medium for essential nutrient resources—carbon dioxide (CO2), nitrogen (N), phosphorus (P), and other nutrients—for algal biomass production. Water has to be pumped to and contained and circulated in mass cultivation systems whether they involve either open ponds or closed photobioreactors. Closed photobioreactors also may use water spraying or submersion to maintain temperature of the culture. Given that the agricultural demand for water in the United States accounts for 85 percent of consumptive water use, large-scale production of biomass, including algae, has the potential for large regional strain on water systems unless nonfreshwater sources are used when possible.
Irrespective of the type of fuel produced, water is an integral element of fuel production, and thus an important nexus exists between fuel production and water supplies (Pate, 2007; Murphy and Allen, 2011). In the case of algal biofuel production, water is necessary for biomass feedstock production, and it can be lost during the processing of the algal biomass to fuels. This section discusses water requirement and consumptive use of fresh water along different steps of the algal biofuel supply chain and throughout the life cycle of algal biofuel production. In this report, water requirement refers to the quantity of water needed throughout the life cycle of algal biofuel production. Consumptive use of fresh water is the quantity of fresh water withdrawn from surface or groundwater sources that is lost to the immediate environment through evaporation or incorporation into products. In a sustainability assessment, the consumptive use of freshwater needs to be assessed in the context of regional water availability. For example, water withdrawn from a fossil aquifer that is declining quickly is less sustainable than the same amount of water withdrawn from an aquifer that replenishes more quickly. Where data are available, estimates for water use are compared to those for other biofuels and petroleum-based fuels.
4.1.1 Water Requirements in the Supply Chain
The water requirements of any algal cultivation system depend on the physical structure and configuration of the system, the local climate, and the ability to reclaim and reuse system water (Table 4-2; Murphy and Allen, 2011). Open ponds are subject to evaporative water losses (Yang et al., 2011) that are influenced by multiple factors including pond area, volume, and water level; water and air temperature; and wind velocity, humidity, and atmospheric pressure (Boyd and Gross, 2000). The average U.S. evaporation rate from a pond system is estimated to be 0.9 cubic meters of water per square meter per year (Murphy and Allen, 2011), but evaporative losses from open ponds vary by geographical region. Moreover, some operation regimes (for example, stirring and sparging) can increase the water loss to levels that are greater than would be predicted by evaporation and purging alone.
In outdoor open-pond algae cultivation, freshwater addition is necessary to compensate for evaporative water loss and to avoid salt buildup. Therefore, fresh water is necessary in any algal cultivation system, irrespective of the type of culture water used (Yang et al., 2011). The linkage between evaporative losses and purging, or blow-down, for an open-pond system is illustrated in Figure 4-2. A significant amount of water (Fout) is lost to evaporation in open ponds thereby concentrating total dissolved solids in the pond water. Whether they are fed with fresh water or with saline water, all algal cultivation systems have a control point for the maximum allowable concentration of dissolved solids that is maintained in the culture. If this set point is based on salinity, evaporation would raise the pond salinity so that steps would have to be taken to compensate for this increase. Addition of water with a lower salt concentration and flushing of water from the pond are two steps that can be taken to maintain salinity below the defined control point (xcontrol). Both steps can increase the water requirement and consumptive water use.