Fit to Drink

Faced with new drinking-water standards that rated Dunedin's water as Ee, the lowest of the low, Dunedin City Council decided it had to replace one of its two main water treatment plants. Fiona Clarkson reports on the new plant, which incorporates New Zealand's first submerged-membrane water treatment process.

Dunedin City Council and project managers and civil works designers MWH New Zealand Ltd are justifiably proud of the Southern Water Treatment Plant. The use of a submerged membrane barrier against cryptosporidium and giardia spores was a first for New Zealand. And, after a lengthy lead-up, the project was completed on time, at 15 percent below budget, and was virtually problem-free from the first day of operation.

Eleven years ago, Dunedin conducted a major strategic review of its water treatment processes, after a change in drinking-water standards meant the city's water went from an A to Ee rating overnight (that's E for treatment and e for reticulation). The review concluded that a 12-year plan of capital works upgrades was needed to achieve an Ac grading, and the city's two main treatment plants, Mt Grand and Southern, would be the focus.

For the Mt Grand plant, constructed in 1977, the review recommended a reservoir to provide more raw water storage, and the addition of dissolved-air flotation and UV disinfection.

At the Southern plant, constructed in 1956 and upgraded in 1971, more drastic measures were needed. The plant only used chlorination and microstraining, leaving a bad taste in residents' mouths and frequently a murky colour in their glasses. It was decided to start again, using a greenfields site adjacent to the existing facility.

The project was undertaken in several stages. Work began in 2000 on building two 20,000m3 treated-water storage reservoirs (representing one to two days' usage) and pipe infrastructure to connect the still-bare site to existing networks.

At the same time, an 18-month-long pilot programme began testing submerged membranes as a possible treatment process. Two submerged membranes were tested, along with a traditional dual-media filter and a pressure membrane.

Finally, construction started on the treatment plant itself, using submerged membranes from Memcor Australia, traditional coagulation and flocculation processes, three UV reactors, and chemical dosing.

The multi-barrier plant started delivering water in July 2005, and now has a provisional A grade from the Ministry of Health.

This case study is reproduced with permission from e.nz magazine. Subscriptions to e.nz are discounted for schools and TENZ members.

Treatment

The Southern plant sources water from the Silverstream, near Mosgiel, and the Taieri Bores, as well as taking surplus water from the nearby Mt Grand plant (which obtains its water from Deep Creek and Deep Stream, up to 60 kilometres away).

The Taieri catchment is heavily agricultural, with run-off that can be contaminated, particularly with dairy effluent which may contain cryptosporidium. No matter which source, the water in Dunedin is cold, with water temperatures of 0.1 degrees Celcius recorded this winter.

Mr Avery says it was a hard call to make, as only one comparable plant (in Bendigo, Australia) could be viewed, and it was still under construction. Since then, however, others have come on line, and many more are now operating or under construction internationally.

Mr Oakley describes the membranes as "14,500 drinking straws in each of 1,440 cartridges". The membranes have walls like honeycombs, with pores of 0.2-micron diameter. They act as an absolute physical barrier to cryptosporidium, which is 15 times bigger than the pores. The 1.5-metre-tall cartridges hang in tanks, and water is sucked through them, removing dirt and bugs. The membranes are cleaned every half hour or so, by backwashing through the membranes for 30 seconds and removing the dirty water to the foul sewer.

UV reactors using a 253.7 nanometre wavelength are the next stage in the treatment process. They affect the DNA of cryptosporidium, so that the micro-organisms cannot reproduce if any make it past the barrier.

Mr Oakley says that chlorine is still added to help protect the water as it travels from the plant to residents, but much less is needed than previously, which has greatly improved its taste. Lime and CO2 are also used to correct pH.

Plant design

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.

Project management

All agree that a key factor in the success was the strong project control group. "We had a good balance between the operators, the executives and the consultants," says Mr Avery. And, Mr Oakley adds, "The focus was always on what was right for the project. It was good honest engineering and contracting."

The team was lucky to keep most of its members throughout the project, partly thanks to the stability of Dunedin's workforce.

A potential obstacle in the team's path was changes to the New Zealand drinking-water standards by the Ministry of Health. The new Southern plant was created to meet the drinking-water standards released in 2000, five years after the changes to the standards which lowered Dunedin's previously acceptable grade of water. However, the updated standards were not due out until after the new plant was to become operational, leaving the design team to make educated guesses about what would be needed to meet the more stringent requirements.

Significant areas of change in the 2005 standards affected the required performance for the membranes, the UV disinfection, and the data collection and reporting. The plant more than meets the new standards, and can be readily upgraded to meet any future standard changes.

Outcome