Renewable energy is key to the UK Government's environmental strategy. The by-product of wastewater treatment, sludge, is most commonly treated via mesophilic anaerobic digestion (AD), in which sludge is mixed with anaerobic bacteria to degrade biodegradable material and produce a methane-rich biogas. Biogas can then be harnessed via combined heat and power technology for energy recovery. Whilst water companies initially implemented the processes for sludge stabilization, the current focus on the role of renewables in securing our energy future means that there still remains a pressing need to optimize digester design and mixing to maximize energy recovery.

Mixing is crucial to digester performance. This project, CoMAnDO, will use the highly innovative approach of coupled hydrodynamic/anaerobic digestion numerical modelling to identify mixing regimes and associated flow patterns that optimise biological activity and biogas output, while minimising energy input and avoiding grit deposition, for anaerobic sludge digesters operating with unconfined gas mixing of municipal wastewater sludge. We will simulate for the first time the complex relationships between hydrodynamic and biokinetic processes in anaerobic digestion to facilitate optimization of process design and operation. The final output will be a methodology for the robust, yet computationally efficient modelling of gas-mixed mixing in anaerobic digesters, together with a set of practitioner guidelines for the most efficient and effective design and operation of digestion systems which can be applied extensively worldwide to provide the maximum renewable energy benefit that can be derived from an anaerobic digestion plant. There are, therefore, tangible outputs and benefits for academics and practitioners to be derived from CoMAnDO. Whilst initially focussed on the water industry, it is believed that the new modelling methodology developed here could lead to the development and widespread adoption of a new class of simulation tools with applications in many other academic disciplines.

This project falls within the EPSRC 'Water Engineering' theme, defined as "Design and optimisation of technologies relating to water resource management, treatment and distribution systems". It addresses EPSRC's stated Resilient, Healthy and Productive Nation Outcomes and specifically addresses several of EPSRC Ambitions; viz: R1: Achieve energy security and efficiency; R2: Ensure a reliable infrastructure which underpins the UK economy; R4: Manage resources efficiently and sustainably.

Potential Impact:
The impacts of this research are significant, and whilst they are impossible to quantify fully at this stage, it is possible to place the work into context. There are 159 anaerobic digestion (AD) plants treating municipal sewage sludge and solid waste in the UK. These plants alone generate approximately 203 MWe-equiv. If CoMAnDO realizes a modest 5% increase at 50% of these sites, the total impact would be 5 MWe-equiv. In addition, 391 plants, including non-sewage sector, are scheduled for design and construction, with a capacity of 435 MWe-equiv. If the outputs of CoMAnDO were to be implemented in each of these, the impact and benefits to our energy future would be even greater.

Of course, that is confining the analysis to the UK only. AD is practised widely throughout the rest of the world and so the impacts of this transformative research will be felt worldwide. Regulators, operators, consultants, and the supply chain all acknowledge the potential of the process for energy futures protection, but at the same time all recognize the inefficiencies in the current process design and operation resulting from the inadequacy of current knowledge. Consequently, they have welcomed this initiative to generate improved integrated understanding and robust management tools. Evidence of this is provided in the strong letters of support from key stakeholders; including an industry-leading contractor that provides engineering solutions across the water industry, and a major UK water utility whose responsibility it is to operate AD plants with maximum efficiency. In addition, CoMAnDO has the strong support of the Fluid Group, one of the leading computational modelling companies in the UK who drive efficient numerical solutions to complex water industry problems. CoMAnDO also has support from leading international academics, with research groups at Karlsruhe Institute of Technology, Germany (Prof Krause); Politecnico Milano (Prof Malpei); and leading CFD modeller, Prof Binxin Wu of Jiangsu University, China, all accepting invitations to the Steering Committee.

By influencing design standards and approaches, CoMAnDO's deliverables will aid planners, regulators and other decision makers in ensuring more efficient, sustainable and resilient infrastructure is constructed, ultimately benefiting the societies that they serve, the UK economy and international research agendas. The societal and economic impacts and benefits to be realised by CoMAnDO are clear. There are significant financial savings to be made, whilst helping the issue of securing our energy future, without compromising environmental benefits secured from a stable sludge product. The enhancements to our knowledge base and the associated impacts arising from this proposal will contribute to the UK establishing itself as an innovation powerhouse in the global water technology sector, which a recent UKWRIP report estimates amounts to >$50Bn in the period to 2020.

How will the benefits arise?
- Regulators, utilities, consultants and contractors will benefit directly from the new understanding of mixing in AD and improved, validated modelling methodologies that will lead to better-informed plant design, maintenance and management decisions.
- Academics and practitioners will benefit from rigorous methodologies for numerical mixing and residence time characterisation that will have generic value for future research and development activities relating to many types of mixing processes.
- Research staff - The PDRF will work within a high calibre research environment, with strong international links, undertaking fundamental research through to applied pilot scale studies and state-of-the-art model development, whilst interacting with key stakeholders. This provides a unique, highly valued skillset that will equip them to progress authoritatively into academic or practitioner roles in the global water sector.