Build food scrap digesters to produce energy from compost

Assumptions

• Digesting food scraps, in addition to producing compost, has an energy benefit of 730-1300 kWh per dry ton

Analysis

Digesting food and green wastes in closed tanks – something akin to wastewater treatment – is a technology worth San Francisco’s attention. Not only might it bring the composting process closer to home, reducing emissions from trucking, but it also produces electricity and reduces air pollutants. Our estimate of emissions reductions is conservative, given these other benefits. The City should explore opportunities to build digesters for food scraps, either as part of its Sewer System Master Plan or separately with the waste collection service Norcal Waste Systems. However, the lack of data on costs makes it impossible to know if this is a cost-effective climate mitigation strategy.

What we do now
Yard trimmings, food scraps and other compostable material from San Francisco is collected by Norcal Waste Systems and hauled to Norcal’s composting facility in Vacaville. There, through a 90-day process of mixing, grinding, aeration and aging, these wastes are converted to high quality compost that can be used in organic farming and viticulture. The facility processes more than 7,000 tons of food and yard waste a month. This process beneficially reuses waste green materials, and the City should continue its efforts to increase the diversion to this facility of the large percentage of food and organic wastes that currently go to the landfill with our other trash.

What we could do
Although the aerated composting process is far environmentally superior to putting organic waste in the trash — beyond just the global warming impact — new technologies have been piloted to make the process of composting even more sustainable. Food waste can be composted in a closed-system anaerobic digester, similar to the way sewage sludge is broken down into “biosolids” at wastewater treatment plants. This process more efficiently captures other atmospheric emissions from aerobic composting, such as volatile organic compounds, that contribute to ground level ozone pollution. It also saves energy, allows the gas byproducts to be recaptured and used to generate electricity, and reduces the need for large areas of land for processing. Although it is not a widely used technology in the United States, the East Bay Municipal Utility District has conducted a pilot program for digesting food waste using excess capacity in the digesters at its wastewater treatment plants. A study conducted by EBMUD for the U.S. Environmental Protection Agency found that food scrap digestion produced more energy — with three times the methane production — and more biosolids for reuse than municipal wastewater digestion. It found an energy benefit of 730 to 1,300 kilowatt hours per ton of dry food waste applied.¹

San Francisco could consider sending some portion of green waste to maximize capacity at EBMUD wastewater digesters or other digesters in California.² This would reduce the distance that heavy green waste is hauled for processing – more than 60 miles – and reduce emissions of polluting volatile organic compounds.

Cost
No data are available to estimate costs to use or build local digesters to process the city’s organic wastes.

Carbon savings potential
If 100 percent of the energy recaptured from food scrap digestion is used to offset energy generated by PG&E, based on 400 tons of food scraps being anaerobically digested per day from San Francisco, a range of 25,328 to 45,104 tons of carbon could be offset each year. The range is based on the range of electricity output from EBMUD’s study. This quantity would be expected to increase if San Francisco’s waste diversion rate improves.

The cost per ton reduced cannot be computed due to a lack of data on costs.

Endnotes
¹ “Anaerobic Digestion of Food Waste,” East Bay Municipal Utility District, Donald Gray P.I., Funding Opportunity EPA-R9-WST-06-004, March 2008.
² According to the 2008 EBMUD study, there are 137 wastewater treatment plants with anaerobic digesters in California, with an estimated excess capacity of 15 to 30 percent.