Evaluation of biological processes performances using different stability indices

The EU's Circular Economy Policy Package aims to minimize waste production in accordance with the Circular Economy concept. In this respect, organic waste valorization through composting and anaerobic digestion process represents a proven solution for converting biomass in bio-based products with high fertilizing and amendment properties. In order to optimize process' performances and reuse the bio-products, biological stability index evaluation of the final products plays a key role. Several respiration tests are currently used for the determination of biological stability and the need to define homogenized regulations both at national and European level, in order to outline harmonized rules for biological stability determination, is becoming increasingly indispensable. Among these, some approaches measure respirometric activity by estimating oxygen uptake rate (SOUR and DRI test) and by recording the maximum temperature achieved by the biomass during the degradation process (Self-Heating test). Others assess the decomposition degree of organic waste by determining the residual biogas potential production (BMP test). This work aims at evaluating biological stability of different organic matrices such as compost, digestate and mixture of them in order to compare different respirometric techniques and define possible correlations between them and their suitability depending on the substrates analyzed. The results show that among the different stability tests analyzed in this work, there is a good linear correlation that allows to affirm a direct proportionality both between DRI and Self-heating test and BMP and SOUR test. The results suggest that all the respirometric methods considered in this study could be used as indicators of the biological stability degree of an organic substrate and are therefore interchangeable with each other, providing the same value of biological stability degree. Nevertheless, future studies on stability index determination could be carried out in order to confirm the results obtained from this work.