Food to Fuel
Waste Management District looks at food scraps to generate electricity—and profit.
Thursday, November 18, 2010
With an eye for what could be “next generation of waste management,” Monterey Regional Waste Management District is developing a pilot program to build anaerobic digesters that will convert organic waste, like food scraps, to methane and compost.
In a district that already diverts 60 percent of its waste through recycling and composting, the digesters can be used to power generators and provide electricity – and hopefully make some money, too, according to MRWMD General Manager William Merry. He hopes to sell the resulting methane to the anticipated regional desalination plant next door to the district’s headquarters just north of Marina.
The pilot, slated to begin as early as summer 2011, will feature several large “biocells” up to 200 feet long and 20 feet tall on top of a landfill. Bacteria in the biocells will break down organic matter to produce methane, imitating the anaerobic digestion process in natural wetlands.
Waste will be ground up, screened for contaminants and heaped into the cells. Added moisture and heat will help the mixture to “cook” – a process Merry likens to making soup. The remaining organic matter will be composted, as yard clippings are today.
The new anaerobic digesters should allow MRWMD to extract more energy and value out of organic waste before composting it, according to district spokesman Jeff Lindenthal.
MRWMD already captures methane from its landfill through a gas-to-energy program that’s been in effect since 1983. But while anaerobic digestion takes 10-20 years in a landfill, the new digesters can shorten the process to 12-16 months by controlling decomposition conditions for an ideal mix of organic inputs like yard trimmings, food waste and treated sewage.
The landfill gas-to-energy program is more break-even than reliably profitable, Merry says, because of a drop in energy prices offered by Pacific Gas & Electric. That’s made the district more cautious about the financial outlook for the proposed organic waste digesters. MRWMD will start with only a few pilot biocells, and will seek federal and state grants to minimize costs, which are still undetermined. If the pilot is successful, district officials hope to construct a larger commercial facility.
Merry estimates the anaerobic digesters could divert about one-fifth of the district’s waste stream while also cutting down on the pollution that causes global warming. Methane accounts for about 15 percent of greenhouse gas emissions but is 25 times more potent than carbon dioxide in its capacity to trap heat.
Several similar projects are already in effect, including a program in Oakland that supplements a digester at a solid waste treatment plant with food scraps. The agriculture industry is also familiar with methane digesters, often used to extract energy from animal waste. Early findings suggest dry matter, like food scraps and yard clippings, could be more efficient than sewage.
Lindenthal sees the pilot as a step toward sustainable waste management: “We look at waste as a resource.”