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Algae cultivation complements wastewater treatment (WWT) principles as the process uptakes nutrients while assimilates CO2 into biomass. Thus, the application of algae-based WWT is on the upward trajectory as more attention for recovery nutrients and CO2 capture while reducing its economic challenge in the circular economy concept. However, the complexity of wastewater and algal ecological characteristics induces techno-economic challenges for industry implementation. Algae-based WWT relies totally on the ability of algae to uptake and store nutrients in the biomass. Therefore, the removal efficiency is proportional to biomass productivity. This removal mechanism limits algae applications to low nutrient concentration wastewater. The hydraulic retention time (HRT) of algae-based WWT is significantly long (i.e. > 10 days), compared to a few hours in bacteria-based process. Phototrophic algae are the most used process in algae-based WWT studies as well as in pilot-scale trials. Application of phototrophic algae in wastewater faces challenges to supply CO2 and illumination. Collectively, significant landscape is required for illumination. Algae-based WWT has limited organic removals, which require pretreatment of wastewaters before flowing into the algal process. Algae-based WWT can be used in connection with the bacteria-based WWT to remove partial nutrients while capturing CO2. Future research should strive to achieve fast and high growth rate, strong environmental tolerance species, and simple downstream processing and high-value biomass. There is also a clear and urgent need for more systematic analysis of biomass for both carbon credit assessment and economic values to facilitate identification and prioritisation of barriers to lower the cost algae-based WWT. Graphical abstract

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Tilapia is one type of freshwater fish that has a lot of demand in the market. Therefore, the production of tilapia aquaculture is increasing. Increased production of aquaculture causes a decrease in water quality due to the large number of metabolic wastes. Fish farmers usually require a large amount of money to overcome the decline in the quality of aquaculture pond water. For this reason, it is necessary to apply techniques to improve the quality of aquaculture pond water at an affordable cost. Azolla sp and Ipoemea aquatica are plants that can absorb environmental contaminants. The research method used was experimental method with untreated pool water samples (control), pool water samples treated with kale (Treatment 1), pool water samples treated with Azolla sp. (Treatment 2) and pond water samples with the combination treatment of kale and Azolla sp. (Treatment 3). Ammonia levels in Tilapia pond water samples were statistically analyzed using the SPSS application. The results showed that kale and Azolla sp. tends to be effective in reducing ammonia levels in Tilapia rearing pond water. The water spinach treatment had a higher value in reducing ammonia levels in the sample water of the tilapia rearing pond, which was 5.99 compared to the Azolla sp. of 3.22 and the combination treatment of 5.95.