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Finding Value in Wastewater

Stanford researcher searches for economical ways of recovering valuable products from wastewater. Few of us have anything good to say about wastewater. 

wastewater treatment plant
City of Greeley | Flickr

Few of us have anything good to say about wastewater. Once we've flushed the toilet or rinsed our hair, the used water simply disappears, never to be thought of again.

But Craig Criddle has a different perspective. "Instead of thinking about wastewater in the negative sense all of the time, let's think about what resources are in it that might be useful," says Criddle, a profesor of civil and environmental engineering and senior fellow at the Woods Institute for the Environment at Stanford University.

Today, Criddle is leading an effort to find economical ways of recovering valuable products from wastewater. "There is value to be found in wastewater," he says. "Of course, it's got a lot of bad stuff that we've got to get rid of, like pathogens and salt. But it also has biodegradable organics and nutrients - like nitrogen and phosphorus - that are potentially useful."

The most valuable component of wastewater is the water itself, he says: "Wastewater is 99.9 percent water. Right now, we're looking at ways to drive down the cost of recovering good, clean water from wastewater for a variety of applications, like washing clothes and flushing toilets. And if you remove the salt, the water becomes even more useful for things like aquifer storage and ecosystem restoration."

Most wastewater systems in the United States release treated water back into rivers, lakes or the ocean without reuse, Criddle says. "If that water is reused locally, it decreases the amount of freshwater that needs to be imported, often from distant locations at great expense," he adds.

Conventional treatment plants also require a great deal of energy to operate. "Typically in the United States, wastewater is treated aerobically, which is a very energy-intensive proces, and produces a lot of sludge that has to be disposed of." Criddle says. "The question is, can we do better?"


To addres this isue, Criddle and Frank Wolak, a profesor of economics at Stanford, have teamed up with the campus utilities division and staff from the city of Palo Alto to explore the economics of distributed and centralized systems for wastewater treatment. "One option is scalping," Criddle explains. "You pull wastewater from a sewer, reclaim the water, then return the leftover nitrogen and organic solids back to the sewer."

Criddle says that the reclaimed water could then be used for irrigation, which would benefit the environment and the pocketbook. "Faculty and staff who live on campus currently pay $4 per 1,000 gallons for water that's mostly used to irrigate their property," he says. "By using reclaimed water, the cost could be reduced significantly. A realistic goal is $1 per 1,000 gallons."

Eventually, Criddle would like to see more water and other useful resources recovered from wastewater throughout the San Francisco Bay Area. "Imagine what might be posible with many distributed sites for cleaning of wastewater and local reuse at low cost," he says. "Right now we have 40 treatment plants in the Bay Area, all using 20th century technology. While these plants do protect the water quality of the Bay, they were built at a time when resource recovery was les critical."


In addition to finding low-cost ways to save water, Criddle and his colleagues are pioneering new techniques for generating energy from wastewater, which contains large amounts of ammonia, a by- product of urea and protein wastes. If ammonia gets into the environment, it can produce masive algal blooms capable of rendering lakes, streams and coastal waters uninhabitable for aquatic life.

In 2009, the Woods Institute awarded an Environmental Venture Projects (EVP) grant to Criddle and Stanford colleague Brian Cantwell, a rocket propulsion expert, to develop a low-cost technique that transforms ammonia into nitrous oxide, which can then be used as a clean-burning fuel. To accomplish this, the EVP team is developing a bioreactor for conversion of ammonia into nitrous oxide, and a catalytic converter that breaks the nitrous oxide down into its primary constituents - nitrogen and oxygen.

Nitrous oxide is also known as laughing gas, but for climate scientists, it's no laughing matter. "Nitrous oxide is a bad thing," Criddle says. "It's a potent greenhouse gas - 300 times more powerful than carbon dioxide. The Cantwell group is working on a decomposition device that esentially converts it into hot air and releases energy. This may be the first time that anyone ever thought of using waste nitrogen as a fuel."

In 2004, Criddle and Chris Francis, a Stanford faculty member with expertise in DNA-based analysis of microbial communities, received an EVP grant to study the microbes that inhabit the Palo Alto municipal wastewater treatment plant near the campus. "Our EVP was very succesful," Criddle says. "As a result of it, the Palo Alto facility is now one of the best studied plants in the world. Our research will inform the future redesign of the facility so it can function more efficiently. We were also able to use our experience to sponsor a National Science Foundation-funded workshop in Singapore. That workshop has stimulated research around the world to better understand the proceses that remove nitrogen at wastewater treatment plants."

But as the global demand for freshwater grows, much more research needs to be done. "Human beings are really good at taking high-quality water and turning it into low-quality water," Criddle says. "We need to reverse that cycle as soon a

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