Energy Vision

Case study - Sacramento, California: The first U.S. city to power refuse trucks on renewable natural gas (RNG) made from separated food


Courtesy of Energy Vision

California has long been a pioneer in implementing sustainable transportation strategies, and yet again, the State can claim another first-of-its land project in the United States as its own. By collecting and diverting food waste from a local landfill, two Sacramento-area companies, CleanWorld and Atlas Disposal Industries, have opened the Sacramento Biodigester — the first closed-loop organic waste project in the country, in which a portion of Atlas Disposal's refuse fleet is powered by the veiy waste that it hauls.

Atlas Disposal hauls food and organic waste from local restaurants, supermarkets, food processing companies and households to Sacramento's South Area Transfer Station, where CleanWorld has built a commercial scale facility to convert the waste into electricity and vehicle fuel — a process known as Anaerobic Digestion. This process involves the work of microorganisms in the digester's airless tanks consuming the organic waste and producing biogas, a mixture of primarily methane and carbon dioxide. This biogas is 'refined' using a biogas upgrading system - designed and built by BioCNG, LLC - at which point it is referred to as renewable natural gas (RNG), also known as biomethane, the near chemical equivalent of pipeline natural gas.

The RNG is stored in tanks and is dispensed into vehicles via the adjacent compressed natural gas refueling station, built by Clean Energy Fuels (also the first renewable natural gas refueling station in California). A small portion of biogas is used to generate the electricity that powers the refueling station.

One of the major advantages of RNG is that it's what is known as a 'drop-in' fuel, meaning that it can directly replace or be blended with fossil-based compressed natural gas (CNG). RNG can travel through the same pipelines as CNG, can be delivered through the same refueling infrastructure and power the same engines that currently run on CNG. Since Atlas Disposal had already replaced 10 (roughly 25%) of its older diesel refuse vehicles with Autocar CNG trucks (and has since added 10 more), the transition to waste-based renewable natural gas was a logical next step. Atlas Disposal's Sustainability Director Andrea Stephenson said, 'As our fleet ages we're going to continue to replace our conventional diesels with compressed natural gas vehicles that can run on either pipeline gas or waste-derived fuel.'

At present, the Sacramento Biodigester is open and operating, converting 25 tons of food waste a day into RNG, but is scaling up the digester to a capacity of 100 tons per day by the fall of 2014. Once fully scaled, the facility will generate more than 700,000 diesel gallon equivalents (DGEs) of RNG annually, which will be distributed, via the public access refueling station, to CNG vehicles owned by Atlas as well as other public and private fleets, including some of Sacramento's municipal vehicles.

The environmental attributes of RNG are second to none — it has the lowest-carbon intensity of any commercially available fuel that exists today. Recent lifecycle analyses conducted by the California Air Resources Board (CARB) conclude that the production and use of renewable natural gas as a transportation fuel via high solids anaerobic digestion of food waste is a net-negative greenhouse gas emissions process. This means that unlike any other commercially available fuel, the production and use of RNG actually removes greenhouse gases from the atmosphere.

Combined with the fact that once the biogases are extracted from the decomposing food waste, what's left is a high-grade compost/soil amendment, the project illuminates the best practice for sustainable management of organic wastes. CleanWorld CEO Michelle Wong said, 'Anaerobic Digestion is a commercial technology, and eveiy city in the United States has the opportunity to capitalize on this strategy to close the loop on organic waste.'

The project's environmental and economic benefits include the following:

  • Diversion of nearly 40,000 tons of food waste from landfills annually
  • Greenhouse gas reductions of 5,800 tons per year
  • Yearly diesel displacement of 700,00 diesel gallon equivalents
  • The creation of 16 long-term green jobs
  • Annual combined municipal tax revenue of more than $1.1 million
  • The sale of high value organic soil amendment co-products

The project received $6 million in grant funding from the California Energy Commission to help offset the total $12 million price tag (digester, biogas upgrading, refueling station, construction costs, etc.). Synergex, Five Star Bank, Central Valley Community Bank, CalRecycle and the California Office of the State Treasurer provided the additional financing.

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