Abstract
Microalgae are promising feedstock to produce biodiesel and other value added products. However, the water footprint for producing microalgal biodiesel is enormous and would put a strain on the water resources of water stressed countries like South Africa if freshwater is used without recycling. This study evaluates the utilization of pre-chlorinated wastewater as a cheap growth media for microalgal biomass propagation with the aim of producing biodiesel whilst simultaneously remediating the wastewater. Wastewater was collected from two wastewater treatment plants (WWTPs) in Durban, inoculated with Neochloris aquatica and Asterarcys quadricellulare and the growth kinetics monitored for a period of 8 days. The physicochemical parameters; including chemical oxygen demand (COD), total nitrogen (TN), and total phosphorus (TP) were determined before microalgal cultivation and after harvesting. Total lipids were quantified gravimetrically after extraction by hexane/isopropanol (3:2 v/v). Biodiesel was produced by transesterification and characterised by gas chromatography. The total carbohydrate was extracted by acid hydrolysis and quantified by spectrophotometric method based on aldehyde functional group derivatization. Asterarcys quadricellulare utilized the wastewater for growth and reduced the COD of the wastewater effluent from the Umbilo WWTP by 12.4%. Total nitrogen (TN) and phosphorus (TP) were reduced by 48% and 50% respectively by Asterarcys quadricellulare cultivated in sterile wastewater while, Neochloris reduced the TP by 37% and TN by 29%. Although the highest biomass yield (460 mg dry weight) was obtained for Asterarcys, the highest amount of lipid (14.85 ± 1.63 mg L−1) and carbohydrate (14.84 ± 0.1 mg L−1) content were recorded in Neochloris aquatica. The dominant fatty acids in the microalgae were palmitic acid (C16:0), stearic acid (C18:0) and oleic acid (C18:1). The biodiesel produced was determined to be of good quality with high oxidation stability and low viscosity, and conformed to the American society for testing and materials (ASTM) guidelines.
Highlights
Microalgae are photoautotrophic organisms utilizing sunlight and carbon dioxide to generate energy which is stored as lipids
The salinity of the wastewater from the Northern wastewater treatment works (NWWTW) increased after treatment from an initial value of 0.38 up to 0.43 in the unsterilized wastewater inoculated with Chlorella spp., while at the Umbilo wastewater treatment plant (UWWTP), salinity was relatively stable and unchanged (Table 1)
At the UWWTP, total phosphorus (TP) and Total nitrogen (TN) were reduced by 32% and 44%, respectively, by Asterarcys quadricellulare cultivated in sterile wastewater while 29%
Summary
Microalgae are photoautotrophic organisms utilizing sunlight and carbon dioxide to generate energy which is stored as lipids. It has a large water footprint that is unsustainable for a semi-arid and water stressed country like South Africa, with low average rainfall of 465 mm, which is below the global average of 860 mm [2]. Demand for this important scarce resource is expected to increase due to rapid industrial development, increasing human population, per capita consumption increase, and the resulting impact of human activities on the environment [3]
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