Compost or Biogas?

In the era of global warming, it may make less sense to compost food waste than to turn it into renewable energy.

For years, environmentalists have been trying to convince people to
turn their food scraps into compost — with some success. The City
of Oakland’s curbside composting program, for example, allows residents
to combine table scraps with their yard trimmings, which is then
trucked to the Central Valley and turned into fertilizer. Similar
programs are growing in popularity around the nation. But in the era of
global warming, this type of traditional composting seems so … 20th
century.

Indeed, it may make a lot more sense during these days of melting
polar ice caps and rising sea levels to stop composting food waste and
start turning it into renewable energy. The East Bay Municipal Utility
District, for instance, has been quietly doing just that for the past
few years at its giant West Oakland sewage treatment plant. EBMUD takes
food waste from restaurants around the Bay Area and turns it into
methane gas, which it then uses to power three on-site generators. The
resulting clean electricity, in turn, helps power the treatment plant.
“I live in Oakland, and sometimes I think: ‘What a shame, all of this
food waste should be going to East Bay MUD,'” Ed McCormick, manager for
support services at the utility, told Eco Watch during a recent tour of
the sprawling West Oakland plant.

The agency’s food waste/renewable energy program, for which it
received a patent last year, begins inside eleven giant anaerobic
digesters. Most of us have probably noticed the massive, round tanks
near the Bay Bridge Toll Plaza without realizing what they’re used for.
Inside the 2 million-gallon behemoths, EBMUD feeds food waste to
bacteria that “digest” it and convert it to biogas in a completely
enclosed process starved of oxygen. The agency then pipes the gas to
its generators, which convert all of it into electricity and heat. The
electricity powers the plant while the heat is piped back to the tanks
to keep them at 125 degrees Fahrenheit, the ideal temperature for
anaerobic digesting, thereby saving the utility from wasting
energy.

The only byproduct from EBMUD’s generators is carbon dioxide, and
although it’s a greenhouse gas, the agency’s process appears to be much
less harmful than traditional composting. Why? Because composting food
waste, which is done aerobically (that is, outside in the open air),
creates methane gas and nitrous oxides as by-products that are released
into the atmosphere and are far more potent greenhouse gases than CO2.
In fact, methane is more than twenty times more potent. EBMUD’s
process, by contrast, releases no methane into the air. In addition,
food waste composting in the Bay Area consumes fossil fuels because
most of it has to be trucked to outdoor facilities in the Central
Valley. Oakland’s, for example, ends up at a giant facility near
Modesto.

EBMUD’s program was a relatively simple and natural progression for
the utility because it actually began generating renewable energy more
than two decades ago. In 1985, the agency became one of the first in
the nation to convert raw sewage into methane gas. In fact, it built
its huge digesting tanks and generators for that purpose. During its
primary and secondary sewage treatment phases, the utility removes
heavier organic solids from the treated water and sends them to the
digesters. The combination of sewage and food waste digesting generates
a total of about 5 to 5.5 megawatts of renewable energy at any given
time for the agency, enough to power the entire West Oakland plant and
sell electricity back to PG&E. And by 2011, EBMUD’s
renewable-energy generating capacity should increase considerably. The
agency is building a turbine generator that could allow it to produce
more than 10 megawatts of power, McCormick said.

So, should we stop composting entirely? No, composting yard
trimmings still makes sense. EBMUD has yet to come up with a way to
break down plant material efficiently so it can be converted to biogas.
At the same time, turning yard waste into compost produces natural
fertilizer for both agriculture and home gardens, thereby reducing the
need for fossil fuel-based fertilizers.

But food composting is a different matter. Clearly, more wastewater
treatment facilities should be following EBMUD’s lead. McCormick said
the Central Marin Sanitation Agency is about to begin a program, while
both San Jose and Los Angeles are looking into the possibility.
Unfortunately, turning food waste into biogas has been far more popular
in Europe than the United States.

Regardless, East Bay cities should be talking to EBMUD about sending
food waste to the West Oakland plant. Currently, EBMUD accepts about 15
tons of food waste a day in a contract it has with NorCal Waste, but
its facility is underutilized. McCormick said that with the equipment
it has today, it could handle to 60 to 80 tons. And with the closing of
the Oakland Army Base, the utility has already been granted access to
land that could allow it to expand significantly.

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