Microalgae biomass using wastewater
The clusters obtained in the first step showed significant clustering matching to microalgae bioremediation of dairy, cassava, and coffee wastewater themes. The main link among the clusters is due to the term “microalgae”. The keywords of the articles were as follows: (i) dairy wastewater related to the removal of nutrients and associated with C. vulgaris and C. pyrenoidosa (red cluster); (ii) cassava starch to obtain lipids related to C. protothecoides (yellow cluster); and (iii) use of coffee wastewater associated with anaerobic digestion and cyanobacteria (blue, green, and purple clusters) (Fig. 1).
Understanding the connections among the groups is important because they refer to the use of microalgae for the bioremediation of agro-industrial effluents.
Studies that used microalgae for dairy wastewater treatment aimed to develop a technology to produce raw materials for low-cost biodiesel production. For instance, Woertz et al. (2009) investigated the lipid productivity and the removal of nutrients by green microalgae cultivated in dairy wastewater, which was supplemented by CO2 due to carbon limitation that accelerated microalgae growth. In addition, Johnson and Wen (2010) cultivated Chlorella sp. in dairy wastewater using foam to perform cell fixation, which resulted in better biomass and fatty acid yield. Additionally, Kothari et al. (2012) used C. pyrenoidosa in two stages: in the first stage, the wastewater quality parameters were evaluated, and nutrient removal was assessed for nitrogen and phosphorus; in the second stage, high oil and fat production was verified. Labbé et al. (2017) reported that Chlorella sp. and Scenedesmus sp. were capable of growing in different dairy farm effluents, showing that there is potential in using microalgae growth for treating these effluents and improving the finances of small and medium dairy farms.
There are few publications on the cultivation of microalgae in cassava wastewater (“manipueira”), aiming at the treatment of this effluent through algal biomass production. Yang, Ding, and Zhang (2008) used cassava powder as a raw material for C. pyrenoidosa cultivation in undiluted wastewater from ethanol fermentation to generate biomass, regulate the pH, and reduce the chemical oxygen demand (COD). However, the focus of some related studies on cassava is on organic carbon supplementation in the microalgae culture medium to increase biomass production. The use of this organic carbon source is justified by the reduction in costs, in addition to increasing biomass production and lipid accumulation (Wei et al., 2009).
Publications address the use of microalgae in the industrial process of manufacturing cassava, aiming at the improvement, simplification, and optimization of production steps; for example, a study implements the simultaneous saccharification of cassava starch (using enzymes) and fermentation (using C. protothecoides) to avoid hydrolysis in several stages of the process (Lu et al., 2010). Another study reported that when C. vulgaris was grown mixotrophically in hydrolyzed cassava waste powder, the protein content and protein productivity of the biomass increased (Abreu et al., 2012). A study using Scenedesmus sp., which was cultured to enhance the lipid production and nutrient removal from tapioca wastewater (Romaidi et al., (2018) showed the potential of using this microorganism to produce raw material for bioenergy and wastewater bioremediation.
Using different exogenous sources of organic carbon in heterotrophic growth, such as cassava starch, the C/N ratio appears to be a significant factor affecting the metabolism performance of cyanobacterium Aphanothece microscopica Nägeli (Meireles dos Santos et al., 2017); therefore, this parameter should be carefully examined to gather valuable information on how to optimize and control the performance of cultivation systems.
The feasibility of increasing bioenergy production by fermentation of non-detoxified cassava bagasse hydrolysate as an alternative carbon source for microalgae biomass production was highlighted by Lu et al. (2010) using C. protothecoides and by Liu (2018) with a consortium of C. pyrenoidosa and red yeast Rhodotorula glutinis. Using different residues, Sun et al. (2019) showed that the addition of C. pyrenoidosa biomass to rice residue and in thermo-chemical hydrolysis and biological acidification processes enhanced gaseous biofuel production during the anaerobic digestion of the raw material mixture in a short time.
Among the publications that address microalgae growing in coffee wastewater, a study by Posadas et al. (2014) was identified that evaluated a consortium of microalgae (Phormidium, Oocystis, and Microspora) and bacteria from activated sludge in five distinct fresh effluents from different agro-industries, one of them being from a lyophilized-coffee manufacturing factory. The authors detected low biodegradability, but found interesting results for nutrient recovery and microbial biomass generation.
Economic and environmental analyses associated with microalgae cultivation
The clusters obtained in the second search show the different approaches identified by the keywords related to terms such as “economic viability” and “environmental impacts.” Four groups were identified: (i) blue cluster: wastewater as a nutrient source for biodiesel generation; (ii) yellow cluster: microalgae for energy production, and clean and renewable energy sources; (iii) green cluster: microalgae cultivation to increase biomass and oil productivity, carbon dioxide biofixation, cost terms, large-scale production, techno-economic analysis, and nutrient removal; and (iv) red cluster: biodiesel production from biomass generated through microalgae cultivation (Fig. 2).
The integration of microalgae cultivation using the treatment of agro-industrial wastewater in the production of biofuels is a promising solution. The main link among the clusters is due to the term’s “growth” and “biodiesel production”. In addition to microalgae cultivation, the growth term is associated with the selection of strains that best adapt to the medium and thus, obtain higher biomass productivity; therefore, the other prominent term is “biodiesel production,” which is directly linked to the microalgae biomass acquisition process. This is because, with the decrease in fossil fuel reserves and environmental deterioration, studies involving microalgae and renewable energy sources are gaining prominence because they offer more economic and sustainable technologies.
Algae biodiesel has been the target of numerous studies because of the reduction of greenhouse gases compared to fossil fuels (Benemann et al., 2012). In addition, microalgae can be used to generate other derived chemicals, such as bioethanol, biokerosene, bioplastics, hydrogen biofuels, and biogas (Chisti and Yan, 2011).
Biofuels derived from microalgae are still not commercially viable because their costs are higher than gasoline (Cruce and Quinn, 2019). Thus, the sustainability of projects that aim to cultivate microalgae for the production of biofuels and other bioproducts is generally evaluated using technoeconomic analysis and/or life cycle assessment (LCA) (Grierson et al., 2013). One of the main “bottlenecks” highlighted by several authors with respect to the implementation of microalgae cultivation systems are the high costs arising from these processes. These can be defined as the sum of used energy, installation, pond downtime, capital costs (investment), operational, maintenance, and environmental issues, among others (Dasan et al., 2019; Strazza et al., 2015), and determinants for the implementation of algal biomass production systems (because they can result in negative economic performance).
Aiming increase the production of biofuels from microalgae, future studies should focus on the areas of biotechnology and synthetic biology related to the efficient production of several bioproducts of economic interest, overcoming the previously mentioned bottleneck (Chen et al., 2019). Besides the economic aspects, microalgae projects are garnering interest due to the reduction in their environmental impacts. Agro-industrial residues are abundant and easily available. When not treated, wastewater contains nitrogen and phosphorus, which can lead to eutrophication and environmental problems, affecting bio-system recycling (Umamaheswari and Shanthakumar, 2016). The irregular disposal of wastewater compromises the environment because the soil, when receiving constant loads above the necessary, can change its characteristics and consequently the water bodies that its holds. The changes in water quality are mainly due to the polluting agents in the water; changing the water quality from the presence of nutrients leads to the eutrophication process (disordered growth of algae and macrophytes) that interferes with water use and ecosystem balance.
Microalgae are photosynthetic microorganisms and reduce greenhouse by CO2 fixation, even when they are growing mixotrophically using organic carbon from wastewater (De Bhowmick et al., 2014); for instance, the production of 1.0 kg of microalgae biomass can fix up to 1.83 kg of CO2 (Jiang et al., 2013).
The integration of microalgae cultivation with wastewater treatment significantly reduces the environmental impacts because it is an emerging technology, and the use of agricultural and industrial waste for microalgae cultivation ensures sustainability and reduces the high costs of cultivation. Agroindustry integration through microalgal cultivation is an economically feasible and ecologically sustainable approach for wastewater treatment, bioenergy production chain, and the food industry (Andrade et al., 2020; de Carvalho et al., 2020).
Geographical distribution of publications
The importance of research on the treatment of agro-industry effluents using microalgae is represented in Figure 3, which shows the distribution of communities of countries that published studies in this area and the advancement of these publications over time. Before 2014, the USA, China, and Brazil had a higher density of publications based on the two searches. Subsequently, in 2016, India and Finland were relevant in the studies. In 2017, countries such as Greece and Iran gained interest and, finally, between 2018 and 2019, England, Qatar, Brunei (Asia), and Australia showed a high density of publications.
Overall, the number of papers showed that China, USA, and Brazil accounted for 45% of the total publications (16.5%, 16.5%, and 12%, respectively), which can be explained by the importance of the agro-industrial sector in these countries.
Researchers’ cooperation among countries and institutions highlights the importance of research involving microalgae and renewable energy sources of the 143 records found; 110 papers were written and developed by researchers of the same nationality. Moreover, 33 articles were elaborated in cooperation with researchers from other nationalities.
Four publications were developed in cooperation among different countries, highlighting the cooperation among Spanish and Brazilian researchers belonging to the Cadis University (UCA), Spain, and Federal University of Bahia, Brazil. The USA, in turn, developed cooperation with researchers from the Netherlands and Norway (Europe), in addition to studies developed together with Chinese researchers. China presented cooperation with researchers from Finland, in addition to its cooperation with USA.
Future research trends on microalgae cultivation
Studies that associate microalgae life cycle evaluation and economic technical analysis are essential to identify the paths to follow and achieve sustainability in bioenergy generation and bioproducts. The particularities and diversity of agro-industrial effluents can provide economic, environmental, and social resources from the use of microalgae in bioremediation and biomass production. The challenge of making the production of microalgae biofuels more accessible is due to the integration of biorefineries with respect to exploring other bioproducts of higher value, thus compensating the process production costs. For algae biofuels, electricity coproduction and high protein value products are the most studied in the literature, especially the study of algae flour as a food source (Cruce and Quinn, 2019).
According to Roth, Hoeltz, and Benitez (2020), Brazil is considered a pioneer in the development of technologies to produce renewable biofuels, although the country has fewer investments compared to the USA and European countries.
Fossil energy use is the main contributor to greenhouse gas emissions (GGE), and carbon dioxide emissions are the most common gas released by human activities, representing three-quarters of the global emissions of GGE (Dasan et al., 2019). Therefore, there is a need to develop renewable energy sources to meet the energy demands of the world.
In addition, public policies that benefit the cultivation of microalgae in agro-industrial effluents, through taxes on production (subsidies), financing for the sector, and carbon credits, are important to stimulate research, development, and innovation and integrate universities and public and private research agencies, while adding more and more research efforts to explore the cultivation of microalgae and their bioproducts.