Degradation of Lignocellulosic Complex Through Production of Struvite from Digestate

Anaerobic digestion (AD) is already contributing to global climate change efforts both from feedstock and end-use side but there is still room for an improvement. Digestate is a by-product of AD process that has been neglected during the 2000s and beyond biogas market uptake in the EU. In areas densely populated with AD plants, the same challenges occur as intensive livestock production: feedstock for AD process and lack of available land for nutrient application, in compliance with the good agriculture practice. If digestate is to become a storable fertilizer, it could also replace fossil-based fertilizer and add to GHG emission savings. This paper examines a viability of thermo-chemical digestate treatment to utilize it as feedstock in the AD process by crystalizing nutrients from digestate to allow long distance nutrient application. Digestate was treated with thermo-chemical treatment, consisted of mostly components (heat, sulphur) available on site. The treatment proved the degradation of lignocellulosic walls, allowing more access of bacteria to monosaccharides concentrations, i.e. sugars, but nutrients crystallization also occurred, producing struvite and β-calcium phosphate tetrahydrate. Treated digestate was re-circulated in AD to record an increase in methane concentrations by 3–5 vol%, compared to conventional mixtures. The results lay foundation for further research in GHG emission savings from digestate, improved nutrient cycle and digestate management as well as potential in improvement of economic feasibility of an AD plant by expanding the feedstock base to locally available plant materials with tighter lignocellulosic bonds.