After defining the system boundary, all the relevant environmental, economic, and social impacts associated with the microalgae biodiesel supply chain stages have to be identified. These can be the following:

• Energy consumption

• Net GHG emissions

• Freshwater consumption

• Wastewater treatment

• Nutrients consumption (e. g. nitrates, phosphates, carbon source)

• Chemicals for oil extraction (e. g. n-hexane) and biodiesel production (e. g. CH3OH, NaOH)

• Residual biomass management

• Land use

• Potential chemical risk

• Net cash flow generated

• Employment

Then, the relative significance or insignificance of these impacts are identified (for each supply chain stage) based on the authors current knowledge of the pro­cesses involved. For example, energy consumption is a significant impact in many supply chain stages, since it is needed for algae cultivation, harvesting, biomass processing, oil extraction, and biodiesel production, while land use is more significant for microalgae cultivation than in the remaining stages. Freshwater con­sumption is significant for the cultivation stage unless wastewater from another source, which can also be used as a source of nutrients, is used instead to cultivate microalgae.

After a careful analysis of the system under study, involving the identification of the impacts on the domains of sustainability that are significant for each supply chain stage, candidate indicators are selected for the sustainability analysis of microalgae biodiesel. The selected indicators have to fulfill the following conditions [10]:

• Form a coherent set of quantifiable variables consistent with the principles of sustainability

• Be representative of the physical system under study

• Be clear, simple, unambiguous, and not biased

• Be independent of each other and form a small set

• Be directly and easily calculated from system data

The dimensionality of metrics (3D, 2D or 1D) can then be determined [19, 20]. For example, “energy intensity” is a sustainability or 3D indicator, since it takes into account aspects of the three sustainability dimensions. The higher it is, the more negative the impact is for the environment, because of the waste generated in energy production. Yet, as it is positive for the economy, because it is essential for value creation and higher standards of living, it has both positive and negative societal impacts, as future generations will be deprived of currently used sources of energy because of their depletion but lower emissions of pollutants will result from the consumption of biodiesel. Similarly, “land use intensity” contributes to soil degra­dation and biodiversity loss with a negative impact in the environment. It is positive for the economy, because of the value creation from crops produced. Yet, it may have a positive or a negative societal impact depending on how it contributes respec­tively to employment or land competition with other crops, in particular food crops. On the other hand “Contribution to Global Warming” also called as carbon foot­print, can be seen as directly related to global warming and to the environmental effects of it. Also, it is expected that it will create an economic impact because of carbon trading or carbon taxes, if GHG control regulations are in place, this way being a 2D indicator.

After the candidate indicators have been selected, prioritization of the set of indi­cators follows, based on technical input and data availability. Only if the informa­tion exists it is possible to quantify the indicators and perform the sustainability analysis. The conclusions from the analysis will be more reliable if the data are of good quality.

The set of indicators that are of high priority for the sustainability evaluation of microalgae biodiesel are the following:

1. Life cycle energy efficiency (dimensionless), 3D

2. Fossil energy ratio (dimensionless), 1D

3. Land use intensity (m2/MJ biodiesel/year), 3D

4. Contribution to global warming (kg CO2-eq/MJ fuel), 2D

Table 1 synthesizes the sustainability indicators selected for evaluating the sus­tainability of microalgae systems for biodiesel production.

Similar metrics have been proposed by other authors in a biofuels or conven­tional fuels context. For example, Pradhan et al. [ 13] compared four biodiesel energy balances using two indicators: the net energy ratio and the renewability fac­tor. Zhou et al. [22] proposed four indicators for a sustainability assessment of con­ventional fuels during their life cycles. Kim et al. [7] considered the land use change and GHG emissions associated with the production of biofuels.

The purpose of this article is to describe how to perform a sustainability analysis of microalgae biodiesel system comprising the entire supply chain, and also pro­pose specific sustainability indicators. To the authors’ knowledge, no full scale com­mercial plant for microalgae biodiesel exists at present. So, in the absence of commercially relevant data, the reliability of the values of the metrics used at pres­ent would be somewhat limited.

Although some suggestions can be found in literature [1, 6, 8, 9, 16, 21], there are no complete LCA studies with reliable data on biodiesel produced from microal­gae. Also, the majority of the published studies analyzed hypothetical scenarios.

For example, Kadam [6] conducted an LCA to compare the environmental impli­cations of electricity production via coal firing vs. coal/algae co-firing, using 50% of the flue gas from a 50 MW power station as the carbon source to grow microalgae. Results of this study show that when recycling CO2 toward microalgae production it is possible to achieve an overall life cycle CO2 saving of 36.7%.

Table 1 Indicators for the sustainability evaluation of microalgae biodiesel Indicator Definition

Life cycle energy It is the ratio of the total energy produced (energy output) to the total

efficiency (LCEE) energy consumed (energy input)

(dimensionless)

Lifecycle energy efficiency (LCEE) = energy output/energy input

The energy content of by-products may be accounted for in the energy output if they are used for energy production in substitution of fossil fuel or electricity. LCEE also called net energy ratio (NER) measures the relative amount of energy that ends up in the final fuel products

Fossil energy ratio It is the ratio between the amount of energy that goes into the final fuel (FER) product (fuel energy output) and the amount of fossil energy input

(dimensionless) (non-renewable energy) required for the fuel production

Fossil energy ratio (FER) = Fossil energy output/Fossil energy input

FER is also called the renewability factor (RF) since it measures the degree to which a given fuel is or is not renewable. Larger the value of FER less fossil energy is used (assumed to be non-renewable) for the same energy output. A FER greater than one can be used to replace the energy used in producing it. Also, theoretically FER can be infinite if no fossil energy is needed for the fuel production meaning it is “completely” renewable It measures the area of land occupied per unit energy of product

(e. g. the land needed for the biodiesel feedstocks cultivation, which affects biodiversity and life support functions)

It measures the potential contribution of different GHG emissions (e. g. CO2, CH4, N2O) to global warming (or greenhouse effect), expressed as equivalent CO2 emission per unit energy of fuel product

Contributiontoglobalwarming GWP; x E. t,

і

where Ei is the mass of greenhouse gas i emitted to the air and GWP i is the Global Warming Potential of the substance i The total GHG emissions ( E ) through the fuel life cycle are calculated as [3]

■ e „ + e. + e + e + e — e — e„

where e are emissions from the extraction or cultivation of raw

ec

materials; el are annualized emissions from carbon stock changes caused by land use change; ep are emissions from processing; etd are emissions from transport and distribution; eu are emissions from fuel usage; eccs are emission savings from carbon capture and sequestra­tion; eccr are emission savings from carbon capture and replacement; and eee are emission savings from excess electricity from cogeneration

Lardon et al. [8] performed an LCA on the production of biodiesel from Chlorella vulgaris, showing that when this algae is grown in nitrogen-deprived conditions and the oil is extracted directly from the wet biomass without the need for drying, the biodiesel would have a GWP lower than fossil diesel but higher than biodiesel produced from rape seed oil or palm oil.

Lehr and Posten [9] estimated the energy needed for operating a photo-bioreactor compared to the possible chemical energy harvested. According to these authors the economical feasibility of biofuel production by algae cannot be obtained in the short term. The outstanding problems of cost and efficiency of microalgae cultivation for biodiesel need critical attention.

Rodolfi et al. [16] evaluated the biomass productivity, lipid content, and lipid productivity of 30 microalgal strains cultivated in 250 mL flasks. They suggested that in order for microalgae cultures to become an economic, renewable, and car­bon-neutral source of transportation fuel, biofuels production needs to be combined with that of production of higher value co-products.

Clarens et al. [ 1] compared from a life cycle perspective, conventional crops (rapeseed, switch grass, and corn) with microalgae cultivation for biofuels production, concluding that microalgae have higher environmental impacts than these conven­tional crops in terms of energy use, GHG emissions, and water consumption regard­less of cultivation location. These authors suggested that to reduce the impacts, flue gas could be used as a carbon source for producing algae near power plants and also waste­water treatment could be combined with algae cultivation as a source of nutrients.

Stephenson et al. [21] investigated the life cycle global warming potential (GWP) and the fossil energy requirements, for a hypothetical operation in which biodiesel is produced from the freshwater microalgae C. vulgaris. These authors concluded that for a more environmentally sustainable cultivation of these algae in open ponds instead if closed photo-bioreactors, it should be possible to achieve a lipids produc­tivity target of 40 tons/ha/year. This way the GWP of microalgae would be about 80% lower than fossil diesel on a net energy content basis.

The utilization of microalgae, at least from a land use intensity point of view should be a viable option for substituting current feedstocks, as the former has much higher productivity when compared with existing feedstocks. However, the state of development at present precludes a more extensive utilization of microalgae for biodiesel production which could have a real impact in the fossil fuel market.

We can summarize the reasons for the delay of a more widespread usage of microalgae as a feedstock for biofuel production. First, there are still some hurdles to cross concerning their cultivation at large scale. Although some strains are already identified as promising for biofuel production, only a few of them have been attempted at an industrial scale. Critical information about the conditions in which microalgae give the highest yield of lipids or other components of interest is still lacking, especially the nutrient mix and sunlight necessary for inducing it [ 11 ] . Second, there are scaling problems when going from laboratory and pilot scale units to fully commercial plants. When growing microalgae in large open ponds, one has to ensure that all microorganisms receive an adequate amount of energy and nutri­ents, a difficult task when the cell concentration is very high. Third, other challenges are posed by the growth cycle of the microalgae, which should be better understood in order to know when it is the best time for harvesting them, and that during the decline or death phase microalgae cultures are more susceptible to potential contaminations by other organisms that will compete for food and space.

Potential solutions for these problems include the selective growth of particu­larly resistant strains or even their genetic engineering in order to produce species better fitted for biofuels production. Fourth, the microalgae harvesting and process­ing steps, before the biodiesel production, are still under development. As described above, due to high water content of the algal biomass, some of the processes may require high quantities of energy, making the production of biodiesel from microal­gae an energy intensive process, thus increasing its environmental impact [11 ] .

Notwithstanding the possible difficulties, microalgae are seen as one of the most viable options in the medium to far future. Some of the reasons are directly related to their physiology. As simple organisms, they can grow very fast and produce lipids among other metabolites of interest, only requiring water with a given salinity and pH, sunlight, carbon dioxide and a source of nitrogen and other nutrients. This is clearly an advantage over agricultural feedstocks and even future cellulosic raw materials, which normally require pesticides, fertilizers, tillage, and other treat­ments for their production. Also, due to their diversity the probability of finding or engineering the most adequate microalgae strain is high and is easier to do than with the more complex plants.

2 Conclusions

In this work a set of sustainability indicators is proposed in order to assess the utilization of microalgae as a feedstock for biodiesel production from a supply chain point of view, the supply chain stages being cultivation of microalgae, harvesting, and further processing for biodiesel production. Microalgae can be a sustainable option as a feedstock for biofuels production, combining high productivity with high oil content. In particular, the land use intensity is clearly smaller when com­pared to other feedstocks, minimizing the questions directly linked with land use, and the loss of biodiversity. Also, microalgae have the potential to be used in the production of other chemicals of high added value or integrated in existing indus­trial processes for other beneficial purposes, such as carbon sequestration or waste­water treatment. However, more research is still needed to develop more economical industrial production systems and to fully explore the microalgae potentials.