The safety of recycled wastewater compared to groundwater-based sources has long been accepted, leading to large-scale implementation of water recycling technologies around the world. However, a recent study published by the University of California, Riverside has compared these two water sources along a new metric – flavour.
After years of drought, the notion of drinking recycled wastewater has gained momentum in California. However, consumers have been quick to flag the euphemism of "recycled." Some have even branded the technology "toilet to tap."
"It seems that this term (wastewater), and the idea of recycled water in general, evokes disgust reactions," said Daniel Harmon, a graduate student in psychology and the lead author in the study on water taste. The study published in print in the February edition of the journal Appetite.
However, Harmon added: "It is important to make recycled water less scary to people who are concerned about it, as it is an important source of water now and in the future."
The wastewater is treated using reverse osmosis and then integrated into existing groundwater or surface water sources in a process known as indirect potable reuse (IPR). Studies have found IPR removes virtually all contaminants. But no one has considered its relative taste; at least, not in a blind taste test, and not in a scientific study.
The UCR study included 143 people, who were asked to compare IP
R-treated tap water with conventional tap water and commercially bottled water. The waters were presented in similar cups and were unlabelled, hence the participants were "blind" to the source of the water. After tasting the water, participants ranked the samples' taste from one to five, then also in categories including texture, temperature, smell, and colour.
The researchers also weighed factors that influence taste perception. There are genetic differences in taste sensitivity, which were gauged using a common screening technique consisting of paper strips coated with the chemical phenylthiocarbomide, or PTC. Those who find the strip's taste to be bitter are considered to have more sensitive taste.
Researchers also considered two personality traits that help determine water preference. These traits are referred to as "Openness to Experience" and "Neuroticism." Openness is how receptive people are to novel and diverse experiences. Neuroticism refers to anxiety and insecurity.
At the outset, researchers hypothesized the three waters would score equally. In fact, one emerged the least preferred.
"The groundwater-based water was not as well liked as IPR or bottled water," said Mary Gauvain, a professor of psychology at UC Riverside and co-author of the study. "We think that happened because IPR and bottled water go through remarkably similar treatment processes, so they have low levels of the types of tastes people tend to dislike."
The more nervous, anxious people in the study expressed the preference for IPR and bottled water, and were more negative about the more mineral-rich tap water. People more open to new experiences liked the three samples about the same.
Another surprise: Women were twice as likely to prefer bottled water as men. The researchers' best guess: Women register higher "disgust reactions" than men, which means their reactions to tastes they dislike are more extreme. These disgust reactions are the subject of the team's next research paper.
In its conclusion, researchers suggest that favourable comparisons between reverse osmosis and bottled water may make consumers more amenable to drinking recycled wastewater. In particular, they suggest, marketing to women, who make most consumer purchasing decisions, should focus on these similarities, and also cater to women's demonstrated openness to new experiences.
"We think this research will help us find out what factors people pay attention to in their water decisions, and what populations need to be persuaded to drink IPR water and how to persuade them," Harmon said.
This research has been noted as quite timely, as California’s water regulation agency recently approved new measures to allow for recycled sewer water to be added to the state’s reservoirs via IPR. These measures are expected to come into effect by 2023 and could affect all 36 of the state’s reservoirs.