The WCI is calculated using Equation (15). Figure 3 plots the second — order water intensity of transportation (consumption and withdrawal) us­ing algal biofuels produced in this system (bio-oil and methane) for the two cases considered. These data are shown alongside equivalent results for a variety of transportation fuels, including fossil fuels, electricity for electric vehicles, and biofuels reported previously by King and Webber [33] (note the logarithmic scales).

As shown, the Experimental Case water intensity (which includes sig­nificant research-scale artifacts, no recycling, evaporation from the ponds, and relatively low biofuel yields) far exceeds any of the other transpor­tation fuels. Meanwhile, the Highly Productive Case water consumption intensity is lower than that of biofuels from irrigated crops, while its water withdrawal intensity is similar to, or slightly greater than, that of biofuels from irrigated crops. Still, the Highly Productive Case, which assumes very efficient water use (no evaporation and 95% recycling), is much more water intensive than traditional fossil fuels or non-irrigated biofuels from conventional feedstocks. While the WCI and WWI metrics are useful to evaluate the magnitude of water required for fuel production, they do not consider water quality (that is, algae can be grown in degraded, brackish, or saline sources, for which the concerns about water quantity are muted as compared with freshwater). The relationship between water require­ments (considering magnitude and quality) and water availability (includ­ing precipitation, which is not considered here for the algal biofuel cases) is more important than the water intensity, alone. However, this relation­ship is dependent on location and must be evaluated on a case-by-case, site-specific basis for all of the fuels shown.

Several other studies have been conducted to determine the water in­tensity of algal biofuel production and the system boundaries used in each study vary [9,11,15,17,54,55]. Analogous to energy inputs, the water in­puts for a production pathway include direct and indirect parts. Addition­ally, the water consumption required to produce capital equipment can be included (e. g., water required for producing glass bioreactors [54]). Finally, the water intensity is dependent on co-product allocation, as the

FIGURE 3: Second-order water intensity of transportation for several fuels [33], including the bio-oil and methane co-products from the two algal biofuel cases: the Experimental Case and the Highly’ Productive Case. *Note the logarithmic scale. To evaluate sustainability, the water intensity and required water quality must be considered in conjunction with water availability.

total water consumed to operate the production pathway should be allo­cated between the bio-oil and co-products (e. g., methane).

The WWI is calculated according to Equation (16). The WWI for the Experimental Case and the Highly Productive Case are 87,000 L/km and 220 L/km, respectively. Like the nutrient analysis presented below, the water analysis underscores the advantages of using nutrient-rich low-qual­ity water, like waste water or agricultural runoff [28]. In these cases, the incremental water usage is minimized and the discharge water can be of higher quality (e. g., higher purity) than the water input.