## 4 Economic Analysis of Microalgae Wastewater Treatment Technologies  
### Microalgae-Based Wastewater Treatment Technologies  
Conventional wastewater treatment procedures consist of four main steps: preliminary, primary, secondary and tertiary treatments when physical, chemical and biological processes are used as the traditional methods [152]. Ion exchange, membrane filtration, coagulation or flocculation, bacteria-based remediation techniques and adsorption are only a few of the widely utilized techniques in traditional wastewater treatment (Figure 9).  
Figure 8: Heavy metals (HMs) mechanisms of bioremediation by microalgae.  
Our literature review demonstrates that microalgae monoculture systems have shown to be beneficial as an innovative wastewater treatment technology. Recently, microalgae have been co-cultured with other microorganisms such as bacteria, fungi and yeast to enhance pollutant removal efficiencies and reduce the cost of the treatment process [153; 154; 155; 156; 157; 158].  
Microalgae-bacteria co-culture system was used by [PERSON] et al. [155] to optimize ammonia nitrogen removal from actual rare earth element tailings (REEs) wastewater, using _Chlorococcus robustum_ as microalga species. [PERSON] et al. [155] found that this strategy of treatment achieved a 97.46% removal of ammonia nitrogen at a ratio of 1:3 of microalgae-bacteria consortium. [PERSON] et al. [158] used _Scenedesmus_ sp. with a native wastewater yeast to treat municipal wastewater and achieved a 96% removal of nitrate and a 100% removal of total ammonia with a 93% removal of orthophosphate [158]. A filamentous fungi (_Aspergillus oryzae_) was cultured with _Chlorella pyrenoidosis_ to valorize starch wastewater into high-value biomass [157]. [PERSON] et al. [157] reported that this technology increased the removal efficiencies of major pollutants, with the chemical oxygen demand, total nitrogen, and total phosphate removal efficiencies reaching 92.08, 83.56 and 96.58%, respectively. [PERSON] et al. [159] summarized these technologies and reported that microalgae such as _Chlorella vulgaris_, _Chlorella variabilis_, _Scenedesmus obliquus_ and _Scenedesmus capricornutum_ were co-cultured with Fungi species, among them _Mucor circinelloides_ UMN-B34, _Ganoderma lucidum_, _Aspergillus_ sp., _Aspergillus niger_, _Pleurotus gesetranus_ and _Pleurotus ostreatu_ to remove nutrients and cadmium by forming biopellets as well as harvesting a microalgal biomass. Furthermore, they indicated that _Chlorella sorokiniana_, _Chlorella vulgaris_, _Scenedesmus obliquus_, _Selenastrum capricornutum_ and _Spirulina platensis_ were used in microalgae-bacteria consortium with _Proteobacteria_, _Pseudomonas_ sp., _Acinetobacter_ sp., _Rhodococcus_ sp., _Mycobacterium_ sp., _Burkholderia cepacia_, _Alcaligenes_ sp. and _Azospirillum brasiliense_ for organic chemical pollutants and heavy metal removals from wastewater and greenhouse gas mitigation [159]. _Chlorella vulgaris_ was cocultured with _Yarrowia lipolytica_ for ammonia nitrogen and sulfate removals, but with _Saccharomyces cerevisiae_ for in situ CO\({}_{2}\) mitigation along with a reduction in the aeration costs of microalgae-yeast technology [159].  
In addition to bacteria, fungi and yeast, nanoparticles such as iron oxide and ZnO were also used as a consortium with microalgae to remove pollutants from wastewater [160; 161]. [PERSON] et al. [160] used an integrated approach microalgae-ZnO nanoparticle association to improve the nutrient removal from sewage wastewater and biodiesel production, and demonstrated that the microalga _Chlorosarcinopsis_ sp. MAS04-ZnO consortium achieved  
Figure 9: Traditional wastewater treatment technologies.  
the maximum biomass of 3.43 g L\({}^{-1}\) in primary treated wastewater with a 1.9-fold increase in cellular lipid compared to the BG11 medium.