Hydrogen Generation From Organic Waste Water In Microbial Electrolysis Cells And Function Analysis Of Anodophilic Communities

Nowadays energy consumption and environmental pollution is a pair of paradoxical problems in the world. We are confronted an increasing energy requirement and an inevitable environmental damage from fossil energy utilization. An all-round solution is one of the important issues for the subject of environmental science and engineering. New technologies are developing on waste water treatment and energy recovery. Microbial electrolysis cells (MEC) have recently been developed as a new technology for organic waste water treatment and energy recovery. Exoelectrogenic bacteria are employed to effectively degrade wide organic substrates and produce hydrogen by given a small external voltage.Targeting at efficiency improvement of waste treatment and hydrogen conversion, this study was carried out as follows: Reactor improvement and key factor analysis for higher hydrogen production; Dominant bacteria analysis during reactor operating using single strand conformation polymorphism (SSCP); Methane production and methane control to reduce hydrogen lose in single chamber MECs; Effects of different startup modes on MEC performances based on functional gene analysis, and to find out the connections among reactor performances and community structures; To elucidate microbial ecology questions in functional bacteria of MEC and explain the determination of community structures to reactor performances.Two-chamber reactor architecture was improved and reactor performances were analyzed to improve treatment efficiency and hydrogen conversion. Coulombic efficiency increased from 56% to 70% by reducing electrode distance and enlarging proton exchange membrane (PEM) area. Furthermore, coulombic efficiency was updated to ~80% with 62% internal resistance reduction when enhancing effective utilization ratio by stuffing activated carbon granule in anode chamber. Hydrogen generated from 0.5 mol/mol acetate to 1.4 mol/mol acetate. Energy efficiency increased from 60% to 110% based on ratio of hydrogen generation and electrical input. Microbial anode potential (MAP), an indicator to hydrogen process, was analyzed as functions of operational factors. The results indicated that hydrogen production was obtained at pH 6.0-7.0 and minimal substrate of 50 mg·L-1 when MAP was lower than -200 mV (vs. Ag/AgCl reference electrode). Moreover, planktonic bacteria contributed to electron transport and reduced the minimum MAP for hydrogen production.In the study Pseudomonas were main species with abilities of electron transport in two-chamber MEC reactors, which inoculated from activated sludge. Other dominant bacteria have not been reported on abilities on electron transporting. Based on anodic community analysis, there was similar microbial structure among planktonic and biofilm communities, but some differences were shown on amount signals and ratios in SSCP results. Diversity of PEM biofilm communities on the side to anode chamber was quite low, and few were related to electron transport or hydrogen consumption. Great part of non-exoelectrogenic bacteria are one of reasons for low efficiencies, however, exoelectrogens did not further overwhelm other groups when new biofilm formed after inoculated by MEC/MFC anode biofilm communities. Hence, it was concluded that exoelectrogens and other dominant communities were cooperating rather than opposing or competing.After MEC reactors were updated to single chamber structure, COD removal and coulombic efficiency were increased to 90% and hydrogen yield to 3 mole/mole acetate when applied voltage was higher than 0.5 V. Energy efficiency came up to 180%. Targeting at methane production and damage to hydrogen loss in MEC, it was possible to increase external voltage to control methane production. Results showed that methane could be controlled below 4% when voltage enhanced to 0.6 V or more, but over 10% methane was got in gas production when voltage was lower than 0.4 V in the experiment.In different startup modes of single chamber MECs, higher coulombic efficiency and hydrogen yield could be obtained at direct MEC startup mode compared to MFC startup mode. Based on functional gene analysis by Geo-Chip higher voltage contributed to enhancement of some parts of functional genes, including carbon degradation and metal resistance. Based on cytochrome genes, key important bacteria, Shewanella,Geobacter,Pseudomonas,Desulfovibrio, were typical exoelectrogens in MEC reactors, however, their functional genes were only a small part of all functions in the system. Although acetate was used as sole carbon source, community functions performed abundantly and involved in over ten processes in the system, including carbon cycling, nitrogen cycling, phosphate utilization and metal reduction. Most carbon degradation genes were related to simple substrates, and some genes on complicated carbon degradation, e.g. cellulose. These results supported that various functional community groups were cooperating in the system rather than formed a specialized function. Connecting reactors performances (coulombic efficiency, COD removal, gas production (H2, CH4, CO2)) and community structures, analysis indicated that coulombic efficiency and COD removal had significant relationship to community structure, but hydrogen and methane were not significant factors.In the study, we tried to understand MEC process from community mechanism and inner regularity. Different reactor performances were analysized in related to anodophilic community structures and it was tried to explain the phenomenon of alternative stable states in the view of microbial ecology. It was found that community assembly presented a stochastic process and formed various stable states in numbers of MEC reactors under the same operations and conditions. Analysis indicated that reactor performance on energy gas production significantly reflected community structures by canonical correlation analysis (CCA). Moreover, the results disclosed microbial function structures determined reactor performance accordingly. These results were meaningful to build high efficient functional communities and apply to practical waste water treatment in future.