Improving circular economy by biogas plantsValorization of agricultural feedstocks

Abstract

Abstract Anaerobic digestion is a biotechnological process operating at the boundary between of livestock effluent and agro-industrial byproducts management, bioenergy generation and food production. Anaerobic digestion is an efficient solution to mitigate greenhouse gas emissions and to improve the circular economy of the agri-food sector via renewable energy generation from biogas/biomethane utilization and nutrient recycling using digestate as soil improver. An inspiring example of how food, feed and biomethane production can be coupled comes from a set of practices developed in Italy under the initiative Biogasdoneright®, introducing the double cropping system along with digestate fertilisation and minimum tillage of soil. The “first crop” is grown to supply the food and feed sector, while the “second crop” is grown for biomethane production. More in particular, farmers introduce a second crop immediately following the first crop harvest, thereby producing additional feedstock for biomethane production on lands that would otherwise remain bare through the winter period. Biomethane is also produced using animal manure, lignocellulosic agricultural residues, and agro-industrial byproducts. For the application of the Biogasdoneright® model, the correct management of agricultural feedstock, the optimization of operational and biochemical parameters into the digesters, as well as the improvement of the global biological efficiency of the anaerobic digestion process are necessary. Starting from the presented challenges, the main goal of this PhD thesis is to assess biotechnological aspects to improve the anaerobic digestion of agricultural feedstock, generating additional knowledge to attain a fully sustainable and circular system to produce food, feed, and bioenergy. In Chapter 2 and Chapter 3, the cultivation of sorghum and triticale as promising second crops for biomethane production has been investigated. The several parameters affecting the methane yield per unit of cultivated area (methane hectare yield) such as the specific methane yield, the harvesting time, and the varieties of these crops has been studied. In Chapter 2, different sorghum phenotypes (forage, high-tonnage energy, sweet and grain with tall size) have been evaluated for biomethane production. In Chapter 3, nineteen varieties of triticale harvested at milk and dough development stages has been assessed for biomethane production. The lignocellulose in second crops, manures and agro-industrial byproducts acts as a barrier preventing anaerobic degradability. In Chapter 4, the effects on physical modifications, biomethane formation, and anaerobic degradability of agricultural feedstocks after different mechanical pretreatments have been studied. Four commercials mechanical pretreatment technologies (knife milling, hammer milling, extrusion, shredding + hydrodynamic cavitation) commonly found at full-scale biogas plants have been investigated, including their electrical consumptions. To reach environmental goals and to optimize economic output, biogas plants should be operated at high biological efficiency, obtaining high biomethane yield per reactor volume, with a fast anaerobic conversion of agricultural feedstock, and resulting into a well-digested material that will decrease greenhouse gases emissions associated with digestate storage. In Chapter 5, the monitoring of 16 full-scale biogas plants fed with different agricultural feedstocks has been carried out to identify important parameters leading to the lowest residual methane potential of digestate. Among such parameters, the concentration of essential trace elements (selenium, nickel, cobalt, molybdenum) in relation to the hydraulic retention time and the organic loading rate has been investigated. Other metals of environmental interest have been also monitored due to the importance of a safe agronomic utilization of digestate on soil.