Use of microalga and filamentous fungi in the treatment of industrial effluents (whey and petroleum produced water)

Abstract

This work aimed to obtain information regarding the use of microalgae and the cocultivation of microalgae with other microorganisms (bacteria, filamentous fungi and yeasts) in the treatment of wastewater, specifically the dairy wastewater, and petroleum produced water. For this, a literature review regarding open system cultivation revealed information about the ability to remove contaminants (mainly Chemical Oxygen Demand (COD), Total Nitrogen (TN) and Total Phosphorus (TP)) present in industrial effluents by microalgae and their consortia with other microbial groups. Microalgae-bacteria consortia are used mainly due to the great capacity to remove organic matter from bacteria and better assimilation of nitrogen and phosphorus by microalgae. On the other hand, lines of research using the consortium of microalgae with yeasts and filamentous fungi have gained attention, first because yeasts can accumulate a high lipid content, as well as microalgae, and can therefore be used, for example, in the production of biodiesel, and in the case of filamentous fungi to increase the capacity of effluent treatment containing molecules complex organic compounds (the metabolic power of fungi is superior to that of microalgae) and helps in the process of harvesting microalgal biomass. But it is recognized that the biotechnological applications for these two groups can be expanded, therefore, research is necessary. The type of bioreactor and mode of operation significantly influence the effluent treatment process using microalgae. Thus, a review of the operating modes showed that batch and fed-batch cultivations present lower risks of contamination, while the continuous and semi-continuous modes have higher productivity rates. Combining the use of effluents, reactors and mode of operation in conjoint with the nutritional and environmental requirements, can reach removal rates for COD, nitrogen and phosphorus greater than 90%. After, a kinetic model was developed to describe simultaneous removal of organic carbon, nitrogen and phosphorus, and microbial growth in wastewater treatment using microalgae, applying the n-order model for contaminant consumption, Monod (a limiting substrate) and Silva and Cerqueira (multiple substrates) for microbial growth. The results demonstrate the ability of these models to predict the treatment behavior of different industrial effluents. Additionally, an experimental procedure was performed, in which the cultivation of the microalgae Tetradesmus obliquus in the treatment of whey in open ponds was evaluated through experiments carried out at different organic loads (0.5-4% v/v) and light intensities (25- 200 µmol m-2 s -1 ). It was possible to efficiently remove COD, nitrogen and phosphorus with rates greater than 80% at higher light intensities (25-200 µmol m-2 s -1 ) in all analysed organic loads, with emphasis on the lower ones (0.5 and 1% v/v) because they have a final concentration of contaminants in accordance with the legislation. In addition, a literature review on the biological treatment of oil produced water was carried out, and it was shown that there are few published works when compared to other effluents, mainly due to the characteristic of this wastewater, such as xenobiotic substances, high oil and grease content, and high salinity. In this sense, physical and chemical processes are more applied, although they are more effective when the treated water is used for reinjection in the wells, but due to the high salinity and the presence of toxic compounds, it is suggested the bioremediation of the effluent when the water produced it can be used for other noble purposes such as irrigation. Studies using bacteria, microalgae, filamentous fungi and yeasts were found, the first one being the most prominent, but showing the potential and need for further studies with other microbial groups. In this sense, experiments were conducted to treat produced water in a bubble column reactor using the co-culture of microalgae (Tetradesmus obliquus) and filamentous fungi (Aspergillus niger, Penicillium oxalicum and Cunninghamella echinulata). The species C. echinulata achieved higher TOG removal rates (90-95%), initially with 312-2500 mg L-1 , being more efficient than microalgae for this parameter. At different salinity concentrations (5-50 g L-1 ), and T. obliquus remained alive up to 25 g L-1 , while the fungus C. echinulata grew at all salinity concentrations and removed TOG at rates between (80-95%). Finally, the co-culture of T. obliquus-C. echinulata removed up to 63.4 and 36.58% of nitrogen and phosphorus, with initial concentrations between 50-150 and 30 mg L-1 , respectively.