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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.

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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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