Outstanding energy reduction of nitrogen recovery by biohythane concept introduction by 3D-weaved anode network in microbial electrolysis cell

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Abstract

This work achieved nitrogen recovery from the wastewater treatment by the bioelectrochemical system with the concept of biohythane generation. In this study, 3D network electrode could improve the biohythane generation to reduce energy consumption of the nitrogen recovery, achieving the highest biohythane production rate at 0.123 m3 m−3 treated wastewater to obtain energy consumption at 0.77 kWh kg−1 N under 0.8 V, significantly lower than other traditional technologies (at about 1.3 to 14 kWh kg−1 N). Further microbial analysis was processed for voltage optimization, and 0.8 V was proved as best voltage for high abundance of Geobacter over Methanosarcina. Correlation factor and heatmap analysis indicated that the suitable voltage could benefit Geobacter growth for higher energy recovery. Furthermore, genera of Bacteroides and Azospirillum were found as key species. This study proved that the biohythane generation concept could outstandingly reduce the energy consumption for higher nitrogen recovery from wastewater treatment.

Introduction

Bioelectrochemical system (BES) is important tool to treat wastewater and simultaneously recover resources, such as energy, nutrient and water. Ammonia recovery, instead of removal, will be of strong interest. Ammonia nitrogen is a key fertilizer component for agricultural applications, and >90% of the world ammonia production is currently from the Haber-Bosch process, which consumes 1–2% of world energy (Qin et al., 2017). Furthermore, the demand for nitrogen-containing fertilizer was continuously increasing by 3–4% per year (Bicer et al., 2016), but majority of the nitrogen was lost as ammonium ions in the wastewater, equivalent to about 19% of the annual ammonium production from the Haber-Bosch process (Bodirsky et al., 2014). Therefore, nitrogen recovery was significant strategy to simultaneously control the nitrogen loss and reduce the energy consumption for nitrogen production.

Traditional wastewater treatment uses activated sludge to remove the ammonium ion under nitrification-denitrification process instead of recovery process (Cruz et al., 2019). Actually, waste ammonium contained great amount of recoverable energetic potential. It was reported that, if the wastewater ammonium was recovered as the gaseous form, ammonia (NH3), and subsequently burnt in the fuel cell system, the energy release could reach 7.42 kWh kg−1 N (Lan and Tao, 2014). The energetic content of recoverable NH3 could even triumph over the energy consumption of nitrification-denitrification process for the nitrogen removal of 2.6-6.2 kWh kg−1 N (Schaubroeck et al., 2015). Thus, the waste nitrogen, especially the waste NH3, was actually the wrongly-placed resource, and new technology was needed to recover the NH4+-N for the sustainable wastewater treatment.

Microbial electrolysis cells (MECs) were proved to be feasible to convert the chemical energy from the organic to electrochemical power to separate ammonium ion from the wastewater (e.g., urine, agricultural wastewater) to recover nitrogen (Qin et al., 2018). For example, the MEC was proved to recover ammonia from the wastewater under nearly all N concentrations, from 50 mg L−1 to 2 g L−1 NH4+-N, to achieve > 70% recovery efficiency (Cruz et al., 2019). However, due to the high power input into MEC system, the nitrogen recovery process was generally not economically attractive, especially for the domestic wastewater of low nitrogen concentration (e.g., about 40–60 mg L−1 N) (Cruz et al., 2019). Most importantly, it was significantly difficult to avoid the energy consumption when using MEC to recover nitrogen resource from the wastewater. Multiple previous studies indicated that though efficient strategies had been extensively applied, there was still energy consumption of > 1 kWh kg−1 N required to support the MEC operation for nitrogen recovery from various wastewater, due to external power supply and pumping energy consumption (Qin et al., 2017; 2018). Thus, energy and cost consumption was still a currently big obstacle to restrict the development of wastewater nitrogen recovery. Searching a new strategy was needed to solve the above problems to recover nitrogen from the domestic wastewater.

Currently, the integration of biohythane generation into the MEC operation was reported to maximally convert the chemical energy potential to usable fuel and achieve the positive net energy recovery (Luo et al., 2017; 2021). It was reported that dual-chamber MEC could stably produce the methane and hydrogen gas together, to increase energy production to 7.5  ×  10−3 kWh kg−1 degraded COD to achieve positive net energy recovery (Luo et al., 2021). Thus, the dual-chamber MEC provided suitable environment to consistently convert the organic to valuable biohythane for energy recovery. Also, the enhanced ionic migration efficiency by 3D anode network in MEC system was confirmed in the previous study (Luo et al., 2020), and 3D anode network had high competency to separate NH4+-N from wastewater to collection side. Thus, the concept of nitrogen recovery was promising to be added into biohythane-generating MEC system, to further integrate the MEC functions with wastewater treatment, energy and resource recovery together.

This study was to comprehensively compare impact of various anodes on the MEC system performance in biohythane generation and nitrogen recovery, including innovative 3D anode network and other traditional anodes. Feasibility analysis was achieved by tracing the energy and mass balance through the entire experimental process. Also, the applied voltage adjustment on MEC (e.g., 0.5, 0.8 and 1.1 V) was processed to evaluate voltage impact. Microbial community and related statistical analysis could give direct evidence to interpret the dynamic change of microbial community and detect the key species in this study, as the reference to know the key species and change the microbial structure for better performance. The significance of this study was to first integrate the concepts of wastewater treatment, biohythane generation and nitrogen recovery together into single fuel cell unit to break the limitation of only energy consumption in the wastewater nitrogen recovery. Instead, this study was to achieve a historic leap to achieve the positive energy recovery in the wastewater nitrogen recovery in the BES application for the sustainable wastewater treatment and resource recovery.

Section snippets

MEC construction and configuration

All MEC reactors were constructed with one anode and cathode for each other, with anodic and cathodic volume at 1.56 L and 0.94 L, respectively. Cation exchange membrane (CEM, Shanghua, Shanghai, China) was inserted into the middle of MEC reactor, with the surface area at 156 cm2 (dimension: 26 cm x 6 cm). All four MEC anodes were installed with various anode materials. The first MEC anode was installed with six carbon brushes (CB, height: 26 cm; diameter: 3 cm, Haote New Material Inc., Wuhan,

Wastewater treatment performance

CMT had higher current generation than other MECs under all external voltages. For example, under 0.8V, CMT could stably generate about 61 mA during 20-day experimental process, while other anode materials, CB and GAC, produced the current only at 46-50 mA (Fig. 2A), indicating the better effect of CMT to enhance electrochemical performance. Also, the CMT MEC could generate higher current density than other materials (CMT at 36.7 ± 0.3 A m−3 anode volume, higher than CB at 28.0 ± 0.3 A m−3 and

Conclusion

This study applied the innovative 3D anode network and integrate the concept of biohythane generation to reduce the energy consumption of nitrogen recovery into the MEC system. This study successfully proved that the 3D anode network could improve biohythane generation efficiency, and further decrease the energy consumption of the nitrogen recovery in BES. The introduction of biohythane generation could make breakthrough to reduce energy consumption of nitrogen recovery over the traditional

Credit author statement

Great appreciation for the help of project administration from Xianzheng Zhu, Xiaoyuan Zhang and Xia Huang; for the methodology and investigation from Boya Fu, Fubin Liu and Lequn Sun; for the data curation from Kai He and Heng Yang; for the writing reviewing & editing from Xianzheng Zhu and Xia Huang.

Declaration of Competing Interest

It is noted that the authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. This paper is our original work. Neither the entire paper nor any part of its content has been published or has been accepted elsewhere. The manuscript is not being submitted to any other journal.

Acknowledgement

The authors would like to thank the financial support from China Postdoctoral Science Foundation Funded Project (No. 2018M640143) and Natural Science Foundation of Beijing Municipality (No.8204066).

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