Fate of Potential Contaminants Due to Disposal of Olive Mill Wastewaters in Unprotected Evaporation Ponds

Cataloging olive oil mill wastewater sludge based on toxicological profiles and functional microbial diversity

Olive oil mill wastewater (OMW) sludge, a byproduct of olive oil production stored in evaporation ponds, poses significant environmental and agronomic challenges due to its toxicity, heterogeneity, and high content of recalcitrant organic compounds. While most studies focus on the composition of OMW, research on its functional potential and practical applications for agriculture and sustainability remains limited. This study aimed to characterize the physicochemical, toxicological, and microbiological properties of OMW sludge from different geographical locations to identify patterns that could help standardize effective valorization strategies for agricultural use. Results showed considerable variability in pH (5.73-8.12), organic matter content (8.06-94.98 %), ecotoxicity (1.08-59.65 TU = Toxicity Units), and phytotoxicity (0.17-124 % GI = Germination Index) across samples. Some sludges, such as TED 19 and 25, exhibited phytostimulant properties (GI > 120 %), highlighting their potential to be used in sustainable agriculture. Functional biodiversity analyses revealed metabolic activities linked to the degradation of complex polymeric compounds and carboxylic acids, consistent with the recalcitrant nature of the sludges. Specifically, the samples that showed the highest values of microbial functional intensity were TED15, TED19 and TED25. A taxonomic analysis identified microbial families such as Moraxellaceae, Longimicrobiaceae, Ruminococcaceae, Sphingomonadaceae, Microbacteriaceae, and Lactobacillaceae, which may play a role in reducing toxicity and enhancing agronomic potential. Principal component analysis (PCA) classified the samples based on their characteristics, offering valuable insights into their functional potential. These findings support the development of tailored bioremediation strategies to mitigate the environmental risks of OMW sludge and harness its microbiological potential for sustainable agriculture.

Comparative risk assessment of different agro-industrial wastewaters in an arid environment: lessons from Antinaco-Los Colorados Valley, Argentina

Agro-industries generate significant volumes of wastewaters that cause environmental pollution due to they are discharged in soil or surface water. In particular, arid environments are especially vulnerable to this impact as they are characterized by water scarcity, high temperature, and unproductive soils. Thus, this study aimed to assess the comparative toxicity of winery, olive oil mill, table olive, tomato processing, and walnut shelling wastewaters from an arid region on aquatic and terrestrial organisms, and its relationship with physicochemical characteristics, and sodification and salinization indexes. The seed germination and root elongation toxicity test on Raphanus sativus and the immobilization Daphnia magna test were carried out in the whole effluent toxicity framework. Also, the salinity and sodicity risks of these wastewaters were evaluated. The most toxic wastewater was produced in the table olive industry, whereas the most harmless wastewater was produced in winery. Even after treatment, the wastewaters derived from table olive industry presented the highest risk of sodification and salinization. Toxicity was associated with high levels of sodicity, salinity, and polyphenols, but low values of BOD/COD ratio. The best wastewater quality for soil irrigation was found in the wine and tomato processing industries. Therefore, the comprehensive use of salinity and sodicity risk indicators together with toxicity tests improves the wastewater assessment to reuse them in food production systems as an alternative water source in arid lands.

Air quality in olive mill wastewater evaporation ponds: Assessment of chemical and microbiological pollutants

Olive mill wastewater (OMW), a pollutant residue from the olive oil industry, is typically stored as sludge in evaporation ponds. This study examines the long-term emissions of OMW sludge and its impact on local air quality, analysing chemical pollutants like PM2.5, volatile organic compounds (VOCs), and trace elements (TEs), along with microbial communities (bacteria and fungi). The study also considered meteorological conditions and back-trajectories to identify sources of these elements. The ecological risk index (ERI) was found to be over 720 due to high Hg levels in the sludge (19.0 ± 0.9 ng/g) and air (0.28 ± 0.13 ng/m³), indicating a significant ecological threat. VOCs, particularly oxygenated compounds such as aldehydes and phenol, contributed to the area's strong odour. Meteorological conditions and Sahara dust intrusions influenced bioaerosol loads and seasonal bacterial diversity, whose composition is closely associated with VOC concentrations. The results could contribute to a better understanding of the environmental dynamics in the OMW sludge evaporation ponds, and they could also assist in formulating effective management strategies.

Increases in the soil ammonia oxidizing phylotypes and their rechange due to long-term irrigation with wastewater

Wastewater irrigation is a common practice for agricultural systems in arid and semiarid zones, which can help to overcome water scarcity and contribute with nutrient inputs. Ammonia-oxidizing bacteria (AOB) and archaea (AOA) are key in the transformation of NH4+-N in soil and can be affected by variations in soil pH, EC, N and C content, or accumulation of pollutants, derived from wastewater irrigation. The objective of this study was to determine the changes in the ammonia oxidizing communities in agricultural soils irrigated with wastewater for different periods of time (25, 50, and 100 years), and in rainfed soils (never irrigated). The amoA gene encoding for the catalytic subunit of the ammonia monooxygenase was used as molecular reporter; it was quantified by qPCR and sequenced by high throughput sequencing, and changes in the community composition were associated with the soil physicochemical characteristics. Soils irrigated with wastewater showed up to five times more the abundance of ammonia oxidizers (based on 16S rRNA gene relative abundance and amoA gene copies) than those under rainfed agriculture. While the amoA-AOA: amoA-AOB ratio decreased from 9.8 in rainfed soils to 1.6 in soils irrigated for 100 years, indicating a favoring environment for AOB rather than AOA. Further, the community structure of both AOA and AOB changed during wastewater irrigation compared to rainfed soils, mainly due to the abundance variation of certain phylotypes. Finally, the significant correlation between soil pH and the ammonia oxidizing community structure was confirmed, mainly for AOB; being the main environmental driver of the ammonia oxidizer community. Also, a calculated toxicity index based on metals concentrations showed a correlation with AOB communities, while the content of carbon and nitrogen was more associated with AOA communities. The results indicate that wastewater irrigation influence ammonia oxidizers communities, manly by the changes in the physicochemical environment.

Olive mill waste sludge: From permanent pollution to a highly beneficial organic biofertilizer: A critical review and future perspectives

Olive mill wastewater sludge (OMWS) is a by-product of the olive extraction process that is attracting substantial attention due to its extremely hazardous effects on aquatic and terrestrial ecosystems. OMWS is a product of the common disposal method of olive oil mill wastewater (OMWW) that accumulates in evaporation ponds. It is estimated that approximately 10 × 10⁶ m³ of OMWS is generated worldwide each year. OMWS is characterized by its significantly variable physicochemical properties and organic pollutant constituents, such as phenols and lipids, which are dependent upon the environmental features of the receiving ponds. Nonetheless, many related studies have recognized the biofertilizer potential of this sludge owing to its high mineral nutrient and organic matter load. OMWS exhibits promising valorization potential in several fields, including agriculture and energy production. Compared to those of OMWW, studies of OMWS are still lacking concerning its composition and characteristics, which are necessary for the future implementation of efficient valorization strategies. The main purpose of this review paper is to fill the gap that exists in the literature by providing a critical analysis of the available data on OMWS production, distribution, characteristics, and properties. Additionally, this work sheds light on important factors affecting OMWS properties, including the variability of the indigenous microbial communities regarding bioremediation. Finally, this review addresses the current and future valorization routes, from detoxification to the development of promising applications in agriculture, energy, and the environment, which could have significant socioeconomic implications for low-income Mediterranean countries.

Biorecovery of olive mill wastewater sludge from evaporation ponds

Olive mill wastewater (OMW) resulting from the olive oil extraction process is usually disposed of in evaporation ponds where it concentrates generating a sludge that pollutes the ponds nearby area. In this study, four bio-treatments were applied for the in-situ bioremediation and valorization of OMW sludge: Landfarming, phytoremediation, composting and vermicomposting. In all cases, the OMW sludge was added with organic residues (mushroom compost, rabbit manure, and chicken manure). The bio-treatments were carried out in duplicate, inoculated and non-inoculated, to determine the effect of a specialized fungal consortium (Aspergillus ochraceus H2 and Scedosporium apiospermum H16) on the efficacy of the bio-treatments. The evaluation of chemical parameters, toxicity, and functional microbial biodiversity revealed that the four techniques depleted the toxicity and favored the stimulation of functional microbiota. Landfarming and phytoremediation allowed the decontamination and improvement of soils. Composting and vermicomposting also offered high-quality products of agronomic interest. Inoculation improved the bioremediation effectiveness. Biological treatments are effective for the safe recovery of contaminated OMW sludge into high-quality services and products.

Microbial Community Succession and Organic Pollutants Removal During Olive Mill Waste Sludge and Green Waste Co-composting

Olive mill wastewater sludge (OMWS) is the main by-product of the olive industry. OMWS is usually dumped in landfills without prior treatment and may cause several eco-environmental hazards due to its high toxicity, which is mainly attributed to polyphenols and lipids. OMWS is rich in valuable biocompounds, which makes it highly desirable for valorization by composting. However, there is a need to understand how microbial communities evolve during OMWS composting with respect to physicochemical changes and the dynamics of pollutant degradation. In this study, we addressed the relationship between microbial community, physicochemical variations and pollutants degradation during the co-composting of OMWS and green wastes using metagenomic- and culture-dependent approaches. The results showed that in raw OMWS, Pichia was the most represented genus with almost 53% of the total identified fungal population. Moreover, the bacteria that dominated were Zymobacter palmae (20%) and Pseudomonas sp. (19%). The addition of green waste to OMWS improved the actinobacterial diversity of the mixture and enhanced the degradation of lipids (81.3%) and polyphenols (84.54%). Correlation analysis revealed that Actinobacteria and fungi (Candida sp., Galactomyces sp., and Pichia manshurica) were the microorganisms that had the greatest influence on the composting process. Overall, these findings provide for the first time some novel insights into the microbial dynamics during OMWS composting and may contribute to the development of tailored inoculum for process optimization.

A critical review of environmental and public health impacts from the activities of evaporation ponds

Evaporation ponds (EVPs) are among the most cost-effective, and simple wastewater treatment technologies used in many regions/countries with high solar radiation levels. However, its operational limitations, which include the overflow of wastewater, leakages via liners, and large surface area of the EVP that is exposed to atmosphere, creates a negative feedback to the environment. Therefore, the main aim of this review study of more than a hundred works published a little all over the continents is to provide a summary of various contaminations that are associated with EVPs activities through different environmental compartments. In addition, the impacts of EVP on fauna, human health including the current on-site sustainable mitigation strategies were also reviewed. The first conclusion from this study shows that the most commonly contaminants released into surface waters, groundwater, soil and sediments were heavy metals, pesticides, herbicides, selenium, including several major anions and cations. Non-methane hydrocarbons (NMHCs), volatile organic compounds (VOCs), and particulate matters (PMs) were the main air pollutants emitted from the surfaces of an EVP. Limited data is available about the emissions of atmospheric greenhouse gas (GHGs) especially carbon dioxide (CO₂) and methane (CH₄) from EVP surfaces. Migratory birds and aquatic organisms are the most vulnerable fauna as EVP wastewaters can cause obstruction of movements, affect diversity, and causes mortalities following the exposure to the toxic wastewater. The study revealed limited data about the potential health risk associated with occupational and environmental exposure to radiological hazards and contaminated drinking water from EVP activities. On-site EVP treatment strategies using bioremediation and electrochemical treatment technologies have shown to be a promising sustainable mitigation approach. Knowledge gaps in areas of GHGs monitoring/modeling, pollution exposure estimation and health risk assessments are urgently required to gain deeper understanding about the impact of EVP activities, and incorporate them into future EVP designs.

Long-Term Impact of Wastewater Irrigation on Soil Pollution and Degradation: A Case Study from Egypt

There is consensus on the impact of wastewater irrigation on soil properties and heavy metal accumulation. The studies that show the impact of temporal changes as a result of different long-term additions of wastewater on the heavy metal accumulation and degradation of soil are extremely limited. This study was carried out to assess heavy metal contamination in soils irrigated with wastewater for more than 30 years in Egypt. A total number of 12 irrigation water samples and 12 soil profiles were collected during 2020 and were chemically characterized. The results showed that soils irrigated with wastewater over the long term contained significantly higher concentrations of heavy metals compared to fields irrigated with fresh water. Heavy metal levels in water and soil samples were within the permissible limits, with the exception of Cd concentration in water (0.03 mg L−1). Continuous cultivation for a long period of time (30 years) using raw urban wastewater application has led to the adverse effect of increasingly available Pb concentration (5.44 mg kg−1). Similar temporal behavior was seen for Cd and Fe, which increased by 0.98 and 11.2 mg kg−1, respectively, after 30 years. The heavy metals in wastewater-irrigated soils significantly increased in clayey soils, as compared to sandy soils irrigated from the same source. Our findings provide important information for decision makers in Egypt and similar countries for the development of a strategy for the use of wastewater in irrigation for sustainable agricultural management.

Microbial communities of the olive mill wastewater sludge stored in evaporation ponds: The resource for sustainable bioremediation

Olive Mill Wastewater (OMW) is a polluting residue from the olive oil industry. It is usually stored in open-air unprotected evaporation ponds where their sediments accumulate. This study compares the characteristics of OMW sludges stored for long-time in evaporation ponds and assesses their impact on the underlying soil layer. Physicochemical parameters, toxicity bioassays, and full characterization of the microbial community were analyzed. The extension of the polluting effects was assessed by analysis of toxicity, microbial biomass carbon, and respiration. Geostatistics was used to predict their spatial distribution. Organic matter and polyphenol content besides toxicity levels determine variations between OMW sludges and have a high impact on the microbiota they contain. The microbial community was abundant, diverse, and functionally active. However, the biodegradability of the sludges was hindered by the toxicity levels. Toxicity and biomass carbon were higher on the surface of the ponds than in the soil layer revealing a reduced leach flow and depletion of contaminants. The natural microbiota might be biostimulated by means of applying sustainable and feasible biological treatments in order to favor the OMW sludges bioremediation. These results open up the possibility of solving the environmental concern caused by its storage in similar scenarios, which are common in olive oil-producing countries.

Olive mill wastewater-evaporation ponds long term stored: Integrated assessment of in situ bioremediation strategies based on composting and vermicomposting

During the last two decades, the method most widely used to manage olive mill wastewater (OMW) derived from olive oil production has been its disposal in evaporation ponds. Long-term storage of OMW leads to the accumulation of toxic sediments (OMWS) rich in recalcitrant compounds with phytotoxic and antimicrobial properties, which limit their use for agronomic purpose. The aim of this study was to compare the effect of two in situ bioremediation strategies (composting and a combination of composting followed by vermicomposting) to remove the potential toxicity of the sediments derived from long-term stored OMW. The results obtained showed that the composting method assisted with the earthworms enhanced the depletion of phenolic compounds and OMWS ecotoxicity more than composting, especially during the maturation stage. Moreover, vermicomposting was more effective in the reduction of the OMWS salinity. However, a pre-composting process to the OMWS is necessary prior to vermicomposting to provide the suitable conditions for earthworms survival and activity. Furthermore, the final compost showed a phytostimulating effect. Therefore, these in situ bioremediation strategies can be considered potential tools for decontamination and recovery of long-term stored OMWS in evaporation ponds, which currently poses an unsolved environmental problem.

Evaluating Earthworms’ Potential for Remediating Soils Contaminated with Olive Mill Waste Sediments

The olive-oil industry generates large amounts of residues that, in the past, were accumulated in evaporating ponds in many Mediterranean countries. Currently, these open-air ponds pose a serious environmental hazard because of toxic chemicals that concentrate in their sediments. Bioremediation of olive mill waste (OMW) sediments has emerged as a viable option for managing this environmentally problematic residue. Here, we postulate that inoculation of an OMW-soil mixture with earthworms may be a complementary bioremediation strategy to that using native microorganisms only. A laboratory study assessed the ecotoxicity of OMW-amended soils (10%, 20%, 40% and 80% w/w) combining earthworm biomarker responses and soil enzyme activities. The doses of 40% and 80% were toxic to earthworms, as evidenced by the high mortality rate, loss of body weight and signs of oxidative stress after 30 d of soil incubation. Conversely, doses ≤ 20% w/w were compatible with earthworm activity, as indicated by the significant increase of soil enzyme activities. Total concentrations of phenolic compounds decreased by more than 70% respect to initial concentrations in 10% and 20% OMW treatments. These results suggest that OMW sediments intentionally mixed with soils in an up to 20% proportion is a workable bioremediation strategy, where earthworms can be inoculated to facilitate the OMW degradation.

Bioremediation of Olive Mill Wastewater sediments in evaporation ponds through in situ composting assisted by bioaugmentation

The common method for the disposal of olive oil mill wastewater (OMW) has been its accumulation in evaporation ponds where OMW sediments concentrate. Due to the phytotoxic and antimicrobial effect of OMW, leaks from ponds can pollute soils and water bodies. This work focuses on the search for microorganisms that can be used as inocula for bioremediation of polluted matrices in OMW ponds by means of in situ composting. Two fungi isolated from OMW sediments, Aspergillus ochraceus H2 and Scedosporium apiospermum H16, presented suitable capabilities for this use as a consortium. Composting eliminated the phyto- and ecotoxicity of OMW sediments by depleting their main toxic components. Inoculation with the fungal consortium improved the bioremediation efficacy of the technique by hastening the decrease of phytotoxicity and ecotoxicity and enhancing phytostimulant property of compost produced. This procedure constitutes a promising strategy for bioremediation of OMW polluted sites.

Characterization and monitoring of subsurface contamination from Olive Oil Mills' waste waters using Electrical Resistivity Tomography

This work describes the efficiency and ability of Electrical Resistivity Tomography (ERT) to map and monitor the subsurface contamination caused by the wastes created during the production of olive oil. The spatial distribution and temporal variation of these wastes are investigated through an integrated methodological flowchart composed of numerical modeling tests and field data collected from an active waste disposal site. An Olive Oil Mills' Wastes (OOMW) real site was chosen to monitor the subsurface flow of the wastes that are disposed of in an artificial pond for 1.5 years. Synthetic modeling was used to simulate and reconstruct the movement of the OOMW as a conductive target within a layered resistive medium. The results of the ERT data show a high degree of correlation between published ERT, geochemical, and IP geophysical results. This indicates that ERT can be a powerful tool for mapping and monitoring the byproducts of the olive oil industry, in the form of subsurface contamination, as demonstrated by the synthetic modeling. The electrical signature of the OOMW was also verified through the identification of in situ wastes within an excavation trench along the monitoring ERT line. The results show that ERT can be used as a stand-alone tool to characterize the subsurface pollution in OOMW sites.