In 2017 Central Otago District Council upgraded their Cromwell oxidation ponds. This included installation of fifteen aerators. Over the years these proved ineffective and resulted in low dissolved oxygen being recorded in the ponds almost all year round.
The equipment had extremely low oxygen transfer efficiency and exceptionally high-power consumption. It also proved unreliable, expensive to maintain and prone to ragging.
The system consisted of fifteen diffused air bubblers, arrayed around edge of the two ponds. Each unit was fitted with two side-channel blowers. The total energy consumed by the thirty blowers was 93kW!
Upon takeover by Fulton Hogan, CODC’s operations contractor, it was apparent the bubblers were unable to lift the DO above 0.2mg/L, even with all 93kW in operation.
Considering the situation, Aeris Global offered to provide CODC with two 7.5kW aerdisc® Directional Aerators as part of a nationwide Pilot Study, which was about to commence, even though all other plants were provided only one aerdisc®.
CODC duly accepted the offer and during June 2022 the units were installed, and the 6-month Pilot Study commenced.
Within 24-hours of the first aerator starting, the DO climbed to 2.5mg/L. After the second unit came online the DO jumped to 5mg/L and continued to rise until automated control was applied.
Ian Evans, Water Services Manager, Central Otago DC comments as follows:
“We have had two 7.5kW aerdiscs running in our primary pond at Cromwell for seven months. Dissolved oxygen levels have increased significantly, and we have seen no evidence of ragging on the discs even though the plant has been running without an efficient inlet screen for some time. At the end of the trial, we purchased the two aerators and ordered an additional three: four duty and one standby. We are confident these will provide us with all aeration requirements moving forward”.
