Biogas Recycling

Biogas Recycling

Harnessing methane from our treatment process

Carson Ice-Gen Project

Regional San’s biogas recycling program is just one of the ways Regional San works to reduce impacts to the environment by innovatively reusing valuable resources. Methane gas, or “biogas,” is a by-product of the solids digestion process that is used to produce steam and electricity at the Carson Ice-Gen Project, a cogeneration facility owned by Sacramento Municipal Utility District (SMUD) and located at the Sacramento Regional Wastewater Treatment Plant (SRWTP).

Sending gas to the cogeneration facility for recycling purposes instead of open flaring at the waste gas burners (which was the practice prior to 1996) greatly reduces emissions of air pollutants as well as greenhouse gases (GHG) from the SRWTP.

Power Generated

The SMUD cogeneration plant has a capacity of 100 megawatts of power for local residential and industrial use. The electrical power produced with the SRWTP digester gas is enough to provide energy for approximately 7,500 households annually.

Regional San also buys some of the steam produced by the recycled methane to heat the wastewater digesters at the treatment plant. In the event of a local power failure, the cogeneration plant serves as a backup power supply to keep the treatment plant running.

Estimated Annual Emissions Reductions

Volatile Organic Compounds (VOCs): 18.5 tons/year
Oxides of Nitrogen (NOx): 16 tons/year
Carbon Monoxide (CO): 75 tons/year

The primary GHG reduction (for methane) is 96%—from 10.76 tons/year to 0.454 tons/year.

Natural Gas vs. Coal

The Carson Ice-Gen Project uses both natural gas and digester gas to make electricity, which results in an overall emissions reduction that benefits the customers of Regional San and SMUD. A lower amount of air pollutants and greenhouse gases are emitted when natural gas is used to produce energy, when compared to burning coal. Typical values for pounds of emissions per billion BTUs of energy produced are:

  Natural Gas Coal
Carbon Dioxide 117,000 208,000
Carbon Monoxide 40 208
Nitrogen Oxides 92 457
Sulfur Dioxide 1 2,591
Particulates 7 2,744
Mercury 0.000 0.016

Source: EIA Natural Gas and Trends 1998

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