Co-composting as a Management Strategy for Posidonia oceanica Residues and Dredged Sediments

Abstract: A significant example of circular economy application concept is the recycling of wastes through co-composting to obtain techno-soils and composts. The objective of this paper was to demonstrate that the co-composting of Posidonia oceanica residues with decontaminated sediments is an effective method to manage two kinds of waste and transform them in a suitable material for the preparation of growth substrates. To demonstrate the effectiveness of the co-composting process, a protocol, involving physical-chemical, biochemical, and toxicological parameters, has been followed. Moreover, to evaluate the quality of the different Posidonia-based composts, the characteristics have been compared with threshold values proposed in the Italian regulation for plant growth substrates and to specific parameters from the scientific literature. All the obtained composts responded to the main characteristics for composting and growing media preparation provided in national and international regulations, in terms of pH (6.7-7.6), electrical conductivity (0.13-0.30 dS/m), and total organic carbon content (6.62-27.9 C%). Moreover, results about humic carbon (16.6-21.9 C%), germination index (139-158%), enzymatic activities (B-glucosidase: 3538-5367 mmol MUB/kg ds H), bulk density (0.30-0.83 g/cm), and cation exchange capacity (43.3-126 meq/100 g ds) proved the quality of the P. oceanica and sediment based composts. These results are an endorsement of Posidonia with sediment-based composts use as a constituent of techno-soils and, as a component for the preparation of nursery growth substrate. It is possible to claim that the co-composting process is an alternative management strategy for decontaminated sediments and P. oceanica residues to be recommended. Adequate policies are needed in order to recycle Posidonia oceanica residues and decontaminated sediments, namely two kinds of waste, into a secondary raw material, namely techno-soil, thus implementing the concept of the circular economy. Graphic Abstract: [Figure not available: see fulltext.].