**Abstract**

This book chapter demonstrated that the filamentous algae could be used as a promising phycoremediation approach to purify municipal tertiary wastewater. Initial screening of 25 algae strains across multiple genera revealed that *Spirogyra* sp. and *Klebsormidium* sp. were suitable to treat the tertiary effluent from a modern wastewater treatment plant (WWTP), and their co-culture was validated in three consecutive outdoor pilot tests. In the first two pilot tests, the nutrient concentrations of phosphorous and ammonium were depleted close to zero within 24 hours, whereas the pH value increased from 7 to 9 in the wastewater. Therefore, CO2 was added for pH control in the 3rd batch, but the nutrient removal efficacy indicated that fresh algae inoculum was critical to maintain treatment efficiency. The biomass accumulated notable amounts of Ca, Mg, K, Fe, Al, and heavy metals from the effluent, while the algae production increased by two to three times over 7 days with an average algae biomass productivity of 1.68 g m<sup>2</sup> d<sup>1</sup> . The derived biomass can be used for biogas production and biofertilizer applications based on the biochemical constituent. Given a great potential for further optimization and improvement, we provide a new insight to use phycoremediation approach to facilitate the green transition of wastewater treatment plants.

**Keywords:** filamentous algae, treated wastewater, phycoremediation, co-culture, *Klebsormidium*, *Spirogyra*
