Effect of hydraulic retention time on biohydrogen production and anaerobic microbial community

The effects of hydraulic retention time (HRT) on biohydrogen production and its mixed anaerobic microbial community grown with glucose were investigated in a continuous stirred tank reactor culture. Starting from a HRT of 50 h the culture was acclimated by stepwise HRT reduction until a steady-state was reached at a HRT of 8 h after 19 days. After that the culture was run with HRTs of 6 h, 8 h, 10 h and 12 h, each lasting 9–15 days. Hydrogen, CO2 and dissolved products were daily measured. The species composition of the culture was ascertained using 16S rDNA genes separated by denaturing gradient gel electrophoresis (DGGE). While a hydrogen yield of 1.6 mol/mol glucose was found during the first steady-state at 8 h HRT it stabilized at 1.9 in the following steady-states. The predominant dissolved products were butyrate and acetate in a ratio of about 2.1:1 and amounted to 82–94% of the total products. In the first 8 h HRT period also propionate was found in an amount of 9%. It is concluded that the increase of the hydrogen yield after transition from 8 h to 6 h HRT is caused by a washout of the propionate producing population during the 6 h HRT period. In fact the fading of two 16S rDNA gene fragments was noticed in the DGGE profiles at the same time.