Greywater—nonpotable water from showers, handwash sinks, and laundry—has become a water-efficiency buzzword in recent years, especially for its potential in outdoor irrigation. However, little research has been done on the long-term health impacts of greywater reuse, and as a result, it is only legal in about 20 states in the United States.

That is what prompted researchers at Colorado State University to undergo a three-year study that investigated the effects of using household greywater for residential landscape irrigation. Funded by the Water Environment Research Foundation and the Soap and Detergent Association, the $370,000 greywater study sampled soil, plants, and water at homes equipped with greywater systems.

According to Sybil Sharvelle, lead researcher and a professor with the Urban Water Center in Colorado State’s College of Engineering, the project was initiated by several regulatory agencies within health departments and utilities that wanted solid, scientific data about greywater irrigation that would help them formulate regulations. “These constituents were finding it very difficult to make science-based regulations,” Sharvelle says. “There just wasn’t the information out there to try to make regulations that made sense.”

The three-year study, which was just completed in April, had three main components. First, the team analyzed plant health and soil quality at homes that have been using greywater systems for irrigation for at least five years. The team then found homes that were willing to install new greywater systems, taking samples before and after the greywater irrigation was in place. The final component was a set of controlled greenhouse experiments, in which the team focused on greywater constituents leaching through soil and ending up in groundwater.

The study analyzed homes in Arizona, California, Colorado, and Texas—four states where governments have taken interest in greywater systems or where regulatory processes have been established. California, for example, has detailed regulations for greywater irrigation systems, targeted at minimizing human interaction with greywater due to concerns about pathogens. In Colorado, however, greywater reuse is currently not regulated, but the state is working to develop regulations.

Although the final report will not be available until August, Sharvelle says that overall, most of the plant species that the team sampled were not sensitive to greywater irrigation. However, there were some species—such as citrus and avocado trees—that were sensitive.

As for soil health, Sharvelle says there was too much “noise” (i.e., bird droppings) in the data to confirm the presence of pathogens of indicator organisms that would negatively affect human health. In fact, she said sometimes pathogen levels were higher in the control areas that were irrigated with potable water than in the greywater-irrigated areas. However, Sharvelle does recommend that homeowners use drip irrigation covered with mulch to minimize human contact in those areas treated with greywater.

Sharvelle’s team also measured the sodium absorption ratio (SAR)—a measure of the salts accumulating in the soil—which is often a major concern in greywater reuse. “We have found that [SAR levels] have the potential to be elevated over time, but they are not exceeding levels of concern to impact soil quality or plant health,” she says.

The homes that were studied used a range of different greywater reuse systems and methods, ranging from a simple hose connected to a washing machine that discharged the water outdoors, to a highly engineered irrigation system with sophisticated filter systems. Interestingly enough, Sharvelle says there were no notable differences among the different types of systems.

It is notable that the research included a small sample of only seven homes, but Sharvelle feels the data is comprehensive enough for regulatory agencies to safely move forward with policy development. “Early results have not suggested that there is an area of major concern, ” she says.

Sharvelle believes that greywater will be an important part of future water conservation efforts, but only if regulation is in place to ensure that people know how to do it safely. She hopes her team’s research will help put that piece of the puzzle in place. “When there is no regulation or even when it’s completely prohibited ... people tend to do whatever they want, especially since it is at their own house,” Sharvelle notes. “If there is a system in place to make it legal and to educate people about best practices, generally it’s been found that people will follow those best practices.”