**1. Introduction**

Bioelectrochemical systems (BES) are devices that drive electrochemical reactions using biological agents as catalysts. Primarily, there are two types of BES. They are microbial fuel cells (MFCs) and Microbial electrolysis cells (MECs). Both depend on two electrodes where anodic and cathodic electrochemical reactions occur and both types are driven by microorganisms. In MFC systems, organic substrates, most often organic pollutants are microbially broken down at the anodic compartment. The electrons released are harnessed by the anode electrode and are driven towards an external circuit where it can produce a usable current. These electrons are then accepted by a cathode electrode where it is coupled with protons and an electron acceptor such as molecular oxygen to produce water (**Figure 1a**). MFCs contain a proton exchange membrane to exchange protons between anode

**Figure 1.**

*Schematic diagrams showing the operating principles of (a) MFC and (b) MEC systems (modified figure adapted from [1], with permission).*

and cathode chambers. On the contrary, MEC systems rely on a small exogenous supply of electrons to the cathode or anode electrode to produce combustible gases hydrogen or methane. It can therefore be termed as an MFC system run in reverse to produce gases with fuel value or other value added products. Both types of BES come in many different designs, shapes and sizes [2–4]. Out of the two BES types, MFCs are the most ubiquitously used type for laboratory and pilot scale studies and full-scale operation.

The primary utility of MFCs is to be used as a wastewater treatment technology and simultaneous recovery of electrical energy as a fringe advantage. However, the amount of recoverable electricity still remains low for large-scale practical applications of MFCs as electricity generation units. Until a research breakthrough in MFC electrical power generation is made, the focus therefore, firmly lies within wastewater and waste treatment capabilities of MFCs. One of the main benefits of MFCs in this regard is that they are demonstrated to possess better degradation kinetics for biodegradation of many recalcitrant organic pollutant types over their conventional counterpart systems such as activated sludge systems, anaerobic sludge blankets and constructed wetland systems [5–7]. Some of the refractory organic pollutants such as azo dyes and nitrophenolic compounds are known to be problematic due to their poor or non-degradable nature in conventional wastewater treatment systems such as activated sludge systems. These compounds are therefore known as highly recalcitrant organic pollutants. Earlier studies that use of MFCs however, have demonstrated that some of these compounds can be effectively degraded, transformed or mineralized into simpler intermediates with the use of BES. This chapter examines the utility of such BES systems for enhanced degradation and removal of such recalcitrant environmental pollutants.

### **2. Removal of azo dyes in BES**

Azo dyes by far, are the most widely studied class of environmental pollutant remediated by electro active microorganisms. Azo dyes are characterized by one or more of the azo moieties, which are of oxidative nature. Therefore, the most obvious conversion mechanism of azo dyes by electro active microorganisms is by the azo pollutant acting as an electron acceptor and undergoing reduction into their constituent amines. Azo moieties are flanked by R groups which may contain various electron-rich or electron-poor substituent groups, which would influence the redox potential of the dye [8]. In recent years, Microbial Fuel Cell (MFC) technology has been explored extensively for their innovative features and environmental benefits [9]. At the anode, organic co-substrate is oxidized by electrochemically active microorganisms. Subsequently, the microorganisms transfer the electrons


*Expedited Biodegradation of Organic Pollutants and Refractory Compounds… DOI: http://dx.doi.org/10.5772/intechopen.99229*

#### **Table 1.**

*BES studies involving azo dyes and the bioelectrochemical characteristics of the BES systems during azo dye remediation.*

resulting from this oxidation to the anode via extracellular electron transfer which then passes through an external circuit to the cathode, thus producing current. Protons migrate through an ion exchange membrane to the cathode where they combine with azo dye and electrons and lead to the degradation of azo bond. Reduction of azo bond results in the formation of colorless and biodegradable aromatic amines [10]. It has been demonstrated in earlier studies that BES can be


#### *Biodegradation Technology of Organic and Inorganic Pollutants*

**Table 2.**

*BES studies involving PAHs and the bioelectrochemical characteristics of the BES systems during PAH remediation.*

*Expedited Biodegradation of Organic Pollutants and Refractory Compounds… DOI: http://dx.doi.org/10.5772/intechopen.99229*

successfully coupled to other wastewater treatment technologies such as activated sludge systems and up-flow anaerobic sludge blanket reactors (UASBs) in order to fully mineralize azo dye pollutants [6, 11]. The use of BES has shown higher kinetic rates of azo dye biotransformation rates compared to other conventional treatment methods [12, 13]. Many other studies have hitherto demonstrated this ability of BES to effectively biodegrade many different azo dyes (**Table 1**).
