Dark fermentation, also known as acidogenesis, involves the transformation of a wide range of organic substrates into a mixture of products, e.g. acetic acid, butyric acid and hydrogen. This bioprocess occurs in the absence of oxygen and light. The ability to synthesize hydrogen, by dark fermentation, has raised its scientific attention. Hydrogen is a nonpolluting energy carrier molecule. However, for energy generation, there is a variety of other sustainable alternatives to hydrogen energy, e.g. solar, wind, tide, hydroelectric, biomass incineration, or nuclear fission. Nevertheless, dark fermentation appears as an important sustainable process in another area: the synthesis of valuable chemicals, i.e. an alternative to petrochemical refinery. Currently, acetic acid, butyric acid and hydrogen are mostly produced by petrochemical reforming, and they serve as precursors of ubiquitous petrochemical derived products. Hence, the future of dark fermentation relies as a core bioprocess in the biorefinery concept. The present article aims to present and discuss the current and future status of dark fermentation in the biorefinery concept. The first half of the article presents the metabolic pathways, product yields and its technological importance, microorganisms responsible for mixed dark fermentation, and operational parameters, e.g. substrates, pH, temperature and head-space composition, which affect dark fermentation. The minimal selling price of dark fermentation products is also presented in this section. The second half discusses the perspectives and future of dark fermentation as a core bioprocess. The relationship of dark fermentation with other (bio)processes, e.g. liquid fuels and fine chemicals, algae cultivation, biomethane-biohythane-biosyngas production, and syngas fermentation, is then explored.

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The large water consumption (1.9-5.9 m(3) water per m(3) of biofuel) required by biomass processing plants has become an emerging concern, which is particularly critical in arid/semiarid regions. Seawater, as a widely available water source, could be an interesting option. This work was to study the technical feasibility of using seawater to replace freshwater in the pretreatment of date palm leaflets, a lignocellulosic biomass from arid regions, for bioethanol production. It was shown that leaflets pretreated with seawater exhibited lower cellulose crystallinity than those pretreated with freshwater. Pretreatment with seawater produced comparably digestible and fermentable solids to those obtained with freshwater. Moreover, no significant difference of inhibition to Saccharomyces cerevisiae was observed between liquids from pretreatment with seawater and freshwater. The results showed that seawater could be a promising alternative to freshwater for lignocellulose biorefineries in coastal and/or arid/semiarid areas.

Mixed culture anaerobic fermentation generates a wide range of products from simple sugars, and is potentially an effective process for producing renewable commodity chemicals. However it is difficult to predict product spectrum, and to control the process. One of the key control handles is pH, but the response is commonly dependent on culture history. In this work, we assess the impact of pH regulation mode on the product spectrum. Two regulation modes were applied: in the first, pH was adjusted from 4.5 to 8.5 in progressive steps of 0.5 and in the second, covered the same pH range, but the pH was reset to 5.5 before each change. Acetate, butyrate, and ethanol were produced throughout all pH ranges, but there was a shift from butyrate at pH < 6.5 to ethanol at pH > 6.5, as well as a strong and consistent shift from hydrogen to formate as pH increased. Microbial analysis indicated that progressive pH resulted in dominance by Klebsiella, while reset pH resulted in a bias towards Clostridium spp., particularly at low pH, with higher variance in community between different pH levels. Reset pH was more responsive to changes in pH, and analysis of Gibbs free energy indicated that the reset pH experiments operated closer to thermodynamic equilibrium, particularly with respect to the formate/hydrogen balance. This may indicate that periodically resetting pH conforms better to thermodynamic expectations.

The present manuscript highlights the economic profit increase when combining organic waste anaerobic digestion with other mixed culture anaerobic fermentation technologies, e.g., lactic acid fermentation and dark fermentation. Here we consider the conversion of 50 tonnes/day of food waste into methane, power generation (from CHP of biomethane), lactic acid, polylactic acid, hydrogen, acetic acid and butyric acid. The economic assessment shows that the basic alternative, i.e., anaerobic digestion with methane selling to the grid, generates 19 USD/t_VS (3 USD/t_foodwaste) of profit. The highest profit is obtained by dark fermentation with separation and purification of acetic and butyric acids, i.e., 296 USD/t_VS (47 USD/t_foodwaste). The only alternative that presented losses is the power generation alternative, needing tipping fees and/or subsidy of 176 USD/t_VS (29 USD/t_foodwaste). The rest of the alternatives generate profit. From the return on investment (ROI) and payback time, the best scenario is the production of polylactic acid, with 98% ROI, and 7.8 years payback time. Production of butyric acid ROI and payback time was 74% and 9.1 years.