New research from Stanford University is telling us something that some of us may not want to stomach: recycled wastewater, a new paper reports, is not only safe to drink but may actually be less toxic than water from most other sources.

We can’t survive very long without water, and that period won’t be very enjoyable either. But with the number of people living on Earth continuously growing, we have to start thinking about managing our drinking water supplies in such a way that all of us get access to this vital liquid.

Wastewater recycling could help ensure that nobody needs to go thirsty, but there are concerns regarding how safe such water is for human consumption. That being said, new research now comes to explain that such concerns aren’t grounded in fact. Not only is recycled wastewater safe to drink, the team explains, but it can actually be safer and less toxic than drinking water obtained from a wide range of other sources.

Cleaned to perfection

“We expected that potable reuse waters would be cleaner, in some cases, than conventional drinking water due to the fact that much more extensive treatment is conducted for them,” says Stanford professor William Mitch, senior author of the paper. “But we were surprised that in some cases the quality of the reuse water, particularly the reverse-osmosis-treated waters, was comparable to groundwater, which is traditionally considered the highest quality water.”

The paper focused on comparing samples of conventional drinking water to purified wastewater, also known as potable reuse water. They did this by applying the water to hamster ovary cells, which are known to be very similar to human cells. They would then be monitored to see if their development slowed or stopped altogether compared to untreated cells. This approach, the team explains, allows them to gauge the toxicity of the chemicals contained in each water sample as a whole, including compounds specified in and monitored by Environmental Protection Agency (EPA) guidelines as well as those that are not.

Such an approach was preferred as, although the Environmental Protection Agency (EPA) aims to protect public health from toxic chemicals in drinking water, not all substances that are present in drinking water have been identified or analyzed by researchers. And, if the EPA doesn’t know that a compound can be present in water, or that it can be harmful, they can’t try to pass legislation against it.

One of the first key findings of the study is that the compounds regulated by the EPA accounted for less than 1% of the damage sustained by the ovary cells from conventional water samples. This was to be expected in the sense that such regulated substances would be present in trace amounts and, as such, their cumulative effect would be quite small. But the findings go to show that even conventional drinking water can carry a significant amount of toxic chemicals that we have yet to identify.

“Even if we include all these other unregulated compounds that a lot of us in this field have been focusing on, that still accounted for only about 16% of the total,” Mitch said. “It really says we’re not necessarily focusing on the right contaminants.”

The team believes that wide-scale dumping of untreated wastewater is a key source of such toxic compounds in drinking water supplies. Even pristine-looking rivers are used for the dumping of used water somewhere upstream. Even if that wastewater is treated, conventional wastewater treatment plants aren’t equipped to ‘deep clean’ the same way that potable reuse systems are. This means that many organic contaminants, from soaps to medicine, aren’t removed by conventional treatment plants, and find their way into drinking water.

The second key finding was that potable reuse water was cleaner than conventional drinking water overall and showed a much lower degree of toxicity for the hamster cells. Potable reuse treatment plants follow more stringent treatment courses, removing harmful pathogens and a wide array of contaminants through processes such as reverse osmosis, ozonation, and biofiltration.

Water from plants using reverse osmosis was found to be especially pure, being as clear if not cleaner than groundwater (the current gold standard for water purity, the team explains). Even water reuse plants that did not employ reverse osmosis produced water that was less toxic to the hamster cells than conventional drinking water sourced from US rivers, the team found.

Disinfectant used to keep water pipes clean of pathogens might be an unwitting source of toxicity in conventional drinking water, the team believes. While the use of disinfectants such as chlorine is essential to our current system, these chemicals may be interacting with chemicals already present in water and converting them to new products that are toxic to living cells. Although the EPA does regulate some disinfection products, Mitch explains that “maybe the toxicity exerted by these byproducts regulated by the government may not be so important”, Going forward, he and his team plan to further investigate this interplay between disinfectant products and chemicals in the water such as pesticides, proteins, and other organic compounds.

“We can’t get to zero contaminants. That would be ridiculously expensive, and probably unwarranted from a health point of view,” Mitch explains, adding that disinfecting water is essentially a balancing act between killing off dangerous pathogens and minimizing exposure to dangerous byproducts.

Several potable reuse treatment plants are operational in the US, with the Orange County Water District running the world’s largest such plant since the 1970s. As water uncertainty increases due to climate change, such treatment plants may become critical in the future in order to ensure that everybody has access to safe drinking water. Furthermore, the use of such plants takes some of the strain off of natural ecosystems, allowing them, too, greater access to the water they need to survive and continue performing key functions for human society.

The paper “Toxicological assessment of potable reuse and conventional drinking waters” has been published in the journal Nature Sustainability.