Investigating the operational conditions for enhancement of dark fermentative hydrogen production in batch and sequencing batch reactors

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2015
Tunçay, Ekin Güneş
The aim of this master thesis study was to investigate the dark fermentative hydrogen production in batch and sequencing batch reactors (SBRs), and to investigate operational conditions leading to its maximization. Batch reactor studies were conducted to determine the initial operational conditions for the subsequent SBR operation. Two batch reactor sets, conducted with either sucrose or molasses as substrate, were operated to investigate the effect of initial pH, chemical oxygen demand (COD) and volatile suspended solids (VSS) concentrations and maximize the hydrogen production. These reactor sets were designed with Response Surface Methodology. Results revealed that maximum hydrogen yield (HY) of 2.3 mol H2/mol sucroseadded was achieved at an initial pH of 7 and initial COD concentration of 10 g/L. The studied initial substrate to VSS ratio (S/Xo) values of 4, 12 and 20 g COD/g VSS had no effect on hydrogen production yield. For batch studies conducted with molasses, the change in HY and productivity could not be explained with the studied ranges of three variables; initial pH, COD and VSS values. Maximum HY was achieved at 10 g/L initial COD, as 2.88 mmol H2/g sucroseadded. The decrease in the H2 and CO2 percentages of the headspace gas and suction observed in the reactors were attributed to homoacetogenic activity. Molasses, for containing potential intrinsic microorganism, might be more suitable to support and trigger the homoacetogenesis than sucrose. The SBR experiments consisted of 5 studies with different operational conditions. The objective of these studies was to increase the hydrogen production via each study, by modification of the operational parameters like pH and hydraulic retention time (HRT), with an attempt to maximize the HY. In the course of the research, two more objectives presented themselves. The first one was to suppress the homoacetogenenic activity and, the second appeared as to investigate the effect of Solid Retention Time (SRT) on dark fermentative hydrogen production. Results indicated that continuous dark fermentative H2 production from sucrose was significantly influenced by pH, HRT, homoacetogenic activity and SRT. Long HRTs (>12 h) and SRTs (>5 days) enhanced homoacetogenic activity and caused low HYs. The pH values higher than 5.5 were inefficient in suppressing the methanogens. The physical interventions done on the reactor increased the hydrogen production significantly. The maximum H2 yield achieved in SBR studies was 2.52 mol/ mol hexose (13.11 mmol H2/ g COD) at the conditions of pH of 5.5, HRT of 12 h, cycle time 8 of h, OLR of 22.4 gCOD/L.day and average SRT of 9.5 day. The HPR calculated at these operational conditions was 7.07 L H2/ (Lrxr.day).Yet, dark fermentative SBR systems are hard to stabilize at high HY. The ways to improve the stability of hydrogen production and the dominant microbial population remains to be studied.
Citation Formats
E. G. Tunçay, “Investigating the operational conditions for enhancement of dark fermentative hydrogen production in batch and sequencing batch reactors,” M.S. - Master of Science, Middle East Technical University, 2015.