East Bay Energy

Here’s a nice recap of Oakland’s uber-progressive food-waste-to-energy plant. East Bay Municipal Utility District (East Bay MUD) has an anaerobic digestion plant that converts Oakland’s food waste to energy, which it then use to power the West Oakland sewage treatment plant.

photo by mattdork via creative commonsCool!

I think anaerobic digestion is a sound destination for that inedible food waste that’s always going to occur. It’s not on the EPA’s food waste recovery hierarchy, but I’d put it right above composting.

There are just two things I’m not sure about from the article. One, does the East Bay MUD plant really produce a carbon dioxide byproduct? When I worked at an anaerobic digestion company, there was no talk of a CO2, just an organic sludge that could be turned into a soil amendment.

The other bone to pick: When food is composted, if done properly, it does not release methane. As long as the pile or row is turned often enough so that all parts receive oxygen (and don’t go anaerobic), there won’t be any methane emissions.Anyway, I hope more municipalities follow MUD’s path.

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6 Comments

  1. Posted April 16, 2009 at 7:44 am | Permalink

    Jon,
    This sounds great. I think what’s going on here is that, while your compost wouldn’t ordinarily produce much methane, they have produced a system that intentionally converts the food waste into methane through this bacterial process. I guess it’s like, trillions of bacterial farts. Then, I suppose that when they burn the methane to produce electricity and heat, that’s when they produce CO2 as a byproduct. I’m no scientist, but I think that’s what’s going on here.
    Peace and Love,
    Dan

  2. Posted April 16, 2009 at 8:59 am | Permalink

    It is a very good idea, but only if it is not used as an excuse to ‘waste’ food by people.

  3. Robert
    Posted April 16, 2009 at 11:58 am | Permalink

    The CO2 resleased by this system is just CO2 that was taken in by plants in the food. It is a closed system and does not add any NEW CO2 to the atmosphere. The problem is fossil fuels that are releasing CO2 that has been trapped in the Earth for millions of years and adds new CO2 to the air. This system certainly does not add more greenhouse gases than a coal or natural gas fired power plant supplying the West Oakland treatment plant would.

  4. Posted April 16, 2009 at 10:11 pm | Permalink

    Thanks for the insight, guys. Robert, my question is where in the process the CO2 is released? Do you agree with Dan’s guess that it’s when the methane is used to create electricity and heat?

  5. Posted April 17, 2009 at 4:17 am | Permalink

    Robert,

    The food turns into methane through a series of steps called collectively “anaerobic digestion”. In a number of these steps, CO2 is produced.

    For example, the step “acidogenesis” in part takes sugars and starches and converts them to alcohols. Example: plain old fermentation! This releases some CO2, as you may be familiar with in beer-making or wine-making.

    The next step, “acetogenesis” breaks down the alcohols in the presence of oxygen into various types of acids. Example: making vinegar from wine. This steps also produces some CO2.

    The last step, “methanogenesis” converts the acids plus other things into CO2, methane, and other basic organic things.

    The end results is that anaerobic digestion produces a gas that is about 60% methane, 30% CO2, and the remaining 10% is a mix of nitrogen, hydrogen, oxygen, and hydrogen sulfide. This is known as “biogas”. Biogas does not burn that well, as the mix is not right, plus the hydrogen sulfide has a nasty, nasty rotten egg smell.

    But, biogas can be reformed (upgraded) into something that is chemically very similar to natural gas that we get from fossil fuels. Basically, the CO2 and hydrogen sulfides are removed, and the remaining gasses are left in the mix. The resulting “biomethane” can be used directly in any natural gas appliance you have at home, or can be burned to run an electrical turbine to generate electricity. The CO2 removed from the biogas can be captured and used for various purposes such as welding or wine-making, or can be released to the atmosphere. It can also be used to sparge a newly filled anaerobic digestion tank of all of its air, creating the necessary anaerobic conditions to start the next batch of digestion.

    Finally, when you burn the biomethane with plenty of oxygen, the product of combustion is CO2 and water.

    The cool part, as you mentioned, is that all of this CO2 released during the production and burning is already part of the carbon cycle, so it does not add “new” CO2 to the atmosphere like mined natural gas does.

    Another advantage of this process is that the “digestate” that is left over after the digestion makes great compost. It needs to sit for a month or two to finish off, and the result is a wonderful, rich, organic compost.

    The water used in the digestion is highly oxygen deficient and cannot be released into the environment as it would suffocate many organisms on its way to the ocean. But luckily, this water is perfect for use in the next batch of anaerobic digestion — it contains the right bacteria already, and creates a nicely anaerobic environment for them. The same water can be used over and over again. If the water does need to be released, it should be reoxygenated via bubbling first to avoid eutrophication.

    Basically, when this is done correctly, there is zero waste, as all the byproducts have uses. (Even the hydrogen sulfide!)

    References:

    http://en.wikipedia.org/wiki/Anaerobic_digestion
    http://en.wikipedia.org/wiki/Combustion
    http://en.wikipedia.org/wiki/Biogas
    http://en.wikipedia.org/wiki/Biogas_upgrader

  6. Posted April 17, 2009 at 4:25 am | Permalink

    Oh, forgot to add that traditional aerobic composting that you might do in your backyard does produce some methane. Methane is 20 times more potent as a greenhouse gas than CO2 is, so anaerobic digestion is actually a better way of disposing of food wastes than composting because the methane is captured and burned into CO2 instead of being released to the atmosphere.

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