. It could be considered efficientmixing is achieved when the minimum velocity of any particle is within a certain percentageof the maximum velocity for the whole tank.This is important when reviewing pumpedcirculation because it is highly conceivable that as the fluid is reintroduced to the tank, aflow regime could present where the flow is laminar and very little mixing occurs at all(Bridgeman 2012).8 | P a g e 1.9 ConclusionMechanical agitation has been reported as the most efficient method, followed by gasmixing then pumped circulation, however the designs of the systems for each of themethods was not extensively explored (Wu 2011).It must be recognised that the rheology ofthe fluid will have a significant impact on the way the fluid flows within the tank andtherefore how it is mixed. It is therefore fundamental that for any CFD model to beaccurate, it must consider and accurately represent the rheology of the fluid beingdigested(Terashima et al. 2009).Much commercially available CFD computer softwareprovides different mathematical models for turbulence and these provide varying resultswhen applied to the same hydraulic situation. It is important that the correct model beapplied for the specific hydraulic situation (Wu 2011). It is clear from the literature that CFDcan play a vital part in analysing the appropriate mixing technology for maximising biogasyield whilst minimizing the energy required. However this literature review has identifiedfurther research is required to develop techniques for utilising CFD in order to optimise theresultant mixing in a complete system design. 9 | P a g e 2 The Research Methodology2.1 The Data Needed and the Means for Obtaining the DataObserved sludge data from three WWTP digesters will be used to develop the standard fluidcharacteristic to be applied to the CFD model. Proprietary data will be used to develop thesimulated mixing infrastructure within the CFD model. Test data will be generated using CFDand this data will be analysed to assess the efficiency of each mixing technique.2.2 Systematic Description of Solving the ProblemA tracer study using lithium chloride will be conducted over a range of mixing intensities at anumber of municipal WWTP using different mixing techniques and sludge concentrations toobtain empirical data. The energy input for the mixing intensities and gas production will berecorded. A relationship between tracer concentration and gas production will then bedetermined. A mathematical model of the mixing systems will also be developed and themixing uniformity of each technique will be analysed over a range of mixing energy inputs.Using the recorded energy input from the empirical evidence a relationship between tracerconcentration and mixing uniformity will be determined. Finally a relationship between themodelled mixing uniformity and gas production will be determined.The research will be broken down into three distinct sub-problems listed below.2.2.1 How to solve Sub-Problem 1: Trace Concentration and Gas ProductionRelationshipA lithium chloride solution will be added to digesters employing each mixing technique. Theenergy input to the mixing system will be altered and the concentration of the lithiumchloride will be measured at different locations within the digester. Gas production will alsobe measured for the different energy inputs.By graphing the lithium chloride concentration variance against the energy input andgraphing the gas production against the energy input a relationship between lithiumchloride solution variance and gas production will be achieved.10 | P a g e "