The combination of both federal government-mandated and individual state renewable energy standards, and the increasing body of evidence that feedstock-based biofuels are unsustainable, have led to the critical evaluation of the feasibility of microalgae as an alternative biofuel source. While microalgae show great promise in their relatively low land requirements, high growth rate, and CO2 absorption abilities, there remain outstanding questions regarding water consumption rates, especially in comparison to current feedstocks. In their 2011 study, Yang et al. use known measurements of algal growth parameters, such as evaporation rate, growth rate, and nutrient usage, to quantify the water footprint and nutrient balance of Chlorella vulgaris, a species of microalgae. They found that microalgae are competitive with traditional feedstocks in terms of total water footprint, and that freshwater and nutrient consumption could be significantly reduced by using seawater and waste water as the base water source. The researchers additionally analyzed spatial variation of microalgae growth in terms of solar radiation and temperature, and found that the water footprint would be lowest in the states of Florida, Hawaii, and Arizona.—Karen de Wolski
Yang J., Xu M., Hu Q., Sommerfeld M., Chen Y., 2011. Life-cycle analysis on biodiesel production from microalgae: water footprint and nutrients balance. Bioresource Technology 102, 159–165.
Yang et al. outline the feasibility of microalgae as a biofuel source in terms of water footprint and nutrient usage by drawing from previously gathered metrics to estimate the water and nutrient consumption of Chlorella vulgaris. The 2007 Energy Independence and Security Act requires that renewable fuel production increase to 36 billion gallons per year by 2022. The first generation biofuels derived from corn and sugarcane (Brazil) are limited in that they require significant arable land, increase food prices internationally, and do not necessarily significantly reduce carbon emissions. Microalgae is promising as an alternative fuel source because of its high growth rate, smaller land usage (15–300 times more oil per land unit), high lipid content, and CO2 absorption abilities. However, microalgae are not yet grown on a mass scale, and several outstanding questions remain regarding the life-cycle impacts of large-scale cultivation and biodiesel production. This study seeks to elucidate the water footprint of biodiesel production from microalgae through quantitative measurements and comparisons with current biodiesel production from feedstocks. The researchers calculate the differences between using seawater, wastewater, and freshwater as a culture base, and they additionally account for nutrient usage and the effects of solar radiation and temperature variation in microalgae cultivation.
Microalgae biodiesel production necessarily entails culture, harvest, drying, extraction, and esterification. Microalgae are initially grown in culture water (sea, waste, or fresh) in an open pond which must be constantly replenished with freshwater to accommodate evaporation and maintain salinity. When sufficiently grown, the microalgae are harvested, dried, and the lipids are extracted to be esterified for biodiesel. Culture water can be partially recycled directly back into a culture pond, and/or discharged into a wastewater treatment system.
Evaporation is the main source of water loss during the culture process, and the authors used lake evaporation rate to approximate open pond evaporation. Microalgal growth is affected by temperature and solar radiation, and data from the national solar radiation database were usd to make the appropriate calculations. Additionally, culture ponds need to be supplied with nutrients, and necessary nutrient concentrations were based on measurements from previous studies. Both harvesting and drying, the second and third steps of the process respectively, are quantified by solid content (ratio of microalgae to water) and recovery rate (ratio of harvested mass to mass after culture). These values were derived from several already established parameters. Because the extraction and esterification of microalgae is similar to that of soybeans, this study substituted water usage rates for biodiesel production from soybean oil (2–10 liters water used per liter of biodiesel produced) for the analogous water usage of biodiesel production from microalgae-derived oil.
The researchers found that, in the absence of recycling harvested water, the water footprint of microalgae biodiesel production is 3726 kg-water/kg-biodiesel. This value can be decreased to 591 kg-water/kg-biodiesel if all harvest water is recycled. The amount of harvest water recycled does not affect the water footprint of the other production processes. Additionally, they found that using seawater or wastewater can reduce the life-cycle freshwater usage by up to 90%. Harvest water recycling can also decrease nutrient (nitrogen, phosphorous, potassium, magnesium, and sulfur) usage by approximately 55%. The use of sea/wastewater for algal culture can additionally reduce nitrogen usage by 94% and abolish the necessity of adding potassium, magnesium and sulfur. A sensitivity analysis revealed evaporation rate, algal lipid content, and slurry content as the most sensitive parameters, while variations in these factors show growth rate to be the most sensitive factor.
When these results are compared to the water footprint of feedstock-based biofuel, microalgae are demonstrated to be extremely competitive. The authors discuss how differences in species could cause significant variation in these parameters. Increases in both lipid content and growth rate, the two most important parameters, result in the reduction of the water footprint. These two parameters are, however, usually negatively related to each other, and vary depending on species. This study calculated the water footprint for eleven other species using the same methodology and found that the water footprint could be 1–6 times higher than that of C. vulgaris.
Geographic variations in solar radiation, temperature, and evaporation must be accounted for when estimating microalgae growth. While microalgae tend to prosper in high temperature/high solar radiation environments, both of these factors are proportionally related to evaporation rate, and therefore cause an increase in water footprint. Taking these factors into account, the researchers calculated that Florida, Hawaii, and Arizona would have the lowest water footprints for microalgae biodiesel production in the United States.
They conclude that, in terms of water footprint and nutrient usage, microalgae could be a feasible alternative to current feedstocks for biodiesel production. The advancement of technology, especially of photobioreactors for cultured growth, could enhance water conservation and increase cultivation efficiency. The use of sea/waste water for culture water would decrease water usage by 90% and greatly reduce the need for nutrient supplementation. Phosphate could prove to be a limiting factor, as it is not found in either of the aforementioned water types, and global phosphate sources are decreasing. However, experimentation with different microalgae species types and phosphate-rich water could overcome this barrier. While much remains to be done before microalgae can be used as a global renewable energy source, freshwater and nutrient usage does not appear to be a limiting obstacle.