Controlling dark fermentation of agro-industrial waste for valued organic acids production

In recent years, our planet has been facing severe environmental degradation and climate change with consequences for the health of people and repercussions on the social and economic balance. Organic waste contains valuable compounds, worth recovering. Traditional waste management methods are typically based on aerobic treatment and anaerobic digestion (AD) of the organic substrate, or a combination of them. From these traditional processes, it is possible to obtain compost and biogas that have a relatively low economic value. In a context characterized by finite resources, the wastes represent both an environmental issue to be properly managed and a potential resource of secondary raw materials. In this context, the present Ph.D. thesis will pay attention to the possibility to produce valuable mixtures rich in organic acids through controlled dark fermentation (DF) of sheep cheese whey (CW). The main goal is the identification of the operating parameters which affect the reactions occurring during DF to engineer the process itself to produce specific organic acids (such as lactic, acetic, propionic, and butyric acid) useful in several applications. For this purpose, two specific applications were investigated: polyhydroxyalkanoates (PHA) production n and metal leaching. 1) Polyhydroxyalkanoates are biopolymers synthesized by several microorganisms as carbon and energy reserves in the event of carbon excess and nutrient deficiency. Industrial production of PHA remains constrained by production costs up to five times higher than those of petroleum-derived plastics. In this framework, to reduce production costs, PHA can be produced starting from waste material as feedstock and using mixed microbial cultures (MMC). 2) Critical metal leaching and recovery. Besides PHA production, organic acids produced through DF were used to recover metals from waste from electrical and electronic equipment (WEEE) through leaching process. Electrical and electronic equipment (EEE) is increasingly part of our life and contributes to improving the quality of life by providing benefits and opportunities in various sectors. At the same time, EEE production requires a high number of resources. Specifically, printed circuit boards (PCBs) and small IT equipment samples were treated for metal recovery (i.e., lead, tin, iron, nickel, zinc) using a leaching mixture produced from an appropriately controlled DF of CW. These scraps, indeed, contain a large amount of base, noble, and "rare earth" metals that can have a high value for the market or can be dangerous for human health and the environment when improperly managed.