Wastewater Treatment Plant Turns Gaseous By-Product Into Profit

By Warren Tenney

Arizona’s largest wastewater treatment plant already cleans and re-uses nearly all of the waste it receives from 2.5 million people in five AMWUA cities. Now, the cities that own the treatment plant have found one more way to re-use its products. As of spring 2018, the 91st Avenue Wastewater Treatment Plant will stop burning off the mostly methane gas it creates as a by-product. Instead, the plant will transform the by-product into renewable biogas and sell it for more than $1.2 million a year.

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91st Avenue Wastewater Treatment Plant   Photos: City of Phoenix

The 91st Avenue Wastewater Treatment Plant’s effluent, or treated wastewater, is re-used to irrigate crops, create a wildlife wetlands project called Tres Rios, and provide cooling water for the Palo Verde Nuclear Generating Station, 36 miles west of the plant. The wastewater plant treats and transforms solid waste – all the stuff ground up in garbage disposals and flushed down toilets – into fertilizer for non-food crops, such as hay, alfalfa and cotton. The City of Phoenix, which operates the plant, and its four AMWUA city partners have found a way to produce yet another marketable product – biogas. Here’s how it will work.

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Tres Rios Wetlands

  • The plant has 16 large digester tanks with a capacity of 1.5 million gallons to 3 million gallons. These digesters operate like an industrial stomach, digesting solid sewage waste as a step toward turning it into fertilizer. Just like our own digestive system, these industrial digesters expel gas. That gas is now vented through three flares and burned off into the atmosphere.
  • Last week, Phoenix and its partners broke ground for a new facility next to the plant that is about as big as a football field. The new facility is being built and operated by Ameresco, a vendor selected by the cities that own the treatment plant. It is expected to be completed and operating by spring 2018. When completed, the plant will be fully automated and will be operated by one person. The stacks that flare the gas will remain on-site to act as a backup if needed.
  • The new facility will scrub and pressurize the plant’s gas into clean biogas. The gas will be compressed and travel through an underground pipe to a large commercial gas pipeline three miles west of the plant.
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This is a similar Ameresco bio-gas plant in San Antonio, Texas.

The 91st Avenue plant was built in 1968 by a partnership of AMWUA’s five original member cities, Glendale, Mesa, Phoenix, Scottsdale and Tempe. This partnership is known as the Sub-Regional Operating Group or SROG. The plant treats an average of 140 million gallons of wastewater a day but has the capacity to treat 230 million gallons. Both the plant and the biogas facility are built to accommodate what is expected to be a growing market.

For 48 years, Arizona Municipal Water Users Association has worked to protect our member cities’ ability to provide assured, safe and sustainable water supplies to their communities. For more water information visit www.amwua.org.

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