Monitoring benzotriazoles: A 1 year study on concentrations and removal efficiencies in three different wastewater treatment plants

Benzotriazole (BTri), 4- and 5-tolyltriazole (4-TTri, 5-TTri) were monitored over 1 year in three wastewater treatment plants (WWTPs) with a membrane bioreactor (MBR-MH) and two conventional activated sludge systems (CAS-E, CAS-M). The influent/effluent concentrations and treatment stages removal efficiencies were monitored. 5-TTri was removed best (mean removal 80%) in the WWTP mainly by biodegradation followed by BTri (mean removal 45%) and 4-TTri (mean removal 15%) that showed a significant lower elimination. High removal fluctuations for all three benzotriazoles occurred over the four seasons with lowest removal during winter. All three WWTPs constituted a point source for BTs in the aquatic environment as concentration measurements in the receiving rivers upstream and downstream of the WWTP proved. While MBR-MH and CAS-M significantly increased the downstream concentrations, CAS-E only slightly increased the downstream concentrations as the receiving river was already contaminated with benzotriazoles from hydropower. 5-TTri was detected in lowest concentrations due to its good removal compared to BTri and 4-TTri that contribute to high effluent concentrations with the potential to accumulate due to insufficient self-purification.