Effect of Hydraulic Retention Time on Nutrient Removal in a Microalgae-Based Tertiary Treatment: A Pilot-Scale Study in Winter Conditions

The wastewater treatment (WWT) industry is currently facing challenges imposed by the revised urban WWT directive, particularly in terms of nitrogen (N) and phosphorus (P) removal. This implies the need for mandatory tertiary treatment, for which microalgae cultivation shows great sustainability promise. This study investigated the impact of hydraulic retention time (HRT) on nutrient removal in open-air microalgae cultivation for tertiary WWT under winter conditions. Two pilot-scale semi-continuous raceway systems were operated with indigenous microalgae, natural sunlight, and no pH control. HRT values of 4, 5.5, and 7 days were tested, and N, P, and carbon (C) removal and recovery were measured. All conditions allowed nitrogen removal, complying with the revised urban WWT directive. Regarding P, only the 7-day HRT condition consistently complied with the directive’s lowest limit (<0.5 mg P·L−1) in the treated water, while 5.5 and 4 days left up to 0.7 and 1.0 mg P·L−1, respectively, in up to 25% of the samples. A stable microalgae consortium was established under variable light, pH, and dissolved oxygen conditions, albeit with variable biomass productivity. Elemental mass balances revealed that nutrients were mostly recovered in the produced biomass, particularly at high HRT, including effective CO2 capture from the atmosphere.