Every Drop Counts

Renewable energy projects involving digester methane and concentrated solar power help a Nevada regional authority save money and conserve water

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Water is a common topic of conversation when you live in the middle of the Mojave Desert. With less than 10 inches of precipitation a year, the fast-growing area of Las Vegas, Nev., places a premium on water conservation and reclamation.

Serving a population that has doubled since 1990, the Las Vegas Water Pollution Control Facility treats 65 mgd using trickling filters, activated sludge, and biological nutrient removal. The treated water is sent back to Lake Mead — the region’s source of drinking water — to be used again.

To make the best use of available energy resources the Las Vegas treatment plant, part of the Southern Nevada Water Authority, uses digester methane as a power source for process blowers. SNWA has also made a major commitment to solar energy that includes a new concentrated solar installation. Today, 12 percent of SNWA’s power comes from renewable energy. Its goal is 25 percent by 2025.

Regional approach

SNWA, formed in 1991, delivers 377 mgd for people in the Las Vegas Valley, Laughlin and Boulder City along with the 40 million visitors to the area every year.

Two-thirds of the district’s annual water allocation is returned to Lake Mead from wastewater treatment plants in Las Vegas, Henderson, and the Clark County Reclamation District. SNWA is limited to 300,000 acre-feet a year from Lake Mead, but gets credits for putting treated water back into the reservoir.

“For every gallon we clean, treat and put back into Lake Mead, we are able to take another gallon out,” says John Bettencourt, water pollution control facility engineering project manager. To reduce the demand on Lake Mead, the region’s water reclamation plants recycle wastewater for irrigation and for electric power plant cooling.

Major power draw

Lake Mead, a 9.3-trillion-gallon reservoir behind the Hoover Dam on the Colorado River, provides nearly all of the water for 22 million people in Arizona, California and Nevada, about 80 percent of it for farm and urban irrigation.

Despite all the bright lights of the Vegas strip, pumping and treating water is the single largest power user in the region. The SNWA estimates pumping and treatment for drinking water consumed 853 million kWh in 2008. Wastewater agencies in the region’s Clean Water Coalition report that treating wastewater uses 119.2 million kWh per year.

The Las Vegas wastewater plant is undertaking several projects to increase use of digester gas, according to Bettencourt. At present, two 900 hp Dresser Waukesha engines powered by digester gas drive process blowers in the nitrification and biological nutrient removal sections of the plant. “Our studies show that the engines save us $1,000 dollars a day per engine over using electric-driven blowers,” says Bettencourt.

The engines, installed about 15 years ago, are available about 60 percent of the time, but a new gas scrubbing project will help remove even more impurities from the gas, reducing maintenance needs and increasing availability to about 90 percent. A pilot program to add FOG (fats, oils and grease) to the digesters will also start soon. “We hope FOG will increase our gas production without giving us too many headaches,” says Bettencourt.

Meanwhile, the plant has undertaken a gas utilization study to analyze the best use for the plant’s methane. That may lead to installation of microturbines or fuel cells. The plant has also issued a design-build contract to Martin-Harris Construction for a $20 million, 20-acre, 3 MW solar energy project. The feasibility study says the project will pay for itself in 20 to 30 years, depending upon how fast other energy costs increase.

Concentrated solar

SNWA has undertaken many renewable energy projects to save money and conserve water. In an area where watering lawns is illegal in the afternoon for five months of the year, everyone is assigned to a “watering group.” People can get rebates for replacing water-intensive grass with desert landscaping. These and other conservation efforts have saved 7 billion gallons a year since 1999.

SNWA has four solar installations, according to Gary Wood, renewable energy program manager. The newest is a concentrated photovoltaic system from Amonix Inc. at the River Mountains Water Treatment Facility in Henderson. It began operation in June 2009.

Conventional photovoltaic cells capture the sun’s energy and convert it directly to electricity. Concentrated solar energy multiplies the sun’s energy. Each of the six 40- by 55-foot solar panels has Fresnel lenses, like those used in lighthouses, to magnify the sun’s energy 500 times. The lenses concentrate the sunlight onto one-square-centimeter multijunction solar cells made of three photovoltaic materials that capture a broader spectrum of wavelengths and produce more energy than a standard solar cell.

A dual-axis tracking system follows the sun to maximize production. In single-axis tracking systems, panels follow the sun from east to west. Dual-axis systems also adjust to the elevation of the sun through the seasons.

“On our fixed solar systems, we’re seeing capacity factors in the neighborhood of 20 to 22 percent,” says Wood. “Our single-axis trackers are about 25 percent, and this dual-axis system has a capacity factor of about 28 percent.”

More to come

The system produces about 600,000 kWh per year. That’s less than 1 percent of the electrical load at River Mountains, but it serves a valuable research and development function. “We’re looking to add to our renewable portfolio, and we want to use technology we’re comfortable with,” says Wood.

SNWA also has three hydroturbines in water pipelines that provide 2 MW of capacity, and another 0.3 MW turbine is being added to the system. They replace sleeve valves that would normally regulate flow. “We’re recovering energy that otherwise would be lost,” says Wood.

SNWA has just begun an energy-efficiency retrofit performance contract with Ameresco Inc., and signed a power purchase agreement with a company that will convert waste oils into biodiesel to fuel 3 MW generating capacity.

Another project under consideration would use waste sludge to fuel 4 MW of capacity beginning in 2013. “We’ve secured a lease in northern Nevada for a potential geothermal power plant, and we are in discussions about some wind power,” says Wood.

It’s not a complete solution, but it will help keep clean water running to homes, farms, and businesses in the Mojave Desert.



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