Tracer studies on a New Zealand waste stabilisation pond and analysis of treatment efficiency

This paper presents data collected from tracer studies undertaken at a facultative waste stabilisation pond in New Zealand. The dye used was rhodamine WT. The tracer studies were conducted using the stimulus-response technique to produce normalised curves of the hydraulic retention time distribution. Two studies were undertaken. Comparison of the results shows a rapid rise to a high peak, followed by a slow, steady decrease with a long tail. The curves are similar to what would be expected from a mixed reactor suffering from short-circuiting, Although very similar for the majority of the study period, the two runs vary during the initial period. The method described by Levenspiel, for analysis of non-linear reactions in reactors having non-ideal flow, was used to calculate treatment efficiencies from the tracer data. It is evident from these calculations that the initial period of the tracer data, corresponding to small amounts of flow exiting with very short retention periods, accounts for the majority of the pollutant that escapes treatment in the pond system. For example, over half of the coliforms discharged untreated are due to the fraction of now that exits the pond in a retention time of 24 hours or less. This highlights the potentially severe impact that short-circuiting has on attempts to achieve high treatment efficiencies. More generally it illustrates the importance of sound hydraulic pond design that avoids premature release of the wastewater. Further, in the context of pond hydraulics research it shows the importance of acquiring adequate data from the initial stages of a tracer study.