Integration of Fungal solid-state fermentation for the optimization of anaerobic digestion of agrifood waste

Abstract

Residues from vegetable cultivation and processing account for a significant proportion of available biomass. In Europe, this amounts to almost 88.5 million tonnes annually. Disposal is costly for many companies, and uncontrolled decomposition releases unnecessary CO2 into the atmosphere and is economically unfavourable. At the same time, biogas plants are looking for alternative substrates to reduce the proportion of energy crops. Residual materials from vegetable production can also be used here. In Germany, however, such biomass accounts for only just under 5% in biogas plants. This is due, among other things, to the difficult-to-degrade structures in the residual materials. The aim of the project was to develop a new process for improving the use of vegetable waste as a substrate in biogas plants. This was demonstrated using the example of residues from pea production. A sub-fraction of the biomass (max. 15%) was disintegrated using a combination of physical and thermochemical methods and subsequently used for on-site enzyme production with special fungi. The treated fraction, enriched with hydrolytic enzymes, was then mixed with the untreated main fraction and the effects in terms of degradation and biogas potential were investigated. The technical optimisation focused on pulsed electric field (PEF) technology and thermochemical disintegration methods. In parallel, solid-state fermentation (SSF) based on residual materials was developed for the production of high-performance cellulases. Within the scope of the project, the parameters of the pretreatment methods for structural disintegration of the residues were significantly optimised. With a view to minimising energy consumption for thermochemical treatment, a process approach was developed in the unpressurised range (<100 °C) using low concentrations (< 0.5 %) of nitric acid. For the subsequent on-site enzyme production, an SSF process was developed in a drum reactor using a species of the genus Trichoderma. Both enzyme production and significant effects on biogas potential were demonstrated. When treating a 15% fraction with the combined process under optimised conditions, an increase in biogas potential of up to 20% was determined in relation to the total biomass. In biogas plants with combined heat and power units, the required process energy can essentially be provided by the waste heat from the cooling water. The project thus provided important process engineering principles for the energy-efficient pre-treatment of difficult-to-degrade residues based on an innovative combination of physical, thermochemical and enzymatic processes.