Fit to Drink
Plant design
the chosen solution – submerged membranes from Memcor Australia
Technical innovations at the Southern plant did not stop with the treatment process. Chemical storage at the plant is unusual because, although it retains traditional bunding for environmental protection, the storage area itself is fully enclosed. This prevents rainwater ingress into the bunded areas, eliminating the need for pumping after a storm; it also slows the deterioration of equipment and creates considerably pleasanter working conditions for the operators.
Downstream from the chemical storage area is a 600m3 chemical "detention" tank. All drains from the treatment plant that could contain contamination are run to this tank, meaning any spills are quickly removed and the plant recovers much faster.
Acoustics was another environmental issue considered in the design. The treatment building has residential neighbours, and a resource consent requirement for a 40dBA night-time noise limit at the boundary.
A double-skin cavity Coloursteel roof was constructed entirely from above the building – a cheaper and faster solution than the conventional sound-proofing system of a plywood ceiling under a Coloursteel roof, and one which prevented hold-ups to construction below.
Mr Oakley says it was a simple and innovative solution, as was the decision to create joint-free floors for the two adjacent 20,000m3 treated-water storage tanks. Contractor Naylor Love cast the floors in a single pour and post-tensioned them, achieving completely crack- and joint-free slabs, increasing their durability and reducing maintenance.
The walls of the tanks were also constructed from post-tensioned panels without sealant, relying on post-tensioning forces and the quality of construction to achieve watertightness.
These innovations meant the tanks were constructed at a cost of less than $100 per cubic metre of storage, which is nearly 10 percent less than any reservoir constructed in New Zealand over the past 10 years, according to an MWH survey.
A risk analysis showed fire was a potential hazard that could shut the plant down for more than the two days' storage available. This was also dealt with using a new approach.
Mr Oakley says that damage to the electrical and electronic control systems was the likely cause of a longer shut-down – "So we put them all in the same room and came up with an inert gas suppression system." Because the room remains habitable after the system is activated, and the equipment is not affected, recovery should be relatively quick.
Should a longer recovery time be needed, a degree of redundancy has been incorporated into the plant by using existing and new pipes from the nearby but higher Mt Grand plant. Raw or treated water can be sent from Mt Grand to, or through, the Southern plant, mitigating the risk to the city.
Gerard McCombie, City Council Water Operations Team Leader, says that being able to send raw water to the Southern plant from Mt Grand has meant savings in the amount of raw water that is spilt from the upper plant. As the city cannot control Mt Grand's intake from Deep Creek and Deep Stream, an average of 15,000m3 a day was being overflowed. A portion of the overflow is now being sent down for treatment at the Southern plant, making better use of the city's resources.
Eventually, the city hopes to be able to pump raw or treated water back up the hill to Mt Grand, providing additional redundancy for that plant too.
