Anaerobic digestion of agricultural and livestock residues is an appealing technology to produce a renewable energy carrier, biogas, contributing to the achievement of the independence from fossil fuels to contend global warming. The economy of biogas plants and their sustainable operation depend on the effective substrate utilization and consequent low residual biogas yield in the digestate and reduced risks of methane emissions during digestate storage. Organic matter degradation can be hampered by the recalcitrance of lignocellulose and inappropriate operational conditions, such as insufficient hydraulic retention time.
The aim of this Ph.D. project was to assess the residual biogas potential of digestate from operational biogas plants and investigate solutions to recover it. In the first part of the study, digestate and its solid and liquid fractions were characterized in terms of chemical composition and residual methane potential. Since considerable contents of fermentable polymers and residual methane yields were discovered, two strategies were evaluated as methods to recover the residual methane potential of digestate: a) extension of the hydraulic retentions time by the addition of a new digester to the system, and b) recirculation of the solid fraction of digestate to the digester. Economic analyses showed that gains are possible with both strategies, depending on the biogas plants considered. Additionally, the potential agronomic use of digestate was evaluated based on the contents of nutrients in liquid and solid fractions.
In the second part of the study, innovative post-treatment methods were evaluated as technologies to accelerate methane production and increase methane yields from digestate. The application of ultrasonication and electrokinetic disintegration on digestate did not affect the ultimate methane yields but increased maximum methane production rates in some of the treated samples, resulting in positive energy balances, depending on the duration of the digestion process. These post-treatment methods were proven efficient on reducing digestate viscosities, showing the potential of the technologies to reduce the energy demand for stirring in the digester and to enable a higher organic load, currently limited by the mixing capacity. The alkaline post-treatment of the solid fraction of digestate with KOH was demonstrated as an efficient method to increase ultimate methane yields and maximum methane production rates and reduce the lag-phase duration. Additionally, the final digestate could be a valuable fertilizer, enriched in potassium. However, economic analyses showed that the current high cost of KOH would limit the application of this treatment.