Microbial electrolysis cells for wastewater treatment using inexpensive and sustainable recycled carbon fibre anodes : innovation report

Vital to human survival and the protection of the environment, is the wastewater treatment industry. The practices employed work very effectively to protect human populations from pathogens and to safely treat water for discharge to the environment; securing future water supplies and protecting the rivers, lakes and seas. However, the drawback of this process is the high energy use, making the process expensive and a contributor to climate change. In the UK, approximately 3% of all energy is used by the water industry. The high energy costs also pose a problem for the developing world, as many nations cannot afford to treat their wastewater streams, resulting in unsafe discharge to the environment of raw untreated effluents. The pollutants (organic and inorganic compounds) are a potential source of value and the industry is failing to recover the majority of this value. Some is recovered via the use of anaerobic digestors for sludge treatment and biogas production, but the majority of the recoverable energy is lost. A potential solution for this problem is the Microbial Electrolysis Cell (MEC), a subtype of a bio-electrochemical system. The MEC is a system comprised of an anode, cathode and optionally; a membrane that separates the two electrodes. The MEC can sustain a biofilm on its anode which is electroactive and is able treat wastewater by facilitating the oxidation of the organic compounds, producing H2 – a renewable and potentially sustainable energy source when produced by this method. The system requires the addition of a small voltage as the reaction is not spontaneous. However, this technology is not ready to solve the challenges the wastewater treatment industry faces, primarily due to its high capital costs but also its low energy efficiency recovery. To address the capital costs, a recycled carbon fibre material (used for components in the automotive industry) was tested and shown to have electrocatalytic properties within an electrochemical cell, comparable to a commercially available graphite battery felt. The recycled materials were then used as anodes in 100 mL MECs using real wastewater, demonstrating potentially superior performance to graphite at a significantly reduced cost. This was confirmed at a larger scale (10 L) at a wastewater treatment plant, where hydrogen gas production and wastewater treatment performance were significantly superior but with a 96% reduction in the anode cost relative to the graphite felt used. A detailed cost benefit analysis using multiple TotEx scenarios confirmed the potential cost savings attributed to the use of the recycled carbon fibre anode, where an equally scaled MEC has the potential to be cost-competitive or less expensive than an activated sludge pool during a 20- or 50-year period. A placement abroad with a water technology consultancy did highlight that there are other technologies that are far more developed and are closer to commercial availability (i.e. sludge destruction via pyrolysis). The MEC offers something different and potentially, better, but larger scales are required to prove the technology. The use of the recycled carbon fibre as the anode now makes larger-scale deployment of MECs far more likely. The significantly reduced capital cost but lack of performance compromise, mean that academia and industry alike can seriously consider the construction and testing of larger and more ambitiously scaled MECs. The material is a lower environmental impact (relative to virgin graphite and carbon), meaning that the life cycle impact of an MEC using the recycled carbon would be more less and more likely to have a positive impact, assuming performance optimisation. This could increase knowledge around the problems associated with upscaling and therefore, dramatically increase the likelihood of the technology becoming a commercially available product for water industries