Contaminants removal and bacterial activity enhancement along the flow path of constructed wetland microbial fuel cells

Abstract

Microbial fuel cells implemented in constructed wetlands (CW-MFCs), albeit a relatively new technology still under study, have shown to improve treatment efficiency of urban wastewater. So far the vast majority of CW-MFC systems investigated were designed as lab-scale systems working under rather unrealistic hydraulic conditions using synthetic wastewater. The main objective of this work was to quantify CW-MFCs performance operated under different conditions in a more realistic setup using meso-scale systems with horizontal flow fed with real urban wastewater. Operational conditions tested were organic loading rate (4.9±1.6, 6.7±1.4 and 13.6±3.2 g COD/m 2 .day) and hydraulic regime (continuous vs intermittent feeding) as well as different electrical connections: CW control (conventional CW without electrodes), opencircuit CW-MFC (external circuit between anode and cathode not connected) and closed-circuit CW-MFC (external circuit connected). Eight horizontal subsurface flow CWs were operated for about four months. Each wetland consisted of a PVC reservoir of 0.193 m 2 filled with 4/8 mm granitic riverine gravel (wetted depth 25 cm). All wetlands had intermediate sampling points for gravel and interstitial liquid sampling. The CW-MFCs were designed as three MFCs incorporated one after the other along the flow path of the CWs. Anodes consisted of gravel with an incorporated current collector (stainless steel mesh) and the cathode consisted of a graphite felt layer. Electrodes of closed-circuit CW-MFC systems were connected externally over a 220 Ω resistance.Results showed no significant differences between tested organic loading rates, hydraulic regimes or electrical connections, however, on average, systems operated in closed-circuit CW-MFC mode under continuous flow outperformed the other experimental conditions. Closedcircuit CW-MFC compared to conventional CW control systems showed around 5% and 22% higher COD and ammonium removal, respectively. Correspondingly, overall bacteria activity, as measured by the fluorescein diacetate technique, was higher (4% to 34%) in closed-circuit systems when compared to CW control systems. KeywordsConstructed wetlands, urban wastewater, microbial fuel cells, bacterial activity, hydraulic regime, organic loading rate removal. Microbial activity was higher in all three transects in closed-circuit mode when compared to control conditions, which could be one of the reasons for the observed enhancement of treatment performance. Differences between closed-circuit and control systems were not significant in the first transect but very significant in the second and extremely significant in the third, possibly indicating that the lower organic load along the flow path benefited the activity of electrogenic bacteria over competing non-electrogenic bacteria. ACKNOWLEDGEMENTSThis project has received funding from the European Union´s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 676070. This communication reflects only the authors' view...

Effect of sulfonamides on the electricity generation by Shewanella putrefaciens in microbial fuel cells

“…The activity of electrogenic microorganisms has a significant impact on the electrical performance of MFCs. 12,25 According to our work, S. putrefaciens, 26 and these results were consistent with other reports. 12,[27][28][29] Microbial activity affects the electrical performance of MFCs mainly through electron transfer.…”

“…The activity of electrogenic microorganisms has a significant impact on the electrical performance of MFCs 12,25 . According to our work, the addition of SAs reduced the electrical production of MFCs, and production decreased with increased concentrations of SAs; this effect might be associated with the toxic effects of SAs on S. putrefaciens , 26 and these results were consistent with other reports 12,27‐29 …”

A new concept in constructed wetlands: assessment of aerobic electroconductive biofilters

“…[25][26][27] Other studies have integrated MFC concepts into CW with the purpose of harvesting energy, although the strategy of burying electrodes in the gravel of conventional CW exhibited a minor impact on the wetland performance from the water purification perspective. [28][29][30] In contrast, the METland® concept followed an alternative approach by replacing the classical biofilter material (gravel and sand) with an EC material so bacteria can interchange electrons with the bed. The first METland® was based on horizontal subsurface flow (HSSF), 25 a saturated system in which anaerobic conditions prevailed.…”

Constructed wetlands operated as bioelectrochemical systems for the removal of organic micropollutants

“…For the purpose of this work, six unplanted meso-scale horizontal subsurface flow (HF) systems were used (a duplicate of systems per treatment: two systems per CW-control, CW-MFC and CW-MEC). The setup of these systems is detailed in a previous study (Hartl et al, 2019). Briefly, the systems consisted of a PVC reservoir of ca.…”

“…Up to date, there are only a few publications dealing with the use of CW-MFC for OMP removal, and to the authors' knowledge, there are none on the use of CW-MEC systems. Generally, CW-MFC systems have been described to increase the microbial activity (determined by means of fluorescein diacetate (FDA) hydrolysis) in CW-MFC (Hartl et al, 2019), and some studies showed that CW-MFC enhance microbial community richness and diversity as compared to an open-circuit control (Song et al, 2018;F. Xu et al, 2018a).…”