The wastewater treatment system for Whangarei city was in need of maintenance, including a wetlands upgrade and pipeline build, ensuring more capacity and an improvement in water quality discharged from the ponds.
Heavy rain water had flooded overflow ponds due to sludge build-up and vegetation that had built up over the years, so Stage 1 of the project comprised of dewatering the wetlands, displacing the sludge and vegetation and removing 300mm from the base of two ponds. The scope of works included the inlay of two timber retaining walls providing an area for disposal of the sludge, removal of two 600mm diameter inlet pipes running the length of the wetlands, construction of concrete weirs, erection of a timber boardwalk and landscaping.
- United Civil actively researched a cost effective solution for dewatering the wetlands and sludge removal, electing to use a series of EnviroSieve dewatering tubes, which capably contain the sludge while resisting clogs. A complex pumping system was set up to siphon the sludge from the wetland ponds into the tubes, where the fine grained particles were captured and separated from the excess water.
- United Civil successfully pumped over 6 million gallons into 9 large tubes, leaving approximately 1500 dry tonnes contained within the tubes that were later incorporated into the landscape.
- The combined volume of the wetland ponds was in excess of 25,000m3, which was drained at rates of up to 60,000 gallons a day.
Close liaison with Whangarei District Council was essential throughout the project, particularly during extreme weather events, which on occasion caused the treatment plant to require emergency overflow capacity into the wetland ponds. As part of the Stage 2 Pipeline, United Civil concurrently constructed a 140m long 1050mm diameter culvert, at depths of up to 3 meters, increasing the capacity of water that could be discharged from the treatment plant to the ponds.
The upgrade has ensured the WWTP has more capacity (even in severe weather conditions) to pass treated wastewater through the wetlands, and produce a higher quality water discharge.