Two-stage biohydrogen and biogas production using agro-food waste biomass

To address climate change and the increasing energy demand, several studies have been conducted over the past to find ways to produce energy from renewable sources. Hydrogen is an excellent energy carrier, as it has a high calorific value and, unlike hydrocarbons, does not release CO2 during combustion. Another environmental issue is waste management. One way to tackle both this problem and the sustainable production of energy-rich gases is through metabolic processes carried out by various microorganisms. These processes include dark fermentation and anaerobic digestion, which can be conducted individually or in a combined process called two-stage anaerobic digestion. This thesis focused on experimenting with different types of biomasses derived from agro-industrial waste to achieve high yields of hydrogen and methane. The biomasses included milk waste, vegetable waste, cow manure, industrial sugar waste, and vinasses. Agro-food waste has a high lignin content, which slows down and sometimes inhibits fermentation processes. In order to allow complete fermentation, various physical and chemical pretreatments were studied to degrade lignin and other high molecular weight compounds such as polyphenols, to make them more accessible for consumption by microorganisms. The inoculum used to set up the systems was taken from mesophilic cow manure digestate and it was treated thermically, in the case of dark fermentation, to inhibit methanogenic microorganisms. The analyses included daily sampling of gas produced in bags connected to the production system and analysis of its composition using a gas chromatograph. The processes were tested with a total solids content of 6% and a volatile solids ratio of biomass to inoculum of one. The dark fermentation of the milk and vegetable waste configuration resulted in yields of up to 50 mL/gVS, while the anaerobic digestion of manure and milk produced 250 mL/gVS. The two-stage process yielded 50 mL/gVS and 100 mL/gVS of methane. The pretreatments for the vinasses consist of grinding and ultrasound for mechanical treatment. Chemical treatment instead in extraction in an 80% ethanol solution to remove polyphenols or lignin degradation in 3M NaOH. These were not sufficient to make vinasses usable biomass for hydrogen production. The effectiveness of polyphenol extraction through pretreatments was evaluated by measuring COD, where mixing in an 80% ethanol solution proved to be the most effective treatment. In conclusion, manure is not suitable for hydrogen production but is effective for methane production when combined with milk waste in a single-stage anaerobic digestion process. On the other hand, configurations containing milk and vegetable waste allow the production of both hydrogen and methane through a two-stage process. The next steps are focused on finding operating conditions that allow for higher yields of hydrogen and methane. Specifically, experimenting with the process at temperatures different from 35 °C and at varying pH levels, monitoring daily fluctuations, and adjusting them by adding acidifying or basic agents.