Chlorella vulgaris growth on anaerobically digested sugarcane vinasse: influence of turbidity

Mayara L Serejo,Marc À Boncz,Graziele Ruas,Paula L Paulo,Gabriel B Braga

doi:10.1590/0001-3765202120190084

Abstract

This paper shows the influence of turbidity (in Nephelometric Turbidity Units - NTU), chemical oxygen demand (COD) and aeration (CO2 supply) on the productivity and growth rate and lipid content of microalgae (a mixed culture predominantly composed of Chlorella vulgaris), using anaerobically digested vinasse as a culture medium. The microalgae can be cultivated in anaerobically digested vinasse, at turbidity and chemical oxygen demand of 690 NTU and 2.5 gCOD L -1, respectively, according to the modified Gompertz model, and removal of turbidity by filtration did not influence the microalgae productivity (≈ 77 mg L1 d1). Furthermore, aeration increased the productivity up to 139 mg L1 d1, with a biomass dry weight of 2.7 g L-1. Finally, a maximum lipid content of 265 mg L -1 was obtained, while a nitrogen removal of 98% was recorded for all conditions. Thus, the combination of anaerobic digestion followed by the use of the digestate for the cultivation of microalgae may be an efficient way to treat large quantities of this residue, in turn yielding large amounts of microalgae biomass, which can be transformed into fertilizer and biofuel.

Highlights

Microalgae cultivation is considered a promising process for obtaining products of great interest, such as biofuels, as well as for producing raw materials for the cosmetic, food and pharmaceutical industries, as well as for the bioremediation of heavy metals, pathogens and organic pollutants in wastewater (Munõz & Guieysse 2006, Christenson & Sims 2011)
Digested vinasse as a growing medium According to the chemical characteristics of the anaerobically digested vinasse (ADV) (Table II), the estimated C/N ratios for ADV-NFT, ADV-FT and ADV concentration was aerated (ADV-AE) ranged from 21 to 27, suggesting a lack of nitrogen for all
A microalgae consortium (97% of Chlorella vulgaris) shows similar productivity of 77 and 76 mg L1 d1 for undiluted ADV-non-filtered and ADV-filtered, respectively, which suggests that the ADV turbidity removal is not necessary

Summary

Introduction

Microalgae cultivation is considered a promising process for obtaining products of great interest, such as biofuels (hydrogen, methane, bioethanol and biodiesel), as well as for producing raw materials for the cosmetic, food and pharmaceutical industries, as well as for the bioremediation of heavy metals, pathogens and organic pollutants in wastewater (Munõz & Guieysse 2006, Christenson & Sims 2011). When compared to conventional terrestrial crops, the mixotrophic and photo-autotrophic cultivation of microalgae has advantages due to their fast growth and the fact that the cultivation neither requires arable land nor competes with food production This process does not require large amounts of fresh water, since the microalgae can be grown in saline water or domestic and industrial wastewater like pig manure, distilleries, dairy, fish and cassava processing, among others (Muñoz & Guieysse 2006, Ji et al 2013, Marques et al 2013, Posadas et al 2014). Mixotrophic microalgae require light, inorganic and organic carbon, water and nutrients (macro and micro) for growth; the latter can be provided by organic wastes, such as the vinasse produced in ethanol distilleries This wastewater has a low pH (3.55.0) and a high chemical oxygen demand (COD) of ≈ 30 g L-1 on average (reaching up to 150 g L-1), which can be harmful to the soil and groundwater when applied for fertirrigation. The high turbidity from vinasse can cause a shading effect, decreasing the photosynthetic activity by limiting light availability (Escudero et al 2014)

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Conclusion