Composting of Olive Mill Pomace, Agro-Industrial Sewage Sludge and Other Residues: Process Monitoring and Agronomic Use of the Resulting Composts

New olive-pomace fertilizer tested with a 2-tiers approach: Biomarkers on Eisenia fetida, physiochemical effects on Solanum lycopersicum and Olea europaea

Every year, the olive oil industry generates a substantial amount of pomace, a semi-solid residue made up of skin, pulp, pit, and kernel fragments. Rather than being disposed of, the pomace can be dried and transported to an extraction facility where pomace oil can be extracted. Utilizing its high thermal capacity, the extracted pomace can be used as a supplementary fuel in the drying process, resulting in the production of ashes. In this study, the effect of pomace waste applied to the soil was investigated by testing two mixtures with different proportions of de-oiled pomace flour and kernel ash (50:50 and 70:30, respectively) in powder and pellet form. We used a dual approach, evaluating the effects of the mixtures on both soil communities and plant physiology and productivity, to assess the actual usability of the fertilizer in agriculture. The biomarker approach was valuable in assessing the sublethal effects of the two mixtures in powder form in soil. After 30 days of exposure, the bioindicator organism Eisena fetida showed lipid peroxidation, glutathione S-transferase and lactate dehydrogenase levels similar to the control, while lysozyme activity was reduced in all treatments. The powder mixture was lethal to the tomato plants, while there was no evidence of any damage to the olive trees. During 60 days of monitoring, both mixtures in pellet form showed a slight increase in physiological parameters, suggesting a benefit to the photosynthetic system. The improved carbon assimilation in tomato plants treated with the mixtures results in increased plant productivity, both in terms of number and weight of fruits, while maintaining the antioxidant content. This study paves the way for the use of the pomace mixture as a soil improver, thus increasing the value of this waste product.

Physicochemical Characterization of Pectic Polysaccharides from Rose Essential Oil Industry By-Products

The rose essential oil industry generates large quantities of solid byproducts yearly. These by-products, usually discarded, could yield valuable substances, such as pectic polysaccharides, widely used in the food industry as jelling agents. Seven industrial by-products were investigated as a source of pectic polysaccharides: four samples resulted from the treatment of Rosa damascena, two from Rosa alba, and one from Rosa centifolia. Three by-products were from steam-water distillation, two from CO2-supercritical extraction, and two after extraction with hexane and 1,1,1,2-tetrafluoroethane. The by-products were pretreated with 70% ethanol and extracted with 0.1 M HCl. The highest polysaccharide yield was observed for 1,1,1,2-tetrafluoroethane-extracted (RD_F) Rosa damascena by-products (13.98 ± 0.14%), followed by hexane (RD_X) and CO2-extracted (RD_CO2) Rosa damascena (12.68 ± 0.11 and 12.66 ± 0.10%, respectively). The polysaccharides were middle-methoxylated pectins, except RD_F and RD_X, having 26.68 ± 1.14 and 31.39 ± 1.39 mol % degree of methoxylation (low-methoxyl pectins). The polysaccharides had molecular masses in the 2.3-2.6 × 104 Da range. The rheological studies suggested RD_F formed a strong high-sucrose gel, while the others yielded weak gels. RD_F and RD_X formed strong Ca2+-mediated gels, comparable with commercial low-methoxylated citrus pectin. This study suggests that rose oil industry by-products could be successfully valorized and yield pectic polysaccharides with gelling properties, comparable with commercial citrus pectins.

Application of Thermo-Malaxation Followed by Three-Phase Centrifugation to Enable the Biorefinery of Alperujo, the Main By-Product of Olive Oil

The pomace olive oil sector needs to improve its use of the main olive oil by-product, called alperujo, which is currently used mainly for combustion after extraction of pomace oil, with all the problems this process entails due to the high degree of humidity, organic load and phytotoxic substances. In this work, a solution at an industrial level that uses thermo-malaxation at a temperature close to 65 °C for one or two hours followed by centrifugation in three phases is proposed. In this way, over 40% of the pomace oil that is rich in minor compounds, a solid with a lower degree of humidity (55%), and a liquid aqueous fraction that is rich in bioactive compounds such as phenolics and sugars are obtained. This aqueous fraction can be treated through subsequent storage stages to increase its content of the main phenolic, hydroxytyrosol, to up to 1.77 g/L, decreasing its percentage of insoluble solids by up to 1.9%, making it possible to obtain extracts that are rich in hydroxytyrosol using systems that are commonly in place at the industrial level. The aqueous fraction, without phenolics, could be used for energy production. A solid with a slightly higher fat content than the initial alperujo remains, thus the rest of the oil content can be extracted from it using solvent, making it, once defatted, suitable for application in subsequent bioprocesses.

High-rate in-vessel continuous composting of olive mill byproducts

The increasing production of agro-industrial organic residues in modern society is extremely concerning. One of the most polluting procedures in the agricultural industry is the production of olive oil. This process creates a large amount of waste with high organic load and phytotoxic components. In this study, composting of two-phase olive pomace (OP), olive leaves (OL) and dewatered anaerobic sludge (DAS) from an olive mill wastewater anaerobic digestion process was conducted in a pilot-scale in-vessel high-rate continuous composter. Five different feed scenarios were studied with different OP/OL ratio in the feed material, while the effect of the addition of pine tree bark pieces (PB) and DAS was examined. The OP:OL 95:5 % w/w ratio exhibited the best results in terms of product quality, while OL proved capable of acting as a bulking agent for the better aeration of the material. The final product in the optimum feed ratio was free of Salmonella spp., was stable in terms of static respiratory index (lower than 0.5 g O₂ kg^-1 VS h^-1) but contained elevated E. coli levels (3.5 × 10⁴ CFU g^-1 with a limit of 1 × 10³ CFU g^-1), which was the only EU proposed compost quality criteria not met. The addition of a more easily degradable material in the feed mixture is expected to lead to elevated composting temperature and amend the presence of pathogens.