Tahuna Plant upgrade - stage one
There are three parts to stage one - the new pump station, odour control and the outfall.
Existing Process
The pump station at Musselburgh collects all of urban Dunedin's water, from Concord to Kaikorai Valley and Brockville, Belleknowes and Mornington to North East Valley, along the west side of the Harbour from Port Chalmers and up the eastern side of the Harbour from Portobello.
An average 40,000 cumecs of water per day is screened, with approximately 300kg of screenings collected each day and taken to landfill. The water is then pumped to Tahuna, where it goes through sedimentation tanks to remove the sludge.
The sludge is dewatered and incinerated at around 850 degrees centigrade, with some sludge going to landfill.
The new pump station
Normal flow of about 0.5 cubic metres (cumecs) will gravitate through the outfall. The new pump station is required for flows greater than this (for example, a storm event which may increase flow to around 3.5 cumecs). It will be activated with flows above 1.0 cumecs, which will happen daily for outfall flushing purposes and during wet weather events. The pump station will also pump primary treated wastewater to the secondary plant for stage two.
Odour control
Odour is managed with biofilters, basically large containers full of pine bark. The moisture in the layers of bark ensures the aerobic bacteria, which essentially eat the smell, survive and do their work. The odour control system includes large ventilation fans.
The outfall pipe
The new outfall pipe takes treated wastewater off shore 1100m from the mean high water mark (the top of the beach). At the discharge point, treated effluent will be dispersed through diffusers. Both the discharge and the pipe will be checked regularly and carefully monitored to ensure discharges meet all environmental regulations, and all life forms below the surface are growing and behaving as they should.
Where it's at
By mid-June 2008, stage one of the Tahuna upgrade is almost finished.
The odour control, pump station, control room and the contact tank (where excess flow is stored) are completed. The odour control has been described by project manager, Brian Turner, as 'excellent' and the other areas are functioning well.
There are still 20 metres of the outfall pipe to be installed. When it is in place, there will be rigorous testing of both the marine section and the plant. When the results of the tests are good enough, the entire outfall will be pressure tested.
Mr Turner expects the outfall to be commissioned (working) in August 2008.
Last reviewed: 12 Jul 2008 7:27pm
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