6.1.1 Reference Pathway–Raceway Pond Producing Drop-in Hydrocarbon
Most algae for commercial products have been cultivated in open-pond systems because of their low costs compared to photobioreactors (Earthrise Nutritional, 2009; Milledge, 2011). Ensuring a high level of productivity of the desired algal species also could improve economic viability and reduce resource use and environmental effects per unit of fuel produced. Some of the key concerns for resource use and environmental sustainability include:
Availability of suitable land for installing large ponds for algae cultivation.
• Evaporative loss of water from ponds, particularly in arid regions with low rainfall.
• Social perception and acceptance. They could be a key barrier if genetically modified organisms are to be cultivated in open ponds.
In the reference pathway, the nitrogen (N) and phosphorus (P) requirements are not a key sustainability concern because the lipid-extracted algae undergo anaerobic digestion to produce energy and these nutrients are returned to the algal culture. Energy generation from anaerobic digestion contributes to reducing energy input and hence GHG emissions. Other potential concerns that could be avoided if care is taken to maintain the algal cultures and the cultivation ponds include:
• Ground and surface water pollution.
• Presence of waterborne toxicants from contaminants.
• Potential for increasing mosquito-breeding grounds if ponds are not properly managed.
Some of the unknowns with respect to environmental sustainability include:
• Emissions of air pollutants from open ponds, which could be monitored to determine the extent of such emissions.
• Effects on terrestrial and aquatic biodiversity, but such effects could not be assessed unless the site of deployment for the algal biofuel production system and the cultivation system to be used are known.
• Site-dependent effect of open ponds on local climate.
The air quality emissions associated with drying, extraction, and processing to fuels could be mitigated by engineering solutions, particularly if most steps are performed indoors. Technology improvements in those steps and in harvesting could reduce energy use and hence reduce GHG emissions. The reference pathway produces a drop-in biofuel that can be used in the existing fuel distribution and vehicle infrastructure.
6.1.2 Alternative Pathway #1–Raceway Pond Producing Drop-in Hydrocarbon and Coproducts
The ability to meet various sustainability goals and the potential concerns for this pathway are similar to the reference case. The only difference lies in the production of coproducts other than energy from anaerobic digestion. That change could affect energy