Developing large scale microbial electrolysis cells (MECs) for the treatment of sludge return liquors

Microbial electrolysis cells can simultaneously treat wastewater and recover energy in the form of value-added products such as hydrogen. The system utilises a biofilm on an anode consisting of electrogenic microbes which anaerobically break down organic matter into electrons and H+ which react at a cathode to produce hydrogen when an external voltage of >0.14V is applied1. The technology's performance at lab-scale has been proven, however, pilot-studies have demonstrated major limitations that need to be overcome to enable its commercial application in the areas of energy recovery and treatment performance, system variability and costs. This research project aims to optimize performance against these barriers and move the technology into position to critically assess its feasibility as an energy neutral or positive wastewater treatment asset at a larger scale.

Energy recovery and treatment performance
To date, few of the pilot-trials have produced sufficient hydrogen to achieve net-positive energy performance, with electrical recovery efficiencies reported to be as low as 3%2. This has been attributed to hydrogen scavenging microbes in the cathode compartment2-4. Hence, this research project will investigate modes of sterilization (e.g H2O2 generation) and electrolyte recirculation within the MEC to inhibit microbial competition and assess their benefits on hydrogen generation and energy efficiency. Additionally, an input voltage optimization experiment on a pilot MEC using real wastewater will be considered to assess the influence of varying voltage on hydrogen recovery and volumetric treatment rates at a larger scale. The novelty here is that input voltages have only been optimized at lab-scale, where studies have used synthetic wastewater and neglect the challenges of increasing overpotentials on performance at scale.
Reactor design will be explored to optimize performance using scenarios tested by an existing computational flow dynamics (CFD) model. Hence, the real effect of narrowing channel width on current, H2 production and treatment rates will be investigated using the pilot MEC at Howdon Sewage Treatment works or the BEWISE MEC facility (Northumbrian Water Ltd/Newcastle University) which allows for electrode cassettes to be adjusted. Sludge return liquors will be further explored as an avenue for MEC application as they provide sufficient COD to minimize concentration limitations on current and hydrogen production. Cost savings between MEC and activated sludge treatment of the liquor line will also be compared with ongoing reactor optimization.
System Variability
Performance variability within pilot-MECs has been observed in a number of trials which raises questions about its ability to produce consistent hydrogen2-4. Hence, another expect of this research will focus on reducing this variability by first identifying its origins in the MEC system by verifying if differences in biofilm development, cell internal resistance or both causes it. This will be tested by operating a high number of replica reactors under identical conditions and using microbial analysis and electrochemical impedance spectroscopy (EIS) to understand the differences between good and bad performing reactors. Following this, strategies for seeding reactors during start up and re-seeding those that fail will be developed.
Costs
Aiken et al highlighted anode and current collector material costs need to be reduced by 90% along with other targets to make MECs financially viable5. Therefore, this research will aim to test the practicalities of using low-cost recycled carbon fibre as an anode material by monitoring material degradation in wastewater and treatment performance overtime at larger scales. This, along with the treatment and variability optimizations, will enable a full market analysis to be undertaken for the MEC.

Graduates from the WRIC programme will produce new knowledge across the disciplinary landscape and graduate to occupy professional roles of influence and authority which require a thorough understanding of the pathways by which knowledge and technology are adopted and put to socially significant use. The people and knowledge delivered through the CDT will improve the efficiency and effectiveness of the nation's >£5bn annual spend on water and water related infrastructure (OFWAT, 2017), improving its resilience and securing its value for society for generations to come. With ambitions to nurture domain experts who can flourish at the interfaces of scientific disciplines and economic/industry sectors, the impact imperative is a significant but stimulating challenge for the WRIC CDT. Our impact strategy seeks to; (i) ensure rapid dissemination of scientific insights, (ii) maximise awareness and uptake of research sponsored through the CDT, and (iii) improve professional and lay understandings of the water infrastructure challenges facing society and the science behind candidate solutions. This strategy has been developed with project and Centre stakeholders so as to leverage additional resources, and maximise impact.
Improving the resilience of water infrastructure systems will be of benefit to a wide range of stakeholders. Given the CDT's bold intention to tackle knowledge gaps at the interfaces between disciplines and problems, new scientific understandings generated through WRIC will be of value to the knowledge users in the public sector (local authorities, regulators) and private sector (utilities, consultancies, technology providers), ultimately benefiting both lives and livelihoods across the UK and beyond. The UK economy will benefit from robust and resilient water infrastructure, in-line with the UK Government's Industrial Strategy for cleaner economic growth, the efficient use of resources, and building a regenerative circular economy. In the next Price Review PR19 (2020-25), water companies will be financially rewarded for implementing enhanced system resilience and innovation. Research outputs from WRIC will enable water companies to be able to meet these demands, alongside ambitious industry targets for zero water and wastewater quality failures, demand reduction and chemical recycling (OFWAT, 2017; UKWIR, 2017). These developments will facilitate inward international investment, development of new technology providers and supply chains, and opportunities for exporting intellectual property and know-how worldwide, further benefiting the UK economy. Project partners, including Thames Water, Severn Trent Water, Atkins, Stantec, Datatecnics also benefit from access to high quality graduates and facilities. Furthermore, regulatory agencies (Environment Agency, Drinking Water Inspectorate) and the European Commission will see benefits from improved compliance to regulations and sustainability agendas (Water Framework Directive 2008/32/EC and Drinking Water Directive 2017/0332(COD)).
The CDT programme will benefit the UK Collaboratorium for Research on Infrastructure and Cities (UKCRIC) government investments (£138M). Sheffield, Cranfield and Newcastle Universities have all received capital grants through UKCRIC to fund industrial scale test facility and observatory facilities to form an Urban Water Hub. The CDT will supply the resources to use and maximise the benefits and outputs from these facilities. Cooperation with other UKCRIC CDTs will help students better understand contemporary challenges for infrastructure and cities will catalyse horizontal innovation transfer and elevate the transformative potential of WRIC graduates.