Comparative study of sewage sludge co-composting with olive mill wastes or green residues: Process monitoring and agriculture value of the resulting composts

The viability of co-composting as a treatment approach for sewage sludge with organic wastes was investigated in the context of management aspects and the agricultural value of the final product. Two composting cycles (P1 and P2) were performed, in which sewage sludge (SS) was used to form two mixtures either with olive mill waste (OMW) or with green waste (GW), respectively. Co-composting of SS with both used organic wastes allowed obtaining hygienic compost with sufficient agronomic quality. Phenols accumulation from the mixture containing OMW caused an important decrease of pathogens within the compost. These products were characterized by a content of P and K that meet similar quality compared to commercial composts and showed an advanced maturation useable directly in agriculture. FTIR analyses and phytotoxicity tests were assessed to evaluate compost maturity. In fact, the germination index (GI) values for maize and tomato seeds, treated with P1, were 79.68% and 97.36%, respectively. However, they decreased to 74.45% and 81.45%, respectively using P2. Furthermore, compost application in peat amended at ratios equal to 30% and 50% improved the growth speed and fresh biomass of maize and tomato plants.