More of Perth's sewage could be treated and injected back into the city's groundwater supply, with tests on metropolitan sewerage plants to work out their suitability for the process already underway.

The Water Corporation said while there were no firm timeframes, it was feasible that eventually 100 per cent of Perth's sewage could be recycled and put back into drinking water.

Just 10 per cent of the city's 134 billion litres of wastewater is recycled into drinking supplies via a landmark treatment plant at Beenyup in Craigie, in the northern suburbs.

The utility is currently expanding the plant's capacity from 14 billion litres to 28 billion litres a year, in a move that will cost $262 million and is set to be completed in 2019.

Water Corporation assets planning general manager Ashley Vincent said the success of the Beenyup plant was proof more of Perth's sewage could be recycled in this way.

One-tenth of Perth's 134 billion litres of wastewater is currently recycled at Beenyup. ( Supplied: Water Corporation )

"We've demonstrated through trials and with the opening of the treatment plant at Beenyup that technically it's quite achievable," Mr Vincent said.

"As we look forward, we look around at our other wastewater treatment facilities in the metropolitan area, and we certainly have a view that's it's possible to re-use that water as well.

"Very similar treatment technologies and processes could see the vast majority of Perth's wastewater being treated to a very high standard and being reinjected back into the aquifers.

"Over the next couple of years we'll be investigating the nature of the groundwater around our major treatment facilities and Woodman Point and Subiaco, or Shenton Park, looking at how suitable they are for highly treated water."

Other existing wastewater treatment facilities are located in Alkimos and East Rockingham.

Mr Vincent conceded that with some of the facilities close to industrial areas — where there was the potential for exposure to risks such as groundwater contamination — extra vigilance was needed.

However, he was confident the corporation could overcome such challenges.

"We've got strong processes to monitor the quality of water that comes into our network," he said.

The Corporation is already conducting a drilling program to investigate the viability of different aquifers for re-injection.

Climate change, population driving need

Mr Vincent said timing for when more of Perth's wastewater would be tapped for drinking was dependent on how the city grew, as well as any changes to the climate.

Recycling more wastewater will help future-proof water supplies, authorities say. ( iStockphoto: danielle71 )

"So if we continue to see our inflows to our dams reduced, and we continue to see the impacts of climate change on groundwater levels, then we might need to speed up the rate which we build new sources of water," he said.

Perth, the Wheatbelt and the Goldfields — which make up the state's biggest water supply system — typically use almost 300 billion litres of drinking supplies a year.

While desalinated drinking water now accounts for the lion's share of supply, groundwater still meets almost half of the demand, with dams used to provide less than 10 per cent.

Mr Vincent said recycled wastewater would be a good way to help future-proof Perth against the effects of a drying climate.

"It's unlikely we'd see the complete volume of water in our wastewater treatment facilities being recycled in the next 10 to 15 years, but if you start to think 20 to 30 to 40 years, recycling all of our wastewater makes a lot of sense," he said.

"It's about taking proven technologies and treatment processes and applying them to meet your ongoing water needs."

'There will be a limit', expert says

Hydrogeologist and UWA adjunct professor Don McFarlane said he supported pumping highly treated wastewater into aquifers, saying it was the best way for people to overcome the feeling they were drinking "toilet water".

"People get that yuck factor and don't want to drink it," Dr McFarlane said.

"There's a human perception that if the treated water goes back into the environment in some way, even if that's going into a river, the environment is cleaning up the water even better than what we can in a factory."

"The reality is it's very, very clean when it comes out of this highly intensive treatment process, and putting it back can make it dirty again."

But Dr McFarlane said he did have some reservations about 100 per cent of Perth's wastewater eventually being turned into drinking supplies, saying it could become very expensive.

Groundwater still meets almost half of the demand for drinking supplies in Perth. ( ABC News: Ian Cutmore )

"It takes a lot of energy and money to clean the water up and then inject it, and then pump it out again," he said.

"So I think there will be a limit for the Water Corporation, and that may come sooner than they expected."

Dr McFarlane said he was also concerned that allocating all of the city's wastewater for drinking supplies would put the utility in direct competition with other users of Perth's aquifers.

"Increasingly it will be competing with local government, horticulturalists and industry for that water supply," he said.

He said once the treated water went back into the aquifer, it became the property of the crown.

"That means the Water Corporation would actually have to go to the Department of Water and Environmental Regulation and get a license to take that water out," he said.

"Once we start getting competition for the water, the community might say, 'we can't afford to be getting water from the sea', and ask the Water Corporation to find it somewhere else."