The Rational Design of a Bio-electrochemical System A quantitative investigation into the design of a commercially viable microbial electrolysis cell for domestic wastewater treatment and hydrogen production

Abstract

Reducing energy use is a key challenge for the wastewater industry. Microbial electrolysis cells (MEC) may provide a solution. No commercially viable bio-electrochemical has yet been developed, despite over 40,000 publications. This thesis seeks to serve as a guide to a commercially viable MEC design. Cost-performance targets are presented for MEC to be financially competitive with activated sludge treatment (AS). By reducing the cost of the anode and current collector by 90%, a viable organic loading rate (OLR) is shown to be between 800 and 1,400g-COD/m3/d, if MEC serve a 20 year lifetime. An order of magnitude greater than previously achieved in pilot reactors - although acetate concentrations, a known direct food source for electrogenic species, are typically low (c.10mg-COD/l). A high performance MEC will be dependent upon sufficient concentrations of readily-digestible substrate close to a colonisable anode surface. To quantify a viable MEC design, a macro-model was developed using the Navier-Stokes equations and Monod kinetics. The model predicts MEC performance over a range of acetate concentrations and anode interstices distances, the space between anodes. To calibrate the model and differentiate biofilm kinetics from total microbial kinetics, the concept of current degradation rate is introduced. Models were calibrated against empirical observations of MEC performing under controlled conditions, analogous to those found in domestic wastewater, with a mean accuracy of 85.6%. Acetate concentrations (10- 50mg-COD/l), conductivity (770 15 S/cm), temperature (10 0.5 C) and pH (7.50 0.05) were carefully controlled. Maximum acetate degradation rates of 10mg-COD/l were observed. Coulombic efficiencies and current densities were both observed to increase with concentration. To achieve the current produced in the laboratory, biofilms must have had higher local ORR than found in the bulk microbial community. As such, decreasing anode interstices distances significantly improved ORR for all modelled concentrations. In combination with pre-treatment technologies, MEC may surpass target ORR. However, material costs to break-even are low due to the requirements for additional anode materials.