Biogas digestate typologies

Anaerobic digestion (AD) is a well-known technology for waste treatment and bioenergy production, but digestate management is still a key challenge. Defined simply as the digested residue of AD, digestate refers to a set of heterogeneous matters featuring different biochemical compositions and physical properties inherited from greatly diversified AD processes and feedstocks. This situation induces a blurry scenario for policy-makers, digestate producers, marketers and consumers regarding digestate treatment and valorization. In this context, the objective of this study was to establish a digestate typology with available data on literature and internal databases. Common fertilizing-value parameters were used in the analysis, which are also present in many policies: dry matter (DM), volatile solids (VS), C/N ratio, C/N org ratio, Total N (TN), Total Ammoniacal Nitrogen (TAN), P and K contents, TAN/TN ratio. Heavy metals contents were also separately assessed. In a first approach, a Principal Component Analysis was performed including raw digestates and mechanical separation fractions. With the selected variables, it was found no statistical difference between raw wet (Wet AD) and liquid fraction and between raw dry (Dry-AD) and solid fraction. Later, Hierarchical Clustering Analysis (HCA) was performed on raw digestates resulting in 8 fertilizing-value groups among a dataset of 91 raw digestates. The groups presented variable nutrients and organic matter contents that could be qualitatively linked to the AD feedstock and to the type of process in terms of moisture (Wet or Dry-AD). HCA was performed separately for liquid and solid fractions after mechanical separation and in both cases, digestates were found to be categorized into two groups depending simultaneously on the separation technique and AD feedstock. With regard to heavy metals content, a typology was found to be similarly grouped by AD feedstock.