Measuring the organic amendment properties of the liquid fraction of digestate.

The liquid fraction (LF) of digestate has usually been proposed as a substitute for mineral fertilizers because of the presence of high N content, above all in easily available form (ammonia). The LF was reported to contain about 66% of dry matter from the digestate. This study reports the characterization of the organic carbon (OC) contained in the LF of digestates obtained from full scale plants by screw-press solid/liquid separation, to find out about their organic amendment properties. Results indicate that LF contains stable OC because of the concentration during anaerobic digestion of recalcitrant molecules, and that its biological stability, measured by oxygen uptake rate, was similar to that of compost, i.e. 40 ± 15 mg O2 g DM-1 20 h-1 and 41.1 ± 5.1 mg O2 g DM-1 20 h-1. 13C NMR indicated that LFs were similar each other and were constituted of recalcitrant Alkyl-C (34.82 ± 5.28% OC) derived from plant and fecal material, Aromatic-C (11.10 ± 2.2% OC) derived from lignin-like structures and O/N-alkyl (44.91 ± 4.87% OC) derived from cellulose/hemicelluloses and protein material. A simple simulation of the real C dosed by using LF as N-fertilizers indicated that amendment properties cannot be ignored. All these results seem to indicate good amendment properties for LFs, contrary to the common opinion.