Among renewable H2-producing biotechnologies, dark fermentation (DF) of ”negative value” Organic Waste Streams (OWS) has received significant attention in recent years, since it combines sustainable waste management with pollution control and with the generation of a valuable clean energy product. Fermentative H2 production provides only a partial oxidation of the organic substrate, therefore, it is likely to be industrially viable only if integrated within a process that can utilize its end-products. Microbial Electrolysis Cell (MEC) is an emerging technology that could utilize the metabolic byproducts generated by DF to produce further H2. Combining DF with MEC in a cascade two-steps process can result in a complete exploitation of OWS, which maximizes at the same time energy recovery and effluent depollution. Although there is an intensive activity in optimizing process conditions through the variation of physicochemical factors, little research investigates the fundamentals of microbial interactions in such microbial ecosystems, that are at the basis of the conversion of organic matter to energy. Bridging the knowledge gap regarding the metabolic interactions within microbial fermentative/electrogenic communities could allow us to manipulate the microbial communities to maximize the efficiency of substrate conversion. This project adopts an interdisciplinary approach to deal with mixed cultures, from microbial engineering to microbial ecology and microbial physiology: this will establish new knowledge derived from microbial ecology, to the modern industrial and environmental biotechnologies. Besides the acquisition of fundamental knowledge on microbial metabolic interactions occurring in mixed cultures, the technical aim of the project is the development of a sustainable, cascade two-step BioH2 production process from OWS, combining DF and MEC. Such coupling of DF and MEC constitutes a technological cornerstone within the concept of an environmental biorefinery.