Tracer studies at a full-scale lagoon used as pre-treatment facility for a water treatment plant

The water treatment plant (WTP) of Turin (Italy), fed by water of river, has a lagoon as pre-treatment facility. Since the effectiveness of the lagoon treatment towards the removal of contaminants from water depends on the residence time of the system, and the mixing aspects for the limitation of peak concentration are also very important, this led to an investigation of the internal hydraulics of the lagoon. A tracer study using sodium fluoride (NaF) was performed to determine the stimulus response output and an extensive internal sampling of the tracer was also performed to better understand the movement of the tracer through the system. The lagoon investigated in the study had a surface of 151600 m2, a volume of about 1.8•106 m3 and a nominal residence time of 18 d at average daily flow (1150 L/s). An amount of NaF equal to 180 kg was diluted with about 50000 m3 of the lagoon water and introduced in the lagoon in 12 hours, in order to simulate a pulse injection. The amount of tracer added was fixed in order not to exceed the threshold concentration of 1.5 mg/L (maximum concentration value allowed in drinking water) under the hypothesis that the lagoon was not mixed at all (plug flow modality). Samples were taken from the outlet channel every 45 minutes and analyzed using a fluoride selective probe. The sampling campaign at the outlet channel was stopped after 29 days from the introduction of the tracer, when the outlet concentration dropped under the detection limit of the fluoride selective probe. Samples were also taken at 15 points (over 3 depth values, for a total of 45 sampling points) throughout the lagoon, 1, 7 and 14 days after the tracer introduction. From the interpretation of the tracer response and the tracer distribution in the lagoon in the three days of sampling, it appears that the system was efficiently mixed without relevant short circuits. In fact the first detection of the tracer at the outlet channel occurred after very few hours from the beginning of the test and the mean residence time of the lagoon, determined using the Levenspiel formula, in the presence of the flow rate used for the test, was about 13 days compared with a theoretical hydraulic residence time of 18 days. This assured that dissolved contaminants, like fluoride, from the river were efficiently diluted by the lagoon before entering the WTP.