Due to the different production materials and manufacturing technology, wastewater possesses different physicochemical properties. For example, most of nitrogen in silk industry wastewater is in the form of organic compounds, but that in swine manure is in the form of NH₄ ⁺-N (Deng et al., 2017; Deng et al., 2020). Wastewater collected from different processing units of the silk industry (i.e., cooking cocoon wastewater, reeling wastewater, and frigon wastewater) have different physicochemical characteristics (Deng et al., 2020). Moreover, some toxic compounds, such as pesticides, insecticides, antibiotics, and toxic metals, are present in agricultural wastewater (Wang et al., 2018). Thus, most wastewater need to be pre-treated or diluted before its treatment using microalgae, which accounts a fraction of the treatment cost. The frequently-used pretreatment methods are anaerobic digestion, chemical oxidation, filtration, autoclave, and UV-radiation (Sharmila et al., 2020). In the future, it is urgent to seek for simple and effective pretreatment methods, which could be used to pretreat wastewater in low cost.

Different species of microalgae have different characteristics, such as nutrient removal efficiency, tolerance to toxic compounds, adaptability and cell composition (Lu et al., 2020). For example, López-Sánchez et al. (2022) summarized the maximum biomass concentration, COD removal, total nitrogen removal, and total phosphorus removal from different microalgae species cultivated in primary livestock wastewater as a medium, and pointed out that selection of an algal strain with high performance for wastewater treatment and resource recovery was very important. In future, selection of algal strains with excellent growth characteristics and nutrient removal capacity should be carried out by different methods, such as genetic engineering, adaptive laboratory evolution, and random mutagenesis (Hassanien et al., 2023).

At present, the frequently-used microalgae cultivation facilities have been directly substituted into the field of microalgae-based wastewater treatment. For example, some open systems, such as waste stabilization ponds, raceway reactors, and sophisticated high-rate algal ponds, have been applied in this field because they are relatively inexpensive to build and easy to scale up (Leong et al., 2021). But the open systems have some main disadvantages, such as poor light utilization, evaporative losses, diffusion of CO₂ to the atmosphere, contamination by predators, heterotrophs, and the requirement of large areas of land (Cheng et al., 2021). In order to address the above disadvantages, researchers have developed various closed photobioreactors for the treatment of wastewater using microalgae, such as vertical column, tubular, flat plate, membrane, and biofilm-based photobioreactors (Sathinathan et al., 2023). But their installation and maintenance costs are much higher than that of the open ones, suggesting that they are also not suitable facilities for wastewater management. Based on the above discussion, economical, effective, and practical in situ treating facilities have not been designed and constructed for microalgae-based wastewater treatment, indicating that more attentions should be paid to this field in future.

It is well known that microalgae are rich in fatty acids, proteins, carbohydrates, lipids, and various micronutrients (e.g., vitamins, chlorophylls and carotenoids) (Tang et al., 2020). Nowadays, microalgae have been applied in various industrial areas, such as food, feed, pharmaceutical, and cosmetic industries, when they are cultivated in tap water, well water, and other clean water (Okeke et al., 2022). Studies on microalgae as animal feed and human food have been started since the early 1950s (Kusmayadi et al., 2021). In the past decades, microalgal biomass in the forms of capsule, powder, and tablet has merged in the market mainly as health food products (Kusmayadi et al., 2021). In 2018, the Food and Agriculture Organization of the United Nations (FAO) registered 87.0 tons of microalgae cultivation in 11 countries, with 86.6 tons only from China (Ferreira de Oliveira and Bragotto, 2022). However, the algal biomass could not be suitable to produce some typical products (e.g., food, feed, and biofertilizer) when microalgae grow on wastewater due to the exceeding permitted levels of heavy metals, pollutants, and pathogens. As reported previously, cadmium in the microalgal biomass was higher than the threshold established by the European regulation for fertilizing products, and three contaminants of emerging concern (hydrocinnamic acid, caffeine, and bisphenol A) were detected in the microalgae biomass used as biofertilizer (Álvarez-González et al., 2023). Although microalgae could remove pathogenic bacteria effectively from wastewater (Bhatt et al., 2023), Escherichia coli with a number of 400 CFU were found in 1 g algal biomass (Álvarez-González et al., 2023). In the light of above discussion, application scenario of algal biomass obtained from microalgae-based systems for wastewater treatment and resource recovery is restricted. Therefore, more and more application modes (e.g., algal biorefinery) should be investigated in the future.