Based on the results of this study, targets can be set for producing algal bio­fuel that will enable a 2nd O EROI and PFROI equal to 1 (i. e., break-even) without exceeding water availability constraints or drastically increasing national fertilizer consumption. Since these targets are devised only with consideration of operating inputs, productivity would need to be increased and/or expenses would need to be decreased significantly to achieve an overall EROI and overall FROI (including capital costs) greater than 1 for the delivered energy carriers (a requirement for fuels to make a net ener­getic contribution), or greater than 3 (for practical purposes). The guiding targets for research stakeholders for comparison to the Experimental Case and the Highly Productive Case are:

1. Algal concentration of 3 g/L with a lipid fraction of 0.3, which would yield approximately 25 kJ of bio-oil and 25 kJ of methane per liter of processed volume (which is about 800 L BO/MLp and 450 kg methane/MLp, estimated to be roughly $600 of revenue per million liters of growth volume (assuming $0.66/L BO ($2.50/gal BO) and $3.8/GJ ($4/MMBtu) of methane));

2. In conjunction with item 1, an energy input for growth, processing, and refining that is less than 50 kJ per liter of processed volume enables a 2nd O EROI > 1 and requires using discounted inputs;

3. The FROI is dependent upon market prices, and therefore can vary substantially depending on market conditions (e. g., oil price). However, based on the price assumptions used in this study, if the targets listed above can be achieved, the PFROI would be greater than 1 if the cost of growth, processing, and refining is less than $600 per million liters of growth volume processed (which is equivalent to $0.20/kg of grown mass). Achieving a total cost less than $600 per million liters of growth volume processed would yield an overall FROI greater than 1 for this scenario (assuming no subsidy revenue);

4. A fresh water consumption intensity on the order of 2.4 L/km (1 gal/mi), achieved by consuming roughly 25 liters of fresh water per thousand liters of processed volume (which corresponds to no evaporation during growth, minimal processing water use, and greater than 97.5% recycling for fresh water cultivation). This con­sumption corresponds to about 33 liters of fresh water per liter of bio-oil produced (with a methane co-product of about 0.58 kg/L BO). Using saline water or waste water could also enable a low fresh water consumption intensity;

5. A net nutrient consumption that would enable large-scale produc­tion while only marginally increasing the national fertilizer con­sumption. For example, to produce 5 Bgal of fuel per year (19 GL/ yr), the net nitrogen consumption for each liter of fuel produced should be less than about 26 g to prevent a national increase in ni­trogen fertilizer consumption of more than 5% (which is about 6 x 108 kg N/yr [43]). In this scenario, one liter of bio-oil is produced from about 4 kg of algae, and therefore the nitrogen consumption should be less than about 7 g per kg of algae, which is roughly 10% of the minimum possible nitrogen requirement for algae (~70 g of nitrogen per kg of algae). Therefore, nitrogen recycling or utiliza­tion of waste nitrogen of 90% or more is required.