Microalgae for Bioremediation of Distillery Effluent

Abstract

Microalgae are single-celled autotrophic photosynthetic microorganisms, and constitute a heterogeneous group of unicellular microorganisms. Microalgae are valuable sources of food and feed products, high-value oils, biofuels, chemicals, medicinal products and pigments. Microalgae are potent candidates for bioremediation of a large number of pollutants. Distillery effluents, also referred to as spentwash/stillage/slop/vinasses, are one of the most environmentally aggressive industrial effluents. Distillery effluents often have low pH, strong odor, dark brown color, and extremely high nutrients content. They often have chemical oxygen demand (COD) ranging from 80,000 to 100,000 mg L⁻¹; biological oxygen demand (BOD) of 40,000–50,000 mg L⁻¹; and nutrients like nitrogen of 1,660–4,200 mg L⁻¹; phosphorus of 225–3,038 mg L⁻¹; and potassium of 9,600–17,475 mg L⁻¹. With the development of economies and resultant growth of distillery industries, large volume of spentwash is produced which is likely to cause extensive soil and water pollution due to the presence of high amount of organic matter and dark brown colored recalcitrant compounds. There have been many isolated studies for treatment of distillery effluents and related compounds using microalgae. This review tries to weave these isolated studies in a string to reflect a clear picture of the utility of microalgae in bioremediation of distillery effluents. In view of the wider applicability of microalgal strains in remediation of domestic wastewaters, inorganic nutrients like nitrogen and phosphorus, heavy metals, pesticides, phenols, aromatic hydrocarbons, textile dyes, and detergents; we reviewed the potentiality of these unicellular microorganisms for bioremediation of distillery effluents.

For treatment of wastewaters native microalgal strains are a favorable alternative to the traditional wastewaters treatment systems. The traditional physico-chemical methods of waste water treatment are costly, energy expensive, environmentally unfriendly, unsustainable, and have tendency to form toxic intermediates. Moreover, the anaerobic degradation of aromatic amino acids by many heterotrophic bacteria and fungi may further aggravate the pollution problem due to further liberation of phenol and cresol. However, biological processes using microorganisms are comparatively less expensive and can lead to almost complete mineralization of the compounds. The environment friendly approach of activated sludge process using consortium of microorganisms is regarded more efficient for mineralization of toxic organic compounds. A dark brown recalcitrant pigment called melanoidin, formed in the molasses as a result of Maillard reaction, can form stable complexes with metal cations. Oscillatoria boryana can utilize melanoidins as a carbon and nitrogen source; and decolorize pure melanoidins by about 75% and crude pigment by 60% in 30 days. A consortium of Oscillatoria, Lyngbya and Synechocystis decolorized melanoidin by 98% by absorption followed subsequently by degradation of the organic compounds. The microalgal strains like Anabaena cylindrica, Phormidium foveolarum, P. valderianum, Synechococcus, Ankistrodesmus braunii and Scenedesmus quadricauda have been reported instrumental in degradation of phenol and its derivatives, whereas the performances of Phormidium ambiguum, Chroococcus minutus, Oscillatoria, and Anabaena azollae were found satisfactory for degradation of lignin. Phormidium ambiguum and Chroococcus minutus were found to reduce lignin by over 73.0% from the pulp and paper mill wastes in 5 days; whereas Phormidium, Oscillatoria, and Anabaena azollae were able to degrade lignin by 89% and hemicellulose by 92% from coir waste.

The ability of microalgae to grow under mixotrophic growth conditions enable them to survive under low light or carbon dioxide shortage and represents an alternative to other biological treatments for remediation of phenol-containing wastewaters. The microalgae carry out ortho-fission of the phenolic substances extracellularly in the dark. This reaction is catalyzed by a protein; however, such transformation is inhibited by heat, proteases and metal ions. Ochromonas danica can enzymatically carry out meta-cleavage of phenol and its methylated homologues and the compounds produced are metabolized as intermediates of the Krebs cycle. Several species of cyanobacteria are also known to possess phenol-degrading enzymes like lignin peroxidase, laccase, polyphenol oxidase, superoxide dismutase, catalase, peroxidase and ascorbate peroxidise. The lignolytic and anti-oxidative enzymatic activities increases in the presence of phenol, if microalgae are subjected to nitrogen limiting condition. Moreover the photosynthetic nature of the microalgae enable them to produce toxic active oxygen species like O^2–, OH⁻, and H₂O₂ which have strong oxidising agent and are involved in degradation of melanoidin. Presence of molecular oxygen is indispensable for enzymatic breakdown of aromatic ring of phenols by microalgae; which involves hydroxylation of the aromatic ring and formation of catechol followed subsequently by oxidation. Moreover, the microalgal growth and the rate of biodegradation can be enhanced under increased light intensities and by addition of carbon dioxide and sodium bicarbonate. The microalgae offers spectacular prospects of their use in bioremediation because of their ubiquitous distribution, cost efficient, central role in nitrogen fixation, turnover of carbon and other nutrient elements, almost complete mineralization of compounds, and ability to scavenge nutrients.