A renewable natural gas plant at the 91st Avenue Wastewater Treatment Plant in Phoenix completes the circle of reusable byproducts for Arizona’s largest clean-water facility.

The 145 mgd average (230 mgd design) plant has long claimed beneficial uses for 100% of its effluent and biosolids. All that goes to the landfill is grit and headworks screenings. The effluent is used for irrigation, supplying the Tres Rios Wetland, and for cooling the nuclear reactors at the Palo Verde Generating Station. Anaerobically digested and dried biosolids are land-applied to crops such as hay and cotton.

Only the biogas remained as an unrecovered resource. For years, a small portion of the gas had been used for boiler fuel, but most had been flared. That changed with the opening last April of a renewable natural gas facility on the plant site. It will clean the biogas to natural gas pipeline standards. 

Major gas producer

The renewable natural gas plant, expected to produce 600 MMBtu of natural gas annually, is owned and operated by Ameresco, an independent provider of services and infrastructure upgrades for energy efficiency, renewable energy and asset sustainability.

The wastewater treatment plant, along with three major sanitary sewer interceptors, is owned by a partnership known as the Sub-Regional Operating Group, or SROG, which consists of the cities of Phoenix, Scottsdale, Tempe, Mesa and Glendale. That partnership was created in 1979. Phoenix operates the wastewater treatment plant, built in 1957 and upgraded several times. It serves about 2 million people in the five cities.

Digested solids from the activated sludge process are dewatered in centrifuges and further dewatered on drying beds before being land-applied.

Cost-benefit analysis

The treatment plant operations team considered a variety of uses for the biogas, including electricity generation, says Patty Kennedy, deputy director of wastewater engineering in Phoenix. “When we looked at the cost-benefit analysis, it made more sense to put it into the pipeline,” Kennedy says. “By entering this partnership, we were able to stick with our core business, which is treating wastewater. Ameresco came in focusing on its core business, which is renewable energy.”

The cities in the SROG partnership send some or all of their wastewater to the 91st Avenue plant, which has two acid-phase digesters and 14 methane-phase digesters. Since the renewable natural gas plant began operating, all of the biogas goes to Ameresco. The plant, the largest of its kind in the country according to Ameresco, treats only biogas from the wastewater treatment plant.

“We sell the raw gas from our digesters to Ameresco,” Kennedy says. “They are in charge of treating it to pipeline quality. They are in charge of their relationship with Kinder Morgan, who is the taker of the gas, and they are also in charge of figuring out off-takers — users downstream.” The gas is expected to be sold to the transportation market through the natural gas pipeline grid.

Environmentally beneficial

Warren Tenney, executive director of the Arizona Municipal Water Users Association, describes the project as a winner for the cities and the environment. In an op-ed in a local newspaper, Tenney says the project ensures the availability of renewable energy, provides income to the SROG member cities and reduces the treatment plant’s carbon footprint.

In a press release, Ameresco estimates that not flaring the biogas from the wastewater treatment plant would reduce the carbon emissions by 45 tons per year, equivalent to taking 70,000 cars off the road or planting 87,000 acres of trees every year. Revenue from the sale of biogas to Ameresco is estimated at $1.2 million per year.

The revenue is significant, but Phoenix officials are encouraged that they finally can beneficially use the last significant resource from wastewater treatment. “That’s what makes this project so exciting,” Kennedy says. “All our water is reused, and so are the solids. The third product is gas. Now we are able to capture and reuse it.”

Wetland in the desert

The 91st Avenue Plant is on a 670-acre site about 16 miles southwest of central Phoenix and close to the Salt River. Almost next door is one of the more visible users of the plant’s effluent: the 700-acre Tres Rios Wetlands.

The wetland, built on former farm fields, is a result of a partnership between the Army Corps of Engineers, the City of Phoenix, the SROG and the Flood Control District of Maricopa County. Construction took place from 2007 through 2012.

The wetland is intended to restore the riparian habitat along the Salt River to early 1800s condition. A 300 mgd pump at the treatment plant sends effluent to the area. The plants and animals in the Tres Rios take what they need, and the rest flows into the Salt River, which at that point is a dry riverbed most of the time.

“We built a wetland in the desert,” Kennedy says. “We’ve got all kinds of wildlife in the area. We even have pelicans.” The Tres Rios Wetlands is home to 150 species of birds, as well as muskrats, beavers, coyotes, bobcats and other mammals. It’s a destination for bird-watchers from all over the world.