The Highly Productive Case is an analytical model that was constructed to represent a system with greater biomass productivity (80 mg/L-d) and a higher neutral lipid fraction (30%) than the Experimental Case (which had productivity of 2 mg/L-d and lipid fraction of 2%). The Highly Produc­tive Case assumes the same basic production pathway as the Experimental Case, but it substitutes bioreactors for growth and an advanced floccula­tion technique in place of centrifugation. In addition, several modifica­tions are modeled to improve energy efficiency in the Highly Productive Case. In addition, it is assumed that there is no water loss from evapora­tion and 95% of the water used for cultivation was recycled. In this sense, the Highly Productive Case is an optimistic, but not wholly unreasonable, scenario for achieving low operating expense in commercial-scale algal biofuel production based on current technologies. The less optimistic as­sumption is the requirement of “full-price” inputs (such as nitrogen fertil­izer and carbon dioxide from ammonia production plants). The ability to achieve each of the specified conditions in the Highly Productive Case in
practice is assumed to be possible and the capital required to do so is not considered in this study. Each assumption used in the Highly Productive Case is compared with those from several literature sources in the “Dis­cussion” section, below.

The ability to utilize discounted inputs, such as waste forms of carbon, nitrogen, and phosphorus and cheap energy inputs, would further improve the return on investment for producing algal biofuels with respect to the Highly Productive Case [11,14,25-28]. The Highly Productive Case is not intended to represent the optimum scenario for algal biofuels nor is it presented as the final arbiter of the fuel’s prospects for success; rather, it is intended to serve as a useful benchmark. The optimum scenario might utilize discounted inputs, high productivity algal strains (e. g., genetically modified organisms), and improved growth, processing, and harvesting methods that might be developed in the future. Instead, the Highly Produc­tive Case models a similar production pathway as the Experimental Case, but with significantly higher fuel productivity and significantly more ef­ficient growth and processing methods.