Comparing Different Biological Stability Indices to Assess the Suitability of Digestate and Compost as Organic Fertilizers

Nowadays, biowaste valorization is a key point in circular economy. Digestate and compost are produced from organic waste treatment and can be used as nutrient-rich fertilizers. In Europe, the use of biowaste-derived fertilizers is promoted by the European Fertilizer Regulation (EU) 2019/1009, which requires verification of their biological sta-bility through regulated indices; however, it is not clear whether the proposed indices and threshold values indicate the same level of stability and what correlations there are be-tween them. This study compared four biological stability indices, namely Oxygen Up-take Rate (OUR), Self-Heating (SH), Residual Biogas Potential (RBP), and Dynamic Respirometric Index (DRI), which were tested on 50 samples of compost and digestate. Overall, results revealed that most of the compost and digestate samples were quite far from European standards, according to OUR, SH and DRI indices. On the contrary, the RBP test seemed to be less stringent than the other indices, since a much larger number of samples was closer to or in compliance with the established threshold. Data analysis us-ing Pearson’s coefficients showed a strong linear correlation between the indices. Nev-ertheless, the linear regression predictive model based on experimental data demon-strated that the indices could not represent the same level of stability, providing poor consistency and variability in the predicted values and established threshold. In partic-ular, the DRI test appeared to be more severe than the other aerobic indices. The results obtained suggest revising the biological stability indices and threshold values, depending on the type of matrix.