A Glass Half Full

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Stanford engineers and scientists are working to turn urban waste water into a valuable resource as part of a larger effort to reinvent the urban water infrastructure.

Sewage treatment plants generally rate low on anyone’s list of preferred recreation sites, and rightly so. The architecture emphasizes utility over aesthetics, and the aroma is – well, pungent to say the least.

But Richard Luthy, the Silas H. Palmer Professor of Civil Engineering, foresees a day when you may well visit a treatment plant to walk the dog, watch birds – even have a picnic. Indeed, the sewage plant of the not too terribly distant future could resemble a large urban park more than an industrial facility: a verdant complex of ponds, marshes, woodlands and pathways teeming with wildlife.

Professor Richard Luthy
Stanford Civil and Environmental Engineering Professor Dick Luthy visits Calera Creek in Pacific, CA. The city used highly treated wastewater to restore the stream, formerly the site of barren rock quarry. He and his interdisciplinary team from ReNUWIt are studying the benefits of water reuse for stream restoration, as well as the decision-making and economics involved. Photo: Thomas Broening

“It will provide multiple benefits, and recreation will certainly be one of them,” says Luthy. “But it will also generate reclaimed ammonia and phosphorous, which can be used for fertilizers. It will yield methane, which can be burned to generate electricity. And perhaps most significantly, it will reclaim water, turning a waste product into a valuable resource.”

Water is, in fact, an extremely valuable resource; the demand for urban fresh water is escalating even as its availability is constricting. In the U.S., problems are occurring not just in the west and southwest, but in cities like Dallas, Atlanta and Tampa and elsewhere. Internationally, water resource issues are occurring in Singapore, as well as in Sydney and Brisbane in Australia.

And population increase and climate change in the dry sunbelt are only expected to exacerbate water shortages.

While this scenario is sobering by any measure, much can be done to mitigate the impacts, particularly in urban areas – and engineers and social scientists are responding to the challenge.

In 2011, Stanford, UC Berkeley, the Colorado School of Mines and New Mexico State formed the Engineering Research Center for Re-inventing the Nation’s Urban Water Infrastructure (ReNUWIt) under a National Science Foundation grant, with Luthy as the project leader; representatives from private industry also were incorporated into the group. ReNUWIt’s mission: identify new ways to supply urban water and treat wastewater, with greater efficiency, resource recovery and environmental mitigation as the foremost goals.

“We’re moving in three planes at once,” explains Luthy. “First, we’re doing fundamental research in the laboratory. Next, we’re devising test beds. Finally, we’re investigating the political and social institutions affecting urban water. This is critical. Federal, state, and local policy, funding issues – they all determine what can be built and where.”

One of the thorniest problems, Luthy and his colleagues quickly acknowledge, is current infrastructure. Existing water supply and treatment systems are – well, old and big.

“At best, current water and wastewater systems incorporate mid-20th century technology,” Luthy says. “They were built with construction costs in mind, not operating costs – so they’re highly inefficient in terms of both energy consumption and operating outlay.”

In the best of all possible worlds, continues Luthy, these antiquated systems would be ripped up and replaced with new technologies. But that’s virtually impossible. The expense would be far too great – and the very ubiquity of water and wastewater treatment and distribution systems makes their wholesale replacement technically infeasible and impractical from the public policy perspective. New technologies that save water, energy and money must therefore be embedded intelligently into existing systems.

“Our goals now are very different than when these systems were built 50, 60 years ago or more,” says Luthy. “Now the emphasis is on saving – even generating – energy, and reclaiming water, not moving wastewater from homes and dumping treated water into the ocean or a river as expeditiously as possible. So we have to figure out how to achieve those goals by incorporating the appropriate technology into existing infrastructure.”

One way to do that, says Luthy, is to decentralize current systems by establishing small “neighborhood” plants to reclaim water from the sewage stream.

The water captured at these satellite facilities could be treated and recycled for irrigation or other uses, Luthy says, while the residual solids would be sent back down the line as thick slurry to existing centralized plants. There, nitrogen and phosphorous would be extracted from the solids for fertilizer, compost would be generated as a soil amendment, and methane gas would be captured as a fuel source.

“You’d burn the methane to generate electricity that could then be used to power the treatment plants, with excess electricity sent to the electrical grid,” observes Luthy. Energy savings would be achieved by using anaerobic bioreactors to treat wastes in place of current energy-intensive aerobic systems, which neutralize contaminants by bubbling oxygen through sewage.

“Ultimately, you could have treatment systems that are energy-neutral or even energy-positive while reclaiming water,” Luthy says.

Another possibility: address wastewater at the point of use.

“Think in terms of a small, robust unit that could treat gray water right in the home or neighborhood,” says Luthy. “Again, the water would be recycled. In any case, the push is to ‘distributed’ technology – smaller, discrete but interconnected systems that provide much greater resiliency, energy conservation, and resource recovery options than the current centralized systems.”

ReNUWIt also is investigating another mode of water treatment and reclamation: Mother Nature. Researchers are convinced that strategically managed wetlands, engineered groundwater replenishment systems, and innovative stormwater basins or street side “filter strips” can augment local urban water supplies – efficiently and with low environmental impact.

Employing wetlands to treat wastewater is not new – the northern California town of Arcata treats its sewage by filtering it through a series of ponds and marshes at the margins of Humboldt Bay. By the time the treated effluent hits saltwater, it’s clean enough to sustain salmon smolts. That’s fine as far as it goes, says Luthy, but the techniques must be quantified and refined.

“We know that it works, but a lot of rigorous science needs to be done so we know exactly how and why it works under various circumstances,” he says. “We want to be able to define these processes so we can scale them up – so ultimately, we can use them not just for a town the size of Arcata (population 17,000), but for truly large cities, such as those in the Bay Area.”

That will involve everything from designing open water and vegetated ponds to determining the types of bacteria and plants needed to break down specific trace chemicals. “These will be natural systems,” Luthy says, “but make no mistake – they’ll also be engineered systems.”

And they’ll do more than clean up water, he emphasizes.

“As we’ve seen in Arcata, these wetlands can be highly attractive, biologically productive places,” Luthy says. “They can provide tremendous recreational and wildlife conservation benefits.”

ReNUWIt is also involved in the Prairie Waters Project in Colorado, an initiative that uses groundwater “infiltration basins” to help store and clean up water in the South Platte River for reuse.

“During the summer, the South Platte’s flow is mostly treated effluent from Denver,” observes Luthy. We’re basically directing the water through a bank filtration system – we percolate it through sandy zones adjacent to the river. The sand acts as a very efficient filter. The water is then put in a groundwater storage basin where it stays in the ground for several months, and then is pumped south to the city of Aurora, which has a perennial water shortfall.”

As with wetland treatment systems, the ERC is researching bank filtration and aquifer storage and recovery closely so the process can be standardized and widely applied. Among other things, the group is investigating ways to make the infrastructure footprint smaller and studying how flows, gradient and geology affect results.

Other ReNUWIt initiatives are establishing storm water infiltration basins in San Francisco East Bay communities for stormwater reuse, and designing test beds that employ mussels and clams as living filtration systems to remove particles from water. In all its projects, ReNUWIt’s overriding goal is to make maximum use of a diminishing resource in an energy-efficient and environmentally sound fashion.

But time, Luthy emphasizes, is of the essence. Decaying water infrastructure, a growing urban population, and unpredictable precipitation patterns resulting from climate change make the ERC’s work a pressing necessity rather than a leisurely academic inquiry.

“Our water treatment systems are at the end of their design life,” Luthy observes. “We need new infrastructure – and it has to be financially, environmentally, and socially sustainable. We’re getting a good idea of the basic components. Now we have to refine and integrate them, and address the institutional pressures that affect their adoption.”

Last modified Fri, 16 Nov, 2012 at 9:19