Anaerobic Digestion

Anaerobic Digestion isn't exactly composting - it's a very different process for decomposing waste.
Anaerobic digestion breaks down organic matter without the use of oxygen, producing 3 outputs:
  • 1 Biogas, similar to natural gas, mostly composed of carbon dioxide and methane, which can be captured and converted to fuel and energy.
  • 2 A digestate mix, which can be a solid, liquid, or slurry of organic matter.
  • 3 Water, which can be used after a purification process.
Anaerobic Digestion Occurs In 3 Phases:
  • 1
    Organic matter is decomposed into smaller, simpler molecules through bacterial hydrolysis.
  • 2
    Bacteria work to convert the simple molecules into amino acids.
  • 3
    The amino acids are converted to methane gas.
Anaerobic Digestion comes with benefits and baggage.
Below are some examples of each side of the conversation.
Advantages
  • When landfilled, organic matter releases methane into the atmosphere, which causes climate change. Anaerobic digestion captures this methane and converts it to energy.
  • An aerobic digester facility can charge waste haulers a tipping fee and sell the energy generated from the methane. Some anaerobic digesters switch between electricity and fuel generation, depending on market prices.
  • In wastewater treatment facilities, fats, oils, and grease clog the water infrastructure. By diverting these substances to an anaerobic digester, it prevents sewer overflows, saving money and improving water quality.
Disadvantages
  • Involves a large capital investment with a long payoff period. The prices of electricity and biofuel fluctuate, so profits will depend on the market.
  • The digestate may only be land applied in certain places because of laws and regulations. The benefits of adding digestate to the soil have also been questioned, because many of the nutrients have already been converted into energy – leaving the digestate potentially nutrient deprived. It may take additional transportation to move the digestate for land application, or additional effort to compost the digestate for land application.
Anaerobic Digestion & The Big Apple
NYC passed legislation mandating that commercial businesses who generate over 1 ton of waste per week cannot send their organic materials to landfill. The purpose is to reduce the amount of greenhouse gases, but also motivate new, local composting facilities and anaerobic digesters to enter the market.

NYC is particularly interested in anaerobic digesters because it would bring an additional renewable and localized energy source to the city. While there are no anaerobic digesters dedicated to food scraps currently in NYC, there are some projects that are piloting the technology.
Newtown Creek Digester Eggs
Located at NYC’s wastewater treatment plant in Greenpoint, Newtown is used to process 1.5 million gallons of sludge each day. Since the digesters have extra capacity, the Department of Sanitation hauls food scraps (turned into a slurry) to the facility for processing in the digesters. The digesters turn the organic material into methane gas, which is used to power the facility, and will soon be used to power some of the city’s homes through a partnership with National Grid.
Long Island Compost
Based in Westbury, NY Long Island Compost is building one of New York’s first anaerobic digesters specifically for food scraps. It will have capacity to accept 120,000 tons of food scraps per year. Long Island Compost has a commercial composting facility, but it does not accept meat or dairy, which limits its clientele, but with the digester they will be able to accept more compostable items.

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