Induction of hydrogen production affects micro and macro structure of granular sludge

Abstract

Mixed-culture dark fermentation is an environmentally friendly bio-hydrogen production process. In this work we study the potential for directing microbial anaerobic mixed communities towards improved hydrogen production. Strategies applied for promoting the selection of hydrogen-producing bacteria in anaerobic granules consisted of Heat treatment and chemical treatment with 2-bromo-ethane sulfonate (BES) and with BES+Chloroform. Three EGSB reactors, RHeat, RBES and RBES+Chlo, where inoculated with each treated granules and fed with synthetic sugar-based wastewater. Hydrogen production was monitored. Morphological integrity and microbial diversity of the granules were studied using image analysis technique and 16S rRNA gene based techniques, respectively. Hydrogen production in RHeat was below 300 mLH2L-1d-1, with the exception of a single transient production of 1000 mLH2L-1d-1, after decrease the HRT. In RBES+Chlo hydrogen production rate never exceeded 300 mLH2L-1d-1. In this sludge, a physical deterioration of the granules was observed along with a decrease of their density and microbial diversity. In RBES, a transient period of unstable H2 production was observed but an additional pulse of BES triggered hydrogen production rate to an average value of 700 ± 200 mLH2L-1d-1, which was kept for 30 days. This strategy did not affect significantly granules structure. Dominant bacterial ribotypes found in RBES were closely related to Clostridium species and to uncultured microorganisms belonging to Clostridiaceae and Ruminococcaceae. This work demonstrates that different methods applied for directing granular sludge for hydrogen production can cause changes in the macro- and microstructure of granular sludge, which can be incompatible with the long-term operation of high-rate reactors.