Environment

From Toilet to Tap

Rio Rancho plans to pour effluent into the aquifer

plinkk / CC BY-SA 2.0

 

Rio Rancho’s waste is being wasted. The same is true for most cities, which treat their sewage well enough to be used for gray water purposes but then send it downriver. Due to the plight of the desert and a rapidly growing population, Rio Rancho no longer wants to send off its sewage.

The city plans to inject it into the aquifer instead.

The project sounds scarier than it is, says Bruce Thomson, professor of civil engineering and director of the Water Resources Program at the University of New Mexico. “It’s extremely low-risk,” he says, adding that the project is environmentally friendly since it conserves resources. Still, people have an instinctually negative reaction to the idea of what’s called “toilet to tap,” and some worry about the safety of the plan.

But first, a little groundwork:

Rio Rancho faces an awkward dilemma. The upstart city’s population is booming, and its water supply is shrinking. Even though Rio Rancho’s populace is only one-sixth the size of Albuquerque’s at about 87,500, it grew a staggering 69 percent from 2000 to 2010, says Peter Wells, a city spokesperson.

Many arid New Mexico municipalities grapple with the same problem, and some have found creative solutions to their water woes. Albuquerque, for instance, bought rights to San Juan-Chama water in the ’60s and started purifying and adding it to the water supply about three years ago. Since Rio Rancho is such a new city—incorporated in 1981—it didn’t have the same opportunity. The aquifer is likely the only water supply Rio Rancho will ever have.

With the city’s growth outpacing the aquifer’s recharge rate, the water table beneath the city is shrinking. If nothing is done to help restore the aquifer and the city continues to swell, eventually wells will go dry and pockets of land in the area will sink.

Rio Rancho could drill deeper wells, but “the deeper you go, the poorer the water quality,” says UNM’s Thomson, who has no connection to the project. “It’s saltier, there’s more arsenic and there are other issues.” The only way to save Rio Rancho’s water supply is to either use a lot less of it or recycle it. By injecting treated effluent into the aquifer, the city hopes to raise and then maintain the water table.

The water that flushes down your toilet takes a long journey before it ends up back in your glass, if it ever does. That journey starts with being filtered through a membrane bioreactor. Without getting overly technical, it’s a common purification system that Thomson calls “state of the art.”

Rio Rancho has treated all of its wastewater through the process for the last five years before adding it to the river. Although water treated with a bioreactor alone isn’t used for drinking water, it would “probably meet every drinking water standard that we have,” says Thomson. “The membrane bioreactor filters out virtually all of the bacteria. ... I haven’t drunk it, but if you hold it up to the light, it’s very clear.”

It then goes through additional filtration, says Rio Rancho spokesperson Wells, “to ensure that the water being injected is equal to or better than the water already in the aquifer.”

If the idea still makes your skin crawl, there are two more points to consider, and they both have something to do with the nature of groundwater. “One thing that people don’t appreciate is how slow groundwater flows,” says Thomson. “A thousand years from now, a contaminant would still be within a mile or half a mile of where it was injected.” In other words, even though treated effluent will be pumped into the aquifer, it will likely take years before any of its molecules mix with the molecules sucked up by Rio Rancho’s wells.

This unhurried pace also means that even though Rio Rancho and Albuquerque share the same aquifer, “the chances that Rio Rancho’s water will ever reach any of Albuquerque’s wells are just extremely low,” Thomson says. In fact, concerned Burqueños should take note that since several cities discharge their wastewater into the Rio Grande, such as Española, and since Albuquerque is now drinking from the river, we’re already consuming recycled water.

Rio Rancho’s plan has one more bit of science working in its favor. After the city pours its effluent into the aquifer, that water continues to be purified through a natural soil filtration process. By the time the water reaches a well, Thomson says, that “environmental buffer” will ensure it’s exceptionally clean.

Janet Greenwald, member of Agua es Vida Action Team and a co-coordinator of Citizens for Alternatives to Radioactive Dumping, has her doubts. “The EPA’s drinking water standards are very out of date,” she says. Although Agua es Vida is primarily worried about Albuquerque consuming river water, Greenwald says until the EPA’s standards are updated, “it’s difficult to say whether drinking from the river or doing toilet to tap would be safe.”

Another possible concern is that since Rio Rancho is discharging 3 million gallons’ worth of effluent into the Rio Grande each day, when it starts putting a portion of that into the aquifer instead, downstream users might be affected by the loss. To put that number in perspective, the Rio Grande pushes through an average of about 893 million gallons a day. Wells says that because the city’s contribution is so minimal, the administration doesn’t expect there to be a problem, but it’s working with the Office of the State Engineer to be sure.

Before the project goes online, Rio Rancho is going to test it in March by injecting 2.2 million gallons of potable water a day into the aquifer for 30 days. The test should give the city time to fix any operational issues and provide a better idea of exactly how well it will work.

“I think that this is an important project,” says Thomson, adding that the plan helps protect water from evaporation, maintains the aquifer and conserves resources. “This is a good way of managing our water resources in New Mexico.”