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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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The object of this work was to carry out a kinetic study on the Candida tropicalis cell lysis and to obtain a kinetic model that would describe the inhibitory and lytic effects of phenol on the yeast growth. From the experiments, a model for the growth kinetic behavior of the yeast was evolved. The proposed model describes satisfactorily the inhibitory and lytic effects of phenol on yeast cultures. From the kinetic model constants, it was found that C. tropicalis showed high affinity and tolerance toward phenol. The overall growth yields decreased when the initial phenol concentration increased, and it may be due to an increased maintenance coefficient and to cell lysis.

The manufacturing processes of tortillas, corn chips, tortilla chips, and related products yield a liquid waste called nejayote. This waste causes serious pollution problems since its chemical oxygen demand is very high, from 25000 to 30000 ppm. To reduce the pollution potential of this effluent, the use of a cascade of three continuous flow bioreactors-in-series is proposed in this work. When nejayote was supplemented with KH2PO4 and (NH4)2SO4, at COD:KH2PO4 and COD:(NH4)2SO4 ratios of 87 and 26, respectively, and the pH in the first stage was adjusted at 7.0 every 24 h, high removal efficiencies of COD (86.4%) and nitrogen (80.9%) were obtained. Although the treated water still requires a further treatment prior to its disposal, the proposed system is a potential alternative for nejayote treatment, since it allows a greater COD removal efficiency than those reported in the literature for equivalent organic loads. Key words: nejayote, biodegradation, multi-stage bioreactor, cascade.

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