Great Lakes: How super-fine membranes keep Rotorua’s water clean
Membranes containing tiny perforations just 0.05 of a nanometre across have transformed the performance of Rotorua’s water treatment plant which serves the region’s population of 70,000.
The Rotorua water treatment plant was upgraded in 2011, following legislative changes which required the plant to demonstrably reduce the amount of nutrients which were ending up in the Rotorua lakes system. Rather than building an entirely new plant, the Rotorua council retrofit a filtering system based on GE’s ZeeWeed membranes into the existing water treatment plant. Requiring less space than comparable water filtration technology, ZeeWeed is nonetheless able to remove higher levels of protozoa, bacteria, sediment, and unwanted nutrients.
To the outside observer, the membranes move in a strangely familiar fashion – much like the waving of seaweed.
The flexibility of these membranes is thanks to a filtration plane made with a super-fine nylon fibre that’s coated with a synthetic resin called polyvinylidene difluoride.
And due to their high performance, the ZeeWeed can operate in much smaller spaces than other water treatment technologies, and in many cases can be retrofit into existing facilities, as was the case in Rotorua.
Seeing the results
The plant has had a big impact on Rotorua’s environment where more than a million tourists visit each year and dangle their feet in thermal streams, or swim, fish and water-ski on the district’s 17 lakes.
“The water being released by the previous treatment plant was still very high in nitrogen and phosphorus, and this was increasing the nutrient loads in the surrounding water courses,” explained Chris Harpham, APAC Regional Technology Leader for GE Power & Water.
Extreme levels of nitrogen and phosphorus in fresh water is linked to excessive algal growth, which in turn can reduce the amount of oxygen in the water, suffocating fish and other aquatic life. In some cases, the algae can also be toxic if ingested by humans and animals.
“The Rotorua water treatment plant is the largest ZeeWeed membrane plant currently operating in New Zealand,” said Chris Harpham, APAC Regional Technology Leader for GE Power & Water.
“It’s notable because we were able to retrofit the ZeeWeed membranes into the existing tanks on the water treatment plant site, and now that it’s operating it’s led to a significant reduction in the nutrient load in the lake.”