It’s easy to forget where all that water goes after a toilet flushes or a tub drains. Although the ancient Romans built aqueducts to pipe away foul-smelling water, the wastewater treatment field as it exists today only came into practice in the mid-20th century.
In the 1950s, fish were going belly-up by the millions as insufficiently treated wastewater was piped into local waterways, prompting municipalities to reconsider how they handled water treatment.
“You can talk to people who grew up in San Francisco then, and they will tell you how driving over the Bay Bridge they had to keep the windows rolled up because of the rotting smell in the Bay,” says William Mitch, Stanford associate professor of civil and environmental engineering.
The Clean Water Act of 1972 brought federal funding and oversight to the issue, and resulted in the nationwide deployment of wastewater treatment plants, with projected lifespans of 40 to 50 years. Now, decades later, these facilities are aging out and beginning to fail. Replacing them is necessary, but using technology from that era, when electricity cost next to nothing, would be like buying a ’73 Buick to use as your commuter car.
“We know this is a transformational moment in our sector,” says Sebastien Tilmans, operations director at Stanford’s William and Cloy Codiga Resource Recovery Center, which has become a proving ground for wastewater technology on campus since it opened in 2016. “We have a narrow window of opportunity when we can demonstrate the viability of alternatives.”
In 2014, Professor Mitch and Professor Craig Criddle, colleagues in the Department of Civil and Environmental Engineering, won a seed grant from the TomKat Center for Sustainable Energy to pursue a bench-scale study on anaerobic wastewater treatment capable of producing drinking-quality water using less energy. Their research could shape the future of tap water.
A different approach
To understand their accomplishment, it helps to know more about how wastewater treatment works. Raw sewage typically goes through four main steps to clean it up: (1) screening large solids; (2) settling out the smaller suspended particles; (3) allowing oxygen-consuming (aerobic) microorganisms to convert dissolved human waste into carbon dioxide; and finally (4) chemical disinfection.
Of all those steps, the third one is the energy hog. Treatment plants bubble oxygen into wastewater in order to entice aerobic bacteria to break down the dissolved organic matter suspended in the water, a crucial action to avoid killing fish when the water is released. Yet this constant bubbling requires lots of electricity—nearly half of the treatment plant’s energy consumption.
“The Holy Grail of wastewater treatment is to switch from an energy-intensive aerobic system to an anaerobic system,” says Mitch, allowing the treatment facilities to skip over the bubbling step. Instead of feeding on oxygen, anaerobic bacteria ferment the dissolved organic matter and release methane, which could potentially be captured for electricity production.
Making this switch has eluded scientists for years because anaerobic bacteria grow more slowly than their oxygen-consuming aerobic cousins. Up to this point, their inefficiency has made their use impractical for the millions of gallons that must pass through a treatment facility each day.
So how do you speed them up?
The answer came from another Stanford colleague: Perry McCarty, the Silas H. Palmer Professor of Civil Engineering Emeritus and a past winner of the Stockholm Water Prize. McCarty made a discovery: anaerobic bacteria love to grow on activated carbon particles. They form a biofilm on the carbon, creating a higher density of the anaerobic critters. Think of it as a concentrated form, like bouillon cubes ready to be added to wastewater.
“You make up for their inefficiency by having more hungry mouths to feed,” Mitch explains.
See it to believe it
McCarty has overseen a pilot-scale version of an activated carbon-based anaerobic reactor at a wastewater treatment plant in South Korea for the past three years at the invitation of the Korean government. Despite this success, however, U.S. municipalities want to see the anaerobic method demonstrated here in the United States. They are cautious to adopt a new technology until it is thoroughly proven to U.S. standards.
Convinced of its promise, Mitch and Criddle set out to develop the first anaerobic system tested in the Western Hemisphere, and their results again proved that an anaerobic system can keep pace with the traditional wastewater methods while requiring less energy.
They are also fine-tuning the disinfection step of treatment, which was not part of the Korean study. There the treated wastewater was released back into a local river rather than being tested for reuse.
In the United States, the goal is to harvest methane generated by the anaerobic treatment to generate electricity as well. This captured energy could then be used to power additional advanced treatment steps needed to convert wastewater to potable water. In drought-prone areas, this scenario could allow utilities to retain their precious water resources and generate drinking water without extra energy expenditures.
The TomKat Center grant provided a pipeline to more funding and buy-in from partners in the field. The researchers received a $1.17 million grant from the California Energy Commission to collaborate with Silicon Valley Clean Water, and they are currently discussing a pilot project with Redwood City.
Mitch and Criddle expect interest to grow if their approach continues to scale up. “Once you get 10 big utilities to adopt a new technology, other major cities are willing to take the risk,” says Mitch. Nationwide, water purification costs more than $4 billion annually—and cities often spend one-third of their electricity budgets treating water and wastewater—so the new technology could save municipalities both water and taxpayer dollars.
This is just the beginning; the impact could spread like ripples in a pond.