Biohydrogen is a sustainable form of energy as it can be produced from organic waste through fermentation processes involving dark fermentation and photofermentation. Very often biohydrogen is included as a part of biorefinery approaches, which reclaim organic wastes that are abundant sources of renewable and low cost substrate that can be efficiently fermented by microorganisms. The aim of this work was to critically assess selected bioenergy alternatives from organic solid waste, such as biohydrogen and bioelectricity, to evaluate their relative advantages and disadvantages in the context of biorefineries, and finally to indicate the trends for future research and development. Biorefining is the sustainable processing of biomass into a spectrum of marketable products, which means: energy, materials, chemicals, food and feed. Dark fermentation of organic wastes could be the beach-head of complete biorefineries that generate biohydrogen as a first step and could significantly influence the future of solid waste management. Series systems show a better efficiency than one-stage process regarding substrate conversion to hydrogen and bioenergy. The dark fermentation also produces fermented by-products (fatty acids and solvents), so there is an opportunity for further combining with other processes that yield more bioenergy. Photoheterotrophic fermentation is one of them: photosynthetic heterotrophs, such as non-sulfur purple bacteria, can thrive on the simple organic substances produced in dark fermentation and light, to give more H2. Effluents from photoheterotrophic fermentation and digestates can be processed in microbial fuel cells for bioelectricity production and methanogenic digestion for methane generation, thus integrating a diverse block of bioenergies. Several digestates from bioenergies could be used for bioproducts generation, such as cellulolytic enzymes and saccharification processes, leading to ethanol fermentation (another bioenergy), thus completing the inverse cascade. Finally, biohydrogen, biomethane and bioelectricity could contribute to significant improvements for solid organic waste management in agricultural regions, as well as in urban areas.

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Two types of induction treatments (heat-shock pretreatment, HSP, and acetylene, Ac), inocula (meso and thermophilic) and incubation temperatures (37 and 55 degrees C) were tested according to a full factorial design 2(3) with the aim of assessing their effects on cumulative H(2) production (P(H), mmol H(2)/mini-reactor), initial H(2) production rate (R(i,H), micromol H(2)/(g VS(i) x h)), lag time (T(lag), h), and metabolites distribution when fermenting organic solid waste with an undefined anerobic consortia in batch mini-reactors. Type of inocula did not have a significant effect on P(H), T(lag), and R(i,H) except for organic acids production: mini-reactors seeded with thermophilic inocula had the highest organic acid production. Concerning the induction treatment, it was found that on the average Ac only affected in a positive way the P(H) and T(lag). Thus, P(H) in Ac-inhibited units (6.97) was 20% larger than those in HSP-inhibited units (5.77). Also, Ac favored a shorter T(lag) for P(H) in comparison with HSP (180 vs. 366). Additionally, a positive correlation was found between H(2) and organic acid production. In contrast, solvent concentration in heat-shocked mini-reactors were slightly higher than in reactors spiked with Ac. Regarding the incubation temperature, on the average mesophilic temperature affected in a positive and very significant way P(H) (10.07 vs. 2.67) and R(i,H) (2.43 vs. 0.76) with minimum T(lag) (87 vs. 459). The positive correlation between H(2) and organic acids production was found again. Yet, incubation temperature did not seem to affect solvent production. A strong interaction was observed between induction treatment and incubation temperature. Thus, Ac-inhibited units showed higher values of P(H) and R(i,H) than that HSP-inhibited units only under thermophilic incubation. Contrary to this, HSP-inhibited units showed the highest values of P(H) and R(i,H) only under mesophilic conditions. Therefore, the superiority of an induction treatment seems to strongly depend on the incubation temperature.