FAQ

How do anaerobic digesters work?

Anaerobic digesters use bacteria to accelerate the break down of organic waste in the absence of oxygen. This decomposition yields raw biogas which can then be upgraded to renewable natural gas, also known as biomethane. While this process already occurs in nature, an anaerobic digester helps in optimizing, controlling, and containing said process.

The Process

  1. The waste is mixed in with water and fed into the digester

  2. The mixture sits in the tank and breaks down over a period of a few days to a few weeks

  3. All of the raw biogas produced is captured

  4. The raw biogas can be used to to produce heat and electricity for use in engines, microturbines, and fuel cells. Alternatively, it can be further upgraded into biomethane (also known as renewable natural gas or RNG) which can be injected into natural gas pipelines or used as fuel for vehicles.

  5. The leftover solids from digestion (known as digestate) can be used as a nutrient-rich fertilizer and/or soil amendment

A process flow diagram of how anaerobic digesters turn organic waste into fuel [1]

Why is anaerobic digestion a good choice of waste processing technology?

Diversion of Food Waste Away from Landfills

Anaerobic digesters utilize food waste to produce energy thereby reducing or eliminating the amount of food waste that ends up in landfills.

Food scraps alone make up 21% of the total municipal solid waste that ends up at landfills. These same food scraps being thrown away are a key ingredient in the production of a greener fuel (natural gas) by means of anaerobic digestion.

More than 50% of the solid waste thrown out into the garbage is compostable or can be used in an anaerobic digester [2]

Carbon dioxide and methane make up around 92% of all greenhouse gas emissions [3]

Greenhouse Gas Reduction

The most common examples of greenhouse gases are carbon dioxide, methane, and nitrous oxide. These gases trap heat from the sun and warm up the planet.

When food waste is diverted towards anaerobic digestion processes, local landfills end up with less organic waste. This contributes to lower greenhouse gas emissions.

Reduction of methane emissions derived from tapping all the potential biogas in the United States would be equal to the amount of emissions from 800,000 to 11 million passenger vehicles. The CNG derived from biogas reduces greenhouse gas emissions by up to 91% relative to petroleum gasoline. [4]

Potential Earnings

Anerobic digesters are able to produce 20 - 800 cubic meters (m3) of biogas per tonne of waste. [5] This depends on the quality of the waste, digester design, and operation parameters of the system. Assuming 100 m3 of biogas produced with a 35% conversion efficiency from biogas to electricity, and an average electricity cost of $0.10/kWh [6], the energy in food waste is expected to be about $33/ton.

In 2013, the state of Minnesota threw away more than 900,000 tons of organic waste. [7] According to the food waste energy amount of $33/ton, the state could have generated almost $30 million in electricity; enough to power 25,000 homes in 2013.

Cost-Effectiveness and Sustainability

Anaerobic digestion technology is cost-effective as it requires a very small energy input compared to other methods like composting. In most cases, an anaerobic digester is able to power itself with the energy it produces. Anaerobic digestion also has a much faster processing time (10 - 30 days) compared to composting (3 months - 1 year).

Production of Useful Byproduct

When the food waste is processed in an anaerobic digester to produce raw biogas, leftover solids leave the system as sludge. This is very rich in nutrients and is also known as digestate. With slight enhancements, it can be reused as a bio-fertilizer or for soil amendment.

Ability to Handle Range of Wastes

Anaerobic digesters are able to handle a much wider range of organic waste such as those with varying moisture content and other impurities.

How will this help you?

Reduce your waste bill

Mitigate your carbon inventory

Eliminate or minimize landfill tipping fees

Get access to low-carbon fuel

What are the additional benefits of anaerobic digesters?

Provides clean, renewable, and reliable source of energy

Reduces dependence on fossil fuels

Reduces water pollution from livestock waste

Benefit local economies through lower waste remediation costs

Potential 13,500 biogas systems could add over 360,000 jobs across the US alone [4]

[1] What is anaerobic digestion? Colloide Process Engineering | Water, energy, environmental and facilities engineering solutions. (2021, June 2). Retrieved from https://www.colloide.com/what-is-anaerobic-digestion/

[2] Platt, B. (2018, April 2). Compost impacts more than you think. Institute for Local Self-Reliance. Retrieved from https://ilsr.org/compost-impacts-infographic/

[3] Environmental Protection Agency. (n.d.). Inventory of U.S. Greenhouse Gas Emissions and Sinks. Retrieved from https://www.epa.gov/ghgemissions/inventory-us-greenhouse-gas-emissions-and-sinks

[4] Tanigawa, S. (2017, October 3). Fact Sheet I Biogas: Converting Waste to Energy. EESI. Retrieved from https://www.eesi.org/papers/view/fact-sheet-biogasconverting-waste-to-energy

[5] Biogas FAQ. BiogasWorld. (n.d.). Retrieved from https://www.biogasworld.com/biogas-faq/

[6] Jeff Kuo & Jason Dow (2017) Biogas production from anaerobic digestion of food waste and relevant air quality implications, Journal of the Air & Waste Management Association, 67:9, 1000-1011, DOI: 10.1080/10962247.2017.1316326

[7] Minnesota Trash: What's In It. (2016, February). Retrieved from https://www.house.leg.state.mn.us/hrd/pubs/mntrash.pdf