Introduction
The importance of water reclaim is crucial for human footprint reduction, therefore the European union decided to bet on a variety of technologies to intelligently clean water with an energy and material recovery [1]. The anaerobic digestion (AD) is a consolidated technology capable to transform solid or liquid organic waste into biogas and digestate [2]; biogas is a gas mixture mainly composed by carbon dioxide and methane which has a high calorific power and can be converted in electric power by CHP units [3]. Moreover, biogas can be transformed into biomethane, an analogous of natural compressed gas (NCG), through a purification and upgrading step [4,5]. Those steps permit to obtain biomethane through the impurity's removal (such as NH3, H2S) and increase the CH4 content up to 95% by the selective removal of the CO2 [6,7]. The present upgrading technologies, already available at a commercial level are based on physicochemical properties which usually require important capital and operational costs [8]. The increasing market share indicates that the biomethane industry is open for new technologies [9], furthermore, the use of upgraded biogas in transport applications has increased as result of the new opportunities for the use of biogas and benefited from various support schemes and programs [10]. Technological improvements in biogas upgrading technologies to biomethane could lead to lower energy intensity and improved cost performance that could make biomethane cost competitive with fossil fuel use in transport [11]. The catalytic reduction of CO2 into CH4, also known as Sabatier reaction, has high operational costs, which include Ni based catalysts and high temperature and pressures, however, an interesting and effective approach can be offered using biological methanation of the CO2 which involve the use of the methanogenesis reaction [12]. During the last years an innovative strategy for biogas upgrading and purification has raised thanks to its ability to abate and reduce CO2 into CH4 exploiting the locally produced H2 [13,14]. This technology consists in the utilization of a microbial electrolysis cell (MEC) for its ability to reuse wastewater such as digestate or fermentate to extract the reducing power needed to reduce CO2 into CH4 [15,16]. This is possible thanks to the ability of some microorganisms called electroactive precisely for their ability to use electrodes as electron donor or acceptor [[17], [18], [19], [20]]. To sum up, if the interacting biofilm uses the solid-state electrode as final electrons acceptor this system can be named bioanode, on the other hand a system consisting in a biofilm using an electrode as electron donor can be named biocathode [21]. Biocathodes are used for many applications such as the bioremediation for polluted waters [22,23] and recovery of nutrients such as phosphates and ammonium [[24], [25], [26], [27]]. The reduction of CO2 into CH4 could be performed by the biofilm directly taking the reducing power from the electrode (bioelectromethanogenesis [28]) using particular membrane's proteins and/or conductive pilii [29] or can be mediated by a molecule able to be reduce/oxidize like 2H+/H2 [30]for hydrogenophilic methanogenesis. To make this possible inside a MEC, an external potential must be applied to obtain the necessary reducing power [31]. Several studies were made to couple a bioanodic oxidation reaction to the cathodic reducing reaction to lower the energetic consumption needed to apply the electric potential [32]. Moreover, a second but more significant CO2 removal mechanism, further than its reduction, was studied inside a MEC's cathodic chamber, which exploits the alkalinity generated by the ongoing reactions [28]. Generally, the alkalinity inside the cathodic chamber is generated by the usage of protons, for either hydrogen or methane generation, and for the “non-refueling” of protons caused by the transport of different ionic species through the membrane [33]. Furthermore, many studies suggested to use an MEC to upgrade the biogas outcoming from an anaerobic digestor exploiting its double mechanism to remove CO2 from a gaseous mixture [32]. Here, a 12L micro-pilot tubular MEC has been built to be integrated with an anaerobic digestor in which biogas upgrading is performed inside the cathodic chamber along with the COD oxidation inside the anodic chamber. In a previous study [34], three different nitrogen loading rates (NLR) were tested in the tubular MEC showing a significative change in bioanode performance in terms of electricity production and COD removal. Based on those previous evidence, additional runs at different COD and nitrogen load rate have been conducted in order to study the role of the carbon/nitrogen ratio on the anodic biofilm and in general on the overall tubular MEC's performances, mainly in terms of electricity production, nitrogen recovery, COD removal, CO2 abatement, CH4 production and energetic consumption. The feasibility of this process showed the great versatility of the MEC which allows to couple target anodic and cathodic processes in one process operated with the use of the sole electrical energy. Moreover, in literature the attempts of scaling up a MEC for biogas upgrading are limited, and this work describes many aspects of the bioelectrochemical upgrading approach to be a starting point for further scale up of the process.
Section snippets
Set up of the tubular pilot scale MEC
The reactor consisted in a 12L Plexiglas cylinder, 1.5m high, divided into two concentric chambers separated by a 2355cm2 cation exchange membrane (CEM FKS-PET FUMASEP, Fumatech GmbH 0.013cm of thickness). The inner/anodic chamber having a volume of 3.14L, was filled with graphite granules (diameter ranging between 0.2cm and 4cm) and inoculated with activated sludge coming from a full-scale municipal wastewater treatment plant located in Treviso (Italy). The outer/cathodic chamber was
Electric current generation and COD removal
The electric current generated by the anodic biofilm was measured during the whole experimental period. Considering the COD removal all the parameters were determined to evaluate the anodic performance. Theoretically, a higher concentration of substrates should lead to a higher electric current generation, but there are very few data in literature on the effect of a high nitrogen concentration inside the feeding solution of an electroactive biofilm. For this reason, four C/N ratio were tested,
Conclusions
The herein studied MEC confirmed to be an appealing system to exploit the residual reducing power inside a synthetic fermentate, to recover nitrogen inside the latter, and to simultaneously abate CO2 inside a biogas. The study on the C/N ratio is fundamental due to the extreme heterogeneity of the wastewaters, which are the feeding solution of this type of MEC. The anodic performances do change significantly changing this parameter: with a high NLR the COD abatement is higher than the one
Declaration of competing interest
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.
Acknowledgments
Prof. Mauro Majone is acknowledged for his skillful assistance during each step of this experimentation.
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