Microalgae are a well known group of photosynthetic organisms composed of more than 3000 aquatic species. Most of them are autotrophic, while the remaining are heterotrophic. Microalgae can grow in different wastewaters and convert sunlight and atmospheric CO₂ into biomass. Their cells are able to convert and store energy instead of using it for their growth and development. Therefore, microalgal biomass can be explored as new systems for biofuels production that are a potential substitute to fossil fuels due to renewability, sustainability, and short life cycle of algal growth. Recently, microalgal biomass is recognized as a carbon neutral fuel due to various phytochemical properties of biomass [1]. For these reasons, the development of microalgal biorefineries could be effective for reducing the demand of fossil fuel and lowering greenhouse gas (GHG) emissions, which mitigates the problems associated with global warming and climate changes. Microalgal biomass is considered essential feedstock for biofuel production, because microalgae can be cultivated through the year with higher productivity [2,3,4,5]. Moreover, they are highly potential candidates for resource recovery from different types of nutrient-rich wastewater. Nutrient-rich wastewaters are generated from various industrial sectors including aquaculture, dairy, food, pharmaceutical, swine, and textile industries as well as from municipalities. The wastewater derived from the above-mentioned sectors are rich in organic and inorganic nutrients that stimulates eutrophication, which is a major threat to ecosystems. Eutrophication affects mainly fishing industries and causes an annual loss of almost 2 BUS$ (Billion US dollars) [2]. In addition, wastewater contains several toxic chemicals and pathogens, which affects the ecosystem. Moreover, in irrigation, untreated wastewater causes several issues including unnecessary vegetative growth, causing several plant diseases, leading to a decrease in the quantity and quality of crops [6]. Untreated wastewaters also cause chemical and biological contamination of ground water, leading to other negative consequences. Therefore, wastewater needs to be treated before being used in irrigation or discharged into water bodies. Traditionally, wastewater from various industrial sectors is treated using chemical (disinfection, flocculation, neutralization, and oxidation) and/or physical (floatation, grit chamber, and screening) methods [7]. Chemical/physical treatments remain expensive and generate significant quantities of slurry/sludge, which requires a secondary treatment [8]. Wastewater treatment processes consume a lot of energy (2–4% of total national electric power), and need skilled workers to operate the treatment plants which, in turn, have a high capital cost for infrastructures [9,10,11]. For these reasons, researchers are investigating microalgae-based technologies for resource recovery from wastewater and wastewater treatment. Ren et al. [12] reported that these technologies could represent a green and sustainable approach for wastewater treatment, allowing up to 95% recovery of nutrients from wastewater. During their growth in wastewater, microalgae produce biomass containing lipids, carbohydrates, and other compounds that can be used for biofuel production. Moreover, the treated water can be used in agriculture for irrigation [13,14]. As a result, these technologies can integrate wastewater treatment with biofuel production and water recycling for agriculture. Biofuel can be produced by two-stage wastewater treatment process. In the first step, microalgae are cultivated in wastewater under aerobic conditions, while in the second step they are used for biofuel production under anaerobic conditions [13,15,16,17]. Moreover, lipids can be extracted for biodiesel production from microalgal biomass [18], while the residual/leftover biomass can converted into different liquid and gases biofuel including bio-alcohols through fermentation [19], bio-H₂ through dark fermentation [3], and bio-CH₄ via anaerobic co-digestion [20,21]. Recent studies reported that algae technology has the potential to produce bioelectricity by a technology based on photosynthetic microbial fuel cells (PhotoMFC) [22,23].

Microalgae can be grown in different modes using various cultivation system including open (traditional), close (modern), turf scrubber, and hybrid (advanced) cultivation systems. However, microalgal technology coupled with wastewater treatment process require optimum nutrient load and composition of wastewater for efficient cultivation in different industrial wastewaters. Other parameters such as physical parameters of photobioreactors (PBRs) (design, volume, and volume to surface ratio) [24,25] and operating parameters (temperature, mixing, illumination, and CO₂ supply) play a significant role in nutrient recovery from wastewater [26]. Therefore, operating parameters need to be optimized to overcome key challenges of microalgal wastewater treatment technology. The algae-based resource recovery can allow the removal of pollutants from wastewater, while algal biomass can be explored for biofuel production, which can reduce the capital expenditures (CAPEX) for wastewater treatment process.

Wastewater generated from various industrial sectors contains micropollutants [27], nutrients (nitrogen, sulfur, copper, phosphorus) [28], carbon-based pollutants (antibiotics, aromatic hydrocarbons, biocides, phenolic compounds, and surfactants etc.) and heavy metals (cadmium, chromium, copper, mercury nickel, lead, and zinc) [29]. These pollutants demand an appropriate method for removal from wastewater prior to release into lakes or other water bodies [30]. As mentioned above, different biological, chemical, and physical methods can be applied for wastewater treatment [28]. Among them, biological methods are considered as sustainable and cost-effective to remove these pollutants from wastewater [27]. However, still there is no individual method that can be applied to all/various types of wastewater from different industries due to the problems linked with individual methods and varied nutrient load in wastewater. Table 1 summarizes the advantage and disadvantage of conventional and modern wastewater treatment methods. However, traditional wastewater management systems exhibited several limitations, including larger area obligation, rigorous power requirement, as well as extensive maintenance, which increases the overall costs of the wastewater treatment process [31]. Therefore, alternative wastewater treatment process needs to be explored for efficient resource recovery from wastewater [32]. Therefore, the microalgae-based wastewater treatment process is considered more effective as well as having the potential for atmospheric CO₂. Microalgae uses CO₂ and available pollutants from wastewater for cell proliferation, while algal biomass can be used for biofuel production [33].

Microalgae are potential candidates for wastewater treatment and biofuel production due to faster growth rate, zero waste generation, and can be cultivated on non-agricultural land [4]. Therefore, integration of wastewater treatment process with microalgae cultivation system could be a forthcoming green technology for biofuel and energy generation. The environmental and economic benefits are occurring by the conjugation of wastewater treatment process with microalgae cultivation for biofuel production. Several studies reported that the different types of wastewater have been successfully utilized for cultivation of different species of microalgae, which can substantially decrease the operational costs of wastewater treatment and biofuel production [3,26]. While during microalgae growth water and nutrients, light and CO₂ are necessary, among them, wastewater provides water and nutrients, while sunlight and atmospheric CO₂ can be used for cost effective cultivation [4]. Wastewater is the most suitable resource for microalgae cultivation due to various advantages such as (i) cheaper organic and inorganic carbon rich medium for growth; (ii) support large scale cultivation; (iii) able to provide ample trace elements; and (iv) able to cope up existing infrastructure wastewater treatment [34]. Several studies demonstrated the potential of microalgae-based wastewater treatment [28,30,31,35], and integrated biorefinery approaches have been proposed for biofuel production [36]. Recently, various species of microalgae have been explored for a variety of wastewater treatment, including brewery wastewater [37], domestic wastewater [38], textile wastewater [39], pharmaceutical waste streams [40], slaughter-house industry [41], heavy metal-containing wastewater [42], palm oil mill effluents [32], starch-containing textile wastewater [43], and agro-industrial wastewater [44]. Although a microalgal cultivation system for wastewater treatment offers several advantages, different challenges are also associated, which need to be remitted as summarized in Table 1.