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Application of Integrated Bioelectrochemical-Wetland Systems for Future Sustainable Wastewater Treatment Lei Xu,†,‡ Wenzheng Yu,*,† Nigel Graham,§ Yaqian Zhao,‡ and Jiuhui Qu†
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Key Laboratory of Drinking Water Science and Technology, Research Centre for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, 100085, China ‡ Centre for Water Resources Research, School of Civil Engineering, University College Dublin, Belfield, Dublin, 4, Ireland § Department of Civil and Environmental Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, U.K. kind of “intensified CW” would be suitable to retrofit to existing CWs.1 In this article, we propose that other factors in addition to electricity production should be included, and these are discussed subsequently by reference to future applications which have the most potential and are worthy of future investigation.
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STIMULATED POLLUTANT REMOVAL VIA MANUALLY CONTROLLED E− TRANSFER The anoxic oxidation of reduced substances or the aerobic reduction of oxidized substances in CWs normally relies on the availability of electron (e−) acceptors or donors within the substrate or the influent wastewater. Through the incorporation of BES, the problem of sufficient e− acceptors/donors can be overcome via a manually installed electron acceptor and electron donor (i.e., the electrodes). Thus, the BES can stimulate the respective reactions in the vicinity of the electrodes, and a number of studies have shown its ability in enhancing organic (mostly biodegradable) pollutant removal. However, further work is needed to consider the migrating and transforming pathways of biorefractory compounds, like pharmaceutical and personal care products (PPCPs) and other micropollutants, or even the microplastics (MPs), etc., in the BES-CW process. In addition, electrode based autotrophic nutrient removal, like N or S, in BES-CW also deserves to be evaluated.
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astewater purification plants are transitioning from being centralized and energy intensive to decentralized facilities incorporating energy and resource recovery. As a potential energy producer via direct/indirect energy extraction from wastewater, bioelectrochemical technologies have been undergoing great strides in the last two decades, but have been slow to mature beyond the lab-scale into practice. Arising from the development of the basic bioelectrochemical system (BES), various integrated BES processes have been proposed in recent years. Among these, the combination of BES with a constructed wetland (CW) holds particular promise. The latter (CW) is a well-recognized environmentally benign wastewater treatment process, which has been shown recently to be capable of acting synergistically with BES to enhance its performance by the simultaneous production of bioelectricity, referred to as a constructed wetland-microbial fuel cell (CWMFC). Although as expected, the electrical performance of an integrated BES-CW process is inferior to a basic pure BES, owing to its inherent greater scale, this has not diminished research interest in the process. A review of the literature has revealed that publications on the topic of BES-CW have increased substantially from only 3 in 2012 to over 40 in the last two years (Figure 1). In view of its burgeoning popularity, it is necessary to consider carefully for which applications this © XXXX American Chemical Society
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BIOCURRENT SUSTAINED IN-SITU OR EX-SITU BIOELECTROLYSIS As bioelectricity production is a major benefit of BES-CW, it might be expected that the energy produced by the process can offset the energy consumed in the CW. However, unlike the basic BES, the electrodes in the BES-CW process usually account for a small portion of the substrate in the CW, which indicates that most of the electrons are actually lost, resulting in a lowcoulombic efficiency (CE) (lower than 10%). Rather than seeking to achieve a greater electrical performance in the BES-CW process, with the necessary increase in capital cost (in the case of the electrodes, either physically or chemically), attention could be transferred to the direct utilization of the bioenergy produced. For instance, it was found that the in situ utilization of the biocurrent supported electro-Fenton process Received: January 8, 2019
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DOI: 10.1021/acs.est.8b07159 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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Environmental Science & Technology
Figure 1. Proposed directions for the future development of an integrated BES-CW process. (Publication data was accessed on October 15th, 2018, with “Keywords = (constructed wetland OR treatment wetland AND microbial fuel cell OR microbial electrolysis cell)” in Scopus).
could be regarded as a natural carbon sink (for CO2), methane (CH4) emissions from wetlands are recognized as the single largest natural source in the global CH4 budget, which roughly accounts for a third of total global emissions. Thus, the reduction of CH4 emissions from CWs represents a significant contribution to global warming mitigation. Recently, an anaerobic reverse-methanogenesis method was utilized in a MFC to directly convert CH4 into electricity with high efficiency (over 90%),4 thereby enabling CH4 to be used as the fuel to sustain the anodic reactions. Therefore, the concept of transforming CW to BES-CW processes has the potential to reduce significantly the undesirable CH4 emissions from CWs. In addition, there is the possibility of importing and purifying exogenous waste gas in the BES-CW process as another potentially important application for further investigation.
had 10−100 times greater degradation rate (for carbamazepine) than other biological treatment.2 As such, the biocurrent sustained in situ or ex-situ bioelectrolysis process, together with the electrically related treatment of waterborne pathogens (e.g., through disinfection or stimulated reactive oxygen species release, etc.), represents another promising application of BES-CW. By this means, the extracted “waste energy” can be directly utilized within the wastewater to further enhance the treatment efficiency. To achieve this, further exploration of the synergistic effects/mechanisms among the indigenous bacteria, exoelectrogens and the electrodes will be necessary.
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BIOCURRENT INDEXED BIOSENSOR DEVELOPMENT It is clear that, at present, the amount of power generated via a BES-CW process is substantially below the commercial application standard. Therefore, it is worth exploring the potential indirect use of the micro biocurrent. To some extent, the biocurrent produced relies on the bioactivities of the electrically related bacteria attached to the electrodes. Accordingly, the level of the biocurrent could be utilized as a biosignal to reflect the conditions of the microbes within the system, as the bioactivities of these microbes can be influenced by their surroundings.3 In this regard, the BES-CW system can potentially perform as a biosensor (living sensor) for monitoring water quality, or as an early warning indicator of problems prior to, during or after the treatment processes, which would be valuable to protecting and maintaining the terrestrial-aquatic environment. Laboratory trials for this approach could be done initially as a proof of concept, followed by long-term studies in real situations (field tests) to further examine its reliability and durability.
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HEAVY METAL IONS REMOVAL AND RECOVERY Metal recycling is an essential part of the closed-loop material economy. Wastewater containing heavy metal ions can be treated via CWs through physical, chemical or biological processes. However, the complex removal pathway in CWs makes it difficult to trace and therefore to recover these metal ions. Theoretically, the inclusion of a cathode/biocathode in the BES-CW process could serve as the electron donor for the reduction of the metal ions. To facilitate this process, the concept of the modularized “electrode wall” could be trialed as a “filter” to capture and then accumulate these metal ions in the form of precipitates. Metal recovery can be realized through periodic extraction of the reduced metal ions from the removable “electrode wall”. In terms of the different metal species, owing to the different redox potentials of different metal ion redox couples, the sequence of the selected metal ions removal and recovery (together with the capability and durability of the biocathode) should be fully investigated (using multiple sets or combined with microbial electrolysis),5 as well as the economic effectiveness of the recovery process.
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GREEN HOUSE GAS (GHG) EMISSION REDUCTION GHG emission is of concern owing to its widely believed link to global climate change. Although the vegetation in wetlands B
DOI: 10.1021/acs.est.8b07159 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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Environmental Science & Technology CW has been extensively investigated for several decades with increasing popularity, and the development of BES-CW can further broaden its future application. This nascent technology has shown not only its superior performance compared to conventional CWs for wastewater treatment but also the benefits derived from the simultaneously produced bioelectricity. The foregoing application areas are considered as particularly promising directions in terms of future studies of BES-CW technology. It is reasonable to believe and fully expected that other aspects of this versatile technology will emerge in the near future.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Lei Xu: 0000-0003-2240-0737 Yaqian Zhao: 0000-0002-2449-4370 Jiuhui Qu: 0000-0001-9177-093X Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Wu, S.; Lyu, T.; Zhao, Y.; Vymazal, J.; Arias, C. A.; Brix, H. Rethinking Intensification of Constructed Wetlands as a Green EcoTechnology for Wastewater Treatment. Environ. Sci. Technol. 2018, 52 (4), 1693−1694. (2) Wang, W.; Lu, Y.; Luo, H.; Liu, G.; Zhang, R.; Jin, S. A Microbial Electro-Fenton Cell for Removing Carbamazepine in Wastewater with Electricity Output. Water Res. 2018, 139, 58−65. (3) Jiang, Y.; Yang, X.; Liang, P.; Liu, P.; Huang, X. Microbial Fuel Cell Sensors for Water Quality Early Warning Systems: Fundamentals, Signal Resolution, Optimization and Future Challenges. Renewable Sustainable Energy Rev. 2018, 81, 292−305. (4) McAnulty, M. J.; Poosarla, V. G.; Kim, K. Y.; Jasso-Chávez, R.; Logan, B. E.; Wood, T. K. Electricity from Methane by Reversing Methanogenesis. Nat. Commun. 2017, 8,: 15419. (5) Huang, L.; Wang, Q.; Jiang, L.; Zhou, P.; Quan, X.; Logan, B. E. Adaptively Evolving Bacterial Communities for Complete and Selective Reduction of Cr(VI), Cu(II), and Cd(II) in Biocathode Bioelectrochemical Systems. Environ. Sci. Technol. 2015, 49 (16), 9914−9924.
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DOI: 10.1021/acs.est.8b07159 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
