If you're an Australian and you can't stand the idea of drinking recycled water then now is the time to go to the beach, dig a deep hole, and bury your head.

According to Jurg Keller, director of the Advanced Wastewater Management Centre at the University of Queensland, in Brisbane, in less than 20 years every major Australian city will be get some of its supply from recycled drinking water.

And he should know, Keller is playing a leading research role in the development of a new A$2.5 billion advanced wastewater recycling plant in southeast Queensland.

The project, funded federally and by the Queensland state government, aims to be operational by the end of next year. In partnership with researchers from the Australian government research body, CSIRO, Keller and his team are evaluating technologies that make clean drinking water from reclaimed wastewater.

According to Peter Franzmann, of CSIRO's Urban and Industrial Water division, the research will involve "the full gamut" of technologies. "By the time the water is pumped back into the supply pipeline it will have been through something like seven separate treatment stages," he says.

But how does the process work? Here Cosmos Online guides you through the science and technology of water recycling.

To consume, or not

Recycled, or reclaimed, water falls into two categories: potable and non-potable.

Potable water is fit for human consumption, whereas non-potable is not. Currently only non-potable recycled water is used in Australia. Its applications include irrigation for agriculture, parklands and golf courses.

The new advanced wastewater recycling plant in Queensland, and others that will follow, will generate potable recycled water.

To confuse things further, there are two strategies for making recycled potable water: direct potable and indirect potable. Direct potable refers to reclaimed water that is treated and then pumped straight back into a supply pipeline. Indirect potable is water that has been similarly treated but is injected back into a natural water supply, such as a dam or aquifer, before it is pumped into the mains.

The primary treatment stages are similar for both, but CSIRO only advocates indirect potable recycled water.

Franzmann says the risks associated with indirect potable are considerably less than with direct potable because "the period of natural treatment gives you a great buffer in case there is any non-performance in any parts of the [recycling] system."

So what exactly is the recycling system?

Send me your sewage

The new advanced wastewater recycling plant will receive sewage flowing out of the city of Brisbane. Once the waste arrives it will be subjected to the same initial three-stage cleaning process that would take place at a standard sewage treatment plant.

The first stage is mechanical. Sewage is passed through a giant 'strainer' to remove large solid objects, such as sanitary towels and condoms that could clog up the system. Heavy solids like sand and grit are allowed to settle and then discarded down separate channels.

The effluent is then passed through a series of rotating screens to remove large floating contaminants, including food and plant waste. After this follows a period of sedimentation, which allows faeces to settle and oil and grease to rise to the surface, where they are skimmed off. The sedimentation containers have mechanical scrapers that remove sludge from the bottom.

The second stage involves breaking down and further removing the biological matter found in the effluent. Among the most effective and simple technologies developed to achieve this is the rotating biological contractor (RBC): plastic discs mounted on a rotating shaft are partially submerged in the wastewater. As the shaft slowly turns it lifts organic particles contained in the wastewater out of the liquid, exposing them to oxidation. This exposure to oxygen allows the organisms to multiply. A layer of biomass forms on the plastic discs that in turn accelerate the biological breakdown of organic pollutants in the wastewater. The excess biomass is then filtered from the effluent.

Insane in the membrane

Filtration would usually herald the third and final stage of standard sewage treatment, but at the new Queensland plant, the wastewater will be subjected to a process called ultra-filtration. This can be applied at different stages of water treatment, but in Queensland, will be used to prepare the wastewater for the next stage, which is reverse osmosis.

During the ultra-filtration process tiny particles are captured as the water passes through semi-permeable membranes dotted with miniscule pores. These membranes are capable of removing viruses, bacteria and other particles that are between 0.001 and 0.1 micrometers in size. This is important to ensure the cell membranes used during the next reverse osmosis phase are not clogged up.

Reverse osmosis is a reversal of the natural process of osmosis. In osmosis, pure water is attracted to salt water to the degree that it is drawn through semi-permeable solids to reach it.

During reverse osmosis, pressure is applied to salt-containing wastewater – which is separated from pure water by another semi-permeable membrane. The pressure forces the water component of the wastewater through the membrane, to join the pure water on the other side of the membrane, while leaving anything dissolved in it behind.

The pores of the membranes are so small that only the water molecules can pass through. Experts claim that the cell membrane technologies eliminate up to 99 per cent of viruses, bacteria and organic matter from the treated wastewater.

And finally…

After reverse osmosis the theoretically pure water is given a final clean using the process of oxidation.

With the Queensland plant still under construction, a decision is yet to be made whether to apply 'ozonation' or ultraviolet (UV) oxidation. In ozonation, subjecting oxygen molecules in the water to high electrical voltages destroys bacteria and other microorganisms. In UV oxidation hydrogen peroxide is added to the water, which is then exposed to intense UV light to achieve a similar outcome.

After all that the water should be good to drink. But as an added precaution with the Queensland plant, it will pumped to the state's Wivanhoe Dam for a period of mixing. The mixed water will be continually tested to ensure it is safe before being pumped back into Brisbane's fresh water supply.

Keller believes that water-recycling technology is sound. "I have a bottle of recycled drinking water here from Singapore that was made without advanced oxidation. I have drunk it myself and so far I've still got only one head and two legs – the water is of a very high quality."

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