A yearly spraying of olive mill wastewater on agricultural soil over six successive years: impact of different application rates on olive production, phenolic compounds, phytotoxicity and microbial counts

Response of chemical and biochemical soil properties to the spreading of biochar-based treated olive mill wastewater

Despite the polluting potential olive mill wastewater (OMW) can be a useful source of nutrients and organic compounds to improve soil properties. The aim of this paper was to verify if biochar-based treatment of OMW could be an efficient method to contrast the richness in phenolic compounds and phytotoxicity of OMW making it more suitable. for soil amendment. In this study poplar biochar (BP) was more effective than conifer biochar (BC) in terms of adsorbing phenols and reducing phytotoxicity at different biochar rates (5 and 10 %). In soil amendment BP-treated OMW induced an increase of organic carbon by approximately 15 % and notably BP10 treated OMW enhanced available phosphorous by 25 % after 30 days of incubation. In soil amended with 10 % BP-treated OMW microbial biomass and enzymatic activities were significantly enhanced after 30 and 90 days, with no effect on cress seed germination. Therefore, biochar based-treatment could be cost-effective and able to facilitate the long-term management of OMW in terms of storage and disposal.

Response of chemical and biochemical soil properties to the spreading of biochar-based treated olive mill wastewater

Olive mill wastewater (OMW) is the effluent derived from the oil extraction processes from olives. Despite the polluting potential OMW can be a useful source of nutrients and organic compounds to improve soil properties. OMW could negatively affect soil and water quality as this waste is rich in phenolic compounds and has high COD and BOD5. Biochar-based treatment could be an efficient method to remediate OMW. In this study poplar biochar (BP) was more effective than conifer biochar (BC) in terms of adsorbing phenols and reducing phytotoxicity at different biochar rates (5 and 10 %). BP-treated OMW was used in soil amendment and induced an increase in chemical properties, especially in organic carbon after 30 days of incubation. In soil amended with 10 % BP-treated OMW microbial biomass, enzymatic activities, and cress seed germination were significantly enhanced after 30 and 90 days.

Olive oil industry: a review of waste stream composition, environmental impacts, and energy valorization paths

The olive oil production is a key economic sector for the producing countries, mainly in the Mediterranean region. However, the worldwide increasing oil production led to the generation of huge amounts of wastes detrimental for the environment. Therefore, efficient and sustainable management of olive industry wastes has recently acquired significant interest in the scientific research community. In the actual world energy context, various studies dealt with the valorization of the solid/liquid waste streams obtained from the discontinuous/continuous extraction of olive oil for energy purposes. The application of waste-to-energy treatments to these effluents can turn them out into an important energy resource. This review article presents the main used oil extraction techniques and their related research developments. The characterization of the generated wastes and the factors behind their bad environmental impacts are highlighted. Relevant research works related to biochemical and thermochemical conversion of olive mill wastes are extensively reviewed and discussed in terms of product yields and composition. A recent update of the studies addressing olive industry waste applications for energy production is also given. This investigation revealed a lack of studies in relation to the hydrothermal processing of olive mill wastes. Despite their suitability for this process (e.g., high moisture content), few papers have investigated the hydrothermal conversion of these waste streams. This scientific gap opens a very interesting research direction, which has to be further investigated.

Impact of Wastewater Spreading on Properties of Tunisian Soil under Arid Climate

The environmental impacts of irrigating an olive field with olive mill wastewater (OMW) and treated wastewater (TWW) on soil properties were investigated. The effect of different irrigation treatments of OMW (50 m3 ha−1, 100 m3 ha−1, and 200 m3 ha−1) and TWW at different soil depths was studied. The obtained findings revealed that TWW application augmented soil pH; EC values; and P, K and Ca contents in all soil layers. However, Mg and Na concentrations, as well as organic matter content (OM), were proven to decrease with TWW irrigation throughout the experiment. Whereas soil adjusted with OMW showed a decrease in K, Ca, Mg and Na contents with soil depth, a significant increase was observed with the increase in applied OMW dose. On the other hand, total phenols and OM content increased significantly with the rise in OMW levels in all the investigated layers compared to the control sample.

Olive mill wastewater phytoremediation employing economically important woody plants

In this study two plant species, Punica granatum L. and Myrtus communis L., have been tested as candidates for phytoremediation of olive mill wastewater (OMW) through recirculation in soil pilot units, according to the proposed patented technology by Santori and Cicalini [EP1216963 A. 26 Jun 2002]. Wastewater was treated in batches of low to high organics strength (COD: 2 700-45 700 mg/L) during summer months of two consecutive years. Dynamics of the most important wastewater parameters were investigated, and corresponding removal rates were estimated. During treatment of low organic load OMW, average removal rate of organics, phenolics, total nitrogen and total phosphorus were 0.68 g-COD/kg-soil d, 0.073 g-TPh/kg-soil d, 0.033 g-TN/kg-soil d and 0.0074 g-TP/kg-soil d respectively and plants proved to be tolerant to the OMW. During treatment of high organic load OMW removal rates were roughly 10-fold higher although phytotoxic symptoms were observed. Plants were found to contribute greatly to the OMW treatment process since organics removal rates in pilot units were found to be at least 10-fold higher than in wastewater treatment in non-vegetated soil. Plant species with high added value products such as pomegranate and myrtle trees were used in this study, improving the circular economy potential of the aforementioned technology. Moreover, its efficiency has been demonstrated by quantification of the overall removal rates of key constituents as well as the contribution of the plants in the OMW treatment.

Performance and dynamic modeling of a continuously operated pomace olive packed bed for olive mill wastewater treatment and phenol recovery

The solid waste of olive oil extraction processes (olive pomace, OP) was converted into activated carbon (AC) by treating it with NaOH and then encapsulating it within sodium alginate (SA) in beads by crosslinking (SA-AC beads). The prepared SA-AC beads were utilized as an adsorbent for the elimination and recovery of phenolic compounds (PCs) from olive mill wastewater (OMWW) following a zero liquid and waste discharge approach to implement and promote the circular economy concept. The novel AC and SA-AC beads were characterized by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), Fourier transform infrared spectroscopy (FTIR) and Brunauer, Emmett and Teller (BET) analysis. The adsorption performance of these beads was evaluated in batch and fixed-bed reactors operated in a concurrent flow system. The results revealed that an adsorption capacity of 68 mg g^-1 was attained for 4000 mg L^-1 phenolic compounds. The kinetics of the adsorption process of the PCs fit a pseudo second-order model, and the most likely mechanism took place in two stages. The adsorption isotherm conformed to the Langmuir model, representing the monolayer adsorption of the phenolic compounds. The dynamic models were used, and they accurately represented the breakthrough curves. Considering PC recovery and process reusability, a regeneration experiment of SA-AC beads was carried out in fixed-bed reactors. SA-AC beads showed a high percentage desorption >40% using ethanol and were efficient after several cycles of OMWW treatment and phenol recovery.

Optimization of hybrid treatment of olive mill wastewaters through impregnation onto raw cypress sawdust and electrocoagulation

This research investigation proposes a new method for sustainable olive mill wastewater (OMW) treatment and handling. It is based on the combination of its impregnation onto raw cypress sawdust (RCS) followed by electrocoagulation. The retention of OMW compounds onto various RCS doses show an important decrease of its chemical oxygen demand (COD) and its main cation and anion content. The maximum retention efficiencies of COD, Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl^-, [Formula: see text], and [Formula: see text] were about 51.0%, 75.3%, 28.7%, 77.9%, 84.7%, 41.1%, 98.3%, and 90.9%, respectively, for the highest RCS dose (200 g L^-1). This organic matter- and nutrient-loaded biomass could be thermochemically converted through pyrolysis into biofuel and biochar for energetic and agronomic purposes, respectively. The treatment by electrocoagulation of the pre-treated OMW using mild steel electrodes could be considered an attractive treatment method since 75.6% of COD removal efficiency was achieved. Besides, this approach permits a significant energy consumption reduction by 46% as compared with the electrocoagulation process alone. It allows also a significant improvement of the treated effluent quality in terms of both organic and mineral contents that could be reused for the irrigation of olive trees in the context of circular economy.

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.

Sustainable Environmental Management and Valorization Options for Olive Mill Byproducts in the Middle East and North Africa (MENA) Region

Cultivation of olive trees and olive oil production have been considered as a legacy for the Mediterranean region. This custom represents a very important benefit for many nations in terms of wealth and health. However, huge amounts of by-products and waste are generated during olive oil production. This represents a serious environmental impact on land and water bodies if not properly handled. Olive oil extraction generates two waste streams, a solid waste called pomace and olive mill wastewater (OMWW), which has been considered as highly pollutant and phytotoxic waste. These wastes have high disposal costs and predominantly generated from small-scale enterprises that have limited financial resources to treat them properly before discharge to the environment. Besides being a serious environmental problem, OMWW has potential economic value that remains to be utilized such as: fertilizers, valuable antioxidants agents and fatty acids needed in human diet. Also, Olive pomace is a valuable renewable energy source with an energy density of 23 MJ/kg and has become an inexpensive alternative for fossil fuels. Aiming at adding value to the olive production sectors and potential valorization options for byproducts in the MENA region, international practices applied in olive mills wastes management’s and treatment methods used in major oil producing countries are presented.

The long term effect of Olive Mill Wastewater (OMW) on organic matter humification in a semi-arid soil

This study investigates the performance of soil and OMW microfloras in OMW organic matter humification in soil. In order to highlight the role of OMW and soil microfloras in the processes of OMW organic matter humification, either OMW or soil was sterilized with autoclaving. The study was carried out in microcosms of 1l containing 500 g of raw or sterilized soil, to which was added 200 ml of raw or sterilized OMW. After 24 months of incubation, the amount of phenolic compounds in the different microcosms was statistically indifferent compared to the control. However, TG-DTA and FTIR analysis of soil humic acids showed that recalcitrant OMW phenolic compounds remain in soil humus in the microcosm: sterilized OMW + raw soil, even after 24 months. Results show that humic acids detected in presence of OMW microflora are loaded with proteins and carbonated compounds and deprived of phenolic compounds and aliphatics with long chain, while humic acids detected in absence of OMW microflora are loaded with phenolic compounds.

Study of the phytotoxic potential of olive mill wastewaters on a leguminous plant 'Vicia faba L.'

We investigated the study of the phytotoxic potential of olive mill wastewaters (OMW) on the germination and growth responses of the faba bean 'Vicia faba L.' leguminous plant cultivated under an arid Mediterranean climate. The results showed that the raw OMW blocked the germination of the seeds tested, while OMW treated soil extracts stimulate the seeds' germination rate. The monitoring of the faba bean growth parameters in the different OMW treated soils showed that the optimal growth of the faba bean plants has been recorded for the soil amended with 25 m³ ha^-1 of OMW. Although the 50 m³ ha^-1 dose is the most suitable for the soil studied, our results showed that the 25 m³ ha^-1 dose is the most suitable for the vegetative development, as well as for the productivity of the plant tested 'Vicia faba L.'.

Phytotoxicity Evaluation of Wastewater from Rice Parboiling

The aim of this study was to evaluate the phytotoxicity of raw and treated effluents from a parboiling rice industry in the south of Brazil. The evaluation included 19 physicochemical parameters and a phytotoxicity test using lettuce and cucumber seeds as bioindicators. The germination index (GI) of both seeds was decreased when in contact with the raw effluent (p < 0.05) and with the treated effluent reduced the GI of cucumber seeds (p < 0.05) and the root length (RL) of lettuce seeds (p < 0.05). For lettuce seeds exposed to the treated effluent there was a negative correlation (p < 0.05) between RL and Mn (- 0.82) and between GI and Fe (- 0.68). For cucumber seeds in the raw effluent, the Fe showed negative correlation (p < 0.05) with seed germination (- 0.75), but, in the treated effluent, their GI presented positive correlation (0.61) with the ammoniacal-N (p < 0.05). The results indicated that both effluents showed phytotoxicity for lettuce and cucumber seeds.

Sugar and mineral enriched fraction from olive mill wastewater for promising cosmeceutical application: characterization, in vitro and in vivo studies

Nowadays, agro-food by-products represent a potential low-cost source of biologically active ingredients which have been paid significant attention as nutraceuticals, medicine, food and cosmetics. In a previous study we evaluated the total sugars, metals and polyphenols of olive mill wastewater (OMWW) from a Cerasuola olive cultivar. In the present work we selectively recovered a sugar and mineral enriched fraction (SMEF) from Cerasuola OMWW by a green adsorption/desorption process. The SMEF was mainly found to be composed of monosaccharides and potassium by HPLC-ELSD and ICP-MS. The in vitro cytotoxicity on human fibroblasts, at different concentrations of the fraction, was investigated by MTT and comet assays. In addition, intracellular reactive oxygen species (ROS) production, apoptosis and cell morphological changes were examined. The physical stability of a formulation containing the SMEF (1% w/w) and its in vivo skin effects were also assessed.Our results highlighted that the SMEF showed a toxic effect at higher concentrations (i.e. cell viability reduction, DNA fragmentation and morphological alterations) well correlated with high ROS levels. Conversely, at low concentrations (0.5% and 1% w/w), no significant changes were observed. For the first time, through stability studies and in vivo tests, we also demonstrated that the SMEF formulation is stable and safe for topical application, since skin hydration improvement without negative effects was observed after 7 days of its use. Therefore, the SMEF has great potential to be used for cosmeceutical applications.

Production of organic fertilizer from olive mill wastewater by combining solar greenhouse drying and composting

Olive mill wastewater (OMW) is generated during the production of olive oil. Its disposal is still a major environmental problem in Mediterranean countries, despite the fact that a large number of technologies have been proposed up to date. The present work examines for the first time a novel, simple and low-cost technology for OMW treatment combining solar drying and composting. In the first step, OMW was dried in a chamber inside a solar greenhouse using swine manure as a bulking agent. The mean evaporation rate was found to be 5.2 kg H₂O/m²/d for a drying period of 6 months (February-August). High phenol (75%) and low nitrogen (15%) and carbon (15%) losses were recorded at the end of the solar drying process. The final product after solar drying was rich in nutrients (N: 27.8 g/kg, P: 7.3 g/kg, K: 81.6 g/kg) but still contained significant quantities of phenols (18.4 g/kg). In order to detoxify the final product, a composting process was applied as a second step with or without the use of grape marc as bulking agent. Results showed that the use of grape marc as a bulking agent at a volume ratio of 1:1 achieved a higher compost temperature profile (60 °C) than 2:1 (solar drying product: grape marc) or no use (solar drying product). The end product after the combination of solar drying and composting had the characteristics of an organic fertilizer (57% organic carbon) rich in nutrients (3.5% N, 1% P, 6.5% K) with quite low phenol content (2.9 g/kg). Finally, the use of this product for the cultivation of pepper plants approved its fertility which was found similar with commercial NPK fertilizers.

Environmental impacts in the life cycle of olive oil: a literature review

The production of olive oil is considered to be one of the largest agricultural business sectors in the Mediterranean area. Apart from its significant impact on the economies of countries in Southern Europe, Northern Africa and Middle East, olive oil production also involves considerable social and environmental considerations. However, despite such importance, the environmental effects of olive oil production have not been studied as much other agricultural productions and farming systems, which are more characteristic of central and northern Europe. We present a thorough and systematic literature review of scientific publications with respect to the use of environmental tools in the life cycle of olive oil. The analysis takes into consideration the farming of olive trees, the manufacture of olive oil, packaging, transportation and reverse logistics. To that end, journal publications up to 2015 in this specific field are recorded and, at the same time, the most important environmental impacts are revealed and a gap analysis is carried out. The analysis conducted reveals that farming of olive trees (with pesticide use and waste/by-product production being the 'hottest' topics) and the manufacturing of olive oil (concentrating mostly on waste/by-product production and management) are the phases with the highest environmental focus from the scientific community. Moreover, gaps in the literature are detected mostly with respect to fuel consumption and the use and promotion of renewable energy sources in olive oil production. © 2016 Society of Chemical Industry.

Agronomic application of olive mill wastewater: Effects on maize production and soil properties

This study investigates the effect of direct amendment of olive mill wastewater (OMW) on the fertility of soil, described as poor in the area of Marrakech (semi-arid region) in Morocco. The treated plots were amended with untreated OMW generated by a traditional extraction process at the amount of 10 L/m(2)/year during two consecutive years. Results of these two years treatments with crude OMW at relatively high dose reveal an important increase in soil physicochemical characteristics, namely electric conductivity (EC), Na(+,) K(+), phosphorus, nitrogen, organic matter and soluble phenolic compounds. EC of treated soil was enhanced from 0.34 to 2.91 mS/cm as compared to the control soil. After spreading OMW in soil, the amounts of its nutritive elements increased by 81% for nitrogen, 66% for phosphorus and 88% for potassium. The accumulation of phenolic compounds and the increase of total peroxidase activity in plants provide evidence of their protective role against the physiological stress induced by OMW. However, this enrichment in mineral and nutritive elements decreased three months after OMW application, revealing OMW biodegradation in the studied calcareous soil. In parallel, an increase in the contents of the soluble phenolic compounds on the upper layer of soil was denoted and maize plants growth was efficiently raised. Significant amelioration was obtained notably in terms of fresh and dry weight of leaves, leaves area, spikes fresh and dry weight, 100 seeds weight and straw yield (37, 54, 27, 24, 14 and 9% respectively). Along with the correct choice of convenient soils notably calcareous ones and tolerant crops such as maize, this method could constitute an efficient approach for avoiding problems attributed to the uncontrolled disposal of these effluents and an effective strategy to regenerate degraded soils and represents an economical alternative that provides a local fertilizer.

Shifts in soil chemical properties and bacterial communities responding to biotransformed dry olive residue used as organic amendment

Dry olive residue (DOR) is a waste product derived from olive oil extraction and has been proposed as an organic amendment. However, it has been demonstrated that a pre-treatment, such as its transformation by saprophytic fungi, is required before DOR soil application. A greenhouse experiment was designed where 0 and 50 g kg(-1) of raw DOR (DOR), Coriolopsis floccosa-transformed DOR (CORDOR) and Fusarium oxysporum-transformed DOR (FUSDOR) were added to soil. Analyses of the soil chemical properties as well as the structure and relative abundance of bacterial and actinobacterial communities were conducted after 0, 30 and 60 days following amendment. The different amendments produced a slight decrease in soil pH and significant increases in carbon fractions, C/N ratios, phenols and K, with these increases being more significant after DOR application. Quantitative PCR assays of the 16S rRNA gene and PLFA analyses showed that all amendments favoured bacterial growth at 30 and 60 days, although actinobacterial proliferation was more evident after CORDOR and FUSDOR application at 60 days. Bacterial and actinobacterial DGGE multivariate analyses showed that the amendments produced structural changes in both communities, especially after 60 days of amendment. PLFA data analysis identified changes in soil microbial communities according to the amendment considered, with FUSDOR and CORDOR being less disruptive than DOR. Finally, integrated analysis of all data monitored in the present study enabled us to conclude that the greatest impact on soil properties was caused by DOR at 30 days and that soil showed some degree of resilience after this time.

Polyphenolic compounds progress during olive mill wastewater sludge and poultry manure co-composting, and humic substances building (Southeastern Tunisia)

In Mediterranean areas, olive mill wastes pose a major environmental problem owing to their important production and their high polyphenolic compounds and organic acids concentrations. In this work, the evolution of polyphenolic compounds was studied during co-composting of olive mill wastewater sludge and poultry manure, based on qualitative (G-50 sephadex) and quantitative (Folin-Ciocalteu), as well as high pressure liquid chromatography analyses. Results showed a significant polyphenolic content decrease of 99% and a noticeable transformation of low to high molecular weight fraction during the compost maturation period. During this step, polyphenols disappearance suggested their assimilation by thermophilic bacteria as a carbon and energy source, and contributed to humic substances synthesis. Polyphenolic compounds, identified initially by high pressure liquid chromatography, disappeared by composting and only traces of caffeic, coumaric and ferulic acids were detected in the compost. In the soil, the produced compost application improved the chemical and physico-chemical soil properties, mainly fertilising elements such as calcium, magnesium, nitrogen, potassium and phosphorus. Consequently, a higher potato production was harvested in comparison with manure amendment.

Efficacy of olive mill wastewater for protecting Bacillus thuringiensis formulation from UV radiations

The effectiveness of 10 low-cost UV-absorbers in protecting Bacillus thuringiensis subsp. kurstaki BLB1 toxins against inactivation by UV-A and UV-B irradiation was evaluated in this study. Among them, two by-products, molasses and olive mill wastewater (OMW) were selected for further studies. They were tested at different concentrations of 0.05, 0.1, 0.15 and 0.2% using the para-aminobenzoic acid (PABA) as a common UV protectant. Interestingly, addition of PABA and OMW to BLB1 formulations was found to be most effective in protecting BLB1 spores at 90.8 and 76.4% respectively and in preserving delta-endotoxin concentration at a level of 81.7 and 72.2%, respectively when used at a concentration of 0.2%. The lowest preserved spores (46.3%) and delta-endotoxin level (12.4%) was found using molasses. In contrast, spore count and delta-endotoxin concentration were completely reduced after an exposure of unprotected Bt strain BLB1 to UV radiations up to 96h. SDS-PAGE analysis of protected and unprotected samples revealed that delta-endotoxin bands (130, 65-70kDa) were conserved until 96h of UV exposure in presence of PABA or OMW compared with their disappearance in presence of molasses after 72h of exposure and their dramatically decline from 8h of exposure in unprotected mixture. A complete loss of larvicidal toxicity against Ephestia kuehniella was found after 24h of exposure in absence of any UV-absorber. Addition of OMW or PABA offered the highest levels of insecticidal activity with 63.2 and 74.7% of residual toxicity, respectively. Whereas, molasses addition, as UV protectant retained only 26.3% of residual activity after 96h of exposure. Therefore, addition of OMW by-product to Bt formulation may be a suitable alternative to others synthetic chemical compounds. OMW may also provided added value, be environmentally friendly and less hazardous, when used at low concentration.

Microbial diversity of a Mediterranean soil and its changes after biotransformed dry olive residue amendment

The Mediterranean basin has been identified as a biodiversity hotspot, about whose soil microbial diversity little is known. Intensive land use and aggressive management practices are degrading the soil, with a consequent loss of fertility. The use of organic amendments such as dry olive residue (DOR), a waste produced by a two-phase olive-oil extraction system, has been proposed as an effective way to improve soil properties. However, before its application to soil, DOR needs a pre-treatment, such as by a ligninolytic fungal transformation, e.g. Coriolopsis floccosa. The present study aimed to describe the bacterial and fungal diversity in a Mediterranean soil and to assess the impact of raw DOR (DOR) and C. floccosa-transformed DOR (CORDOR) on function and phylogeny of soil microbial communities after 0, 30 and 60 days. Pyrosequencing of the 16S rRNA gene demonstrated that bacterial diversity was dominated by the phyla Proteobacteria, Acidobacteria, and Actinobacteria, while 28S-rRNA gene data revealed that Ascomycota and Basidiomycota accounted for the majority of phyla in the fungal community. A Biolog EcoPlate experiment showed that DOR and CORDOR amendments decreased functional diversity and altered microbial functional structures. These changes in soil functionality occurred in parallel with those in phylogenetic bacterial and fungal community structures. Some bacterial and fungal groups increased while others decreased depending on the relative abundance of beneficial and toxic substances incorporated with each amendment. In general, DOR was observed to be more disruptive than CORDOR.

Aspergillus niger P6 and Rhodotorula mucilaginosa CH4 used for olive mill wastewater (OMW) biological treatment in single pure and successive cultures

The aim of this study was to investigate the Rhodotorula mucilaginosa CH4 and Aspergillus niger P6 abilities to purify olive mill wastewater (OMW) in single pure and mixed cultures during the treatment. Both fungi were molecularly identified. OMW was used at five dilutions from 5% to 30% with chemical oxygen demand (COD) ranging from 11,600 to 24,600 mg L(-1). Firstly, each fungus was used separately, then they were successively used to treat the OMW. In single pure culture, A. niger showed a better efficiency in OMW purification than R. mucilaginosa. Furthermore, when successively used, the two studied strains exhibited improvements in the decrease of COD, polyphenolic compounds concentration and effluent colour. COD removals were 95.68-56.71% by R. mucilaginosa and 98.02-69.51% by A. niger for OMW dilutions varying from 5% to 30%. Both strains showed an important polyphenolic compounds removal of 83-45% by R. mucilaginosa and 94-58% by A. niger, in accordance with the OMW COD initially used. The COD and phenolic compound removals fitted simple equation models, with high regression coefficients. The strains' growth kinetics decreased according to the OMW concentration, but, when successively used, fungal growth was improved, allowing efficient effluent treatment.