**3. New demand for treating municipal tertiary wastewater**

In the past decades, there have been numerous attempts to employ phycoremediation technologies for the treatment of primary or secondary effluents, while less attention has been given to tertiary treated effluents. Although the advanced physical, chemical, and/or biological techniques used in modern WWTPs can remove the most nutrients, the tertiary effluents still contain considerable loads of N and P contributing to eutrophication. With more concerns of environmental eutrophication and rapid loss of nonrenewable resource of P, how to effectively eliminate the residual nutrients in the final discharge becomes an emerging request for most WWTPs. In Norway, for example, it was reported that more than 900 tons of phosphorus and about 15,000 tons of nitrogen were discharged into the ecosystem via WWTPs per annum [12]. These nutrients are equivalent to approx. 5% and 15% of agriculture P and N fertilizer consumptions as reported at a national level in 2017 [13]. Instead of releasing them to the aquatic environment as pollutants, it is apparently beneficial to recover these nutrients from the released wastewater.

Compared to primary and secondary wastewaters, tertiary treated wastewater has relatively stable pH and less turbidity, and these are acceptable conditions for algae to grow. Furthermore, along with the biological oxidation and denitrification process, tertiary wastewater contains much less organic macronutrients and the major dissolved nutrients are inorganic forms and less bioavailable organic compounds. In fact, they are the preferable medium conditions for algae proliferation. With the potential benefits of biomass valorisation and carbon sequestration, it is conceived that the application of filamentous algae to purify tertiary wastewater could represent a new win-win strategy for WWTPs. However, there is a suspicion on the algae productivity when the filamentous algae are exposed to the treated municipal wastewater. After all, the low concentration of those nutrient residuals (normally in a level of mg L<sup>1</sup> ) seems not optimal for the algae growth. In order to address this concern, the case study introduced in this book chapter will provide more details to demonstrate the possibility of employing filamentous phycoremediation for a Norwegian WWTP.
