Reuse of the digestate obtained from the biomethanization of olive mill solid waste (OMSW) as soil amendment or fertilizer for the cultivation of forage grass (Lolium rigidum var. Wimmera)

Abstract

The principal by-product from the two-phase olive oil production process is olive mill solid waste (OMSW). It is a highly-pollutant by-product, not only because of its characteristics, but also because of the considerable volume of OMSW which is generated, amounting to 2 to 4 million tons per year in Spain. The anaerobic digestion of this by-product is a well-studied process, and results in the generation of biogas, methane and carbon dioxide mainly of high calorific values (20–25 MJ m−3), and an effluent or digestate. The digestate of this by-product has never been characterized. This study presents an informative view on how the composition of OMSW digestate shows promising implications as a soil amendment or fertilizer due to the quality of the biomass from Lolium rigidum, a useful grass specie for the production of forage. Three OMSW digestate alternative applications or treatments were investigated: the digestate and the solid fraction of the digestate for a nutrient-poor soil amendment and the liquid fraction of the digestate as fertilizer. The results confirm that all the OMSW digestate treatments studied presented suitable characteristics for agricultural use, and showed an optimal Carbon/Nitrogen ratio with adequate values for heavy metals which are below the limits established by the Spanish and European legislation in the absence of pathogens. However, fertirrigation was the treatment that provided Lolium rigidum with the best characteristics, improving its shoot biomass, photosynthetic rate and nutritional content.