Avoiding Conflicts between Future Freshwater Algae Production and Water Scarcity in the United States at the Energy-Water Nexus

Abstract

Sustainable production of algae will depend on understanding trade-offs at the energy-water nexus. Algal biofuels promise to improve the environmental sustainability profile of renewable energy along most dimensions. In this assessment of potential US freshwater production, we assumed sustainable production along the carbon dimension by simulating placement of open ponds away from high-carbon-stock lands (forest, grassland, and wetland) and near sources of waste CO 2 . Along the water dimension, we quantified trade-offs between water scarcity and production for an ‘upstream’ indicator (measuring minimum water supply) and a ‘downstream’ indicator (measuring impacts on rivers). For the upstream indicator, we developed a visualization tool to evaluate algae production for different thresholds for water surplus. We hypothesized that maintaining a minimum seasonal water surplus would also protect river habitat for aquatic biota. Our study confirmed that ensuring surplus water also reduced the duration of low-flow events, but only above a threshold. We also observed a trade-off between algal production and the duration of low-flow events in streams. These results can help to guide the choice of basin-specific sustainability targets to avoid conflicts with competing water users at this energy-water nexus. Where conflicts emerge, alternative water sources or enclosed photobioreactors may be needed for algae cultivation.

Balancing Water Sustainability and Productivity Objectives in Microalgae Cultivation: Siting Open Ponds by Considering Seasonal Water-Stress Impact Using AWARE-US

“…Using an earlier version of the BAT model, Jager et al 18 estimated that algae biomass potential is about 24.8 Tg yr −1 , which is lower than our estimate, owing to differences in the number of potential sites and algae strains considered. Compared to annual-based (CF annual ) or monthly based (CF monthly ) water-stress constraint alone, our results indicate that screening sites using a hybrid approach can achieve a better balance between productivity and water savings (Figure 5).…”

“…In these arid regions, if saline water is available, cultivation of saline strains can be an alternative strategy to reduce freshwater demand. Using an earlier version of the BAT model, Jager et al estimated that algae biomass potential is about 24.8 Tg yr –1 , which is lower than our estimate, owing to differences in the number of potential sites and algae strains considered.…”

“…18,23 Previous studies have often set a target of using less than 5% of the long-term mean annual flow per watershed for sustainable algae cultivation. 10,11,20−22 More recently, Jager et al 18 evaluated algal biofuel potential under a set of minimum seasonal flow constraints.…”

“…Recent studies have suggested that the competition for water resources is likely to increase in areas like the high plains and western U.S., , owing to factors like warmer climate, a growing economy, and instream flow restoration. For freshwater algae, the water demand of open-pond algae biomass production varies significantly across regions, depending on local climate. , Several resource analyses have assessed the U.S. algal biofuel potential with consideration of freshwater availability and other factors (e.g., growth rate, CO 2 resources, infrastructure), ,,− but only a few have evaluated the water-stress impact. , Previous studies have often set a target of using less than 5% of the long-term mean annual flow per watershed for sustainable algae cultivation. ,,− More recently, Jager et al evaluated algal biofuel potential under a set of minimum seasonal flow constraints.…”

“…10,17 Several resource analyses have assessed the U.S. algal biofuel potential with consideration of freshwater availability and other factors (e.g., growth rate, CO 2 resources, infrastructure), 10,11,18−22 but only a few have evaluated the water-stress impact. 18,23 Previous studies have often set a target of using less than 5% of the long-term mean annual flow per watershed for sustainable algae cultivation. 10,11,20−22 More recently, Jager et al 18 evaluated algal biofuel potential under a set of minimum seasonal flow constraints.…”