Abstract
Microbial Fuel Cells (MFCs) operated as biosensors could potentially enable truly low-cost, real-time monitoring of organic loading in wastewaters. The current generated by MFCs has been correlated with conventional measures of organic load such as Biochemical Oxygen Demand (BOD), but much remains to be established in terms of the reliability and applicability of such sensors. In this study, batch-mode and multi-stage, flow-mode MFCs were operated for over 800 days and regularly re-calibrated with synthetic wastewater containing glucose and glutamic acid (GGA). BOD5 calibration curves were obtained by normalising the current measured as a percentage of maximum current. There was little drift between recalibrations and non-linear Hill models of the combined dataset had R2 of 88–95%, exhibiting a stable response over time and across devices. Nonetheless, factors which do affect calibration were also assessed. Increasing external resistance (from 43.5 to 5100 Ω) above the internal resistance determined by polarisation curve decreased the calibration upper limit from 240 to 30 mg/l O2 BOD5. Furthermore, more fermentable carbon sources increased the detection range, as tested with samples of real wastewater and synthetic media containing GGA, glucose-only and glutamic acid-only. Biofilm acclimatisation therefore did not account for differences between aerobic oxygen demand determinations and anaerobic MFC responses; these are likely attributable to competitive processes such as fermentation. This further highlights the potential for MFCs as real-time sensors for organic load monitoring and process control in addition to BOD-compliant measurement systems.
Highlights
Determination of water quality is important for treatment and se curity of downstream ecosystems
Re-calibration of batch-mode Microbial Fuel Cells (MFCs) To determine the effect of calibration drift and reproducibility under identical conditions (i.e. glucose and glutamic acid (GGA) medium, RExt of 43.2 Ω, 28 ◦C), BOD5 calibrations of batch-mode cells A and B initiated on days 136, 219, 328, 730 and 792 were collated
Current density normalisation was necessary to make valid comparisons be tween calibrations (Fig. 1) as cathode potentials decreased over time and increased when fresh membrane-cathode assemblies were installed
Summary
Determination of water quality is important for treatment and se curity of downstream ecosystems. A crucial parameter determining water quality is Biochemical Oxygen Demand (BOD); the dissolved ox ygen required by micro-organisms to oxidise organic matter present in the water. BOD is conventionally measured by offline, five-day tests (BOD5; APHA, 1999) or online with expensive transducers requiring frequent re-calibration and maintenance (Jouanneau et al, 2013). The electrical outputs from Bioelectrochemical Systems (BES), including Microbial Fuel Cells (MFCs), have been correlated to the concentration of biodegradable organic carbon present with high ac curacy (Gupta et al, 2019; Sonawane et al, 2020). There is potential to operate sensors with minimal maintenance over multiple years (Kim et al, 2003a), either in batch-fed mode as an offline testing platform or for continuous process monitoring. To prevail against other online BOD sensors (where unreliability has limited their uptake), the stability of the calibrated MFC response must be defined. As membrane fouling or cathode degradation may necessitate recalibration (Kim et al, 2003b; Sonawane et al, 2020)
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