**Abstract**

Industrial fermentation and biological wastewater treatment are usually based on redox processes taking place in living cells and on enzyme processes. The practical application of these redox processes is usually associated with electricity generation in microbial fuel cells or process enhancement in microbial electrolysis cells. The microbial fuel cell approach leads to straightforward wastewater treatment with less energy demand. Additional advantages of these processes are the direct removal of various pollutants and the avoidance of addition of chemical agents with the resulting waste products of treatment as it is familiar with the traditional chemical methods. Another option for the use of bioelectrochemical processes in practice is the approach of microbial electrolysis cells. The application of electric field on fermentation or microbial wastewater treatment processes might result in different aspects: either in purely electrochemical processes on the electrodes or in different types of bioelectrochemical stimulation of enzyme activity in the living cells. These applications are associated with the combination of enzyme activity with electrochemical processes to produce or remove certain compounds rapidly at high concentrations with no additions of other chemicals. In the present chapter, both approaches (microbial fuel cells and microbial electrolysis cells) are presented and discussed. Some practical applications and experimental examples of such bioelectrochemical redox processes stimulated by constant electric field are demonstrated.

**Keywords:** redox systems, microbial fuel cells, microbial electrolysis cells, bioelectrochemical oxidation, bioelectrochemical reduction

## **1. Introduction**

The depletion of the traditional energy resources together with the environmental problems caused by the excessive use of fossil fuels and the resulting emissions of greenhouse gases have prompted humanity to replace fossil fuels, at least partially, by renewable energy sources. Besides the well-known hydropower, wind power and solar energy as well as utilization of biomass, there is another option to remedy the problem of energy demand and the resulting pollution. It is based on bioelectrochemical processes.

During the last decades the concept of microbial fuel cells (MFC) is a subject of significant scientific interest [1–4]. This idea seems to be very attractive, because it offers double benefit: first, to generate electricity without air pollution and second, to carry out wastewater treatment to clean water ponds with considerably reduced

energy consumption [3, 4]. Although the electric power density for these devices is rather modest, the generated energy might reduce the energy consumption of a wastewater treatment plant compared to the traditional ones and to open the way to further improvements.

Another option for bioelectrochemical applications in wastewater treatment and industrial biotechnology is the stimulation of microbial redox processes by electric field.

The present work proposes an overview on the principles, achievements and future trends in these two fields of scientific and practical activity: energy generation by microbial fuel cells and enhanced processes in biotechnology and wastewater treatment by microbial electrolysis cells (MEC).
