SYSTEM B: HETEROTROPHIC
System B is a heterotrophic system in which algae use digestible organic carbon (acetate, glucose, glycerol) as a carbon source to produce lipids, instead of CO2 and sunlight (Lee, 2011). Such carbon sources may be derived from effluents from the agricultural, dairy, and sugar industries.
In this study, the carbon source is acetate that is derived from switchgrass.
Switchgrass is chosen because it would be valuable to know what the impacts would be if the feedstock were specifically cultivated for feeding algae instead of using organic wastes. Cultivation of lignocellulosic feedstocks, such as miscanthus, switchgrass, convolvulus, etc. has been discussed and presented as an alternative pathway for the production of algal biofuel by Solazyme (Solazyme, 2011). The growth media is freshwater and nutrients with no evaporation loss, because it is a closed system. The spent water is sent to a water treatment facility before being discharged into surface water, so there is no recycling of water or nutrients. Further discussion of the implications of not recycling water or nutrients and using a water treatment facility is in the Potential Environmental Impacts, Water and Land Permitting Concerns, and Wastewater Treatment sections of this report. Heterotrophic algae produce CO2 and hence do not sequester it; up to 70% of the carbon consumed as substrate is lost to respiration, cell maintenance, lysis, etc., resulting in actual yield of about 30%, which is comparable to other heterotrophic microbes (Lee, 2011). The algae are grown in large bioreactors in the absence of O2, to allow fermentation to occur.
While
heterotrophic algae can grow in aerobic conditions, this is not considered in this study because data to model such a system is unavailable and a major algae biofuel company, Sapphire Inc., 21