Ultra small particles of the most commonly used plastics tend to move through the water supply, especially in fresh water, settle out in wastewater treatment plants, or end up as sludge in landfills, according to a study that may lead to new guidelines for water purification.

The study, published in the journal Water Research, examined what happens to these tiny, nano scale plastics that are making their way into the aquatic environment.

"We are drinking lots of plastics," said Indranil Chowdhury, study co-author from Washington State University (WSU), who led the research.

"We are drinking almost a few grams of plastics every month or so. That is concerning because you don't know what will happen after 20 years," Chowdhury said.

According to the scientists, every day about eight trillion pieces of micro plastics go through wastewater treatment plants and end up in the aquatic environment.

These little bits of plastic, they said, can come from the degradation of larger plastics or from microbeads that are used in personal care products.

In their study, the scientists studied the fate of nanoparticles of polyethylene and polystyrene, which are used in a huge number of products, including plastic bags, personal care products, kitchen appliances, disposable drinking cups and packaging material.

They assessed how the tiny particles behaved under various conditions, ranging from salty seawater to water containing organic material.

"We're looking at this more in a fundamental way," Chowdhury said.

"Why are they becoming stable and remaining in the water? Once they're in different types of water, what makes these plastics remain suspended in the environment," he said.

While acidity of water has little impact on what happens to nanoscale plastics, the study found that salt and natural organic matter are important in determining how the plastics move or settle.

Chowdhury cautioned that the tiny plastics are staying in the environment with unknown health and environmental consequences.

"Our drinking water plants are not sufficient at removing these micro and nanoscale plastics. We're finding these plastics in the drinking water but we don't know why," he said.

"Reuse plastics as much as possible," the WSU scientist said.