Tracer studies on an aerated lagoon

The city of Palmerston North, New Zealand, has two aerated lagoons as its secondary treatment facility. Interest about treatment efficiency led to an investigation into the hydraulics in the second lagoon to determine if further optimisation was viable. A tracer study using rhodamine WT was undertaken to ascertain the stimulus response output. Samples were also taken at 24 points within the lagoon to determine the tracer concentration profile throughout the lagoon. The mean residence time was determined to be 39.9 hours compared to a theoretical residence time of 55.4 hours. Peak concentration of the tracer at the outlet occurred at 0.44 of the mean residence time. The results of the tracer study pointed to 28% of volume being dead space. A subsequent sludge survey indicated that 26% of the design volume of the lagoon was filled with sludge. The internal tracer sampling revealed evidence of localised concentration hotspots in the first 1/3 of the lagoon as the influent enters and mixes into lagoon. The final 2/3 length of the lagoon, however, showed an almost uniform tracer gradient. No evidence of any preferential flow paths from the inlet to the outlet was identified and comparison with the indices from the literature indicated that the lagoons hydraulic efficiency was on par with a baffled system.