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Pop Quiz: What's the Difference Between Aerobic Decomposition and Anaerobic Digestion?
In San Francisco, the green compost bins are ubiquitous. Even the food trucks are required to place bins alongside their trucks for compost, recyclables, and trash. Result: the city is diverting an enormous amount of trash from the landfill to help meet its zero-waste goal and producing lots of compost in the process - a process which also produces a lot of carbon dioxide, "the primary greenhouse gas emitted through human activities," according to the EPA.
A few cities are taking an alternative, more expensive approach to diverting this organic waste called anaerobic digestion and in the process also producing biomethane that is captured for use in electricity generation or used as a transporation fuel. And from a "carbon intensity" perspective, this biogas, also called renewable natural gas, scores at the bottom of carbon intensity chart [PDF, pg. 49] for California's Low Carbon Fuel Standard, i.e. it is one of the cleanest of all transportation fuels.
But this story is not about San Francisco's compost bins nor about the California's Low Carbon Fuel Standard. It is about Brooklyn's Newtown Creek Wastewater Treatment Plant and its anaerobic digestion done by bacteria in "eight huge, shiny, oval-shaped steel tanks known as digester eggs," writes (and states) Joel Rose [listen here].
"Right now, what these bacteria are digesting is mostly sewage sludge. But they're being introduced to a new diet: food scraps. The hope is that this plant will soon take in hundreds of tons of organic waste from houses and apartments.
A bold plan - but one with huge greenhouse gas emission reduction payoffs as food-waste is one of the world's largest sources of emissions as we noted last year.
Anaerobic digestion is more popular in Europe and in a few cities in Canada and California, including San Jose and Los Angeles. While there clearly is a need for more renewable energy, be it in the form of electricity generation or transportation fuels, it is uncertain whether anaerobic digestion will take hold in the U.S. as widespread as composting.
"Right now, it's all in its infancy," says Samantha MacBride, a former New York sanitation official who's now a professor at the City University of New York's Baruch College. "And it's a huge question mark about whether it can grow."