Surface-to-surface biofilm transfer: a quick and reliable startup strategy for mixed culture microbial fuel cells

Enrichment of Clostridia enhances Geobacter population and electron harvesting in a complex electroactive biofilm

Current research on microbial fuel cell or microbial electrolysis cell dealt with finding new electroactive bacteria and understanding the mechanisms of electronic exchange. Complex consortia allowed to obtain better performances than pure cultures in part thanks to inter-species cooperation. However, the role of each bacterium in a complex biofilm in the electron harvest on an electrode remains unclear. Thus, we combined electrochemical monitoring of electron exchange and high throughput sequencing analysis in order to describe the bacterial composition and the electroactive performance of mangrove mud biofilms. In this study, secondary electroactive biofilms were formed on carbon electrodes from Desulfuromonas-dominated inoculum of pre-formed bioanodes. The performances and the Desulfuromonas-dominated profile were the same as those of primary bioanodes when the planktonic community was conserved. However, a Clostridium enrichment allowed to restore the performance in maximal current densities promoting an increase of Geobacter population, becoming the most dominant group among the Deltaproteobacteria, replacing Desulfuromonas. These results highlight a positive collaboration between Clostridium and Geobacter spp. helping a bacterial population to achieve with the depletion of their environment. Our study provides new insight into relationships between dominant electroactive bacteria and other bacteria species living in an organic matter-rich environment as mangrove sediments.

Anode Surface Bioaugmentation Enhances Deterministic Biofilm Assembly in Microbial Fuel Cells

Microbial fuel cells (MFCs) generate energy while aiding the biodegradation of waste through the activity of an electroactive mixed biofilm. Metabolic cooperation is essential for MFCs' efficiency, especially during early colonization. Thus, examining specific ecological processes that drive the assembly of anode biofilms is highly important for shortening startup times and improving MFC performance, making this technology cost-effective and sustainable. Here, we use metagenomics to show that bioaugmentation of the anode surface with a taxonomically defined electroactive consortium, dominated by Desulfuromonas, resulted in an extremely rapid current density generation. Conversely, the untreated anode surface resulted in a highly stochastic and slower biofilm assembly. Remarkably, an efficient anode colonization process was obtained only if wastewater was added, leading to a nearly complete replacement of the bioaugmented community by Geobacter lovleyi Although different approaches to improve MFC startup have been investigated, we propose that only the combination of anode bioaugmentation with wastewater inoculation can reduce stochasticity. Such an approach provides the conditions that support the growth of specific newly arriving species that positively support the fast establishment of a highly functional anode biofilm.IMPORTANCE Mixed microbial communities play important roles in treating wastewater, in producing renewable energy, and in the bioremediation of pollutants in contaminated environments. While these processes are well known, especially the community structure and biodiversity, how to efficiently and robustly manage microbial community assembly remains unknown. Moreover, it has been shown that a high degree of temporal variation in microbial community composition and structure often occurs even under identical environmental conditions. This heterogeneity is directly related to stochastic processes involved in microbial community organization, similarly during the initial stages of biofilm formation on surfaces. In this study, we show that anode surface pretreatment alone is not sufficient for a substantial improvement in startup times in microbial fuel cells (MFCs), as previously thought. Rather, we have discovered that the combination of applying a well-known consortium directly on the anode surface together with wastewater (including the bacteria that they contain) is the optimized management scheme. This allowed a selected colonization process by the wastewater species, which improved the functionality relative to that of untreated systems.

Performance and inorganic fouling of a submergible 255 L prototype microbial fuel cell module during continuous long-term operation with real municipal wastewater under practical conditions

A submergible 255 L prototype MFC module was operated under practical conditions with municipal wastewater having a large share in industrial discharges for 98 days to investigate the performance of two of the largest, ever investigated multi-panel stainless steel/activated carbon air cathodes (85 × 85 cm). At a flow rate of 144 L/d, power density of 78 mW/m²_Cat (317 mW/m³) and COD, TSS and TN removal of 41 ± 16 %, 36 ± 16 % and 18 ± 14 %, respectively, were reached. Observed Coulombic efficiency and substrate-specific energy recovery were 29.5 ± 14 % and 0.184 ± 0.125 kWh_el/kg_COD,deg, respectively. High salt content of wastewater (TDS = 2.8 g/L) led to severe inorganic fouling causing a drastic decline in power output and energy recovery of more than 90 % in the course of experiments. Mechanical cleaning of the cathodes restored only 22 % (17 mW/m²_Cat) of the power output and did not improve nutrient removal or energy recovery.

Storing of exoelectrogenic anolyte for efficient microbial fuel cell recovery

Starting up a microbial fuel cell (MFC) requires often a long-term culture enrichment period, which is a challenge after process upsets. The purpose of this study was to develop low-cost storage for MFC enrichment culture to enable prompt process recovery after upsets. Anolyte of an operating xylose-fed MFC was stored at different temperatures and for different time periods. Storing the anolyte for 1 week or 1 month at +4°C did not significantly affect power production, but the lag time for power production was increased from 2 days to 3 or 5 days, respectively. One month storing at -20°C increased the lag time to 7 days. The average power density in these MFCs varied between 1.2 and 1.7 W/m³. The share of dead cells (measured by live/dead staining) increased with storing time. After 6-month storage, the power production was insignificant. However, xylose removal remained similar in all cultures (99-100%) while volatile fatty acids production varied. The results indicate that fermentative organisms tolerated the long storage better than the exoelectrogens. As storing at +4°C is less energy intensive compared to freezing, anolyte storage at +4°C for a maximum of 1 month is recommended as start-up seed for MFC after process failure to enable efficient process recovery.

Inhibition of microbial fuel cell operation for municipal wastewater treatment by impact loads of free ammonia in bench- and 45L-scale

A 45-liter microbial fuel cell (MFC) system was integrated into a full-scale wastewater treatment plant (WWTP). The system was operated under practical conditions with supernatant of a pre-thickener for 50days in order to identify, whether higher power output and energy recovery is possible compared to the use of primary clarifier effluent, as used in a previous study. The higher COD (chemical oxygen demand) loading rates of supernatant neither increased power densities, nor energy recovery, but impact loads of total ammonia nitrogen (TAN) in concentrations >800mg/L (free ammonia nitrogen (FAN)>40mg/L) led to an instant collapse of power output and nutrient removal, which was reversed when ammonia concentrations decreased. Investigations in lab-scale under defined conditions verified that the inhibition of the exoelectrogenic biofilm is in fact caused by high levels of FAN. Here, COD removal, power output and energy recovery constantly decreased, when FAN-concentrations were increased above 64mg/L.

An integrated 45L pilot microbial fuel cell system at a full-scale wastewater treatment plant

A 45-L pilot MFC system, consisting of four single-chamber membraneless MFCs, was integrated into a full-scale wastewater treatment plant (WWTP) and operated under practical conditions with the effluent of the primary clarifier for nine months to identify an optimal operational strategy for stable power output and maximum substrate based energy recovery (Normalized Energy Recovery, NER). Best results with the MFC were obtained at a hydraulic retention time of 22h with COD, TSS and nitrogen removal of 24%, 40% and 28%, respectively. Mean NER of 0.36kWhel/kgCOD,deg and coulombic efficiency of 24.8% were reached. Experimental results were used to set up the first described energy balance for a whole WWTP with an integrated MFC system. Energetic calculations of the model WWTP showed that energy savings due to reduced excess sludge production and energy gain of the MFC are significantly higher than the loss of energy due to reduced biogas production.