A biorefinery for valorization of industrial waste-water and flue gas by microalgae for waste mitigation, carbon-dioxide sequestration and algal biomass production

Abstract

Massive industrialization all over the globe is the main cause for the generation of huge quantity of wastes such as flue gas and wastewaters. Mindless release of these hazardous wastes into the environment is threatening the health and survival of the mankind. Judicious use of these wastes for microalgal biomass cultivation is recognized as a plausible approach for the creation of a renewable and sustainable process chain for biofuel production. This study was designed to cultivate microalgae utilizing the organic and inorganic nutrients from the industrial wastewater (IWW) and coal-fired flue gas (FG) for simultaneous waste bio-remediation and biomass production for biorefinery application in closed photobioreactors. The two microalgae, Chlorella sp. and Chlorococcum sp. were cultivated in industrial wastewater where varying concentrations of coal-fired FG from 1 to 10% CO₂, volume/volume percent (v/v) was supplied to stimulate the mixotrophic growth. Performance of the two microalgae was evaluated in terms of nutrient removal (ammonium, nitrate, phosphate and COD), CO₂ fixation, total lipid and carbohydrate content obtained in the integrated mode of process development. The IWW with flue gas (5% CO₂ (v/v)) resulted in maximum growth and CO₂ fixation. The highest biomass growth (1.52 g L^-1) and CO₂ fixation (187.65 mg L^-1 d^-1) of Chlorella sp. with nutrient removal of >70% was observed by 5th day of batch cultivation. Nearly 90% removal of nitrogen resulted in nutrient limitation condition that steered the accumulation of lipid (17-34%) and carbohydrate (21.5-23.1%) in Chlorella and Chlorococcum sp. An overall 1.7 fold improvement in biomass was observed in this process integration compared with control culture. The present study presents a green process for waste remediation, CO₂ fixation and production of biomass rich in lipid & carbohydrate content for the development of a green microalgal biorefinery.

Keywords: CO(2) sequestration; Coal-fired flue gas; Microalgae cultivation; Process integration; Wastewater bio-remediation.