A model-based approach for the development of a bioelectrochemical sensor for biochemical oxygen demand in wastewater

Abstract

Bioelectrochemical systems such as microbial fuel cells and microbial electrolysis cells are currently being researched for many different environmental engineering applications. Some of these applications include wastewater treatment coupled with electricity generation, wastewater treatment coupled with hydrogen production, powering of remote sensors, and as sensors for biochemical oxygen demand or volatile fatty acids. After introducing bioelectrochemical systems and the basic principles and mechanisms of their operation, this thesis will focus on a model-based approach to developing a sensor based on a microbial electrolysis cell to characterize wastewater biochemical oxygen demand. In a microbial electrolysis cell, a microbial biofilm growing on the anode of an electrochemical cell catalyzes the oxidation of organic matter and uses the anode as a terminal electron acceptor for respiration, thereby generating a measurable current. A polarization curve measures the response of the current generated to changes in the anode potential. In this study, four continuously operated microbial electrolysis cells with two different anode materials with differing surface properties have been used to model the relationship between acetate concentration in the bulk liquid and the shape of a polarization curve generated using low scan cyclic voltammetry. In terms of Monod kinetics, the anode potential determines the limitation placed on microbial respiration rate by the affinity of the electron acceptor for electrons, and is analogous to the effect of oxygen concentration on growth rate in an aerobic system. The Butler-Volmer Monod model is used to relate substrate concentration, anode potential, and generated current. Polarization curves generated at a range of different acetate concentrations were used to estimate the kinetic parameters of the model and to test its capability to predict substrate concentration. The results of this characterization of anode respiring biofilm respiration kinetics, presented in Chapter 3, show that this approach can differentiate between different substrate concentrations and that the fit and predictive capability of the model can be improved by optimizing the choice of anode materials and electrochemical techniques. This approach differs from other studies which attempt predict substrate concentration using only whole-cell current or total charge generated. Emphasis has also been placed on developing a sensor that is inexpensive and easy to operate, with the idea that such a sensor could allow better process monitoring and optimization at small or resource-limited wastewater treatment plants. To address this, a control and data acquisition system based on the Arduino Uno, an inexpensive microcontroller, was also developed. This study shows that an approach based on characterizing the kinetic parameters of an anode respiring biofilm in a microbial electrolysis cell and using the Butler-Volmer Monod model to estimate substrate concentration holds promise. To the best of this author’s knowledge, this is the first study to test the predictive capability of a kinetic model for bio-anode polarization curves. Other studies, discussed in Chapter 2, which have developed bioelectrochemical sensors for biochemical oxygen demand, chemical oxygen demand, specific substrates, or other wastewater quality parameters or conditions have shown that sensors based on microbial electrolysis cells and microbial fuel cells can correlate these parameters or conditions with current or total charge generated, and that such sensors have good long term stability and reasonably low response times. However, these biosensors are often subject to thermodynamic limitations on current production, leading to a very low upper detection limit. Furthermore, these previously developed biosensors do not account for the limitations that anode potential can impose on microbial respiration. A sensor based on a three-electrode microbial electrolysis cell and a kinetic respiration model for anode respiring biofilms such as the Butler-Volmer Monod model addresses the issues of thermodynamic limitations and anode potential effects. Future work to include characterization of the effects of other environmental conditions such as temperature, pH, and solution electroconductivity and to further refine the electrochemical techniques and electrode materials could be expected to dramatically improve biosensing capabilities for applications in wastewater treatment process monitoring and optimization