Fit to Drink
Treatment
Submerged membranes pilot programme
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.
Tony Avery, Dunedin City Council's City Environment General Manager, says Council staff travelled the world seeking solutions to its particular needs before deciding on membrane technology. Membranes provide an absolute barrier to the potentially fatal cryptosporidium bug, and would future-proof the plant – hence the decision to use this technology despite the greater cost.
Membranes have been used nationally and internationally for some time in water treatment processes that force water through the equipment under pressure. Submerged membranes, by contrast, work by sucking the water through the membranes, a more energy-efficient method.
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.
