Dark fermentation effluent as substrate for hydrogen production from Rhodobacter capsulatus highlighting the performance of different fermentation systems.

Hydrogen production by biological route is a potentially sustainable alternative. Nowadays, energy production from sustainable sources has become urgent for several countries as well as for international policies. In this perspective, hydrogen has gained substantial global attention as clean, sustainable, and versatile energy carrier. In the current work, the resulting effluent from dark fermentation, rich in organic acids, was used as substrate for the purple non-sulfur bacteria (PNS) Rhodobacter capsulatus. In the first stage, experiments were carried out in bioreactors of 50 mL to check the influence of the composition of the effluent dark fermentation. The results proved that the provision of a sugar source improved bio-H2 production. The lactose and lactic acid concentrations exceeding 4.4 and 12 g/L, respectively, resulted in a productivity of up to 37.14 mmol H2/L days. Based on initial conditions obtained on the previous assays, in the second stage, a photo-fermentation in enlarged scale (1.5 L) was performed with the purpose to monitor the production of hydrogen and metabolites, sugar consumption and growth cells during the process. It was observed that the maximum productivity obtained was 98.23 mmol H2/L days in 26 h of process.

Alternative techniques to improve hydrogen production by dark fermentation.

The production of biofuels as an alternative to the fossil fuels has been mandatory for a cleaner and sustainable process. Hydrogen is seen as the fuel of the future because it has a very high energy density and its use produces only water instead of greenhouse gases and other exhaust pollutants. The biological synthesis of hydrogen by dark fermentation complies with these criteria. In the current work, the use of cheese whey permeate was evaluated aiming hydrogen production by dark fermentation using a microbial consortium in the semi-continuous process, with a reaction volume of 700 mL. The volume of the medium renewal and the frequency of replacements of fresh medium were evaluated to extend the production of H2. It is important to note decreases in the hydrogen production after 84 h. The target-product content became higher particularly when 466 mL of medium were withdrawn, in every 24 h in the first two replacements and, subsequently, in every 12 h. Besides, it was observed lower lactic acid concentration under this condition, suggesting that the shorter removal time of the medium could inhibit lactic acid bacteria, which may secrete bacteriocins that inhibit the hydrogen-producing microorganisms.