Investigation of pond velocities using dye and small drogues: a case study of the Nelson City waste stabilisation pond.

The city of Nelson, New Zealand, has a 27 hectare oxidation pond as its primary wastewater treatment facility. Recent changes in the configuration of the pond and installation of a mixer/aerator raised concerns that pond treatment rates and effluent quality may be affected by high internal pond velocities and short retention times. This paper presents the findings of an investigation into wastewater velocity and movement within the pond using qualitative dye dispersion and tracking of small-scale "holey-sock" drogues. Simultaneous deployment of drogues and dye allowed methods to be compared, since small-scale drogues have not commonly been used in wastewater ponds. Dye dispersion was assessed using low-level aerial photography from a tethered helium blimp to track short term movement and mixing, while a datalogger and fluorometer were used to measure pond retention time. Drogue movement was tracked in conjunction with the dye study from a small boat using hand-held GPS. The dye study found that: (i) the first portion of pond influent discharged from the pond after 37.5 hours, substantially quicker than the theoretical pond retention time of 27 days. However, the measured retention time was with a mixer in place and the theoretical time was without a mixer; (ii) the position of the paddle wheel mixer/aerator was not optimally placed for mixing the influent and a quiescent region existed adjacent to the influent point; and (iii) the low-level aerial photography was an effective method of evaluating larger pond systems. The "holey-sock" drogue studies showed that: (i) the drogues accurately followed the movement and velocity of dyed influent within the pond; (ii) wastewater velocity and movement was dominated by the paddle wheel mixer/aerator; and (iii) wind direction had a minor influence on wastewater velocity and movement in areas not directly affected by the paddle wheel mixer/aerator. The study demonstrated that the combined use of dye and drogues was a relatively low-cost and effective means of determining internal pond velocities and movement. Future studies using similar methods will be useful in helping validate computer-modelled movement and velocity.