Combined effects of digestate viscosity and agitation conditions on the fermentative biohydrogen production

The combined effects of agitation and viscosity on fermentative hydrogen production were investigated in mixed cultures. The hydrogen production potential was studied in a batch fully-baffled cylindrical bioreactor, monitoring gas production and the composition of the gas effluents over time, while the profiles of volatile fatty acids in the digestate were analyzed. Viscosity was varied between 9.0.10(-4) and 6.1.10(-2) Pa.s by adding polyalkylene glycols that could not be used as a substrate by microorganisms. Experimental results demonstrated that hydrogen yield and production varied significantly as a function of viscosity and rotation speed between 0.90-1.38 mol H-2/mol of glucose and 7.4-11.3 L, respectively. Their evolution did not correlate with power input, but was adequately described by a unique quantity, the dimensionless Reynolds number: in laminar flow, mixing enhanced slightly hydrogen production, which peaked in the transitional regime, and finally fell under established turbulent conditions. Hydrogen production was significantly impaired when the Kolmogorov length scale approached the size of sludge aggregates. Butyric acid concentration was strongly correlated to H-2 production, whereas acetic acid content was unaffected by mixing. Finally, this study brings new knowledge on the effects of flow regime and mixing in the dark fermentation process.