**1. Introduction**

Wastewater management has received increasing attention and interest in the context of circular economy. Different from the conventional perception, waste streams become emerging resource for valorisation instead of being treated as a problem [1]. This has gradually become a new consensus for the green transition of wastewater treatment plants (WWTPs), include the request to neutralize carbon emissions. Thereby, it is imperative to introduce new technologies and solutions to

WWTPs to alleviate their environment footprints as part of the green transition. Algae represent a promising tool to recover and recycle the residual nutrients from wastewater into bioproducts, coupled with significant carbon abatement. However, due to several constraints of scalable microalgae cultivation, it is still debatable whether the algae-based treatment technique (or phycoremediation) is a viable approach to facilitate the circular economy development of WWTPs. Therefore, this book chapter will provide a new insight to evaluate the phycoremediation for wastewater treatment.

Based on a case study, this book chapter will elaborate the potentials of using filamentous algae to treat the municipal tertiary effluent from a modern Norwegian WWTP. Both laboratory research and pilot scale tests were employed for the demonstration, since transferring the results from lab-scale R&D to pilot scale is a critical process for phycoremediation studies. Different from the conventional monoculture approach, the filamentous co-culture was exploited on purpose in order to enhance the resilience and viability of proposed phycoremediation strategy. High Rate Algal Pond (raceway) was used for the pilot study as it is an efficient system for algae wastewater treatment [2]. The depletion of nutrients was monitored to indicate the treatment efficiency, while the productivity of algae biomass was detected in the pilot tests. Finally, the biochemical constituent and elemental content of the produced algae biomass were characterized with an attempt to assess the potential valorisation. It is anticipated that this study will shed light on how to effectively deploy the phycoremediation technology to facilitate the green transition of WWTPs.
