Co-composting alperujo with other agro-industrial residues as sustainable practice for its recycling

Abstract

The main by-product generated by Argentinean olive oil industry is a semisolid cake made of olive pulp, stone and vegetation water locally called alperujo. It is reported that composting represents an interesting option to recycle it; however, this practice is rarely implemented by olive industries in Argentina. The aim of this work was to evaluate two low-technology methods, open and covered piles with mechanical turnovers, for their effect on composting a mixture of alperujo, olive leaves and grape stalk (56:22:22) with or without urea addition. Temperature, organic matter content (OM) and phenolic compounds (PC) were measured to follow up the composting process. While electrical conductivity (EC), pH, macroand micro-nutrients, as well as germination index (GI) of Medicago sativa seeds were determined at the end of the experiment in order to assess compost quality. Additionally, to infer the soil and water conditions required to apply the obtained compost, the agricultural index (AgIndex) was calculated. Results showed that all piles presented similar evolution patterns, achieving maturity within 150 days. Temperature rose upwards in the thermophilic phase and next it decreased through the maturation phase. pH increased along the experiment and reached slightly alkaline level at maturity. OM content and C/N ratio decreased, this decrease being higher in covered piles than in open ones. Interestingly, PC was highly reduced in all treatments and was inversely correlated with the increase in GI. Furthermore AgIndex indicated that the obtained compost may be appropriate for amending soils with excellent drainage characteristics, low salt content and good water quality. The results suggest that evaluated methods are suitable for composting alperujo, decrease its toxicity and obtain a product appropriate for amending soils where olive groves are usually implanted. Undoubtedly the long time required to reach maturity may represent a negative feature. In this sense further works, focused on conditions to accelerate the process should be carried out.