Molecular characterization of digestates from solid-state anaerobic digestion of pig slurry and straw using analytical pyrolysis

Straw and pig slurry Solid-State Anaerobic Digestion (SS-AD) was carried out in a pilot-scale apparatus using percolate recirculation technology. The digestion experiments were performed using 1, 2 and 4 recirculations per day; an additional experiment without percolate recirculation was used as control. The initial mixture and the digestates were analysed by means of chemical analyses and Pyrolysis–Gas Chromatography/Mass Spectrometry (Py–GC/MS), a direct analytical technique that allows investigating the changes in the organic matter (OM) composition of digestates and the effect of percolate recirculation frequency. Chemical analyses suggested a positive effect of percolate recirculation on OM degradation. The highest values of OM loss were found with 2 (26%) and 4 (31%) recirculation cycles per day, that also corresponded with the lowest values of the hydrophilic water extractable organic matter fraction (5.5 and 6.3% respectively). Py-GC/MS showed that the anaerobic digestion proceeded with progressive polysaccharide degradation (from c. 19% in the initial mixture to 10–8% with 2–4 recirculation cycles) and selective enrichment of lignin derived compounds (from c. 58% in the initial mixture to 67–69% with 2–4 recirculation cycles). In addition, a shift in the fatty acids distribution was observed with a decrease in the long/short chain ratio of fatty acid methyl esters. These results indicate that under our experimental conditions, percolate recirculation had a positive effect on the OM degradation. Also OM stabilization is observed with relative increases in recalcitrant lignin at the expense of the more liable polysaccharide fraction. This paper represents the first attempt to apply Py-GC/MS to evaluate the OM quality in digestates obtained by SS-AD of pig slurry and straw optimized by percolate recirculation.