Effect of hydraulic retention time and environmental parameters on nutrient removal in a microalgae-based tertiary wastewater treatment: A yearlong study

Continuous development of the wastewater treatment (WWT) sector towards a stricter discharge legislation, particularly concerning nitrogen and phosphorus, is driving the need for updating existing WWT facilities. Microalgae cultivation is proven as an effective technology for the removal and recovery of these nutrients during the primary and secondary treatment stages, but its application as a tertiary polishing step requires further study. In this study, a year-long operation assessed the effects of hydraulic retention time (HRT) and seasonal changes on a tertiary WWT step, using two pilot-scale raceway systems operated semi-continuously with an indigenous microalgae consortium. HRT values of 7, 5.5, 4, and 3 days were tested, varying with the season. The temperature and peak daily irradiance ranged between 3 and 39 °C and 266–2940 μmol.m⁻².s⁻¹, respectively. All tests performed allowed the reduction of nitrogen levels to the required specification (< 8 mg N.L⁻¹). Phosphorus levels below 0.5 mg P·L⁻¹ were achieved depending on season and HRT, higher HRTs being needed under less favourable environmental conditions. Biomass productivity was higher in spring and summer, and discharge targets were achieved under lower HRTs. The maximum values for N and P removal and biomass productivity reached 0.90 g N.m⁻².day⁻¹, 0.65 g P.m⁻².day⁻¹, and 19 g.m⁻².day⁻¹, specifically. Nutrient mass balances showed that the major removal mechanism was biomass uptake throughout the experiment. Principal component analysis identified the most influential operational and environmental factors for this system.