Conductive materials stimulated direct interspecies electron transfer and application in anaerobic digestion

Methanation of complex organic substances needs cooperation between syntrophic microorganisms. During this process, electrons and protons or other reducing equivalents are transferred from bacteria to archaea by intermediate shuttles, such as hydrogen. However, this process cannot be completed unless intermediates are readily removed. The speed of intermediates consumption is related to electron transfer efficiency between functional communities. Thus, the strategy to facilitate intermediates consumption and interspecies electron transfer could play an important role in promoting organics degradation. Direct interspecies electron transfer (DIET) is a newly discovered electron transfer process within anaerobic system, where conductive materials (CM) act as electron conduits between syntrophic partners. Theoretical calculation revealed that the electron transfer rate via DIET is substantially higher than that via interspecies molecular transfer. Therefore, DIET is beneficial to facilitate intermediates consuming rate, and remove inhibition factor such as H2 accumulation. This thesis focuses on the role and impact of CM in four application cases of anaerobic digestion (AD), i.e. thermophilic reactor start-up, toxic compounds (organic matter - phenol or inorganic matter - ammonia) degradation, and organic compounds transformation. The results exhibited CM could greatly promote methanogenic performance, enhance system stability, and improve organic compounds converting efficiency. In thermophilic AD start-up study, methane generation rate increased more than two times resulting in 50% shortened start-up time needed in CM group compared with Control group. The less affected AD performance under high hydrogen partial pressure, greater conductivity and healthy sludge morphology suggested CM trigger DIET between syntrophic partners. Similarly, one-fold higher phenol degradation rate was observed after dosing CM in a phenol degradation system. In CM groups, the electron shuttles, i.e. protein and humic substances, were greatly enriched in extracellular polymeric substances (EPS). In particular, around 2.3 to 20 folds higher protein was observed in CM group. As for organics transformation study, the addition of CM promoted 35% to 86% more methane generation. The study revealed the improved methane generation was attributed to the stimulated hydrolysis of organic matters and greater degradation of humic-like substances. Microbial community analysis showed electro-active strains, and bacteria that are capable of degrading complex organics were greatly enriched in CM groups. However, negative effects of carbon-based CM on methanogenic performance were observed in high ammonia system. This phenomenon was evidenced by a strong negative correlation between specific methane production rate and carbon nanotube (CNT) concentration (Rs = -0.972, p < 0.01). Further investigation found that potassium transportation process of the anaerobes was disturbed at the presence of CNT, and intracellular energy level was reduced probably due to the microbial detoxification activity and decreased ATP production capability. This thesis investigated the role of CM in alleviating the common obstacles of AD technology. The working principles were carefully investigated. The results indicated CM promoted organic degradation via promoting sludge conductivity, acting as electron conduit for interspecies electron transfer, enriching electron shuttles within EPS, and altering microbial community. However, the negative effects of CNT on high ammonia inhibited AD system also alerted that CM may disturb the energy transfer system and worsen inhibition on anaerobic microorganisms. This study provides a basis for future research on application of CM in AD system.Doctor of Philosoph