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As a partner in the Pond Management team with Watercare Services at the Mangere Wastewater Treatment Plant, Fletcher Construction owns dredges, which were purpose designed and built in Auckland for dredging sewage treatment ponds in New Zealand.
PipeWorks were contracted to dredge the number 2 & 3 ponds at Orewa WWTP for Rodney District Council.
How did it work?
The sludge were removed from the bottom of the pond by the 4m wide rotating auger head (or cutter head), the vanes of which brought the sludge to the 200mm diameter PE suction pipe. The discharge pipe floated on the pond surface and delivered the sludge as far as 1 km away.
Sludge removal were maximised through an accurate dredge positioning system. The dredge moved up and down the pond in a similar manner to mowing a lawn.
Alignment buoys of different colours were set up on a rope, which spanned from the corner piles, perpendicular to the direction of travel as a guide which enabled the dredge operator to keep the dredge on line.
The rate of the dredge advance was governed by the pump's ability to remove all the sludge cut.
Visual monitoring of the oxidation pond was essential to ensure that the contracts works did, not adversely impact the stability of the pond operation and the quality of effluent.
Based on the experience gained at Mangere, we were confident that the contract works would not adversely impact on pond operation or the quality of effluent during the project.
The following issues have been considered specifically for this project:
- Discharge of excess suspended solids
- Stability of pond operation
Project facts:
- All quality, environmental and health & Safety objectives were met
- The main bulk of the works were completed from the 20/08/07 thru 16/11/08
- Over this period of 77 days, 1282 tonne of dried sludge was processed at an average of 16.65T per day
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