ReviewVarious potential techniques to reduce the water footprint of microalgal biomass production for biofuel—A review
Graphical abstract
Introduction
Microalgae are photosynthetic organisms with a low doubling time and having extensive applications such as biodegradation, bioremediation, biofuel, fish and animal feed (Ma et al., 2018b; Nagappan and Verma, 2016a; Nagappan and Verma, 2018; Vergara et al., 2016). Moreover, microalgae can fix the CO2 and thus can be used in the industrial set-up for reducing the carbon footprint (Singh et al., 2016; Thawechai et al., 2016). Besides, the adoption of a suitable lipid extraction based biorefinery approach can lead to the production of several high-value products (González-Fernández et al., 2016; Nagappan et al., 2018; Nagappan and Verma, 2016b; Park et al., 2013; Perazzoli et al., 2016). It is also noteworthy that, due to the various reasons, the microalgal products are not yet commercialized and among which the high water consumption is the least addressed issue.
Water is essential for microalgal growth as it helps in maintaining the temperature and also serves as a medium for nutrient delivery. As biofuel applications require large scale biomass production, the water demand is very high for such products (De Bhowmick et al., 2019; Guieysse et al., 2013). Water used for microalgae cultivation competes with human use of water for essential purposes such as drinking and irrigation (Delrue et al., 2012). This, in turn, leads to food vs. fuel scenario, which arises mainly in the case of the use of fresh water. Therefore, water management is essential in the successful commercialization of microalgal fuel. In this context, recycling of growth medium and use of adherent growth systems such as biofilm reactors can possibly overcome the above problem.
Section snippets
Water consumption in industrial production of microalgae
A typical microalgal biomass production for various industrial purposes involves microalgae cultivation (in a suitable growth system such as an open pond, photobioreactor, etc.) followed by drying, harvesting, lipid extraction, and esterification (Batan et al., 2016; Lardon et al., 2009). The loss of water is mainly observed during the stages of drying, harvesting, and cultivation (Delrue et al., 2012; Feng et al., 2016). The replacement or avoiding the loss of water is not possible especially
Calculation of water demand and water footprint
A water footprint is a term adopted by UNESCO in 2002 to signify the quantity of fresh water used by people, organisations or companies to manufacture products or offer facilities that the population needs. In this study, the water footprint means the consumption of water for the production a biofuel. In the case of microalgal biofuel production, the water footprint is comprised of three components: grey water, blue water, and white water. The blue water footprint is the amount of water
Water footprint of conventional crops and microalgae for biomass production
A comparison of the water footprints allied with the conventional crops and microalgae based on biofuel production is presented in this section. Table 1 compares the total water footprint of biomass production using different fuel crops including microalgae. Among conventional crops, the water footprints of sorghum and soybean were far higher (between 13,000 and 16,000 kg of water per kg of biodiesel) than sugar cane and sugar beet, which are estimated to be between 2000 and 4000 kg of water
Location and environmental conditions
Water consumption for microalgal biomass production depends on the location and environmental conditions prevailing in that area. A region with higher solar radiation and temperature will generally have a more giant water footprint than the one with lower solar radiation and temperature. This is primarily due to a higher rate of evaporation, as in the case of tropical and equatorial regions. Microalgae cultivation in arid regions consumes more water than tropical regions. For example, the water
Recycling
Nearly a six-fold reduction in the consumption of water can be achieved if the harvested water is recycled during the microalgal cultivation (Huang et al., 2016). Furthermore, the consumption of water can be reduced also if wastewater and seawater is used along with the recycling process. Besides the advantage of the reduction in the consumption of water, the recycling of water reduces the requirement of the nutrient since recycled medium contains unused nutrients. It has been estimated that
Economics of recycling and surface cultivation vs reduction of water footprint
Surface cultivation, including algal biofilm reactor, generates comparable or even higher biomass than the conventional suspended culture. Moreover, by simple scrapping, biomass can be easily detached from the surface. Since surface cultivation does not have a harvesting burden, the net energy ratio seems more promising. It has been estimated that the biomass density of the harvested biomass in an algal biofilm photobioreactor is 96.4 kg m−3, which is significantly higher than biomass
Key challenges and perspective
Large scale open ponds requiring a massive area for cultivation contribute to high water utilization. Also, water is, to a great extent, lost by evaporation in open pond type cultivation. On the other hand, photobioreactors, even though they do not experience the ill effects of these issues, the expense of establishment is high. Therefore, procedures, including inexpensive reactors expending less volume of water must be developed. In such manner, novel systems, for example, capillary-driven
Conclusion
The use of recycling systems and high density cultures, as proposed in this review, are an efficient technique with a low water footprint. Recycling systems significantly reduce the use of water but also reduce the production of biomass in subsequent recycling steps. While high-density cultivation, including non-suspended methods, such as surface and biofilm cultivation and suspended cultivation, including trophic-based methods, does not encounter these problems. In addition to low water
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors are thankful to the Ministry of Science and Technology, Taiwan (MOST107-2113-M-037-007-MY2), Kaohsiung Medical University (KMU)-Taiwan, Research Center for Environmental Medicine-KMU, and NSYSU-KMU collaboration research project (NSYSU-KMU 107-I004)-Taiwan for research grant support. The authors are thankful to Sri Venkateswara College of Engineering – Sriperumpudur, India for supporting the work. This work also supported by the Research Center for Environmental Medicine, Kaohsiung
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