Tracer study and profiling of a tertiary lagoon in the United Kingdom: II

Monitoring data over a year showed that the performance of a tertiary lagoon system in the United Kingdom was very variable. Distinct seasonal patterns were seen for all the determinands. Tracer studies and profile sampling of one of the lagoons were carried out in order to investigate the factors affecting performance. The tracer studies, using sodium fluoride, indicated that the hydraulic regime was dispersed plug flow. The mean retention time was found to be 26 h, but the peak in tracer concentration occurred after 12 h, showing significant short-circuiting. During the profile sampling, the pH, temperature, DO and conductivity were measured down through the lagoon at 16 positions and four depths. E. coli, enterococci and total coliform numbers were measured in samples taken from the surface and the base of the lagoon. The results showed temperature decreased down through the lagoon, but the other parameters increased with depth, indicating that the short-circuiting was caused by the cooler influent sinking to the base of the lagoon.