A yearlong field experimental campaign was conducted to reveal time scales over which antibiotic fluxes vary in the influent of a wastewater treatment plant (WTP). In particular, sampling was carried out to ascertain the amplitudes of monthly, daily and hourly fluctuations of several antibiotics.A total of 180 samples was collected at the entrance of a WTP in Lausanne, Switzerland. Sample concentrations were multiplied by flow rate to obtain monthly, daily and hourly mass fluxes of six antibiotics (trimethoprim, norfloxacin, ciprofloxacin, ofloxacin, clindamycin and metronidazole). Seasonality in mass fluxes was observed for all substances, with maximum values in winter being up to an order of magnitude higher than in summer. The hourly measurements of the mass flux of antibiotics were found to have a period of 12 h. This was due to peaks in toilet use in the morning and early evening. In particular, the morning peak in flushing coincided with high concentrations (and hence high mass fluxes) due to overnight accumulation of substances * corresponding author. Tel: +41 (0)21-693-8024.Email address: sylvain.coutu@epfl.ch (Sylvain Coutu) April 8, 2013 in urine. However, little variation was observed in the average daily flux. Preprint submitted to Science of the Total EnvironmentConsequently, fluctuations in mass fluxes of antibiotics were mainly evident at the monthly and hourly time scales, with little variation on the day-week time scale. These results can aid in optimizing removal strategies and future sampling campaigns focused on antibiotics in wastewater.

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