**1. Introduction**

From sea ice in the Arctic to microbiotic crusts in deserts, the term "algae" refers to a wide group of (mainly) autotrophic, aquatic creatures found all over the world [1,2]. Algae are usually one of two types such as macro and micro algae. Macroalgae are generally considered terrestrial plants that returned to a damp environment, according to evolutionary theory. They are classified into red, brown, and green algae and are diverse forms of multicellular eukaryotes, each with a respective evolution pathway. They have leaves and branches and may be fixed firmly [3]. Microalgae, on the other hand, are unicellular and range in size from nanometers to millimetres. Microalgae is defined by phycologists as a creature with chlorophyll and a body (thallus) that is not divided into roots, leaves (thallophytes), and stems [4]. They comprise both the prokaryotes and eukaryotes. Microalgae fix carbon dioxide more efficiently than terrestrial plants, and are widely known for capturing both atmospheric and industrial pollutants [5].

According to the research conducted earlier, production of algae biomass are 5–10 times higher than terrestrial plants, indicating a considerable chance of increased biomass production [6]. Microalgae are considered as the most successful feedstock for biodiesel synthesis due to their high photosynthetic activity, effective capture of the emitted carbon dioxide,

**Citation:** Jayaseelan, M.; Usman, M.; Somanathan, A.; Palani, S.; Muniappan, G.; Jeyakumar, R.B. Microalgal Production of Biofuels Integrated with Wastewater Treatment. *Sustainability* **2021**, *13*, 8797. https://doi.org/10.3390/ su13168797

Academic Editor: Alessio Siciliano

Received: 14 June 2021 Accepted: 20 July 2021 Published: 6 August 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

and remarkable environmental adaptation, including high algal production [7–9]. Thus, algae utilize carbon dioxide, along with sunlight and water, to produce sugars through photosynthesis. The products thus obtained are broken down into carbohydrates, proteins, lipids, nucleic acids, etc. The fatty acid content varies with each type of algae. Fatty acids can account for up to 40–50% of the entire mass of some algae species. Table 1 details the general composition of various microalgae.


**Table 1.** General composition of various microalgae on percentage dry matter basis [10].

Current biofuel production from microalgal biomass is limited by a lack of dependable and cost-effective technologies for producing and harvesting algal feedstocks [11]. Muchrecent research has proposed that algal biomass production be combined with wastewater treatment and recycling to equalize the expense of fertilisers and freshwater necessary for microalgae growing [12–15]. This combination of algal biomass generation and wastewater treatment also helps to purify wastewater [16–18]. Microalgal cells- nutrient absorption ability can lower the nitrogen and phosphorus content of wastewater to a relatively low level, allowing it to fulfil the increasingly stricter nutrient discharge standards [19,20]. Most nutrients in the wastewater are eliminated, suggesting the possibility of integrating wastewater treatment with algal biomass production. In addition, the nutrients in the wastewater are not only eliminated from the wastewater, but they are also caught by the microalgae and returned to the environment as agricultural fertilisers. Another benefit of microalgae-based wastewater treatment is that the algal cells fix the greenhouse gas (carbon dioxide) through photosynthesis. Microalgae cultivations have been widely employed for wastewater treatment so far, demonstrating the capacity to remove nutrients from a variety of wastewater types, including wastewater from industries [21,22], municipal wastewater [23,24], cattle wastewater [14,25], and aquaculture wastewater [18]. Microalgae may utilize the nutrients in wastewater to flourish, and the wastewater may be treated at the same time. Large amounts of freshwater and nutrients necessary for algal development might be conserved by utilising wastewater. Using microalgae to remediate wastewater for biofuel production is a more sustainable and ecologically beneficial option. Algal growth is primarily reliant on carbon, nitrogen, and phosphorus. Various wastewater sources, such as urban wastewater, agricultural run-off, animal wastewaters, and industrial water, often provide adequate carbon, nitrogen, and phosphorus for algae to thrive efficiently. Algae may develop in a variety of wastewaters as long as there is enough carbon (both organic or inorganic), nitrogen (in the form of urea, ammonium, or nitrate), and other trace components [26]. Due to its efficiency and requirement, widespread production of algal

biomass for biofuel and other purposes utilising wastewaters is attracting increased interest for study.Nutrient removal efficiency of microalga in different wastewater is tabulated in Table 2.


**Table 2.** Microalgal nutrient removal efficiencies from various wastewaters.

Algae may be harvested from the treatment facility on a regular basis and used to make biofuel. In comparison to traditional wastewater treatment technologies, simultaneous wastewater treatment and algae culture can give a more cost-effective and environmentally friendly wastewater treatment. It has been demonstrated that it is a more cost-effective method of removing biochemical oxygen demand, pathogens, phosphate, and nitrogen than activated sludge [33]. This review paper discusses the various biofuels obtained through the nutrient removal in wastewater using microalgae (as shown in Figure 1). The fuels generated are biodiesel, biomethanol, biomethane, biohydrogen, bioprocessed esters and fatty acids, and synthestic fuels such as Fischer Tropsch Diesel, Dimethyl ether, and methanol.

**Figure 1.** Microalgal wastewater treatment and the application of the produced biomass.

## **2. Biofuel Production Pathways**

Biofuels using microalgae can be produced through various pathways such as (i) physicochemical pathway, which involves transesterification or esterification,(ii) biochemical conversion, which involves alcoholic fermentation, dark fermentation, anaerobic digestion, andbiophotosynthesis, and (iii) thermochemical conversions such as pyrolysis, gasification, hydrothermal processes, or hydro-processing. Biomethane is produced by anaerobic digestion [34,35], biohydrogen is generated by biophotosynthetic process [36,37], bioethanol is produced by fermentation [38,39], liquid fats by thermal liquefaction [40–43], and biodiesel by transesterification or esterification process [40,44,45].
