Member Case Study – June 2018

  • by Will Holbert
  • Jun 15, 2018
  • Newsletters

Nothing Goes to Waste in South Tahoe Public Utility District’s Wastewater Treatment Operation

Everyone knows what runs downhill, but the South Tahoe Public Utility District knows how to recycle it into 381,000 kilowatts a year.

Known for its blue waters, incredible depth, and endless recreation opportunities, Lake Tahoe attracts around 3 million visitors each year.  With a population of 21,000, South Lake Tahoe is the largest city in the Sierra Nevada range. Supplying most of the drinking water and all of the sewage collection and treatment is the South Tahoe Public Utility District (STPUD).

A turbine and supervisory control and data acquisition system inside STPUD’s Hydroelectric Building.

Last year, STPUD became the first wastewater district in the Lake Tahoe Basin to produce its own hydroelectricity. How did a wastewater treatment plant get into the energy business? It all started back in 1969, when every wastewater district around Lake Tahoe was required by law to export treated wastewater out of the Tahoe Basin to keep Lake Tahoe blue.  As such, STPUD pumps all of South Lake Tahoe’s treated wastewater — an average of 3.9 million gallons per day (MGD) — 26 miles to Alpine County, where it’s used by ranchers for irrigation and by STPUD to grow grain for livestock on its own 1,400-acre Diamond Valley Ranch.

Pumping the treated wastewater up the Sierra Nevada requires an enormous amount of energy, but once it reaches the top, 2,200-feet of gravity takes over. Harnessing the resulting water pressure wasn’t a new idea for STPUD, and similar systems operate throughout the world. But it fulfilled a long-standing goal for the district, which prides itself on innovative environmental stewardship. However, cost-benefit analyses of investing in renewable energy didn’t pencil out until recently.

“It’s something we’ve been dreaming about for a long time, but have been waiting for it to make financial sense,” said Shelly Thomsen, the district’s water conservation specialist. “How can we capture energy that would otherwise be wasted?”

In November 2017, STPUD made that goal a reality. Today, at the bottom of Luther Pass on Diamond Valley Ranch inside a small and unassuming building is a hydroelectric turbine, which Thomsen compared to a large hot tub motor.  As the water moves from high to low elevation, pressure builds in the pipe. The turbine converts that pressure into electricity via a generator next to the pipe. The district currently produces and returns to the grid around 381,000 kilowatts of renewable energy each year, enough to power about 55 homes. It also saves 100 tons of the greenhouse gas carbon dioxide from entering our atmosphere every year.

That’s not the only way the district takes the waste out of wastewater.

In addition to recycling 100% of its wastewater, the district also recycles 100% of its biosolids as fertilizer for agricultural land with Bently Agrodynamics in Douglas County, Nevada. This small wastewater plant in the Sierra Nevada range is one of only a handful of wastewater agencies in the world that can claim 100% re-use of both recycled water and biosolids.

A pivot-rig irrigating with recycled water at Diamond Valley Ranch.

The district’s venture into renewable energy stems from a history of innovation. Back in 1963, the district hired CH2M Hill — now Jacobs — to study possible methods to treat and dispose of sewage.  Small scale tests were begun to determine if a treatment process that had never been applied to municipal wastewater could provide a treated water quality adequate to support a recreational reservoir in Alpine County. The tests were successful and a full-scale 2.5 MGD tertiary water reclamation plant was placed into service in 1965. During this period of operation, the district hosted many international visitors who came to observe and learn state-of-the-art wastewater reclamation. The technology created in South Lake Tahoe has since been used throughout the world.

Today, STPUD treats wastewater to filtered secondary, instead of the tertiary treatment originally designed, because ranchers using the recycled water want the added nutrients for their farmland. Now, the small water and sewer district in beautiful South Lake Tahoe continues to make waves as it embarks on its first venture producing hydroelectricity.

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