“What is happening now is the just the beginning of Southwest Gas’ plans for the sustainable energy future,” she said.

Southwest Gas is in the process of linking its system to the Pima County wastewater plant in Marana, the Tres Rios Wastewater Reclamation Facility. 

Sewage to gas

The Tres Rios RNG project has been in the works for some time as Pima County for many years has used raw biogas to fully power the engines used at the Tres Rios sewer plant.

The country “flared,” or burned off, its excess production to avoid venting pure methane, which is a potent greenhouse gas.

After studying the issue as part of a major upgrade to the sewer plant in 2014, the county decided that it would be more economical to build a plant to refine the Tres Rios’ raw biogas into RNG for sale, said Jeff Prevatt, deputy director with the Pima County Regional Wastewater Reclamation Department.

In 2014, the county signed an agreement with a biogas joint venture to build the RNG treatment facility and market the gas. But the deal was canceled a year later when a customer the joint venture expected to buy the RNG – a glass supplier to computer giant Apple – folded and the joint venture couldn’t find another ready customer.

Pima County decided to go ahead and build its own RNG plant and partner with Southwest Gas to link it to the gas system to market the RNG.

In 2019, the county began building the RNG plant at Tres Rios at a cost of about $12 million, up from an earlier estimate of $8 million because the county opted for a higher-purity system in order to market the highest-quality, most valuable gas, Prevatt said.


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