17 th International Conference on Environmental Science and Technology Athens, Greece, 1 to 4 September 2021 CEST2021_00829 Start-up of full-scale UASB-vertical flow constructed wetland for domestic wastewater treatment Evangelos Statiris 1 , Taxiarchis Seintos 1 , Asimina Koukoura 2 , Daniel Mamais 1 , Constantinos Noutsopoulos 1 , Fabio Masi 3 , Anacleto Rizzo 3 , Riccardo Bresciani 3 , Óscar Prado 4 , Albert Bartrolí 4 , Josep Torà 4 , Antonis Mouflouzelis 5 , Ilias Skaltsas 5 , Vasilios Lamprinidis 5 , Simos Malamis 1* 1 Sanitary Engineering Laboratory, Department of Water Resources and Environmental Engineering, School of Civil 2 Water and Air Quality Laboratory, Department of Environment, University of the Aegean, 81100, Greece Engineering, National Technical University of Athens, 5 Iroon Polytechniou, Zografou, 15780, Athens, Greece 3 Iridra Srl, via la Marmora 51, Firenze 50121, Italy 4 Aeris Tecnologías Ambientales SL, Calle Santa Rosa 38, Cerdanyola Del Vallès 08290, Spain 5 Minavra Construction SA. Spartis 6 and Saronikou 4, Kallithea, 17673, Athens, Greece *corresponding author: e-mail: malamis.simos@gmail.com Abstract This work examined the start-up of a full-scale upflow anaerobic sludge blanket (UASB) - vertical flow constructed wetland (CW) for domestic wastewater treatment. The UASB reactors were inoculated with sludge originating from an industrial anaerobic reactor and the start-up period due to the slow growth rate of anaerobic sludge and the low temperatures (<18 o C) during this period, expanded the start-up phase to 3 months. After the start-up period, two different operational configurations were studied on the vertical flow constructed wetlands with the application, or not, of internal recirculation between the saturated and unsaturated CW beds. During the 1 st period of relatively stable operation, the UASB-CW system (with CWs recirculation mode) treated 32.3±2.5 m 3 d -1 of domestic wastewater achieving up to 92±5% of COD removal. During the 2 nd period of operation, the system operated without recirculation between the 2 different types of vertical flow wetlands and under an influent flow of 65±4.0 m 3 d -1 of domestic wastewater and achieved up to 96±1% of COD removal Keywords: UASB, constructed wetlands, domestic wastewater treatment 1. Introduction During the last decades, different wastewater treatment technologies have been applied in remote areas characterized by high seasonal variability of wastewater flow. Such areas are usually adopting the conventional activated sludge process. The activated sludge process is being criticized as an energy intensive process. Remote areas in the Mediterranean are often not able to cope with the burden of the operating expenses of this system. Recent studies are focusing in the application of other processes that offer higher performance and simplicity in terms of operation and maintenance and which can also recover resources (Ruiz et al., 2008; Singh et al., 2015). The anaerobic processes are an attractive prospect for small, decentralized areas since they can achieve sufficient treatment performance under low operational costs having the advantage of energy production in the form of biogas (Capodaglio et al., 2017). The anaerobic technology that can be applied for domestic wastewater treatment is the upflow sludge blanket (UASB) reactor. The UASB technology offers sufficient COD removal even under moderate temperatures, while at the same time biogas is produced. The main disadvantage of this technology is that this process is unable to reach the treated effluent discharge limits of developed countries, mainly due to the inability of the anaerobic bacteria to reduce the nutrient content of wastewater and due to its lower capacity to remove organic matter and suspended solids (Engida et al., 2020). In order to overcome the limited performance of the UASB technology in meeting strict effluent standards, an efficient strategy is to combine the UASB with technologies that are able to further reduce the organic content of the wastewater and at the same time remove nutrients (Tufaner, 2020). A wastewater treatment technology that offers simplicity in operation and is able to achieve high performance when used as a secondary treatment process after a UASB reactor, is the nature based solution (NBS) of constructed wetlands (CWs). The application of CWs after UASB process eliminates the problems of clogging in the wetlands while this combination of technologies offers