{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2025,10,10]],"date-time":"2025-10-10T01:39:55Z","timestamp":1760060395357,"version":"build-2065373602"},"reference-count":34,"publisher":"MDPI AG","issue":"16","license":[{"start":{"date-parts":[[2025,8,21]],"date-time":"2025-08-21T00:00:00Z","timestamp":1755734400000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"name":"FCT","award":["PRT\/BD\/152859\/2021","UIDB\/EQU\/00102\/2020","UIDB\/04565\/2020","UIDP\/04565\/2020","LA\/P\/0140\/2020"],"award-info":[{"award-number":["PRT\/BD\/152859\/2021","UIDB\/EQU\/00102\/2020","UIDB\/04565\/2020","UIDP\/04565\/2020","LA\/P\/0140\/2020"]}]}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Sustainability"],"abstract":"<jats:p>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\u2019s lowest limit (&lt;0.5 mg P\u00b7L\u22121) in the treated water, while 5.5 and 4 days left up to 0.7 and 1.0 mg P\u00b7L\u22121, 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.<\/jats:p>","DOI":"10.3390\/su17167553","type":"journal-article","created":{"date-parts":[[2025,8,22]],"date-time":"2025-08-22T07:41:45Z","timestamp":1755848505000},"page":"7553","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":0,"title":["Effect of Hydraulic Retention Time on Nutrient Removal in a Microalgae-Based Tertiary Treatment: A Pilot-Scale Study in Winter Conditions"],"prefix":"10.3390","volume":"17","author":[{"given":"Sofia","family":"Vaz","sequence":"first","affiliation":[{"name":"A4F\u2014Algae for Future, Campus do Lumiar, Edif\u00edcio E-R\/C, Estrada do Pa\u00e7o do Lumiar, 1648-038 Lisboa, Portugal"},{"name":"University of Coimbra, CERES\u2014Chemical Engineering and Renewable Resources for Sustainability, Department of Chemical Engineering FCTUC, Rua Si\u0301lvio Lima, P\u00f3lo II, 3030-790 Coimbra, Portugal"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-1376-0829","authenticated-orcid":false,"given":"Rui","family":"Martins","sequence":"additional","affiliation":[{"name":"University of Coimbra, CERES\u2014Chemical Engineering and Renewable Resources for Sustainability, Department of Chemical Engineering FCTUC, Rua Si\u0301lvio Lima, P\u00f3lo II, 3030-790 Coimbra, Portugal"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-7846-6546","authenticated-orcid":false,"given":"Helena M.","family":"Pinheiro","sequence":"additional","affiliation":[{"name":"iBB\u2014Institute for Bioengineering and Biosciences, Associate Laboratory i4HB\u2014Institute for Health and Bioeconomy, Department of Bioengineering, Instituto Superior T\u00e9cnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-5232-2018","authenticated-orcid":false,"given":"Laura","family":"Monteiro","sequence":"additional","affiliation":[{"name":"A4F\u2014Algae for Future, Campus do Lumiar, Edif\u00edcio E-R\/C, Estrada do Pa\u00e7o do Lumiar, 1648-038 Lisboa, Portugal"}]}],"member":"1968","published-online":{"date-parts":[[2025,8,21]]},"reference":[{"key":"ref_1","unstructured":"Office of the European Union (2024). Directive (EU) 2024\/3019 of The European Parliament and of the Council of 27 November 2024 Concerning Urban Wastewater Treatment, Office of the European Union."},{"key":"ref_2","doi-asserted-by":"crossref","unstructured":"Derco, J., Gu\u013ea\u0161ov\u00e1, P., Legan, M., Zakhar, R., and \u017dgajnar Gotvajn, A. (2024). Sustainability Strategies in Municipal Wastewater Treatment. 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