Effects of CO2 on South African fresh water microalgae growth

Abstract

South Africa obtains over 80% of its energy needs from fossil fuels. Latest research indicate that South Africa emits over 500 million tons of carbon dioxide (CO2) per year and is therefore ranked number eight as the world's CO2 emitter. Unabated or controlled emission of CO2 and other green house gases has largely contributed to detrimental global warming and adverse climatic change. Microalgae are capable of converting hazardous CO2 into valuable biomass. A high CO2 tolerating microalgae was collected from Johannesburg Zoo Lake and identified as Desmodemus sp. Batch cultures were grown in 1000 mL. Erlenmeyer flasks. CO2 was bubbled into the microalgae culture media. The media contained optimal nutrients, an optimal photoperiod and light intensity and controlled pH. CO2 concentrations of 100%, 50%, 25%, 10%, and 5% and total gas flow rates of 20, 50 and 100 mL/min were used. The aim of this study was to determine the effective flow rate and CO2 concentration that gave optimal microalgae growth. Microalgae dry mass contain 50% carbon and carbon is known to be a limiting factor when all other nutrients and environmental conditions are presents. When atmospheric air was supplied at 50 mL/min the growth rate was 0.1151 per day and it increased drastically by almost 5 folds when 100% CO2 was supplied at 50 mL/min. Using a gas flow rate of 20 mL/min and 10% CO2, growth rate increased to 1.42 per day with a dry biomass yield of 809.96 mg/L after 12 days of growth. At a gas flow rate of 50 mL/min, 5% CO2 and the highest growth rate was 1.993 per day and an overall biomass yield of 1200 mg/L and the average pH was 6.36. At 100% CO2 the growth rate was 1.265 per day and dry biomass yield obtained was 469.81 mg/L while the average pH was 5.12. When gas flow rate was increased to 100 mL/min using 5% CO2 the growth rate slightly reduced to 1.30 per day with a dry biomass yield of 1000 mg/L. A growth rate was 0.34 and dry biomass yield of 279.47 mg/L at an average pH of 5.09 were achieved at 100% CO2. Higher flow rates and higher concentrations resulted in slightly reduced growth due to low pH. These investigations indicate that the species under study is CO2 tolerant and it presents a viable CO2 abatement strategy. © 2011 American Institute of Chemical Engineers Environ Prog, 2012