Research trends on biodegradation of organic pollutants and removal of metals in constructed wetlands integrated to microbial fuel cells (cw - mfc)

The combination of constructed wetlands (CW) and microbial fuel cells (MFC) for wastewater treatment and energy generation is a recent technology that has received great attention due to the advantages that this combination presents over the two individual technologies. CW is a cost effective and environmentally friendly technology that has been used to treat municipal, industrial and agricultural water utilizing several types of macrophytes to promote the removal of carbon, nitrogen and phosphorous. Some macrophytes frequently used in CW are Cyperus alternifolius, Chrysopogon zizanioides, Phragmites sp., Typha angustifolia, Iris pseudacorus and Scirpus validus among several others. The macrophytes Schoenoplectus californicus and Typha domingensis are two of the most utilized in wastewater treatment using the CWtechnology. The main effort to optimize the treatment efficiency is to overcome the availability of oxygen as the main electron acceptor in CW. Several configurations have been evaluated that include aeration, baffling system to modify flow, effluent recirculation, artificial aeration, other electron donor supplementation, etc. In regard to MFC, this is a technology that has been proven to be effective on removal of organic matter from wastewater coupled to energy recovery as electricity. MFCtechnology has also been evaluated for the biodegradation of organic pollutants and removal of metals utilizing electroactive bacteria that can also exhibit capability of biodegrading several organic pollutants that can be taken as electron donors or can be degraded as a result of syntrophy among the bacteria in the consortium. In several cases, biodegradation by-products of organic compounds can also act as electron donors to the electrode, for example, in the anodic compartment of a MFC. On the other hand, removal of several metals has also been demonstrated in MFC while yielding electrical energy. Electroactive bacteria that can biodegrade organic pollutants and promote the removal of metals through several mechanisms have been widely reported as well. The CW-MFC systems offer several advantages for wastewater treatment, such as the improvement of the removal of chemical oxygen demand (COD), nitrogen and phosphorous coupled to the generation of electricity. Some designs of these systems are meant to promote the biodegradation of organics coupled to the removal of heavy metals while yielding electricity. Recently, one of the objectives of CW-MFC has been to evaluate the biodegradation of recalcitrant compounds. Antibiotics, personal care products and other pharmaceuticals that are present in municipal wastewater streams and that pose potential risks to the environment have been the focus of research in CW-MFC. Another important target hasbeen the evaluation of the CW-MFC systems on the treatment of water streams containing pesticides and their derivatives as a result of agricultural practices. The biodegradation of dyes (azo dyes) has been the most studied in CW-MFC systems. Research on the biodegradation of aromatics, halogenated compounds and removal of metals (i.e., Pb, Zn and Cr) in CW-MFC systems has also received attention although it is still in an early stage. The present chapter reviews the current findings on thebiodegradation of pollutants and removal of metals in CW-MFC systems. The review includes the biological catalyzers of this technology, such as he macrophytes and the microbial communities associated with the system. A final discussion on the perspectives and future challenges of this technological approach for wastewater treatment will be presented.