Characterising the stability of the organic matter during anaerobic digestion: a selective review on the major spectroscopic techniques

Digestate landspreading is a key aspect of the circular economy. However, organic matter (OM) stability in digestates is usually either poorly assessed or done through laborious methods. Spectroscopic methods are useful and easy to deploy alternatives to assess several aspects in anaerobic digestion (AD) studies such as process performance, waste classification and both OM composition and transformation. In these studies, a lack of agreement on analytical techniques, indicators and reference values is evident. This unclear scenario brings to the forefront the need for a meta-analytical study providing benchmarking values and trends. This review aimed to fill up this gap through the identification and evaluation of: (i) the most frequently applied techniques, their principles, deployment methods and limitations, (ii) the quantitative spectroscopic indices to define OM stability, (iii) the common trends of these parameters due to AD effect on the OM and (iv) the relevance of each technique based on the frequency of statistically significant results reported. Ultraviolet–visible and fluorescence spectroscopy have been identified as the most relevant techniques for aqueous phase study whereas mid-infrared and 13 C cross-polarisation magic angle spinning nuclear magnetic resonance were the most appropriate for the solid phase. Their most applied indicators and their trends after AD have been summarised. Finally, the research studies that displayed statistically significant findings were described, the representativeness of the indices and the influence of sample preparation on their calculation were discussed and future research lines were suggested. Overall, this review demonstrates that spectroscopic methods provide relevant information for better digestate management. Graphical abstract