Water is essential for all lives on Earth especially for human consumption and agricultural production. Water resources have been used inefficiently as a result of rapid development of human society and the need for water. Water scarcity becomes a major problem and one of current solutions is to recycle wastewater by employing wastewater treatments. Traditional technologies of wastewater treatment are mainly divided into physical and chemical methods. Physical wastewater treatments involve separating impurities from wastewater, especially insoluble impurities. This is based on the principle of sedimentation by gravity, centrifugal force, etc. (Kreissl and Westrick, 1972). On the other hand, chemical wastewater treatment uses chemical reactions such as coagulation and flocculation processes to aggregate sludge or small suspended solids in wastewater for easy settlement and separation. However, introducing chemicals into wastewater causes a negative environmental impact and chemicals used can be expensive. Biological wastewater treatment is an alternative intriguing method that uses living microorganisms to remove contaminants in wastewater especially organic carbon (C), nitrogen (N), and phosphorus (P). These nutrients in wastewater are energy sources for microbial growth (Park and Park, 2021). This principle requires matching suitable conditions with appropriate populations of microorganisms for effective microbial activities and enabling sufficient time for nutrient assimilation (Zhang et al., 2021). For this review, N and P are the focus as they can be transformed into various value-added products such as pigments, biofertilisers, and food supplements (Mehta et al., 2018) that can transform linear economy into circular economy through waste utilisation.

Among the most widely applied nutrient removal processes from wastewater, microalgae-based systems have emerged as an economical solution due to attractive waste conversions via biomass into high-value products for Bio-Circular Green (BCG) economy. The systems can be used as either secondary or tertiary wastewater treatment (Mohsenpour et al., 2021). Other major benefits of using microalgae for wastewater treatment are (I) high biomass production, (II) potential of recycling N and P for circular economy, and (III) reduction in aeration costs by dissolved oxygen through photosynthesis (Casazza and Rovatti, 2019). In this review, tertiary wastewater treatment will be the main focus as the biomass would be easily recovered and there are still useful traces of nutrient in secondary effluents.

Historically, microalgae have been employed for wastewater treatment since the 1950 s (Jia et al., 2017). They are photosynthetic microorganisms which duplicate rapidly up to 100 times faster than terrestrial plants resulting in higher biomass productivity (Cheah et al., 2015). They have received many interests in other applications including biofuel production and CO2 sequestration (Mujtaba and Lee, 2016). Many studies found that microalgae can reduce ammonia, N, and P contents in wastewater significantly (Shawky et al., 2015). Among various microalgae species, Chlorella species have a great potential in wastewater treatment because they can withstand the harsh conditions of wastewater, have fast growth rates, and are able to generate high biomass productivity (He et al., 2013). They are also widely used as baselines in scientific research and practical biological engineering processes (Su et al., 2012). To increase the efficiency of wastewater treatment, microalgae need energy sources to grow and generate biomass without compromising effluent quality while maintaining lower operating costs. These challenges of microalgae-based systems can be solved by using mixed microalgae and bacteria (MMB) communities. The MMB communities have an advantage of complex symbiotic relationships between microalgae and bacteria that can enhance each other metabolic activities including their growth rates, nutrient recoveries, and assisting in taking up rare essential elements from their surrounding environment (Mujtaba and Lee, 2016, Nguyen et al., 2022, Yao et al., 2019).

In the past few years, there are many studies focusing on utilising MMB communities for wastewater treatment. This technique has become an emerging interest for biologically treating wastewater along with removing organic matter and N to be used for other meaningful purposes. In addition, many studies showed that MMB systems increased biomass production (Al-Jabri et al., 2020, Paddock et al., 2020). Most MMB cultivation technologies are intended to increase the removal of nutrients such as N and P during wastewater treatment (Jia et al., 2017). It was established that bacteria supply the required metabolites as an inorganic carbon source for microalgae growth and, in turn, microalgae produce oxygen through photosynthesis for bacteria (Fallahi et al., 2021, Khan et al., 2022, Lee and Lei, 2022). This symbiotic relationship can be manipulated to produce desired products and enhance the homogeneous culture of microalgae.

Recently, there have been various arrangements of MMB cultivations. The cultivations can be grouped into three main approaches: 1) suspension, 2) biofilm, and 3) immobilisation. This review focuses on comparing system arrangements for MMB cultivations particularly for N and P recovery from wastewater. This review describes the symbiotic relationship between microalgae and bacteria focusing on N and P fixation pathways as they are essential macronutrients. An evaluation on N and P removal efficiencies is conducted on existing suspension, biofilm, and immobilisation systems with MMB communities by comparing the total amount of light supplied. The review is expected to inspire future work to utilise the idea of MMB systems for recovering unused polluted nutrients in wastewater. Aspects on biomass harvesting and conversion are emphasised for future directions to advance the current technology toward completing the cycle of BCG economy. The key parameters such as balancing populations between microalgae and bacteria, balancing pH and light distribution are also discussed based on different cultivating systems.