San Antonio has turned its water treatment plant into a gold mine

I love a good sewage treatment plant. The wastewater treatment plant of San Antonio, Texas, USA, resembles its sister plants around the world: a wide expanse of deep, in-ground concrete tanks filled with brown liquid. There’s a faint organic odor, not unpleasant, and noise from big pumps and motors that are moving city-size quantities of water.

I’ve toured a dozen sewage treatment plants in the last couple of years while researching a book about water. I spent a week watching the staff of the City of Galveston struggle to bring the sewage treatment plant back to life after Hurricane Ike. I scrambled down a 20-foot ladder to the bottom of a half-million-gallon treatment tank. It had just been pumped out, and it was wisest not to look too closely at the debris covering the bottom.

Almost by accident, I’ve become a treatment plant tourist. And I’ve discovered that not much has changed in the world of municipal sewage treatment in the last 20 or 30 or 40 years.

Wastewater treatment is effective, but not high tech. There’s no app for that. A municipal water treatment plant has to clean millions of gallons of water every hour, and the only way to do that affordably is to use big tanks, big pumps, and lots of bacteria that like to eat the stuff in dirty water.

However, San Antonio’s wastewater treatment plant is cutting edge. The fact that it doesn’t look any different from most other plants is fitting, because most of what has changed in San Antonio is in the way people think about water there.

They don’t, for instance, think of their Dos Rios wastewater treatment plant as a costly burden. The San Antonio Water System (SAWS) sees Dos Rios as a resource mine and recycles everything the city’s residents pipe in. And it makes money doing it.

“We call this the greenest place in Texas,” says Gregg Eckhardt, a senior analyst for SAWS.

The Dos Rios plant cleans the dirty water and sends it back into a purple-pipe (the designated hue for reclaimed water in the United States) water distribution system to 65 big customers, including a Toyota truck factory, which gets a million gallons of recycled water a day, a Microsoft data center, and the Alamo.

The biggest customer for clean water from Dos Rios is San Antonio’s other famous feature: the River Walk. During the dry months of late spring and summer, the San Antonio River has no natural flow – which means that all those people in the riverside restaurants are dining alongside recycled wastewater.

“Ten years ago,” Eckhardt says, “we couldn’t give the recycled water away.” Today, not only does Dos Rios charge for the water, it’s sold out. It sells every gallon of water it can recycle.

The muddy-colored sediment that remains is called sludge, and most cities separate it from their water stream, dry it, and send it to landfills. This is the “waste” in wastewater.

San Antonio harvests about a ton of sludge from every acre-foot of water. That’s the equivalent of a pound of coffee grounds in 63 gallons, or 1,360 pounds, of water. The folks at Dos Rios wring as much water as possible out of the sludge. Then they spread it out on the Texas turf in a layer about a foot deep and 25 acres across, a field that looks like a chocolaty mud pie, and let it dry under the Texas sun. The sludge – what remains after bacteria eat everything harmful in the water – is rich organic material. It’s trucked to a commercial compost supplier, where it’s processed, bagged, and sold. By recycling the sludge – Dos Rios recycles as much as 98 percent of it – SAWS cut the cost of disposal by more than half.

Smart water management has a kind of spiraling quality: Once you’re turning the sludge into a commercial product (which SAWS has been doing since 1996) and selling the clean water, you start thinking, What else could we be selling?

Dos Rios has five enormous gas burners – the same kind you see flaring off the natural gas on oil fields. At wastewater plants, the bio-solids digesters that process and stabilize the sludge produce natural gas as a byproduct. Until September 2010, the five gas flares at Dos Rios sent enormous flames into the sky 24 hours a day, disposing of that bio-gas by burning it.

Then it occurred to the people at SAWS how silly it was to waste the natural gas. The Dos Rios plant now works with a company called Ameresco to market the gas it generates.

Dos Rios produces 1.5 million cubic feet of natural gas a day – enough to fill seven blimps. Ameresco has built a small cleaning and compression plant at Dos Rios and sends that natural gas straight into a nearby natural gas pipeline. If you live in San Antonio, the black beans that simmer tonight over your stove’s gas burner might be cooked by fuel made from the remnants of the black beans you made three weeks ago.

SAWS gets about $20,000 a month for the natural gas it used to burn. That might seem forehead-slappingly obvious, but San Antonio is the first – and, as far as the folks there know, the only – municipal wastewater plant in the United States to sell its bio-gas commercially.

In San Antonio, they don’t call Dos Rios a wastewater treatment facility – they call it a recycling plant. But what’s so inspired about the operation is how simple it is. Yes, recycling water requires a system to distribute it – but San Antonio has partnered with its biggest customers to sell the recycled water in the large quantities that justify extra spending on infrastructure. Yes, packaging your sludge and your natural gas for commercial sale requires greater effort, but it turns something that has always been regarded as waste into a useful product – and a source of savings and revenue.

No technological breakthrough was required to turn Dos Rios from a “waste” facility into a “recycling” facility. No major overhaul of a plant built more than 25 years ago. The only thing that had to be overhauled was the thinking of the people who run Dos Rios and SAWS, who learned to look around and start asking if their “waste” was really an asset.

That’s true of water in general. Once you start looking at how you use water – at how to use less, and use it more intelligently – that thinking becomes contagious.

Eckhardt walks me out to the edge of Dos Rios, where about 70 percent of the plant’s cleaned water is returned to the Medina River. Dos Rios has a longstanding agreement to supply water to cool power plants downstream. The clean water – a crisp blue-green – cascades down a 45-foot-high spillway from the level of the treatment plant into the Medina below.

Eckhardt grins. “We’re working on this too,” he says. “We’re trying to figure out if we could put a small hydropower generator at this outfall.”