Valorization of olive mill effluent with oleaginous Yarrowia lipolytica

Abstract

More than 90% of world olive oil production takes place in Mediterranean countries. Therefore, the olive mill industry has an enormous role in their economy. However, due to the high amount of water requirement of olive oil production which depends on the preferred oil extraction method, a huge amount of wastewater origins from the olive mill industry which has adverse effects on land, aquatic, and air due to its high polluting structure in terms of organic and phenolic contents. For this reason, olive mil effluent (OME) must be treated in an appropriate way not only due to its flow rate but also since it contains a high concentration of pollutants. Treatment of wastewater should be carried out with the approach of valorization of wastewater during treatment. The sustainable energy requirement is an urgent need due to the dramatic increase in global energy consumption. The predictions indicate that the energy reserves of fossil fuels are not sufficient to meet the increasing future energy demand. What is more, due to the increment of fossil-based fuel, its adverse effects on the environment such as global climate change, deterioration of the ecological balance increase day by day. These negativities have turned the direction of scientific research to the search for clean and sustainable energy sources. Biodiesel is known as a non-toxic, sustainable, and biodegradable carbon source. These specialties provide biodiesel to be an alternative energy source to fossil fuels. Biofuels are produced from biological materials like not only plants or raw materials produced in many different sectors such as forestry or agriculture as a resource, but also some microorganisms such as yeast, algae, etc. Some yeast species have the ability to grow on OME and store extra lipid in their body while metabolizing the organic matter of OME to value-added products such as enzymes and organic acids. The previous studies say that the oleaginous yeast species Yarrowia lipolytica (Y. lipolytica) is capable of lipid accumulation up to 70%. Therefore, OME can be valorized by Y. lipolytica in this view while eliminating its pollution load. Besides, to compete with the conventional energy source in terms of energy yield, economic feasibility, and sustainability, lipid production from OME by using Y. lipolytica needs to be optimized. In this study, optimization of the satisfactory conditions for Y lipolytica to ensure maximum growth and lipid productivity on OME was aimed. Y. lipolytica was cultured in different mediums that contain OME in different concentrations, with/without ammonium and/or yeast extract addition to determine the optimum conditions for growth, and also lipid productivity. In addition to this, pollutant removal efficiencies were evaluated. In all trials, the YPD medium was also used for cultivation as a witness. The pH value of all the mediums was adjusted to 5.8±0.1 before cultivation. During cultivation, the growth of Y. lipolytica was monitored by OD600 measurement continuously. Before cultivation and after cultivation, conventional parameters COD, TKN, NH3-N, TP, VSS were measured. Furthermore, protein, carbohydrate, lipid composition, and FAME composition determinations were conducted. The maximum COD removal efficiency was achieved in the mediums containing 40% OME with ammonium or yeast extract addition with a 77±2% ratio while this ratio was 64±4% in the YPD medium. The average TKN removal efficiency was 51% in the OME mediums. On the contrary, this value was 25±4% in the YPD medium. The maximum TKN removal efficiency was achieved in the 100% OME mediums with yeast extract addition reactors as 78±2% ratio. What is more, the NH3-N removal efficiency of all media was 62% on average. While maximum and minimum ratios were 79±6% and 46±3.5% respectively. The average NH3-N removal ratio was 76±4% in the YPD medium. When ortho-p removal performance was examined, it can be said that the average removal efficiency was 65% in the OME mediums. This value was 87±5% in the YPD medium. The ortho-P removal performance of the 70%-100%+N, 100% OME in different COD/N ratios, and 100% OME with different yeast extract addition batches was higher than the average of all the other reactors while the maximum ortho-P removal efficiency was observed in this mentioned sets as 87±5%. The maximum net VSS value was observed as 7.2±0.5 g/L in 100 % OME with COD/N:75 medium. While average net VSS values were 7.5±0.5 g/L in YPD mediums. The average lipid value of the biomass obtained from OME mediums was 32.9%±1.6 while this value varied between 17.2%±3.6 – 59.7 %±1.3 as a minimum and maximum values respectively. The average lipid value of the biomass cultured in the YPD medium was 25.9±0.5%. The best lipid content was obtained in 100% OME mediums with no addition. It was observed that 100% OME medium without any dilution is appropriate for lipid accumulation of Y. lipolytica. Then it was followed by nitrogen-limited media with different COD/N ratios. Since the original 100% medium was nitrogen-limited with COD/N = 118 ratio, the results were mutually supportive. Although nitrogen deficiency has a negative effect on microbial growth, it triggers lipid storage capacity. The experimental results support this information. Growth of the Y. lipolytica was increased in the trials enriched with ammonium. On the contrary, the maximum lipid accumulation was achieved in the cultured from under the stress condition medium in terms of nitrogen. The produced net biomass in volatile suspended solid concentration, and lipid storage ratio of the bio culture grown on all the OME mediums were examined. In this view, both produced biomass, and lipid storage was quite satisfactory in the COD/N:75 mediums. The maximum lipid yield in terms of sufficient biomass and lipid accumulation was achieved in the COD/N mediums with 7.2±0.5 g VSS/L biomass and 49.7±1.7% lipid content. These results showed that Y. lipolytica has the ability to grow on the OME and store a sufficient amount of lipid while metabolizing the OME. FAME compositions were examined. The dominant FAME species were C18:1 and C16:1. Addition to this, C18:0, C16:0, C14:0, C15:1, C17:0, C17:1, C18:2, C18:3, C20:0, C20:1, C21:0 was also observed in a small portion. It can be said that for all trials of OME mediums, the composition of saturated fatty acid (SFA) is lower than the unsaturated fatty acids (MUFA + PUFA). The maximum SFA obtained from OME mediums was 35±1.5%. Furthermore, the total ratio of MUFA and PUFA produced in the OME mediums varied between 65% to 96% with an average value of 86%. Unsaturated fatty acid produced in YPD medium was averagely 96% which are not efficacious results. However, even the results were unqualified regarding the desired biodiesel quality, these results were in the same way as the previous studies conducted with Y. lipolytica and/or OMEs in the literature. Overall, after all the evaluations, it can be inferred that even though the suitability of OME for growth of Y. lipolytica in a sufficient way was proved, the obtained fatty acid from the studied OME mediums has not satisfactory quality in terms of the biodiesel usage. To improve biodiesel quality, further studies may be conducted.