**8. Conclusions**

Based on a realistic case study, this book chapter reveals that the mixed culture of filamentous algae *Klebsormidium* and *Spirogyra* can act as an effective tool to treat the municipal tertiary wastewater, with notable algae productivity. The co-culture can effectively recover macronutrients, mineral elements, and heavy metals from the wastewater, and their cultivation potentially can consume a considerable amount of CO2 for biomass production. Thereby, this phycoremediation process could significantly reduce or eliminate the environmental footprint of municipal tertiary wastewater in the ecosystem. Future research will focus on the remaining questions derived from the case study, which require proper optimization in areas of algae inoculum preparations, the nutrient depletion vs. carbon supplementation, hydraulic retention time reduction, and new cultivation strategy (e.g. two-stage) for continuous process. In order to improve the viability of proposed concept, the associated techno-economic analysis and environmental impact assessment will need to be deployed prior to the full-scale implementation.

Although the produced filamentous algae biomass will not be suitable for some typical algae economy purpose, their nutrient profiles and the easy scalable production can bring new hopes to other different value-added applications. As highlighted in the case study, the produced bulk biomass can become an optimal feedstock for new green fertilizer production. Although the case study was performed in pilot test, the outcomes endorse the feasibility of extrapolation to a full-scale wastewater purification or deployment. Thus, the proposed wastewater algae will represent a win-win strategy for WWTP and agriculture enterprise, as a typical model of circular economy. Apart from the contribution to the green transition of WWTP, this approach also can alleviate the pressure of soil deterioration and environment pollution due to the vast usage of chemical fertilizers. With a better understanding on the filamentous algae for municipal tertiary wastewater treatment, it is anticipated that this new phycoremediation approach can shape future investment plans of WWTPs or other new business consideration. Overall, this book chapter sheds lights on a new approach for the green transition of wastewater management and provides a new insight on the potentials of phycoremediation technology for WWTPs'sustainable development.
