Chapter 17 - Searching for sustainability in organic matter and nitrogen removal by integrating constructed wetlands and microbial fuel cells

Abstract

Constructed wetlands (CWs) as nonconventional treatment systems is an attractive solution for organic matter and nitrogen removal. Despite the removal efficiencies of over 90% for both organic matter and nutrients, CWs may be a source of greenhouse gases (GHG). Emitted GHGs can be methane (CH4), carbon dioxide (CO2), or nitrous oxide (N2O), depending on the configuration used for the constructed wetland and its operation. In the case of CH4, it is produced in anoxic-anaerobic soil and/or sediment (100 to −350mV) as a result of organic matter degradation. The microbial fuel cell (MFC) represents a new biotechnological approach and is defined as a bioelectrochemical system that converts the chemical energy contained in chemical bonds of organic and inorganic substances to electrical energy through reactions catalyzed by microorganisms, mainly bacteria under anaerobic conditions. Thus, MFCs have been combined with CW technology, giving rise to integrated CW-MFC systems. Due to this, when an anode is incorporated into the CW, it functions as an electron acceptor for bacteria, which helps prevent biochemical reactions that lead to methane formation. The results reported to date can have implications for controlling methane emissions from wetlands, as in most cases a decrease in methane emissions has been observed. The MFC provides an attractive alternative for recovering electrical energy and eliminating GHG emissions from wetlands in a sustainable manner. However, more studies that allow various questions to be answered are required. One such issue is the competitive relationship between electricity generation (electrogenesis) and methane production (methanogenesis). It is also crucial to achieve an understanding of the methane fluxes that occur in a CW-MFC, as they are much more complex than those of conventional MFCs due to the presence of plants. In this way, being able to know how organic matter can be converted from a CW-MFC can be transformed into a virtuous circle explained through the circular economy.