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4 Natural Resource Use F uel production from fossil and biological feedstocks is resource intensive, and algal biofuels require resource inputs in the form of water, energy, land, and nutrients. Algal biofuels have been produced at small scale, sufficient to prove that there are a number of possible production pathways. Although production of algal biofuels is technically feasi- ble, they have to be shown to be economically, environmentally, and socially sustainable to become a practical substitution for petroleum-based fuels. The scaling of the pathways for algal biofuel production that are deemed practical for commercial production poses a new demand on natural resources. The levels of nutrients, water, land, and energy necessary to 99
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100 SUSTAINABLE DEVELOPMENT OF ALGAL BIOFUELS FIGURE 4-1 Resource requirements of algal biodiesel production. NOTE: WWTPs denote municipal wastewater treatment plants. CAFOs are concentrated animal feeding operations. produce alternative biofuels in an economically, environmentally, and socially sustainable way would have to be carefully considered (McKone et al., 2011). This chapter focuses on the current sustainability knowledge of the water, nutrient, land, and energy requirements of algal biofuels at all steps in the photoautotrophic algae-based production process (Figure 4-1). Where relevant data are available, quantitative case studies for at least two potential pathways for algal biofuel production are presented in this Chapter. In addition, potential assessment indicators are provided when appropriate, and current knowledge and data gaps are identified for each of the four resource requirement categories. Major new advancements in the current knowledge base will require multi-hectare scale demonstration facilities to be built and maintained in operation for a period of time sufficient to allow detailed real-time analyses of the key variables required for commercial success (Campbell et al., 2011). Moreover, commercial-scale demonstrations will be nec- essary to assess and to improve algal biofuel technologies and their integration with the existing energy infrastructure (Sagar and van der Zwaan, 2006; Katzer, 2010). Innovations that result in reduced resource use along the entire algal biofuel supply chain will remove some of the existing barriers to the development of large-scale, sustainable, and economi- cally viable algal biofuel enterprises. In addition, improvements in algal productivity and biofuel yield will help to reduce resource requirements per unit of algal biofuel produced. 4.1 WATER Water provides the essential physical environment in which cultivated algae grow and reproduce (Murphy and Allen, 2011). It also acts as a thermal regulator and provides a
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NATURAL RESOURCE USE 101 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 produc- tion, 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 bio- mass 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 sustain- ability 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 struc- ture 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 at- mospheric 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. More- over, 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 compen- sate for evaporative water loss and to avoid salt buildup. Therefore, fresh water is neces- sary 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.
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102 SUSTAINABLE DEVELOPMENT OF ALGAL BIOFUELS FIGURE 4-2 Dissolved solids control in a simplified open-pond cultivation system. NOTE: Make-up water addition (Fin) and water purge (Fout) are used to control the critical concentration of total dissolved solids in the pond water (xcontrol). If the pond is well-mixed, the concentration of total dissolved solids in the purge (xout) is equal to the concentration in the pond (xcontrol). In addition to evaporative losses, water can seep into and out of open ponds, particu- larly if they are clay-lined or if liner failure occurs. Water percolation is strongly influenced by the composition and texture of the underlying soils (for example, clay versus sand). Seepage rates are typically on the order of 5 to 6 millimeters per day (Weissman et al., 1989; Boyd and Gross, 2000), which is low compared to rates of evaporative water loss in many regions of the United States. In contrast to open ponds, closed photobioreactors are not af- fected by surface evaporation and seepage, and the lowest reported values for estimated water use are associated with closed systems. However, the water requirements of a pho- tobioreactor system depend on its actual configuration and operating conditions. The reclamation and recycling of water are key determinants of the total water require- ments of both open-pond and closed photobioreactor systems. Whether and how much of the harvest water can be reclaimed and reused depend on the efficiency of separation processes, the quality of the return water, and the sensitivity of the algal culture itself to changes and impurities in the return water, including any waste products produced by the resident algae (Murphy and Allen, 2011). The water requirement for processing of algal biomass to biofuel is small relative to evaporative losses during cultivation in open-pond systems. Water use for processing algal biomass to fatty-acid methyl ester (FAME) was estimated to be 1 liter per liter of biodiesel produced. Water loss during the drying of algae to prepare the biomass for processing to fuel is unavoidable, and some water also is unavoidably lost during the extraction of oil from algae and esterification of algal oil. However, Pate et al. (2011) stressed that evapora- tive water loss under operating conditions involving the inland use of water with a high salt content will result in salinity increases unless fresh water is used to make up for the loss or steps are taken either to mitigate or adapt to salt build-up. The use of inland saline water in algal biofuel production also could have other potential environmental effects (see Chapter 5).
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NATURAL RESOURCE USE 103 4.1.2 Life-Cycle Water Requirements Quantification of life-cycle water requirements of algal biofuel production would sup- port managing future impacts on water demand and enable comparison of water use for algal biofuel with other fuels. The estimation of life-cycle use of any requirement for algal biofuel production (for example, water, nutrients, and energy), however, is complicated by the developing nature of the technologies. In addition to uncertainty as to how algal biofuel will evolve on the path of commercialization, there is also a lack of data on material and energy requirements of the current technologies. An additional complication in open-pond algae cultivation is that water use varies significantly with climatic differences in temperature, humidity, and rainfall. Other biofuels and agricultural crops show such variability in water use. For example, regional variability in irrigation results in estimates of life-cycle water requirements to make ethanol from corn varying from 5 to 2,140 liters of water per liter of fuel, depending on in which U.S. state the corn is grown (Chiu et al., 2009). If the national average of water demand for corn-grain production is used, Chiu et al. (2009) estimated the water use for corn-grain ethanol to be 142 liters of water per liter of fuel. However, the geographical distribution of additional corn grown to meet ethanol demand is uncertain so that whether their water demand matches the national average for all corn also is unclear. The water intensity of open-pond algae cultivation depends critically on the future geographic distribution of cultivation. This future distribution is difficult to forecast, however, being based on the conflux of uncertain future technological performance, policy, and industry response. In the absence of a reliable forecast, studies of water intensity can clarify relationships between location, climate, and water use. 184.108.40.206 Life-Cycle Water Use of Freshwater Open-Pond Systems A number of studies have analyzed water requirements of biofuel produced from algae cultivated in open-pond systems and include different phases of the life cycle (Harto et al., 2010; Wigmosta et al., 2011; Yang et al., 2011). There are large differences in assumptions and results among studies, which is not surprising given the challenges mentioned above. Table 4-1 summarizes the assumptions and results of three studies on open-pond algae cultivation to highlight differences in results and the origins of these differences. The results span over two orders of magnitude, from 32 to 3,650 liters of water per liter of algal biofuel. As a comparison, 1.9-6.6 liters of water are consumed to produce 1 liter of petroleum-based gasoline from crude oil or oil sands (King and Webber, 2008; Wu et al., 2009; Harto et al., 2010). Resolving the variability and uncertainty in these results is beyond the scope of this report. Instead, the goal of this report is to identify and prioritize issues that could affect the long-term sustainability of algal biofuels. Prioritization of research and development (R&D) for issues of concern could contribute to developing algal biofuels as a sustainable part of the energy future. Harto et al. (2010) analyze life-cycle water requirements of a number of alternative transportation fuels, including corn-grain and switchgrass ethanol, soybean biodiesel, so- lar and wind generated electricity, and algal biofuels with algae cultivation in open-pond systems and closed photobioreactors. The scope of processes analyzed includes embodied water in facilities and vehicles. In most scenarios, water use in evaporation and fuel produc- tion dominate the life cycles. Scenario analyses that combine pessimistic versus optimistic assumptions for productivities with evaporation yield variability from 32 to 656 liters of fresh water per liter of biodiesel. The low-end scenario of 32 liters per liter applies to algae
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104 TABLE 4-1 Summary of Assumptions and Results of Studies for Life-Cycle Water Requirements for Open-Pond Algal Biodiesel Assumptions Estimated Water Loss or Use Fuel Net Fuel Life-Cycle Productivity Evaporation Cultivation Harvest Processing Coproducts Water Use Study Case (L/m2/yr) (cm/d) (L/L) (L/L) (L/L) (%) Other (L/L) Harto et al. Low case 5.5 0 0 01 31 46 1 32 (2010)a Base case 5.5 0.25 165 01 50 0 1 216 High case 2.6 0.42 575 01 80 0 0.58 656 Wigmosta 79.5 gal/yr 0.46 0.06 438 01 N/Ab 0 0 438 et al. production (2011)b 200 gal/yr 0.46 0.20 1613 01 N/Ab 0 0 1613 production Yang et al. 100% harvest 3.2 0.27 450 0 140 0 0 590 (2011) recycle 50% harvest 3.2 0.27 450 1550 140 0 0 2140 recycle No harvest 3.2 0.27 450 3060 140 0 0 3650 recycle aHarto et al. (2010) and Wigmosta et al. (2011) assumed 100 percent recycling of harvest water. bWigmosta et al. (2011) did not estimate water use for processing algal biomass to fuels. They did not include water use upstream of algae cultivation (for example, water use for fertilizer production), but that amount is a small fraction of the total water requirement (Harto et al., 2010). NOTES: L/L = liters water consumption per liter of biodiesel produced. As a comparison, life-cycle water use to produce gasoline from crude oil and oil sands has been estimated to be about 1.9-6.6 L/L.
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NATURAL RESOURCE USE 105 TABLE 4-2 Life-Cycle Water Requirements to Produce Biofuel from Corn and Soybean Water Consumption Crop Study (L/L) Notes Corn Chiu et al., 2009 513-1,402 Average water use when irrigated. Range comes from different U.S. states. Dominguez-Faus 5-2,138 Average irrigated and nonirrigated. Range comes et al., 2009 from state variability. Chiu et al., 2009 142 National average of irrigated and nonirrigated. Soybean Dominguez-Faus 1,400-2,900 Average water use when irrigated. Range comes et al., 2009 from different U.S. states. Harto et al., 2010 133 National average irrigated and nonirrigated. NOTE: L/L = water consumption per liter of ethanol or biodiesel produced. Ethanol has about two-thirds of the energy content of gasoline. Biodiesel has about the same energy content as gasoline. cultivated in regions in which rainfall makes up for evaporative losses. Allocation of water use to coproducts in addition to fuel can significantly reduce water use associated with the biofuel product. Wigmosta et al. (2011) developed a geographically resolved model of variability in water and land requirements in different areas in the United States. They estimated water requirements that range from 22 to 3,600 liters of water per liter of oil depending on location of cultivation. Their assumptions for productivity of algae are much lower than those of Harto et al. (2010) and Yang et al. (2011), highlighting the uncertainty associated with criti- cal factors driving material requirements. Wigmosta et al. (2011) also constructed scenarios that build out the geographical distribution of algal biofuel production, starting with areas with lower evaporation and more rainfall. They found steep increases in water require- ments as production moves to more water-intensive areas. Yang et al. (2011) explored water recycling, use of saline water instead of fresh water, performance of different algal strains, and geographic variability. They find that recycling harvest water is critical in managing water requirements. The committee reviewed what is known about water requirements for other algal bio- fuel pathways. Sapphire Energy estimated that its proposed biorefinery in Columbus, New Mexico, would require 3,500 acre feet (4.32 billion liters) of fresh water to produce 30,000 barrels (4.77 million liters) of green crude each year, or 906 liters of water per liter of green crude. The green crude can be upgraded to drop-in fuels (USDA-RD, 2009). Therefore, Sapphire Energy's production pathway is comparable to either the reference pathway in Chapter 3 or the alternative pathway #1 depending on whether coproducts are included. The estimates of life-cycle water use of algal biofuels (Table 4-1) were compared to those of other biofuels to explore whether algal biofuels are more or less water intensive than other biofuels. Table 4-2 shows results of studies of life-cycle water requirements of corn-grain ethanol and soybean biodiesel. For biofuels produced from corn grain, soybean, and algae cultivated in open ponds, water use depends more on the climate (rainfall in particular) where the biomass is grown rather than the type of biomass. 220.127.116.11 Life-Cycle Water Use in Closed Systems--Photobioreactors Cultivating algae in photobioreactors has the potential to eliminate water consumption from evaporation, which could significantly reduce overall water demand. However, data for closed systems are even scarcer than for open systems. Harto et al. (2010) estimated the
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106 SUSTAINABLE DEVELOPMENT OF ALGAL BIOFUELS 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. 18.104.22.168 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 per- formance, 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 productiv- ity 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 phospho- rus 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 mi- croalgae 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 con- sumption, 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 re- place 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 hun- dred 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
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NATURAL RESOURCE USE 107 optimistic production scenario presented in DOE (2010b) was for the southwestern United States. Those estimates were based on high rates of areal production (31 grams per square meter [g/m2] per day) and high average cellular oil contents (50 percent by dry weight). Taking meteorological conditions into account, Wigmosta et al. (2011) estimated the con- sumptive water use to compensate for evaporative loss from ponds to be 312 trillion liters per year if algae are grown to produce 220 billion liters of algal biofuels. That amount is about twice the quantity of water used for irrigated agriculture in the United States (177 trillion liters in 2005; USGS, 2012). If they limit the algae cultivation to areas with high rain- fall, such as areas near the Gulf Coast, the Great Lakes, and most of the eastern seaboard, then consumptive use of fresh water per unit fuel produced can be reduced by 75 percent. Pienkos (2007) estimated that between 16 and 120 trillion gallons (60 and 454 trillion liters) of water per year would be required to produce the algal oil needed to produce 60 billion gallons (227 billion liters) per year of biodiesel. The amount of petroleum-based fuels consumed by the U.S. transportation sector in 2010 was about 207 billion gallons (783 billion liters) per year (EIA, 2011). Pate et al. (2011) estimated the consumptive use of fresh water necessary to achieve target biodiesel production levels of 10 billion, 20 billion, and 50 billion gallons (37.8 bil- lion, 75.7 billion, and 189 billion liters, respectively) per year from freshwater algae for four regions of the United States. Their estimates of water use for algal biofuel (Figure 4-3) FIGURE 4-3 Algal biofuel scale-up scenarios for four different geographical regions of the United States: the Southwest, Midwest, Southeast, and nineteen lower-tier states. SOURCE: Pate et al. (2011). NOTE: 1 gallon = 3.78 liters; 1 acre = 0.40 hectare; 1 gallon per acre = 9.5 liters per hectare.
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108 SUSTAINABLE DEVELOPMENT OF ALGAL BIOFUELS varied with the geographic region, the volume of feedstock production targeted, and the algal productivity assumed to be achieved. The projections of Pate et al. (2011) suggest that using fresh water only in open-pond algal production systems likely will be a significant sustainability issue if 10 billion gallons (37.8 billion liters) of biodiesel are to be produced each year, depending on the region. Cultivating freshwater algae to 10 billion gallons (37.8 billion liters) per year production appeared to be unattainable in the Midwest and South- east regions due to water requirements, which represent more than 70 percent and more than 170 percent of the total water used for irrigation in the Midwest and in the Southeast, respectively. Although water requirements for algal biofuels are estimated to be higher than those of petroleum-based fuels, sustainable use of freshwater needs to be considered in the context of regional availability and other competing uses (NRC, 2011). For example, a petroleum refinery located in a dry region, where groundwater recharge is low with water shortage could be more detrimental to its local supply than would open-pond algal cultivation systems located in a region with rising groundwater level, even though the petroleum re- finery uses considerably less water than algae cultivation. The total demand on local water resources by algal biofuel production will depend on management practices for individual facilities (for example, the type and quantity of water used), the number of facilities located within a watershed, and both the existing volume and time trends in the volume of local aquifers, as influenced by competing water uses. Water use and freshwater and saline- water withdrawals in the United States have been estimated by the U.S. Geological Survey (Kenny et al., 2009). Pate et al. (2011) suggested that the irrigation water from other agricultural applications will need to be diverted to algal biofuel production if 10 billion gallons of fuels are to be produced from algae cultivated in fresh water. Diverting irrigation water from agriculture to algae cultivation for fuels will raise the concern of water use for fuel versus food and feed. Large water withdrawals from surface waters or from groundwater that is connected to surface water systems can affect ecosystems. Many ecosystems require minimum sea- sonal flows to support life cycles of fish (Jager and Rose, 2003, Nagrodski et al., 2012) and riparian vegetation (Stromberg et al., 1996, Greet et al., 2011). Stream macroinvertebrate communities and diversity also are affected by stream flow (Dewson et al., 2007). In some regions, groundwater depth can affect terrestrial vegetation composition and nutrient cy- cling (Goedhart and Pataki, 2011). Effects of water withdrawals for algae cultivation on ecological populations and ecosystem processes would be important to consider in concert with effects of irrigation, hydropower, industrial water use, and municipal water use. Pate et al. (2011) stressed that approaches are needed for algal biofuel production that use nonfreshwater such as coastal marine water; wastewater from agricultural, municipal, and industrial sources; brackish or saline groundwater; and produced water from oil, gas, and coal-bed-methane wells. Cost-effective approaches for reducing evaporative water loss and for dealing with salinity build-up need to be developed. Such approaches will be more important for inland sites where evaporation and salinity build-up are expected to be higher than in coastal marine operational settings that have high relative humidity. 4.1.4 Sustainability Indicators The sustainability implications of water use are difficult to quantify. Many studies use consumptive water use as a measure. Consumption is withdrawal and subsequent "loss" of ground or surface water through evaporation or runoff. The link between water consumption and sustainability effects, such as ecosystem change or scarcity for human
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NATURAL RESOURCE USE 109 needs, depends on local conditions. As water supplies are increasingly stressed, there is an increasing need for methods to connect different uses of water to sustainability impacts. Indicators of sustainability of freshwater requirements for algal biofuel production include the following (Mulder et al., 2010; GBEP, 2011): · Consumptive freshwater use expressed as kilograms of water per kilogram of fuel produced (biodiesel or ethanol) or liters of water per liter of fuel produced. · Energy return on water invested (EROWI), megajoule per liter (MJ/L). These indicators permit general comparisons among sites, feedstocks, and production technologies, but do not provide information about sustainability relative to local supplies. Additional project-specific and site-specific information--total consumptive water use by a facility relative to current supply at the site and relative to projected future demands for all purposes, including biofuel production--will be required for this purpose. For example, a facility estimated to require 1 percent of available supply in an area that is not expected to experience significant population growth or increased agricultural demand for water is likely to be more sustainable than a facility requiring 50 percent of available supply in an area with a rapidly growing population or agricultural demand. Indicators in addition to water consumption also are used. Water withdrawal refers to the quantity of fresh or ground water withdrawn. The use of green, blue, and gray water footprints are gaining interest in some research communities (Gerbens-Leenes et al., 2009; Hoekstra, 2009). Green water is rainwater evaporated during production such as crop growth. Blue water is irrigation water evaporated during crop growth. Grey water is the quantity of water needed to dilute pollutants from a process to meet water-quality stan- dards. The choice of which of these indicators to use is a matter of debate; it is important that researchers report raw data on water use in addition to processed results for indicators. 4.1.5 Information and Data Gaps Evaporation during cultivation is a major contributor to life-cycle water requirements. Some studies use pan evaporation to approximate water use in algae cultivation (Harto et al., 2010; Yang et al., 2011). Evaporation from algal ponds could, however, behave differ- ently from pan evaporation. Wigmosta et al. (2011) used mathematical models intended to improve upon the use of pan evaporation data. Empirical data from actual ponds in vari- ous operating conditions would enable validation and construction of improved models. The extent to which water can be recycled in harvesting and other process steps also is a critical factor. Empirical data on and actual experience with water recycling in cultivation systems are needed. Water balance and management, along with issues associated with potential salt build- up and salt management, are essential areas for future research, modeling, and field as- sessment (NRC, 2008; Gerbens-Leenes et al., 2009). If nonfreshwater is to be used in algae cultivation to alleviate consumptive use of fresh water, then current knowledge of the extent, the water quality and chemistry, and the sustainable withdrawal capacity of those nonfreshwater resources needs to be expanded (DOE, 2010b). For example, Subhadra and Edwards (2011) described the principal fresh and saline aquifers located in the southwest- ern United States, but comprehensive information on the geography and availability of fresh and saline aquifers in other regions suitable for algal biofuel production is needed. Although saline aquifers in the United States were mapped in the 1960s (Feth, 1965), the depth of the aquifer and other factors, such as aquifer hydraulic conductivity and well
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128 SUSTAINABLE DEVELOPMENT OF ALGAL BIOFUELS TABLE 4-7 Energy Use in Cultivation Stage in Closed Photobioreactors Cultivation Energy Source Bioreactor Type (MJ Input/MJ Biodiesel) Stephenson et al. (2010) Air lift tubular 6 Jorquera et al. (2010) Air lift tubular 14 Jorquera et al. (2010) Flat plate 0.61 Brentner et al. (2011) Annular 19 Brentner et al. (2011) Tubular 57 Brentner et al. (2011) Flat plate 1.4 Coproducts significantly affect the energy analysis. The typical scenario analyzed is anaerobic digestion of algae residuals to produce electricity and recover nutrients. One can see from Brenter et al. (2011) that changing from landfilling of algae residuals to an- aerobic digestion nearly doubles EROI in their calculations. In Sander and Murthy (2010), the energy credit for using algae residuals is 10 times larger than the energy content of the produced biodiesel. This result can be interpreted as an assertion that the algal biorefinery becomes primarily a source of fermentation inputs and biodiesel is a secondary output. There is considerable variability within similar technology or supply pathways--for example, Sander and Murphy's EROI of 1.77 versus Brentner et al.'s EROI of 0.28. This vari- ability is related to the use of different data or assumptions for the same processes and dif- ferent boundary choices for supply chains. Variability in data has two sources. One source of variability is generic in LCA. Different analysts choose different data sources, and there is not a systematic way to realize convergence (Williams et al., 2009). The second source of vari- ability is tied to the emerging nature of the technology. LCA is a method designed to assess existing technologies through chaining process input-output tables. Many processes in algal biofuel production systems are still in the laboratory or pilot phase. There is much uncertainty in how these technologies will evolve and scale up in the future; actual energy use could be much higher or much lower than suggested by the current suite of initial LCA studies. The results in Table 4-6 address open-pond cultivation. Given the potential for closed photobioreactors to mitigate other resource and environmental issues such as water con- sumption (Harto et al., 2010), the energy use of closed systems is important to consider. A few studies estimated the direct energy use for the feedstock cultivation step in algal biofuel production systems that use photobioreactors (Table 4-7; Jorquera et al., 2010; Stephenson et al., 2010; Brentner et al., 2011). Tubular and annular reactors are thought to require far more energy to operate than is contained in the biodiesel product. Flat-plate reactors are thought to require far less energy, though Brentner et al. (2011) reported a net energy input higher than contained in biodiesel output (1.4 megajoules per megajoule of biodiesel). While the caveats for other results apply here as well, these studies suggest that the energy use of photobioreactors could fundamentally affect the net energy balance of algal biofuels (Jorquera et al., 2010). 4.4.2 Energy Requirements in the Supply Chain and Credits for Coproducts Analyses of prior studies provide insight into the current understanding of what pro- duction stages are important contributors to energy requirements despite the large uncer- tainties and variability associated with energy requirements of algal biofuel production. Table 4-8 abstracts results from a meta-analysis of LCA studies to summarize the range in energy requirements of different stages (Liu et al., 2011). This meta-analysis included data
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NATURAL RESOURCE USE 129 TABLE 4-8 Range in Energy Requirements by Process Stage for Open-Pond Cultivation of Algae to Produce Biodiesel (Plus Coproducts) Energy Requirement Production Stage (MJ/MJ Biodiesel) Nutrients (fertilizer + CO2) 0.2-1.6 Cultivation 0.04-3.14 Separation 0.01-0.26 Extraction 0.19-0.51 Conversion 0.03-0.22 SOURCE: Liu et al. (2011). Reprinted with permission from Elsevier. from several studies (Lardon et al., 2009; Clarens et al., 2010; Jorquera et al., 2010; Sander and Murthy, 2010; Stephenson et al., 2010; Campbell et al., 2011). The high end of the range of energy requirements for nutrients corresponds to the use of virgin fertilizers and industrial CO2 in production. Cultivation consumes energy primar- ily for mixing and pumping water. Murphy and Allen (2011) suggested that energy needs for water management could be substantially higher than current LCA studies indicate. Extraction is generally assumed to be via hexane solvent and conversion to biodiesel via transesterification. The treatment of the energy credits for coproducts is critical in the energy balance of algal biofuels. For production of bio-electricity, the energy credit per megajoule of biodiesel ranges from 0.3-1.3 megajoule/megajoules biodiesel (Lardon et al., 2009; Stephenson et al., 2010; Campbell et al., 2011). Sander and Murthy (2010) assume that residual biomass from algae production substitutes for corn used in ethanol production, yielding an energy credit for the coproduct of 10.7 megajoules/megajoule biodiesel. 4.4.3 LCA Issues Related to Algal-Lipid Processing Pathways described in Chapter 3 describe two different ways of converting crude algal lipids to liquid fuels. Both yield fuels suitable for use in diesel applications. The majority of the published LCAs assumed the production of FAME diesels, which are less desirable than "drop-in" fuels because of FAME's incompatibility with existing infrastructure for petroleum-based fuels. Given the lack of LCA work on green diesel products from algae, differences in energy use between FAME and green diesel are addressed by analogy with conventional diesel processing. A number of studies of conventional diesel processing that have been reviewed allow comparisons to be made (Kalnes et al., 2009). Comparison between seed oil-derived diesel fuels treated by esterification and by hydrotreating show that there is little difference in either the energy return or carbon emis- sions. These studies start with the same raw oils, meaning that the differences only reflect the processing to finished fuel. Figure 4-8 shows that these are nearly identical and, there- fore, life-cycle impacts will be similar between esterification processes and hydrotreating. 4.4.4 Opportunities for Mitigation Keeping other factors constant, increasing the productivity of algal growth drives down energy use for cultivation and harvesting. This said, if achieving higher productivity involves major process changes, such as using photobioreactors instead of raceways, the
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130 SUSTAINABLE DEVELOPMENT OF ALGAL BIOFUELS FIGURE 4-8 Comparison of conventional esterified biodiesel production with green diesel (GD) production based on soybean oil (SBO) and rapeseed oil (RSO). NOTE: Each of the three completed studies shows little difference on energy use or carbon emissions between green diesel and biodiesel. By analogy, green diesel production from algal lipids relative to conventional biodiesel processing is likely to have similar life-cycle impacts. SOURCE: Adapted from Kalnes et al. (2009). embodied energy of the process change needs to be assessed and compared to direct energy savings. Water management is clearly an important factor in energy use. Efficient pumps and gravity-driven designs, for example, could mitigate high energy use for water manage- ment. The embodied energy in providing nutrients, including CO2, can be substantial. The extent to which waste products can be used as nutrients has the potential to substantially reduce energy use. Separating algal products from water is a major factor driving energy use in separation and extraction. Drying processes in particular are energy intensive. Brent- ner et al. (2011) called attention to the potential of supercritical processes to reduce energy use for processing algae. Given the importance of coproducts in the net energy balance, developing higher "energy value" coproducts could be an important mitigation strategy. 4.4.5 Sustainability Indicators A number of different metrics are already in use to assess energy systems (Farrell et al., 2006; GBEP, 2011; Baral et al., 2012), including: · EROI. · Net Energy Value (NEV). · Fossil Inputs.
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NATURAL RESOURCE USE 131 Before discussing the different measures in more detail, it is important to first recapitu- late the definition and quantification of energy use. The key issue is that energy inputs and outputs come in different forms having differing utility, in particular chemical (for example, heating value of fuel), electricity, and heat. While one could simply add different energy types by unit conversion, different forms of energy are not interchangeable. For example, it takes more than one unit of heat to make one unit of electricity. One approach to put differ- ent energy forms on a comparable basis is the idea of primary (or source) energy. The pre- cise definition of primary energy varies, but in general it includes the heat or fossil inputs needed to make electricity. In some cases, it also includes indirect energy use associated with delivering fossil fuels. For the U.S. energy system, one common conversion used is 3.4 megajoules of upstream primary energy per kilowatt hour of electricity. Analysts often use different definitions of energy and do not always explicitly state which definition is being used. Care is needed when comparing energy results from different studies. Energy outputs generally include only those utilized; that is, waste heat is not included. Energy output is estimated by direct unit conversion, conversion to source energy, or in some cases, using the energy needed to make products that coproducts replace. NEV forms the difference. NEV = Energy outputs of fuels and products Energy inputs. Fossil inputs measure quantities of fossil fuels used in processing. EROI and NEV refer generically to energy, which could be supplied by fossil or renewable forms. Fossil inputs are thus specific to how heat and electricity are supplied and thus require definition of the enveloping energy system. 4.4.6 Information and Data Gaps Much uncertainty remains as to the current and future energy properties of algal culti- vation systems, pointing to critical gaps. Scarcity of data on material flows at existing scales of algal biofuel production presents a challenge in assessing EROI. LCA studies of algae use process modeling to estimate energy consumption. Additional empirical data can help validate these models. Although there are gaps in data, data collection by itself will not resolve the uncertain- ties of life-cycle energy implications of algal biofuels. The true energy behavior is a result of scale-up and learning processes that bring algae from the laboratory and pilot scale to in- dustrial scale. While future energy behavior is challenging to forecast, given the substantial investment and path dependence associated with bringing an energy technology to scale, due diligence demands that a serious forecasting effort be made. While there are a variety of cost forecasting methods available, such as learning curves and scaling factors, methods to forecast energy and material flows of developing technologies are undeveloped. Efforts need to be made to develop such methods. Increased data availability from laboratory and pilot scales is critical to calibrate and validate the forecasting methods that emerge. 4.5 CONCLUSIONS A review of published literature suggests that the scale-up of algal biofuel production to yield 37.8 billion liters of algal oil (10 billion gallons) would place an unsustainable de- mand on energy, water, and nutrients with current technology and knowledge. Estimated
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132 SUSTAINABLE DEVELOPMENT OF ALGAL BIOFUELS values for EROI range from 0.13 to 3.33. The estimated consumptive use of fresh water for producing 1 liter of gasoline equivalent of algal biofuel is 3.15-3,650 liters. The estimated requirement for nitrogen and phosphorus needed to produce 37.8 billion liters of algal biofuels ranges from 6 million to 15 million metric tons of nitrogen and from 1 million to 2 million metric tons of phosphorus if the nutrients are not recycled or included and used in coproducts. Freshwater use for production of algal biofuel is inevitable because fresh water is nec- essary to compensate for water loss and to avoid salt buildup as a result of evaporation during cultivation. Two key drivers for freshwater requirement in algal biofuel production are evaporative loss in open-pond cultivation and discharge of harvest water in biofuel production systems that harvest the algae. Therefore, water use would be a serious concern in an algal biofuel production system that uses fresh water in open ponds without recy- cling harvest water. Freshwater use also could be reduced by using saltwater or brackish water in algae cultivation. However, information on the availability of inland saline water resources is sparse. Recycling of harvest water also is important in reducing nitrogen and phosphorus in- put to algae cultivation. To promote high yield, algae are cultivated in nutrient-rich media. The nutrients will be lost if harvest water is not recycled. If algal biomass is harvested to process to fuels, there could be another opportunity to recover nitrogen and phosphorus after processing because the fuel products do not contain those two elements. Recycling nutrients via reuse of harvest water or the use of wastewater from agricul- tural or municipal sources provides an opportunity to reduce energy use, as synthetic fertil- izer input contributes to energy input over the life cycle of algal biofuels. Energy inputs for water management (for example, pumping groundwater, and moving water in cultivation systems), harvesting, and water extraction (for example, drying of algal biomass) are two key drivers in the overall energy balance of algal biofuel production systems. A key aspect to sustainable development of algal biofuels is siting (for example, suit- able climate and colocation of key resources) and recycling of key resources. Siting of algal biofuel production facilities needs to account for climate, topography, and proximity to water and nutrients. Siting algae cultivation far away from water and nutrient resources would incur additional costs and energy consumption for transporting those resources to the cultivation facilities. Although several studies assessed land, water, and nutrient re- quirements and energy balance of algal biofuel production, few studies considered all four requirements simultaneously. A national assessment of land requirements for algae cultivation that takes into account climatic conditions; fresh water, inland and coastal saline water, and wastewater resources; sources of CO2; and land prices is needed to inform the potential amount of algal biofuels that could be produced economically in the United States.
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NATURAL RESOURCE USE 133 SUMMARY FINDINGS FROM THIS AND EARLIER CHAPTERS Based on a review of literature published until the authoring of this report, the com- mittee concluded that the scale-up of algal biofuel production sufficient to meet at least 5 percent of U.S. demand for transportation fuelsa would place unsustainable demands on energy, water, and nutrients with current technologies and knowledge. However, the potential to shift this dynamic through improvements in biological and engineering variables exists. (See also Chapters 2 and 3 on improvements in biological and engineering variables.) Sustainable development of algal biofuels would require research, development, and demonstration of the following: ·Algal strain selection and improvement to enhance desired characteristics and biofuel productivity. (See Chapter 2.) ·An EROI that is comparable to other transportation fuels, or at least improving and approaching the EROIs of other transportation fuels. ·The use of wastewater for cultivating algae for fuels or the recycling of harvest water, particularly if freshwater algae are used. ·Recycling of nutrients in algal biofuel pathways that require harvesting unless coproducts that meet an equivalent nutrient need are produced. A national assessment of land requirements for algae cultivation that takes into ac- count climatic conditions; fresh water, inland and coastal saline water, and wastewater resources; sources of CO2; and land prices is needed to inform the potential amount of algal biofuels that could be produced economically in the United States. a U.S. consumption of fuels for transportation was about 784 billion liters in 2010. Five percent of the annual U.S. con- sumption of transportation fuels, which would be about 39 billion liters, is mentioned to provide a quantitative illustration of the water and nutrients required to produce algal biofuels to meet a small portion of the U.S. fuel demand. REFERENCES Alley, W.M. 2003. Desalination of Ground Water: Earth Science Perspectives. Reston: U.S. Geological Survey. Baral, A., B.R. Bakshi, and R.L. Smith. 2012. Assessing resource intensity and renewability of cellulosic ethanol technologies using Eco-LCA. Environmental Science and Technology 46 (4):2436-2444. Batan, L., J. Quinn, T. Bradley, and B. Willson. 2011. Erratum: Net energy and greenhouse gas emissions evalu- ation of biodiesel derived from microalgae (Environmental Science and Technology (2010) 44 (7975-7980)). Environmental Science and Technology 45(3):1160. Batan, L., J. Quinn, B. Willson, and T. Bradley. 2010. Net energy and greenhouse gas emission evaluation of bio- diesel derived from microalgae. Environmental Science and Technology 44(20):7975-7980. Benemann, J., P.M. Pedroni, J. J. Davison, H. Beckert, and P. Bergman. 2003a. Technology roadmap for biofixation of CO2 and greenhouse gas abatement with microalgae. Paper read at the Second Annual Conference on Carbon Sequestration, May 5-8, Alexandria, VA. Benemann, J.R., J.C. Van Olst, M.J. Massingill, J.C. Weissman, and D.E. Brune. 2003b. The controlled eutrophica- tion process: Using microalgae for CO2 utilization and agricultural fertilizer recycling. Paper read at the 6th Greenhouse Gas Control Technologies Conference (GHGT6), October, Kyoto Japan. BLM (Bureau of Land Management) and DOE (U.S. Department of Energy). 2010. Solar Energy Development Draft Programmatic Environmental Impact Statement (Draft Solar PEIS). Washington, D.C.: Bureau of Land Management and U.S. Department of Energy. Boyd, C.E., and A. Gross. 2000. Water use and conservation for inland aquaculture ponds. Fisheries Management and Ecology 7(1-2):55-63. Brentner, L.B., M.J. Eckelman, and J.B. Zimmerman. 2011. Combinatorial life cycle assessment to inform process design of industrial production of algal biodiesel. Environmental Science and Technology 45:7060-7067. Cai, X.M., X.A. Zhang, and D.B. Wang. 2011. Land availability for biofuel production. Environmental Science and Technology 45(1):334-339.
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