Harvest Power’s bacterial mascot, Burp Reynolds, describes the anaerobic digester. “Anaerobic digestion is awesome! You put stuff, like pizza crusts, banana peels and fish guts – into a warm, airtight box. Then tiny bugs eat up the yummy things like the fats, sugars and starches. Then the bugs burp and fart! And if you capture those burps and farts – it’s biogas –you can use it for electricity, heat or fuel. It’s pretty cool.”
Anaerobic digesters are both that simple and that complex. The majority of food waste at this time ends in landfills, where it produces methane and carbon dioxide as it decomposes. These two greenhouse gases are contributing to climate change. Food waste in landfills also attracts vermin and has an odor we don’t even need to mention.
Several cities have banned post-consumer food waste from landfills, but alternative technologies to convert food waste into elements that are not harmful to the environment are thin on the ground.
Garbage disposals in home and commercial kitchens grind up food waste until it can be disposed of into the wastewater system. Once in the wastewater system, it becomes sludge. Other wastewater streams contain chemicals and pesticides, such as storm runoff from yards and farms. Combining this wastewater with our sludge turns it into toxic sludge.
The wastewater system has a series of filters or traps that should prevent fats and grease from sinks from entering into the water. This product, called FOG for fats, oils, and grease, or brown grease, is usually separated and dumped. It is too contaminated to easily be turned into biofuels, like French-fry oil.
So we can’t dump it and we can’t flush it. Backyard compost piles are not an effective option for the millions who live in apartments, not to mention the effects on the raccoon population if the entire world suddenly started throwing their spaghetti scraps into the compost pile.
But assuming we are already working on smaller initiatives to reduce food waste, such as recycling and donation, we still have a massive amount of biologically-active organic material that needs to be dealt with in a safe and tolerable way.
Anaerobic digesters use microorganisms in the absence of oxygen to break down biomass into less complex elements and compounds- specifically, methane, carbon dioxide, and solid or semi-solid waste material called digestate which can be used as soil amendment. Around the world, anaerobic digesters are being used on farms, to treat post-industrial effluent, and as part of a wastewater treatment of sludge.
In the developing world, small home and farm-based anaerobic digesters are producing energy for household needs. In areas of soil depletion, the nutrient-rich solid digestate produced at the end of anaerobic digestion may be as valuable as the energy.
In Oakland, the East Bay Municipal Utility District’s wastewater treatment plant is co-digesting food waste with primary and secondary wastewater sludge. Processed food waste has greater gas and digestate-making capability compared to sludge, because it has greater organic mass.
We need to use an anaerobic digester to turn food waste into gas and organic fertilizer before it enters the wastewater or landfill system. But these challenges remain: transporting food waste to a digester takes energy, manpower, trucks, as does transporting it back to the farm. Restaurants in the city and supermarkets have a dumpster out back and a powerful garbage disposal in the sink. What incentive do they have to haul their food waste to a digester? The industrial components of a large, city-wide anaerobic digester system is not as unappealing as a landfill, but remain an industrial waste processing system.
Development goals might include working smaller rather than larger. People who live and work in supermarkets or apartment communities already are trained to trash disposal in a series of dumpsters, with perhaps recycling bins. An anaerobic digester no larger than a typical dumpster, with the ability to be easily maintained, and without the issues of smell and vermin food waste in the trash usually produces, will be a system easily accepted by consumers for food waste disposal.
Researchers and engineers at the University of Florida have developed a small farm prototype that was made from off-the-shelf components, to keep the cost down. Cornell University’s Small Farms program is also working on models. Bioferm Energy has a ready to use system designed for small applications of between 2000 and 6000 tons of organics per year, a good size for a shopping center that contains a supermarket and a restaurant or two. Washington State University is looking at adapting models for house-hold sized systems.
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