Olive Mill Pomace Research Articles

Fertilization with Olive Mill Pomace Compost Can Moderate Pest Damage in a Superintensive Olive Grove

Olive cultivation is a key agricultural activity in Spain, primarily for producing oil. The extraction process of olive oil from the drupe yields a by-product known as ‘alperujo’, which can be composted and utilized as fertilizer. This research examines the impact of composted ‘alperujo’ on arthropod assemblages in the tree canopy, comparing it to mineral fertilization over the years 2021 and 2022. The study was conducted in two olive groves with different management systems (superintensive and traditional). Two types of sampling methods were used for the canopy: visual survey and sweep net. Under superintensive management, the presence and damage of Eriophyidae (Acari; Trombidiformes) in the shoots was significantly lower in the compost treatment in 2022 (20% of the shoots were occupied/damaged) compared to the mineral treatment (60% of the shoots were occupied/damaged). Araneae abundance was significantly higher (p = 0.033) in the compost treatment compared to the mineral treatment. However, under traditional management, no clear effect on arthropod assemblage was observed. In conclusion, the addition of compost to the superintensive grove helped to limit the presence of some arthropod pests compared to mineral treatment, contributing to a more sustainable crop. Grove type management appeared to play a significant role in the arthropod assemblages and effect of compost addition, but future research utilizing a greater number of groves (replicates) and an extended observation period should be performed to confirm these results.

Carbon and Nitrogen Mineralization of Common Organic Amendments in Olive Grove Soils

Cover crops (CC), pruning residues (PR), and olive mill pomace (OP) are common amendments to enhance soil quality in olive groves; however, there is limited information on the dynamics of carbon (C) and nitrogen (N) during their decomposition under varying conditions. In this laboratory decomposition experiment, we assessed the C and N mineralization of CC, PR, and OP at application rates of 2 and 5 mg C g−1 in soils with varying organic C levels, both with and without the addition of NO3−. The results indicated that C and N mineralization patterns varied significantly between amendments, although the predicted remaining C after one year was similar for CC and PR (46.7–48.9%) and slightly lower for OP (40.0%). Soil organic carbon level did not affect remaining amendment-C. The addition of N accelerated the decomposition rate of labile C by 15% but slowed down that of the recalcitrant C another 13%, with no significant effect on remaining C. Conversely, increasing the C dose led to a 13% overall reduction in amendment-C mineralization across all combinations of factors. CC decomposition released between −8 and 31% of the amendment-N by day 130, while PR and OP showed net N immobilization. Therefore, CC residues contribute to both C sequestration and N availability, while PR and OP residues can improve N retention in the olive groves while also contributing to C sequestration.

The use of olive mill pomace compost increases the population of certain ground/soil organisms in olive groves

Olive is one of the largest crops in Spain, primarily for oil extraction from drupes. This process produces a by-product called ‘alperujo’, which can be composted and used as fertilizer. This study investigated the impact of ‘alperujo’ compost on ground/soil invertebrate inhabitants compared to mineral fertilization in two groves with different crop management types (superintensive and traditional) during 2021 and 2022. Anystidae/Erythraeidae (Acari; Trombidiformes) and Acari (Other) were more abundant in the compost treatment in both groves, but significant only in the superintensive grove. Some other ground/soil inhabitants, such as Anthicidae (Coleoptera), Araneae, and Gastropoda were generally more present in the compost treatment of the superintensive grove. No significant effect of fertiliser treatment was observed for other ground/soil organisms. Future studies with more replicas and over a longer period of time should be performed to confirm these results, but they can be considered of interest to push forward the implementation of ‘alperujo’ compost in the olive fertilization, favouring a circular economy and a sustainable agriculture.

Accumulation of heavy metals from single and combined olive mill wastewater and pomace in soil and bioaccumulation in tissues of two earthworm species: Endogeic (Aporrectodea trapezoides) and Epigeic (Eisenia fetida).

Soil and earthworms are threatened by anthropogenic contamination resulting from olive mill waste dumping on the soil due to their pollutant properties. While several studies have explored the effects of olive mill waste on soil properties and the accumulation of heavy metals in soil, there is currently a gap in the literature regarding the potential bioaccumulation of heavy metals from olive mill waste in earthworms. In this study, soil with earthworms from two ecological categories (endogeic: Aporrectodea trapezoides and epigeic: Eisenia fetida) was treated with increasing doses of olive mill wastewater (OMWW) and olive mill pomace (OMP), applied individually or combined, in an indoor experiment in plastic containers, under laboratory conditions. The results revealed the presence of significant concentrations of heavy metals in the two types of wastes ranging as follows: Fe˃ Zn˃ Cu˃ Cd˃ Cr for OMWW, and Fe˃ Zn˃ Cu˃ Cr for OMP (with Cd below the detection limit). The study demonstrated distinct effects of OMWW and OMP, both individually and in combination, on soil heavy metal content, ranging as follows: soil OMWW > soil Combination > soil OMP for Cd; soil Combination > soil OMWW > soil OMP for Cr and Fe; and soil Combination > soil OMP > soil OMWW for Cu and Zn. Additionally, our investigation showed that both earthworm species exhibited significant uptake of these metals into their tissues, particularly the endogeic species. Interestingly, the most significant difference between species was in the accumulation of Cu, with the epigeic species accumulating significantly lower amounts.

Production of Xylanase by Trichoderma Species Growing on Olive Mill Pomace and Barley Bran in a Packed-Bed Bioreactor.

Xylanases are hydrolytic enzymes that have tremendous applications in different sectors of life, but the high cost of their production has limited their use. One solution to reduce costs and enhance xylanase production is the use of agro-wastes as a substrate in fungal cultures. In this study, olive mill pomace (OMP) and barley bran (BB) were used as carbon sources and possible inducers of xylanase production by three species of Trichoderma (atroviride, harzianum, and longibrachiatum), one major xylanase producer. The experiments were conducted under a solid-state fermentation system (SSF) in flask cultures and a packed-bed bioreactor. Cultures of OMP and BB were optimized by examining different ratios of OMP and BB, varied particle sizes, and inoculum size for the three species of Trichoderma. The ratio of 8:2 OMP and BB yielded the highest xylanase activity, with a particle size of 1 mm at 29 °C and an inoculum size of 1 × 107 spores/mL. Studying the time profile of the process revealed that xylanase activity was highest after seven days of incubation in flask SSF cultures (1.779 U/mL) and after three days in a packed-bed bioreactor (1.828 U/mL). The maximum percentage of OMP degradation recorded was about 15% in the cultures of T. harzianum flask SSF cultures, compared to about 11% in T. longibrachiatum bioreactor cultures. Ammonium sulfate precipitation and dialysis experiments showed that Xylane enzyme activity ranged from 0.274 U/mL in T. harzianum to 0.837 U/mL in T. atroviride when crude extract was used, with the highest activity (0.628 U/mL) at 60% saturation. Xylose was the main sugar released in all purified fractions, with the G-50 and G-75 fractions showing the maximum units of xylanase.

Assessment of natural radioactivity in soil and olive mill pomace utilizing nal (TI) gamma-ray spectrometry and low background alpha/beta counting system

The study conducted in the northwest region of Jordan aimed to assess the levels of natural radioactivity in soil and olive mill pomace (OMP) samples. The researchers used Nal (TI) gamma-ray spectrometry to measure the activity concentrations of 226Ra, 232Th, 40K, and 137Cs in the samples. The average activity concentrations of 226Ra, 232Th, 40K, and 137Cs in the soil samples were found to be 18.624 ± 5.82, 12.276 ± 5.728, 518.33 ± 212.57, and 0.140 ± 0.09 (Bq,kg−1), respectively. In the OMP samples, the average activity concentrations of 226Ra, 232Th, and 40K were 7.272 ± 4.386, 3.454 ± 1.503, and 169.997 ± 81.873 (Bqkg−1), respectively, and no 137Cs was detected.The study also investigated fundamental parameters associated with radon, specifically the radon emanation coefficient (RnEC) and radon mass exhalation rate (Ex). The RnEC values ranged from 0.621 to 0.78 (Bqkg−1), with an average value of 0.71 ± 0.06 (Bqkg−1). The estimated Ex from the soil samples ranged from 65.83 to 124.86 (mBqkg−1h−1), with an average value of 99.74 ± 21.73 (mBqkg−1h−1).Regarding radiological hazards, the study examined various parameters, including radium equivalent activity, external and internal hazard indices, gamma and alpha indices, absorbed gamma dose rate, and excess lifetime cancer risk. All of these assessed values were found to be below the worldwide recommended limits for radiological safety.Additionally, the study analyzed the concentrations of gross alpha and gross beta radioactivities in soil and OMP samples. The soil samples had an average gross alpha activity of 4.642 ± 1.04 (Bqkg−1) and an average gross beta activity of 48.13 ± 14.50 (Bqkg−1). The OMP samples showed an average gross alpha activity of 0.32 ± 0.27 (Bqkg−1) and an average gross beta activity of 59.19 ± 12.94 (Bqkg−1).Overall, the obtained results are crucial for evaluating the radiological risks associated with natural radioactivity in the northwest region of Jordan. The findings establish baseline data for comparison and reference for radioactivity levels in the environment.

Investigation of the Performance of Bio-Oils from Three Different Agricultural Wastes as Rejuvenators for Recycled Asphalt

The aim of this study was to investigate the possibility of using bio-oil obtained from pine cones, olive mill pomace, and wheat straw as rejuvenators for the reuse of aged asphalt binders. Additionally, the biomass used for bio-oil production was selected from waste materials. Therefore, it makes great contributions to both the environment and the economy. B50/70 bitumen was selected as the neat binder. The bio-oils used in the study were obtained as a result of pyrolysis. Bio-oil rejuvenators at 5%, 10% and 20% by the weight of the binder were added to the aged binder obtained from recycled asphalt mixtures to obtaine bioregenerated asphalts. The physical and rheological properties of bioregenerated asphalts were investigated and not compared on neat and aged binders through penetration, softening point, rotational viscometer and dynamic shear rheometer tests. In addition, the effects of temperature and biooil content on complex modulus properties were examined using response surface methods. It was foud that while the bio-oils increased the penetration values of the aged binders, they also decreased the softening point and viscosity values. The bio-oils significantly modified rutting resistance of the aged binder. The addition of bio-oil improved the viscous components and can rejuvenate the viscoelastic properties of aged asphalt binders to that of almost the original level. In addition, response surface methods results showed that the interactions between both independent variables were effective. Finally, high coefficient of determination (R2) values indicated good agreement between the actual and predicted values. It was recommended as a result of the study that 20% concentration of bio-oil should be used to rejuvenate the aged asphalt binder for reuse in pavement construction.

Transfer factors for natural radioactivity into olive mill pomace

The transfer factor (TF) of naturally occurring radioactivity between soil and olive mill pomace (OMP) was calculated in this study. Nine samples were collected from three locations in north Jordan. The study focused on calculating the 226Ra, 232Th, 40K, and 137Cs activity concentrations in both soil and OMP samples using high-purity germanium (HPGe) gamma-ray spectrometry. The average activity levels in the soil samples were determined to be 24.0±10.1, 31.3±9.1, 323.7±68.4, and 1.0±0.4 (Bqkg−1), respectively. In the OMP samples, the average activity levels of 226Ra, 232Th, and 40K were 6.7±1.8, 3.2±1.2, and 185.5±56.6 (Bqkg−1), respectively, while no trace of 137Cs was detected. The activity concentrations of 226Ra, 232Th, and 40K in the soils and OMP samples are within the average worldwide ranges. The calculated values of the TF geometric mean (geometric standard deviation) for 226Ra, 232Th, and 40K were determined to be 0.35 (0.2), 0.11 (0.05), and 0.59 (0.19), respectively. This research offers valuable information about the behavior and movement of radionuclides in the environment.

Effect of Different Supplemental Levels of Discarded Dates, and Olive Mill Pomace on the Performance of Broilers

Effect of Different Supplemental Levels of Discarded Dates, and Olive Mill Pomace on the Performance of Broilers

The Olive Mill Pomace: A Sustainable Biofertilizer to Improve Soil Proprieties and Plant Nutrient Uptake

The Olive Mill Pomace: A Sustainable Biofertilizer to Improve Soil Proprieties and Plant Nutrient Uptake

Ant Community Is Not Influenced by the Addition of Olive Mill Pomace Compost in Two Different Olive Crop Managements.

Industrial production of olive oil generates large amounts of solid waste called 'alperujo'. Its compost can fertilize many crops, especially olives. Furthermore, superintensive orchards are increasing their surface globally due to higher production and savings in different costs. Ants are considered an important part of the arthropod community in olive orchards and could even play a significant role in pest control. The tree canopy and ground were sampled to compare the ant assemblage in plots fertilized with compost and mineral products in two groves with different types of crop management (superintensive and traditional) over two years. The numbers of ants in both types of fertilization in each grove were not statistically different (p > 0.05), indicating that the type of fertilization did not have a significant impact on its populations in the ground or in the canopy, but the number of individuals was significantly higher in the superintensive grove than in the traditional grove (both in the ground and in the canopy, p < 0.01). The most frequent species in the ground were Pheidole pallidula, Plagiolepis smitzii and Aphaenogaster senilis (superintensive grove) and Pheidole pallidula, Tetramorium gr semilaeve, Plagiolepis pygmaea, and Tapinoma nigerrimum (traditional grove). In the canopy, the most frequent species were Plagiolepis spp. in both groves. Differences in ant densities and species between the groves could be due to the different management, especially of the soil, but it must be confirmed using more replicas and longer periods of study.

Does olive cultivation sequester carbon?: Carbon balance along a C input gradient

Currently, there are some initiatives aimed at transforming agriculture from being a source to a sink of greenhouse gases, mainly by encouraging combination of management practices that drive nature-based climate processes (NbCS) resulting in an increase in the stocks of soil and biomass organic carbon. Olive grove cultivation represents a key socio-economic and environmental asset for Mediterranean rural regions. Being a woody crop planted in an orchard fashion, the potential for organic carbon accumulation in the biomass and the soil is very high. In this study, farm, trees and soil carbon balances were analysed in 12 pairs of olive groves located in Southern Spain with different tree densities, age, varieties and irrigation regimens. One from each pair of the olive groves has applied (> 8 years) a combination of management practices that promote NbCS resulting in an increase in the entry of carbon, whereas the other comparable nearby olive grove has not implemented management practices that promote NbCS or C entries (non-NbCS). C balance at the farm level was mainly neutral or positive and averaged + 1.20 Mg C ha−1 y−1. However, C balance in the NbCS olive groves was 5 times higher than that of the non-NbCS. The mean soil C balances were negative (−0.18 Mg C ha−1 y−1; losing soil organic C) in the non-NbCS and positive (+1.48 Mg C ha−1y−1) in the NbCS olive groves thanks to the increase in the carbon entries due to the management practices which boost NbCS. This study highlights the important contribution of olive farming in mitigating climate change, which in turn would be an economic incentive for olive growers. Nonetheless, there is a high potential for improvement by implementing management practices which enhance nature-based processes such as the cultivation of temporary spontaneous cover crops and the application of shredded tree pruning and composted olive mill pomace and/or manure to the soil.

Effect of olive-processing technology on the utilization of olive mill pomace as a soil amendment.

In the Mediterranean basin, the treatment and disposal of olive mill pomace (OMP) remain a salient environmental issue for the olive oil-producing industry. This study assesses the effects of olive-processing technology (three-phase and two-phase systems) on the potential use of OMP as a soil amendment. Samples from 12 Croatian olive mills were analyzed for their total phenolic content (TPC), residual oil fraction, and elemental concentration. The samples were profiled using Fourier transform infrared spectroscopy (FT-IR) and structurally characterized using scanning electron microscopy-energy-dispersive X-ray spectroscopy (EDS). Compared to three-phase samples, two-phase OMP was more acidic (pH 4.5vs. 5.0), with a higher TPC (3835vs. 1576mg/kg fresh weight), oil content (11.7%vs. 7.5% d.w., where d.w. is dry weight), electrical conductivity (EC, 5.1vs. 3.0 mS/cm), and levels of calcium (Ca, 1.34vs. 1.20g/kg d.w.) and copper (Cu, 10.4vs. 7.0mg/kg d.w.). Similar values of carbon/nitrogen (C/N; 61vs. 72), N (10vs. 8.1g/kg d.w.), phosphorus (1040vs. 691mg/kg d.w.), and potassium (K, 13.7vs. 8.1g/kg d.w.) were observed. The amounts of chromium, copper, nickel, and zinc were below EC limits in both cases. The EDS mapping revealed that Ca was concentrated at sharp-edged OMP particles while K was evenly distributed, suggesting that pelletized OMP compost is preferable for amending soil to obtain a homogeneous distribution of nutrients. It was also possible to distinguish between OMPs based on oil and lignin absorption bands in their FT-IR spectra. According to the obtained results, composting is recommended for both types of OMP to produce a safe product for amendment purposes.

Effects of olive mill wastewater and olive mill pomace on soil physicochemical properties and soil polyphenols

Effects of olive mill wastewater and olive mill pomace on soil physicochemical properties and soil polyphenols

Understanding the compositional changes of organic matter in torrefied olive mill pomace compost using infrared spectroscopy and chemometrics

Composting olive mill pomace (OMP), the major by-product of the olive oil industry, is an attractive waste management practice in the context of sustainable food production. Thermal treatment of compost at mild temperatures (torrefaction) can aid to improve its characteristics as a soil amendment. This study aims to understand the chemical changes occurring during torrefaction of olive mill pomace-based (OMP) compost, as well as to evaluate the treatment effects on compost at different stages of maturation. Here, treatments at different temperatures (175, 225, and 275 °C) and duration (from 1 to 5 h) have been employed to obtain a sort of torrefied samples. In general, the H/C and O/C atomic ratios of compost samples decreased with torrefaction temperatures, which suggests an incipient coalification of the organic matter. Furthermore, the results showed that a combination of FT-NIR and FT-MIR spectroscopy using a low-level data fusion strategy is very sensitive to the molecular changes occurring both in the composting process and during heating. Principal Component Analysis (PCA) of the merged spectra revealed that the changes at 175 °C are mainly the loss of water (O–H contributions at 3300 and 5169 cm−1) together with the degradation of proteins (observed in the decrease of amide I and II characteristic bands). Furthermore, the samples heated at this temperature can still be differentiated by their initial maturation stage. On the other hand, thermochemical changes occurring at higher temperatures are more intense and make the samples more alike, independently of the composting time. When heating above 225 °C, the loss of O–H happens together with the decrease of aliphatic moieties, reflected in the bands 2920 and 2850 cm−1 (FT-MIR) and 4258, 4323, 5665, and 5781 cm−1 (FT-MIR). This can be attributed to the thermal degradation of cellulosic materials and, additionally, to the degradation of the residual oil in the case of poorly composted samples. Heated samples are characterized by the presence of carbonyl groups (1709 cm−1) and humic-like complex and polymerized aromatic structures (1579 cm−1). Since the characteristics of the torrefied compost at 275 °C are very similar regardless of the initial maturation stage, torrefaction may be a very interesting way to reduce the composting time of olive mill pomace to obtain a high-quality organic amendment for soil application.

Polycyclic Aromatic Hydrocarbons (PAHs) and Metals in Diverse Biochar Products: Effect of Feedstock Type and Pyrolysis Temperature

Biochar's agricultural and environmental benefits have been widely demonstrated; however, it may cause environmental contamination if it contains large amounts of pollutants such as polycyclic aromatic hydrocarbons (PAHs) and heavy metals (HMs). Therefore, this study aimed to assess the contents of PAHs and HM in a range of biochars generated from different sources and pyrolysis temperatures. A range of feedstock was converted to biochar, including sewage sludge (SS), olive mill pomace (OP), feather meal (FM), soft offal meal (CSM), chicken manure (CM), and date palm residues (DPR). Each feedstock was then pyrolyzed at three temperatures of 300, 500, or 700 °C, thereby producing a total of 18 types of biochar. These biochar products were analyzed for 16 PAHs and eight metals (Cr, Mn, Fe, Ni, Cu, Zn, Cd, and Pb). Benzo[b]fluoranthene, benzo[k]fluoranthene, and benzo(a)pyrene were significantly greater in the biochar produced at 700 °C than in that produced at 300 °C, especially for CM. The concentrations of dibenz(a,h)anthracene were significantly lower at 700 °C but greater at 500 °C and 300 °C in DPR. Increasing the pyrolysis temperature from 300 to 700 °C significantly increased the concentrations of metals, including Cr in SS and OP; Mn in CM; and Fe, Ni, Cu, and Zn in SS. However, the concentration of Cd was significantly lower in the SS when biochar was produced at 700 °C than at 500 or 300 °C. The type of feedstock used and the pyrolysis temperature are key factors influencing the contents of PAHs and HMs in biochar, both of which need to be considered during the production and use of biochar. Further investigations are recommended to establish the relationships between pyrolysis temperature and types of feedstock and the formation of PAH or the concentrations of metals. Monitoring the concentrations of PAHs and HMs before applying biochar to soil is also recommended.

Olive Mill Pomace Extract Loaded Ethylcellulose Microparticles as a Delivery System to Improve Olive Oils Oxidative Stability

The protective effect of olive mill pomace (OMP) loaded ethylcellulose microparticles as an alternative to synthetic antioxidants against the oxidation of olive oils was assessed. OMP extract was obtained by an optimized two-step solid-liquid extraction; encapsulation was performed by double emulsion solvent evaporation technique considering a theoretical loading content in phenolic compounds of 5% (w/w). The changes in the peroxide values, the p-anisidine values, the total oxidation values, the free fatty acids content, the total antioxidant activity, and the total phenolic content were synchronized under storage at 62 °C. The results of oxidative stability were compared with plain oils, oils enriched with synthetic antioxidants, and oils fortified with OMP extract. The encapsulation efficiency of phenolic compounds was 96.0 ± 0.3%. The fortification of olive oils with microparticles retarded the appearance of peroxides, reduced the content of secondary oxidation products, and slowed down hydrolysis processes. The microparticles were efficiently designed to sustain the release of antioxidants to control the oxidative status of oil samples, retarding the free fatty acids formation rather than synthetic antioxidants. The results of this study bring new perspectives regarding the potential use of encapsulated extracts rich in antioxidants as an alternative to synthetic antioxidants to improve oil oxidative stability.

Response Surface Modeling and Optimization of the Extraction of Phenolic Antioxidants from Olive Mill Pomace.

Bioactive compounds from olive mill pomace (OMP) were extracted through a two-step solid-liquid extraction procedure considering four factors at five levels of a central composite rotatable response surface design. The influence of the process variables time of the primary extraction (2.0-4.0 h), solvent-to-sample ratio during the primary extraction (5.0-10.0 mL/g), time of the secondary extraction (1.0-2.0 h), and the solvent-to-sample ratio during the secondary extraction (3.0-5.0 mL/g) were examined. The content of bioactive compounds was determined spectrophotometrically, and the individual phenolic compounds were evaluated by reserved-phase high-performance liquid chromatography (RP-HPLC). The Derringer's function was used to optimize the extraction process, and the best conditions were found to be 3.2 h for the primary extraction, 10.0 mL/g for the solvent-to-sample ratio and 1.3 h for the secondary extraction associated with a solvent-to-sample ratio of 3.0 mL/g, obtaining a total phenolic content of 50.0 (expressed as mg gallic acid equivalents (GAE)/g dry weight (dw). The response surface methodology proved to be a great alternative for reducing the number of tests, allowing the optimization of the extraction of phenolic antioxidants from OMP with a reduced number of experiments, promoting reductions in cost and analysis time.

Analytical pyrolysis (Py-GC-MS) for the assessment of olive mill pomace composting efficiency and the effects of compost thermal treatment

Composting is an interesting solution for recycling the large amount of residues typically generated in olive oil production areas, particularly olive mill pomace. Bulking agents must be added to wet olive mill pomace for composting. The diversity of the materials used (such as olive tree-pruning residues, animal manure, or straw) and the variable proportion, in which they are mixed, are responsible for the heterogeneity and variability of the quality of the produced compost. For this reason, a comprehensive characterization of compost organic matter is necessary to gain information about its stability and maturity useful as well as to predict its behavior as a soil amendment. In this work, fresh olive mill pomace and four composted olive mill pomace samples were characterized using analytical pyrolysis coupled with gas chromatography and mass spectrometry (Py-GC-MS). Results showed a large variety of pyrolysis products, many of which had a specific biochemical origin. Pyrolysis-GC-MS revealed a decrease in fatty acids and aliphatic compounds content with composting, along with carbohydrates degradation and an increase in phenolic and N-compounds levels. Furthermore, the differences observed in organic matter characteristics had an impact on agronomic aspects (water repellency and phytotoxicity). Thus, the occurrence of fatty acids in incompletely composted materials (mainly palmitic and cis-vaccenic acids) was associated with increased phytotoxicity and higher water repellency. The presence of chicken manure in the mixtures was proved to favor the humification process and the production of stabilized compost (characterized by a lower Aliphatic-C/Aromatic-C ratio and higher N-heterocycles content). Furthermore, in this work, the effectiveness of applying thermal treatments to improve compost quality has been evaluated. Analysis of the Py-GC-MS results by means of principal component analysis (PCA) revealed thermal rearrangement, predominantly aromatization, and loss of functional groups of carbohydrates, lignin, and proteins, at temperatures above 225 ºC. Interestingly, the results suggest that materials with a non-effective or incomplete composting process can be transformed by thermal treatment into more stable products, exhibiting similar agronomic characteristics as those have that undergone a more efficient biotransformation.

The advantages of co-digestion of vegetable oil industry by-products and sewage sludge: Biogas production potential, kinetic analysis and digestate valorisation.

The production of edible vegetable oils generates considerable amounts of energy-rich waste, which is usually not utilised fully. Besides, inefficient management of such wastes can have a negative impact on the environment. On the other hand, this waste can also serve as a raw material for the production of high value-added products, such is biogas. The mono-digestion of seven different by-products and wastes from the vegetable oil industry was investigated in this study: Pumpkin seeds press cake (PSPC), grape seeds press cake (GSPC), olive mill pomace (OMP), coconut oil cake (CC), filtration additive (FA), spent bleaching earth (SBE) and sludge from a vegetable oil industry (SOI) wastewater treatment plant. In addition, co-digestion of these substrates was performed with municipal sewage sludge (SS). Besides inoculum, rumen fluid was added to the reactors to enhance biogas production. The biogas production potential of the tested substrates was monitored by measuring various parameters. A kinetic analysis was later carried out and a growth test was performed on the digestates to evaluate their potential for agricultural use. The highest biogas yields in the mono-digestion test were obtained with the substrates with the highest fat content: 1402, 1288, 830 and 750mL of biogas/gVS for SOI, FA, PSPC and CC substrate, respectively. Co-digestion of SS with by-products of vegetable oil industry such as FA, SBE, CC, SOI and PSPC increased the biogas yields by 94.9%, 74.1%, 30.8%, 27.4% and 23.6% compared to SS mono-digestion. Furthermore, the data for mono-digestion of PSPC, GSPC, and FA, and co-digestion of SS with these substrates, CC and SBE, have not been found in the literature to date. The maximum methane content ranged from 61 to 74vol%, while the chemical oxygen demand removal efficiency ranged from 42 to 78%. Relatively high fatty acids contents and ammonium concentrations were measured in the reactors. Kinetic analysis showed the best fit to the experimental data for the Cone kinetic model (R2>0.98). The First order kinetic model, Monod, and the modified Gompertz model also exhibited high R2 values. The digestates obtained from co-digestion proved to be excellent in the cress seeds growth test at digestate concentrations of 5-10wt%, while higher concentrations had a toxic effect.