Enhancement of mass transfer conditions to increase the productivity and efficiency of dark fermentation in continuous reactors

Abstract

Hydrogen (H2) produced by dark fermentation is an alternative to fulfill the requirements of the transportation sector and to be a complementary source in the forthcoming electricity grid. However, the dark fermentative H2 production is limited by the accumulation of H2 in the fermentation broth. In continuous stirred-tank reactors (CSTR), such phenomenon is associated with poor mass transfer conditions. Nevertheless, this parameter has been scarcely considered to enhance H2 production. In this research, the effect of the H2 mass transfer conditions on the productivity and efficiency of continuous H2 production was evaluated using a series of CSTR operated at H2 mass transfer coefficients (kLa) ranging from 1.04 to 4.23 1/h. The results showed that volumetric H2 production rate (VHPR) and H2 yield increased 74 and 78%, respectively, due to enhanced mass transfer conditions. This behavior was driven by 53% decrease of the dissolved H2 concentration. More specifically, the maximum VHPR of 7.66 L/L-d with a H2 yield of 1.1 mol H2/mol hexose was obtained at a kLa = 4.23 1/h. Furthermore, 16S-DGGE analysis and sequencing revealed that Clostridium and Lactobacillus were the dominant bacterial genera in continuous operation. In particular, Clostridium increased its occurrence at kLa of 2.72–4.23 1/h as a response to lower dissolved H2 concentrations. The novelty of this work relies on the demonstration that mass transfer conditions control not only the H2 accumulation and reactor performance (VHPR and H2 yield), but they also influence the metabolic pathways and the composition of the microbial community.