Experimental determination of residence time distribution in the wastewater stabilization ponds from Cabrero, using tritium as radiotracer

The hydrodynamic behaviour of wastewater treatment ponds was studied using radioactive tracers, as part of a technical cooperation programme between Chilean Institutions and the International Atomic Energy Agency (IAEA). A plant in the town of Cabrebo, 500 km South of Santiago, was selected for the study. It is a rectangular pond with a surface of 7100 m2 and a maximum volume of 12327 m3. It deliver fresh water to a population of 7000 inhabitants. The sewage flowing into the pond was labelled with tritium (3H), as tritiated water, injected punctually into the inlet and its concentration measured in the outlet of the pond, at fixed time intervals. The treated sewage samples were distilled and their tritium content measured with a Liquid Scintillation Counter. Sewage flow rate was also measured during the experiment. An experimental Mean Residence Time of 5.3 days was obtained. The experimental Residence Time Distribution analysis curve shows that part of the injected tracers leaves the pond shortly after injection, which indicates that the pond is not operating properly, being characterized as an important short circuit and probable dead zones. (author)