Two-stage systems have been proposed to overcome the drawbacks associated to the impletnentation of the autotrophic biological nitrogen removal process in the mainstream of urban wastewater treatment plants. In this study, an upflow anammox sludge blanket (UAnSB) reactor was successftilly operated for 325 days treating a low-strength synthetic influent mimicking mainstream conditions. A nitrogen loading rate of up to 1.8 +/- 0.2 g N L-1 d(-1), was achieved at 26 degrees C and the nitrogen removal rate obtained (1.7 +/- 0.1 g N L-1 d(-1)) resulted considerably higher than most of the previously reported values for systems treating low-strength wastewater at similar temperatures. Fluorescence in situ hybridization analysis showed a high enrichment in the anammox specie Candidatus BrOcadia anammoxidans during the whole operation. The evolution of the granule diameter was followed throughout the operation of the UAnSB reactor and a direct correlation of the average granule diameter with the liquid upflow velocity (V-up) was established, being the higher the V-up, the bigger the granules. A stable granule diameter of 790 +/- 40 gm was achieved by maintaining a V-up of 1.0 +/- 0.1 m h(-1). The low V-ups applied avoid the use of effluent recirculation which would present a huge inconvenient to implement UAnSB reactors at real scale, however these low V-ups led to external mass transfer problems in the reactor. In spite of the mass transfer limitations, not only a high specific anammox activity (0.26 +/- 0.02 g N g(-1), VS d(-1),) was achieved in the UASB reactor but also a high nitrogen removal (80 +/- 3%). (C) 2016 Elsevier B.V. All rights reserved.