Low-rate trickling filter with natural ventilation: diagnosis in a full-scale WWTP set in southern Brazil

The upflow anaerobic sludge blanket (UASB) reactor effectively removes organic matter from wastewater but does not remove ammoniacal nitrogen, which harms aquatic life in water bodies when in excess. The trickling filter (TF) has been used in Brazil mainly as post-treatment of anaerobic effluents to remove organic carbon and residual solids. Information on N-ammonium removal in UASB-low-rate TF systems at full treatment scale is scarce. Therefore, this work aims to evaluate the performance of a wastewater treatment plant (WWTP), with stone media, featuring natural ventilation and including nighttime effluent recirculation, operating at an average flow rate of 53 L·s^-1 in southern Brazil, which combines a UASB reactor and low-rate TF, as well as the transformations of N in the liquid phase of treatment. A 37-month monitoring period was conducted, along with seasonal 24-h sampling campaigns. Results showed that the system's efficiency was above 90% for the parameters BOD, TSS, and SS, and the results for average N-ammonium removal in the low-rate TF showed a reduction of 18% when compared to the influent. In addition, nitrification was influenced by the N-ammonium load and the recirculation occurring in the system.