Mesophilic and thermophilic biohydrogen and bioelectricity production from real and synthetic wastewaters

Abstract

In the last century, fossil fuels have been intensively used for energy production causing a dramatic increase of CO2 level in the atmosphere and the related environmental issues, such as global warming and ozone layer depletion. In 2015, the increased awareness about climate change led to the signature of the Paris agreement, in which 195 countries committed to cut off their greenhouse gases emission by 40% (compared to emissions in 1990) by 2030. The achievement of such an ambitious target is strictly linked to a gradual switch from fossil fuels to sustainable and renewable sources for energy production. This is driving many industries producing organic and inorganic waste towards a biorefinery concept, in which side streams, wastes and wastewaters are seen as a potential feedstocks for biofuel and/or biochemical production.<br/><br/>Dark fermentation and microbial fuel cells (MFCs) are two emerging technologies for biological conversion of the chemical energy of organic compounds into hydrogen (H2) and electricity, respectively. Although these technologies can potentially replace fossil fuels for energy production, their establishment is hindered by their low energy output. Due to kinetic and thermodynamic advantages, high temperature can be the key for increasing both dark fermentative H2 production and electricity production in MFCs. Therefore, this thesis focuses on delineating how temperature influences biological production of H2 and electricity from organic carbon-containing wastewaters.<br/><br/>Start-up and selection of a suitable microbial community is a crucial phase in dark fermentation. Two heat-treated inocula (fresh and digested activated sludge) were compared, in four consecutive batch cycles, for H2 production from xylose at 37, 55 and 70 °C. At both 37 and 55 °C, a higher H2 yield was achieved by the fresh than the digested activated sludge, whereas a very low H2 yield was obtained by both inocula at 70 °C. Then, four different inoculum pretreatments (acidic, alkaline, heat and freezing shocks) were evaluated, in a single-stage batch experiment, for creating an efficient mesophilic (37 °C) and thermophilic (55 °C) H2 producing community. Acidic and alkaline shocks selected known H2 producing microorganisms belonging to Clostridiaceae at the expenses of lactate producing bacteria, resulting in the highest H2 yield at 37 and 55 °C, respectively. Although a heat shock resulted in a low H2 yield in a single batch, H2 production by the heat-treated fresh activated sludge was shown to increase in the experiment with four consecutive batch cycles. This suggests that H2 producing microbial communities may develop in the long-term as long as culture conditions are optimized for growth of H2 producers.<br/><br/>Heat-treated fresh activated sludge was selected as inoculum for continuous H2 production from a xylose-containing synthetic wastewater in a mesophilic (37 °C) and a thermophilic (55-70 °C, increased stepwise) fluidized bed reactor (FBR). A higher H2 yield was obtained in the thermophilic than in the mesophilic FBR. Furthermore, H2 production at 70 °C, which failed in the earlier batch study, was successful in the FBR, with a stable yield of 1.2 mol H2 mol-1 xyloseadded, by adapting the microbial community from 55 °C to 70 °C stepwise at 5 °C intervals. Operation temperature of 70 °C was also found optimal for H2 production from thermomechanical pulping (TMP) wastewater in a temperature gradient incubator assay, as batch cultivation at 70 °C enriched the H2 producing Thermoanaerobacterium sp. and repressed homoacetogenic microorganisms.<br/><br/>A detailed knowledge of microbial communities, and particularly the active subpopulation, is crucial in order to adjust the conditions to favor the growth of exoelectrogenic microorganisms in MFCs. A RNA approach was used to study the structure and role of the anode-attached, membrane-attached and planktonic microbial communities in a mesophilic (37 °C) and a thermophilic (55 °C) two-chamber, xylose-fed MFC. An anodeattached community dominated by Geobacteraceae sustained electricity production at 37 °C, whereas the establishment of methanogenic and H2 oxidizing microorganisms resulted in a low electricity production at 55 °C. However, the development of a thermophilic exoelectrogenic community can be promoted by applying a start-up strategy which includes imposing a negative potential to the anode and chemical inhibition of methanogens. At both 37 and 55 °C, aerobic membrane-attached microorganisms were likely involved in consuming the oxygen diffusing from the cathodic to the anodic chamber, thus favoring the exoelectrogenic microorganisms, which are strictly anaerobic, but competing with them for the substrate. A mesophilic exoelectrogenic community was also shown to produce electricity from TMP wastewater in an upflow MFC operated at 37 °C.<br/><br/>In conclusion, a higher and more stable H2 yield can be achieved in thermophilic rather than mesophilic dark fermentation. Dark fermentation at 70 °C is particularly suitable for treatment of TMP wastewater as it is released at high temperature (50-80 °C) and could be treated on site with minimal energy requirement for heating the bioreactor. TMP wastewater can be also used as substrate for electricity production in mesophilic MFCs. Electricity production in thermophilic MFCs is feasible, but enrichment of thermophilic exoelectrogenic microorganisms may require a long start-up period with optimized conditions. The detailed RNA-level microbial community analysis performed in this study may help in selecting a start-up and operation strategy to optimize electricity production.