Influence of percolate recirculation on OM stability of digestate from straw and pig slurry

Abstract

Pig slurry is a source of organic matter (OM) and nutrients, nevertheless nowadays its disposal represents a serious problem at national scale due to both the large volume to be managed (ca. 18 million m3 y-1) and the insufficient land on which it could be spread. Anaerobic digestion (AD) could solve this environmental issue as it allows converting organic residues into biogas, a renewable source of energy, and digestate, a by-product with enhanced fertilizer and amendment properties. This is particularly true when the AD is performed at the solid state where the more recalcitrant fraction of OM may be accumulated. In this study, we investigated the changes in the OM composition during AD at different percolate recirculation frequency, thus providing information on the agronomic potential of resulting digestate samples. Solid-state anaerobic digestion (S-SAD) of pig slurry and straw (3:1 w/w) was carried out for this purpose. To avoid inhibition phenomena caused by volatile fatty acids accumulation during S-SAD, percolate recirculation technology was adopted. Three tests were carried out using 1, 2 and 4 recirculations day-1, respectively. A control test (i.e., without percolate recirculation) was also performed. The initial mixture and digestate samples (control, 1S-SAD, 2S-SAD, 4S-SAD) were characterized by elemental (CHNS), thermogravimetric (TG) and differential thermogravimetric (DTG) analysis, pyrolysis coupled to gas chromatography and mass spectrometry (Py-GC/MS) and 13C nuclear magnetic resonance (13C NMR) spectroscopy. Preliminary results suggested an increase of the OM degradation due to percolate recirculation. In particular, among digestate samples, 4S-SAD showed the lowest C/N and O/C values, as well as the lowest TG weight loss between 250 and 350°C (WL1), the latter one due to the greater consumption of sugars and cellulosic materials. 2S-SAD and 4S-SAD exhibited also a slight enhancement of TG weight loss between 350 and 550°C (WL2), which may be ascribed to a relative enrichment in humic-like substances and non-hydrolyzable residues. As a result, the thermostability index (WL2/WL1), generally used to describe chemical changes induced by OM biotransformation, was highest for 4S-SAD, followed by 2S-SAD, 1S-SAD and the control. Pyrolysis products at 400°C were grouped into 8 families according to their chemical nature, i.e. aromatics (Ar), lignin-derived aromatics (Lig), polysaccharides (Ps), ketones, n-alkane/ene, fatty acids (FA), sterols and terpenoids and N-compounds. Digestate samples exhibited lower contribution of Ps (the lowest in 4S-SAD) and higher Ar and Lig contents (the highest in 4S-SAD), as compared to the initial mixture. Likewise, 13C NMR spectra of the digestates evidenced a preferential degradation of O-alkyl C. This was particularly true for 4S-SAD that showed also the lowest O-alkyl C/alkyl C ratio, commonly used as an index of OM stability. On the opposite, this ratio was highest for the sample with 1 recirculation day-1 (1S-SAD), indicating its low degradation degree. In conclusion, atomic ratios, thermostability index, O-alkyl C/alkyl C ratio and preliminary Py-GC/MS results consistently showed a positive effect of the highest number (4) of percolate recirculations per day on the degree of OM degradation, resulting in a more stable OM in the corresponding digestate.Peer Reviewe