Effects of CO 2 on South African fresh water microalgae growth

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 (CO 2 ) per year and is therefore ranked number eight as the world's CO 2 emitter. Unabated or controlled emission of CO 2 and other green house gases has largely contributed to detrimental global warming and adverse climatic change. Microalgae are capable of converting hazardous CO 2 into valuable biomass. A high CO 2 tolerating microalgae was collected from Johannesburg Zoo Lake and identified as Desmodemus sp. Batch cultures were grown in 1000 mL. Erlenmeyer flasks. CO 2 was bubbled into the microalgae culture media. The media contained optimal nutrients, an optimal photoperiod and light intensity and controlled pH. CO 2 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 CO 2 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% CO 2 was supplied at 50 mL/min. Using a gas flow rate of 20 mL/min and 10% CO 2 , 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% CO 2 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% CO 2 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% CO 2 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% CO 2 . Higher flow rates and higher concentrations resulted in slightly reduced growth due to low pH. These investigations indicate that the species under study is CO 2 tolerant and it presents a viable CO 2 abatement strategy. © 2011 American Institute of Chemical Engineers Environ Prog, 2012