Microalgae growth in industrial wastewater for the production of hydrocarbons

Abstract

Microalgae have demonstrated unique abilities to photosynthesise the conversion of biodegradable organic materials and inorganic carbon to value-added biomass because dissolved nitrogen and reactive phosphate are present in the cultivation medium. The absence of a breakthrough in biomass production that would enable it to meet and exceed the existing fossil energy demand has elicited research into technologies and protocols that would yield competing energy output. The financial and energy implications associated with the technology employed for biomass harvesting would significantly contribute to the overall cost of the process. Would the microalgae strains that exhibit high growth rates and lipid content, as well as accommodate culture conditions, enhance biomass and lipid productivity? The goal of this study was to provide microalgae with nutrients from industrial wastewater while also producing hydrocarbon compounds that could have positive social effects. A tailored airlift-raceway photobioreactor was utilised to grow microalgae in industrial wastewater after the wastewater was characterised and the optimum conditions for microalgae development were investigated. The resulting production of hydrocarbon derivatives was optimised. Wastewater from the sugar refinery, brewing industry, and dairy industry was characterised by its physical, chemical, mineral, and biological properties using conventional methods. The different industrial wastewater sources were tested for microalgal growth rate and biomass output. The generated biomass was assayed for carbohydrates, lipids, and protein contents of the microalgae strains, and the wastewater that gave the highest biomass and lipid yields was used for advanced cultivation techniques. After careful consideration, the brewery wastewater was found to be the most effective wastewater for microalgae growth and was thus selected for this investigation. Using a novel airlift-raceway photobioreactor system, Scenedesmus sp. biomass was produced in brewery wastewater using optimised conditions. Also, the biomass of a microalgae consortium, native to Durban, South Africa, was produced, leading to hydrocarbons and hydrocarbon derivatives using nutrient-enriched brewery wastewater. This study investigated these capabilities to sequester heavy metals and other pollutants from brewery wastewater and sparged carbon dioxide gas. The light was sourced from 40 W fluorescent tubes, which were powered by a 210 V supply and used at different electromagnetic frequencies ranging from red to blue in a novel airlift raceway system for microalgae cultivation. The microalgal biomass, which was harvested by filtration, was freeze-dried and the surface morphology was analysed using the scanning electron microscope (SEM). The microalgal lipid was extracted with a hexane-methanol solvent system by the soxhlet technique. The morphology of the extracted biomass was analysed using SEM, and the composition of the microalgae oil was analysed using gas chromatography-mass spectrometry (GC-MS). Investigations revealed that the sugar wastewater (SWW) used did not support microalgal growth. However, dairy wastewater (DWW) only supported microalgal growth to some extent, while brewery wastewater (BWW) was best suited for the growth of Scenedesmus sp. and the microalgae consortium. The BWW was nutrients enriched through the oxidation pond, thus raising the influent NO3 - -N (4.98±0.13 mg/L), PO4 3- (13.34±0.48), BOD (35±19), and COD (3979±3) to NO3 - -N (15.98±0.91), PO4 3- (39.93±1.83), BOD (279±10), and COD (5855±4), respectively. GC-MS analysis of the oil extract of the microalgae biomass showed the presence of saturated, monounsaturated (MU), and polyunsaturated (PU) fatty acids in both Scenedesmus sp. and the microalgae consortium, and the presence of an isolated C4 iv and C8-C38 hydrocarbons and hydrocarbon derivatives, mostly fatty acid esters, in the microalgal oils. Nutrient enrichment of the brewery wastewater enables microalgal growth sustainably, thus encouraging lipid accumulation. Using the novel airlift-raceway photobioreactor in this study changed the mass transfer dynamics due to the enhanced hydrodynamics of the novel reactor. Because of this, it was simpler for light and nutrients to reach every area equitably, which is what propels the formation of biomass. The dominance of fatty acid esters in the microalgal oil demonstrates that the protocols adopted in this study can serve to save on the cost of the transesterification step in the production of biodiesel and other useful bio-products. This serves as a major contribution to the body of knowledge on this subject.