"Student:ID: ContentsThe List of Acronyms .............................................................................................................. 31 The Problem and its Setting ............................................................................................... 31.1 General Background and Importance of the Study .................................................... 31.2 The Statement of the Problem and its subproblems .................................................. 41.3 Limitationsof the study ............................................................................................... 51.4 The Assumptions ......................................................................................................... 52 Literature Review ............................................................................................................... 62.1 Introduction................................................................................................................. 62.2 Gas Circulation ............................................................................................................ 62.3 Mechanical Agitation .................................................................................................. 72.4 Pumped Circulation ..................................................................................................... 82.5 Conclusion ................................................................................................................... 93 The Research Methodology ............................................................................................. 103.1 The Data Needed and the Means for Obtaining the Data ........................................ 103.2 Systematic Description of Solving the Problem ........................................................ 103.2.1 How to solve Sub-Problem 1: Trace Concentration and Gas ProductionRelationship ..................................................................................................................... 103.2.2 How to solve Sub-problem 2: Tracer Concentration and Mixing UniformityRelationship ..................................................................................................................... 113.2.3 How to solve Sub-problem 3: Mixing Uniformity and Gas ProductionRelationship ..................................................................................................................... 113.2.4 Ethical Implications ............................................................................................ 124 An Outline of the Proposed Study ................................................................................... 125 References ....................................................................................................................... 14 2 | P a g e The List of AcronymsCFD - Computational Fluid DynamicsPS - Primary SludgeWAS- Waste Activated SludgeWWTP - Waste Water Treatment Plant1 The Problem and its Setting1.1 General Background and Importance of the StudyThe use of aerobic and anaerobic digestion for the treatment of wastewater sludge is notuncommon. Digestion is the process of utilising the bacteria present within the sludge tobreak down organic material and thereby reduces the volume of waste requiring disposal.For this reason, wastewater treatment managers employ measures which maximisedigestion. This includes introducing additional biological matter (bacteria), promoting mixingwithin the digester or a combination of both. However, it is recognised that promotingefficient mixing is the simplest method of maximising digestion due to the complexbiological interactions between bacteria and their environment.To promote efficient mixing,consideration must be given to the type of mixing and its design. Certain types of sludge willdigest more easily using certain mixing types. Accordingly the design of mixing systems iscomplex and generally undertaken using computational fluid dynamics (CFD).Recognisingthe significance of digestion and hence the integral nature of designing efficient mixingsystems,research which investigates these methods is important. Minimizing waste disposal is important to minimize the operational expenditure ofwastewater treatment plants. This can be achieved through reducing sludge volumes bypromoting digestion of organic material. This also provides a recyclable byproduct throughthe production of gas which can be redirected through the plant, thereby reducing externalenergy demands and providing a more sustainable and energy sensitive solution. Whiledigestion and energy recycling are integrated in to many modern waste water treatmentplant (WWTP), it is acknowledged that more work is needed to understand the relationshipbetween digestion and mixing such that design decisions are better informed which willresult in less trial and error during commissioning. As these assets have design lives in the3 | P a g e order of 20-25 years, assumptions and decisions made during design have long lastingimplications.1.2 The Statement of the Problem and its subproblemsThis research proposal outlines an experiment (to be conducted using CFD) to investigatethe most efficient method of mixing for anaerobic digestion. Therefore the researchquestion is:Using CFD, what method of mixing produces the most uniform continuousmixing of waste activated sludge within a municipal wastewater treatment plantanaerobic digester?This research compares the different mixing techniques currently in use in wastewatertreatment. It is anticipated that a comparable model of each mixing technique can beproduced in order to quantify the energy required to achieve uniform mixing in eachtechnique which will allow a direct comparison and thereby quantify which is the mostefficient for various sludge characteristics. This research aims to investigate which mixingtechnique is the most efficient by firstly defining how a digester’s gas production ismeasured. By redefining gas production as a measure of the degree of uniformity of theshear rate within the flow field of the mixed fluid, a direct comparison can be madebetween each mixing technique and the energy input required. The goal of this research isthe development of two graphs which estimate the efficiency of each mixing techniquerelative to the sludge concentration and energy input.If successful, the data from this research utilising CFD will provide a simplified designreference to inform mixing technique selection as well as other flow on plant selectiondecisions based on incoming sludge characteristics. This will be useful to the wastewaterindustry in the scoping of future WWTP design or existing plant augmentation and upgradeworks.The problem can be classified in to the following three sub problems:1. Compare the results of a lithium chloride tracer study with the recorded level of gasproduction with respect to the energy required to mix.4 | P a g e 2. Compare the results of the lithium chloride tracer study with the degree of mixinguniformity within the digester calculated by a CFD model,for various mixing energyinputs.3. Compare the relationship between the tracer study and the calculated mixinguniformity with the relationship between the tracer study and gas production todevelop a mathematical model for the prediction of gas production from a specifiedsludge concentration for a specific mixing energy input. 1.3 Limitationsof the studyThe dimensions and volume of the digester to be modelled will be based on currentdigesters in use at WWTP’s and the mixing techniques to be analysed will be limited to jetmixing, gas lift mixing and mechanical agitation.In order to simplify the CFD model to reduce computing time and complexity the sludgeconcentrations such as the percentage of primary sludge (PS) and waste activated sludge(WAS)will be defined by the viscosity of the fluid based on viscosity measurements ofdigester sludge from current digesters in use at WWTP’s.The element of the digester operation to be analysed is:? The volume of fluid contained within the digester.While the elements of the digester operation that will not be considered or analysed are:• Secondary mixing effects such as gas generation and thermal stratification.• Any turbulence and/or inconsistencies in the flow field of the fluid within thepipework that is generated by any pumps or pipe fittings such as valves located upstream ofthe digester wall.1.4 The AssumptionsIn this research, the following assumptions are made: • The fluid will be considered as a single phase.• The fluid will be considered at a uniform temperature.• The effects of temperature changes to the fluid in relation to mixing efficiency.• The effects of rheological changes to the fluid in relation to mixing efficiency.• Thermal loss or gain to the fluid created in the digestion process.5 | P a g e "