Increased concentrations of carbon dioxide (above atmospheric concen­tration) have been proven to improve the productivity [45-47] of algal cultivation. Production of synthetic CO2 however is too energy-intensive to generate and a source of waste carbon dioxide is required. Many studies have proven the advantages of using CO2 injection combined with algal cultivation [45, 47, 50, 51, 99]. As producing CO2 synthetically is not sus­tainable, it is necessary for an existing source of CO2 to be situated near to the algae growth ponds. Researchers have considered the plant flue gas from coal-fired power stations as an ideal source of CO2 [48, 50] and flue gases have been shown to be successful as a source of CO2. Nevertheless barriers would need to be overcome to implement the concept in a scaled — up system. It is evident from literature that CO2 concentrations that are too high (above 15%) will cause a decrease in biomass productivity and potentially death of the cells. This may limit the number of possibilities for use of flue gas, although it must be noted that generally flue gases contain CO2 concentrations lower than this [5]. It is not only the CO2 that could be lethal to the cells: other toxins may also negatively impact the biomass. SO2 can have a great impact upon the biomass and the pH of the water and high SO2 concentration cause the pH to drop to very low levels. pH can be adjusted using NaOH but this requires additional materials and energy. In addition, the temperature of flue gas is generally above that of normal culture conditions and is likely to be too high to allow biomass growth. Cooling would be necessary to reduce the temperature to an acceptable level thus requiring water and additional energy for pumping.

Clearly there are many issues related to the use of waste flue gas as a source of CO2 that must be addressed to allow implementation on a larger scale. It may be the case that transporting and treating flue gas prior to in­jection would require too much energy compared to the benefit that could be gained.