A tracer study was conducted in the winter at the Sainte Julie (Quebec, Canada) lagoon facilities to ascertain the hydraulic flow characteristics. The lagoon system is a continuous discharge type consisting of three facultative aerated lagoons and a maturation pond operating in series. The first three are provided with bottom air diffusers, the number of which decreases from the first to the third lagoon. C-diagrams indicate significant short-circuiting for all lagoons. Long tails in the C-diagrams of lagoons 1, 2, and 3 suggest the presence of stagnant or dead regions within them reducing the effective volume of the lagoons and hence the treatment efficiency that could otherwise be achieved. Values of the dispersion number, d, were 0.392, 0.452, 0.283, and 0.487 for lagoons 1, 2, 3, and 4, respectively. The jacknife statistical technique is used to assess the impact of influent flow variations on the dispersion number. Fluid flow pattern in the lagoons does not follow the simple “flow-through” behaviour. Rather, circulatory patterns are superposed on this as indicated by the multiple-peaked C-diagrams. Also, the Polprasert and Bhattarai model gives good prediction of the dispersion number for aerated lagoons.

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