Increasing the efficiency of anaerobic waste digesters by optimising flow patterns to enhance biogas production Rebecca Clare Sindall A thesis submitted to The University of Birmingham for the degree of DOCTOR OF PHILOSOPHY School of Civil Engineering College of Engineering and Physical Sciences The University of Birmingham August 2014 University of Birmingham Research Archive e-theses repository This unpublished thesis/dissertation is copyright of the author and/or third parties. The intellectual property rights of the author or third parties in respect of this work are as defined by The Copyright Designs and Patents Act 1988 or as modified by any successor legislation. Any use made of information contained in this thesis/dissertation must be in accordance with that legislation and must be properly acknowledged. Further distribution or reproduction in any format is prohibited without the permission of the copyright holder. Abstract The wastewater treatment industry makes extensive use of anaerobic digestion to stabilise sewage sludge and produce biogas. Whilst the need to mix digesters, to bring biomass and micro-organisms into contact, is well-recognised, the level of mixing required and its effects on biogas production are not clear. Here, the effects of mixing speed in mechanically-mixed lab-scale digesters on flow patterns, digester stability, microbiological community and biogas production are considered. For the first time, positron emission particle tracking was used to visualise flow patterns in lab-scale digesters at different mixing speeds. Computational fluid dynamics models of the digester were then built to identify the turbulence characteristics present. Four lab-scale digesters were run for a period of four months at different mixing speeds and key indicators of digester stability were recorded alongside gas production. Samples were taken at the end of each retention time in order to analyse the microbiological communities, particularly the methanogens, present in the digester. It has been shown that increased mixing speed leads to higher levels of turbulence. Experimental work has shown that in these digesters, increasing the mixing speed reduces the stability of the methane generation process and accordingly has a detrimental effect on the gas production. Similarly, the abundance of methanogenic communities, dominated by the acetoclastic Methanosaeta, was adversely affected by increased VFA concentrations brought about by increasing mixing speeds. However, the unmixed digester produced less biogas than the digester mixed at a low speed, due to the formation of pockets of different environments in the digester which leads to uncontrolled digestion. As such, in the case of these digesters, minimal mixing represents the ideal scenario. By considering the velocity gradient in the digester as a surrogate for turbulence, a threshold of 6-8 s-1 was identified. Below this threshold, increased mixing was seen to be beneficial but increasing mixing above the threshold was detrimental to both digester stability and gas production. i Acknowledgements Firstly, I must thank my supervisors, Professor John Bridgeman and Dr Cynthia Carliell-Marquet, for their support and insight throughout this research. Additionally, I am grateful for the EPSRC-CASE Award in conjunction with Severn Trent Water that has allowed me to cover the costs of the past four years. Thanks too to Pete Vale, my sponsor at Severn Trent. There have been so many people who have helped with this wide-ranging research. Thank you to Mark Carter for extensive assistance in the lab. Thank you to Goff, Jimmy, Farryad, Raf and Gav for their help with various lab tests and for their camaraderie. Thank you to Mark Simmons and Federico Alberini for allowing me access to PIV equipment and to David Parker, Tom Leadbeater and Joseph Gargiuli for allowing me to play with their PEPT equipment. Both bits of kit helped remind me that science is cool! Thanks too, to Julie Williams for processing my qPCR samples and to Sandra Esteves for letting me take advantage of the microbiological community analysis expertise at Glamorgan. Away from the lab, Nainesh, Dom and Justin have been my technical wizards, dealing with a huge variety of CFD, Excel and Matlab questions. Thank you to my officemates, Chris, Roger, Neil, Sally, Shahad, Ashley, Davide and Martin along with others mentioned above who have distracted me and cheered me up over many cups of tea and long lunch breaks. Last, but by no means least, I owe a great deal of thanks to my family. Dad, the original Dr R. Sindall, deserves recognition as the (probably unwitting) inspiration for this PhD. John has always been willing to remind me that having a “real” job is just as challenging as academic research and is the best little brother I could wish for. Finally, Mum has not only proof-read every word of this thesis but has always supported me through every challenge I have had thrown at me and she has consoled, advised and encouraged me even when she had no understanding of the problems involved. Thank you! ii Contents Abstract .................................................................................................................................................... i Acknowledgements ................................................................................................................................. ii List of figures .......................................................................................................................................... ix List of tables ..........................................................................................................................................xvi Abbreviations and notation ................................................................................................................ xviii Abbreviations .................................................................................................................................. xviii Notation ............................................................................................................................................ xix CHAPTER 1 Introduction .................................................................................................................... 1 1.1 Background and Motivation ................................................................................................... 1 1.2 Research Relevance ................................................................................................................ 3 1.3 Layout of thesis ....................................................................................................................... 3 CHAPTER 2 Anaerobic digestion ........................................................................................................ 5 2.1 Aims of anaerobic digestion .................................................................................................... 5 2.2 Biogas as green energy ........................................................................................................... 6 2.3 Biochemical processes ............................................................................................................ 7 2.4 Factors affecting AD ................................................................................................................ 9 2.4.1 Temperature ................................................................................................................... 9 2.4.2 Retention time .............................................................................................................. 10 2.4.3 Nutrients ....................................................................................................................... 11 2.4.4 Alkalinity and pH ........................................................................................................... 12 2.4.5 Inhibitors ....................................................................................................................... 13 iii 2.5 Mixing and anaerobic digestion ............................................................................................ 15 2.5.1 Purpose of mixing ......................................................................................................... 15 2.5.2 Methods of mixing ........................................................................................................ 16 2.5.3 Industrial application of mixing..................................................................................... 16 2.6 Effects of mixing on biogas production ................................................................................ 17 2.6.1 Effect of mixing regime ................................................................................................. 18 2.6.2 Effect of mixing method ................................................................................................ 35 2.6.3 Review of literature ...................................................................................................... 39 2.7 Effects of mixing on digester microbiology ........................................................................... 41 2.8 Microbiological community analysis of anaerobic digestion ................................................ 46 2.9 Review of previous research and knowledge gaps ............................................................... 49 CHAPTER 3 Fluid flow and CFD modelling ......................................................................................