life-cycle water requirements of a photobioreactor system at 30-63 liters fresh water per liter of biodiesel, though this result is based on expert opinion, not empirical measurement of a functioning system. The Algenol process is a closed photobiorector using sea water and fresh water. In its environmental impact assessment for a proposed biorefinery in Fort Meyers, Florida, Algenol estimated that the facility would require 3.6 million gallons of seawater and 210,000 gallons of fresh water to produce 100,000 gallons of algal ethanol each year (or 36 liters of salt water per liter of ethanol and 2.1 liters of fresh water per liter of ethanol) (DOE, 2010a). The freshwater use equals 3.15 liters of fresh water to produce each liter of gasoline-equivalent fuel. The Algenol estimate does not include upstream water use for inputs to their facilities.
220.127.116.11 Algae Cultivation Using Salt or Brackish Water or Wastewater
Using salt-tolerant algal species would allow the use of alternative water sources such as seawater, saline, and brackish groundwater, or coproduced water derived from oil, natural gas, and coal-bed methane wells (DOE, 2010b). This physiological flexibility of algae implies that locating algae production to areas where alternative water sources are available could reduce consumption of fresh water in cultivation. Cultivating saline algae in inland ponds also could reduce the potential for invasion of the ponds by undesirable freshwater organisms.
Vasudevan et al. (2012) estimated the consumption of fresh water in a saline water, open-pond, algae cultivation facility for three cases that they formulated—a base case (nominal, in their language) with reasonable assumptions in technology and system performance, a case with pessimistic assumptions, and a case with optimistic assumptions. The estimated requirement for freshwater make-up was 1,000 liters of freshwater per liter of oil, with a range of 200-2,000 liters from optimistic to pessimistic cases (Vasudevan et al., 2012). This result suggests that the need for freshwater make-up is significant when saline water is used for algae cultivation. However, the make-up water use depends on productivity and salinity limits of algae used, climate, and other uncertainties and variabilities that have yet to be resolved.
Wastewater also can be used in cultivating algae, thereby reducing groundwater and surface water consumption and treating wastewater by reducing nitrogen and phosphorus content. Pittman et al. (2011) discussed the potential benefits and limitations of using wastewater to produce algae for biofuels cost effectively, and concluded that dual-use microalgae cultivation for wastewater treatment and biofuel production has the potential to use up nutrients in wastewater and reduce the amount of fresh water required for biofuel generation from algal biomass. The potential environmental benefits and concerns of algal biofuel production using wastewater as a water and nutrient feed will be discussed further in Chapter 5 of this report, but this concept has not yet been tested at scale.
4.1.3 Scale-up Considerations
The freshwater demands of algal biofuel production will be high if algal biofuels are used to substitute for a significant fraction of annual U.S. liquid transportation fuel consumption, particularly if open ponds are to be used for algae cultivation. If open ponds are used for algae production, then a significant amount of water will be required to replace evaporative losses from the pond surface and to prevent dissolved salt buildup in the biomass cultivation system (Yang et al., 2011). Recent estimates reported by the U.S. Department of Energy (DOE, 2010b) suggest that water losses on the order of several hundred liters of water per liter of algal oil or algal biodiesel produced would result from the operation of open ponds in arid, sunny regions of the continental United States. The most