Dynamics of antibiotics concentrations in the environment at different time scales

The aim of this study was to assess the most relevant time scales of antibiotics concentration variations at the sewage treatment plant (STP) inlet and to create a model for the simulation of short-term concentration variations. During one year six antibiotics (ciprofloxacin, clindamycin, metronidazole, norfloxacin, ofloxacin, and trimethoprim) were being successfully and repeatedly measured at the inlet of the STP of Vidy in Lausanne. The campaigns were arranged that way that seasonal, daily and hourly mass flow rate variations of the antibiotics could be analysed. The measurement campaigns confirmed the occurrence of seasonal antibiotics variations. The hourly variations showed about the same order of magnitude as the seasonal ones. The day-to-day variations were in general found to be not as heavy as seasonal and hourly variations. A model simulating the antibiotics concentrations at the STP inlet on an hourly basis has been developed. It is applied for three antibiotics (ciprofloxacin, norfloxacin and trimethoprim) in the catchment area of the STP of Vidy in Lausanne. The modelled antibiotics concentrations have in some cases another order of magnitude compared to the measurements. This could be due to seasonal effects that cannot be adjusted by the calibration such as the antibiotics losses during sewer transit that are not known a priori. It could also be due to hypotheses made on the amount of consumed antibiotics. The modelled detailed hourly dynamics are limited in their validity because of the random noise due to the Monte Carlo modelling approach. Moreover, the uncertainties concerning the toilet use attitudes are limiting for these short-term dynamics. The view on the general modelled dynamic throughout the day reveals a first antibiotic peak in the morning between 07:00 and 10:00 and a second one in towards evening between 17:00 and 21:00 which corresponds perfectly to the findings of the measurement analysis. The development of the model allowed identifying the most important parameters contributing to short-term antibiotics dynamics namely the amount of consumed antibiotic and the number of persons administering the medicine, the behaviour of the antibiotics’ accumulation in the urine and the toilet use habits.