Composting of the solid fraction of digestate derived from pig slurry: Biological processes and compost properties

- Anaerobic digestion leads to the production of a biologically stable digestate.
- Solid-liquid separation produces a solid fraction having high fertilizer value.
- Composting process shows low biological activity due to high biological stability of digestate.
- Solid digestate fraction can be composted in a short time or used directly as organic fertilizer.

The aim of this paper was to assess the characteristics of the solid fractions (SF) obtained by mechanical separation of digestate, their compostability and compost quality. To do so, the SF of digestates obtained from anaerobic digestion of pig slurry, energy crops and agro-industrial residues were sampled in five plants located in Northern Italy. Results obtained indicated that anaerobic digestion by itself promoted the high biological stability of biomasses with a Potential Dynamic Respiration Index (PDRI) close to 1000 mgO2 kg V S−1 h−1. Subsequent composting of digestates, with an added bulking agent, did not give remarkably different results, and led only to a slight modification of the characteristics of the initial non-composted mixtures; the composts obtained fully respected the legal limits for high quality compost. Chemical studies of organic matter composition of the biomasses by using CP MAS 13C NMR, indicated that the compost was composed of a high relative content of O-alkyl-C (71.47% of total C) (cellulose and hemicelluloses) and a low alkyl-C (12.42%) (i.e. volatile fatty acids, steroid-like molecules, aliphatic biopolymers and proteins).