Anaerobic Digestion – How Does It Work?

Turning waste materials into energy is a fantastic way to produce renewable power – but how does it really work?  The good news is that the process has been in operation for tens of millions of years – naturally – a variety of bacteria take long-chain hydrocarbons, like sugars, fatty acids and amino acids and break them down eventually producing “biogas,” an energy-rich gas that can be used to produce renewable electricity and heat, upgraded for injection into a natural gas pipeline, or compressed into vehicle fuel.

What are anaerobic bacteria? And what do they do?

Multiple types of bacteria are involved in this process, and indeed the process takes different pathways if there is oxygen present or not.  “Anaerobic” digestion describes this process without oxygen present, a critical factor in producing methane (CH4).  In fact, some believe all of the natural gas present underground was formed through this anaerobic digestion process.

It is important to note that anaerobic digestion does not typically break down certain “tougher” molecules such as cellulose or hemicellulose, key molecules in wood and woody products.  The process is most effective on simple sugars, fats and proteins that are found in food, foodwaste and manure.  However, these anaerobic bacteria can break down cellulose and hemicelluloses over very long periods of time.

What are the chemical and biological stages in anaerobic digestion?

To put it all in simple chemical terms, here is a typical chemical pathway that takes place through anaerobic digestion:  Glucose (C6H12O6) (this means a chemical bond with 6 carbon atoms, 12 hydrogen atoms and 6 Oxygen atoms) is eventually turned into 3 molecules of carbon dioxide (CO2) and 3 molecules of methane (CH4).  This process can happen relatively quickly – in as little as two weeks, depending on the conditions of Ph, oxygen levels and temperature.

There are four chemical and biological stages in anaerobic digestion:

  • Hydrolysis – the first step in which longer chain carbohydrates, fats and proteins are broken into shorter chain molecules in solution. Some of these shorter chain molecules can be directly used in step 3 and 4 below.
  • Acidogenesis – this second step is a biological process, similar to what happens with milk souring, in which these shorter chain molecules produce carbon dioxide, hydrogen sulfide and even more volatile fatty acids.
  • Acetogenesis – in the third step, simple molecules created through the first two steps are digested by specific bacteria to produce largely acetic acid (CH3COOH), as well as carbon dioxide and hydrogen.
  • Methanogenesis – the last stage is a biological process in which a certain class of bacteria, known as methanogens utilize the products developed in stages one to three above and convert them into methane (natural gas), carbon dioxide and water.  This stage is sensitive to pH, and is most prevalent at pH levels between 6.5 and 8.

One of the most fascinating aspects of this process is that it occurs every day inside a cow’s stomach – they have these very same bacteria who do the work of producing energy for the cow, as well as the by-products of methane and carbon dioxide.

Is anaerobic digestion an effective way to extract methane from food waste products?

This same chemical and biological pathway is in use around the world – most notably at waste water treatment plants – as well as over one thousand manure digesters in use at dairy farms.   This process also occurs at landfills (from the waste buried within it).  The process can move faster at higher temperatures (thermophilic, around 55-70 degress Celsius, or at mesophilic temperatures of 35-40 degrees Celsius).

How can anaerobic digestion be optimized?

Key ingredients for faster anaerobic digestion:

  • Water – the molecules must be effectively in solution to be digested
  • Temperature – the process can occur at lower temperatures, but temperatures of 35 to 70 degress Celsius allow it to move much more quickly
  • pH – the initial steps in the process produce significant volumes of acid (namely acetic acid), however the bacteria are most effective closer to a neutral pH of 6 to 8.

It is indeed fascinating that we can use trillions of naturally-occurring bacteria to do the “heavy lifting” of producing renewable energy from waste materials.  Instead of all of the manufacturing and distribution required for oil, coal, or even solar panels, wind turbines or nuclear power plants, we can use these microscopic bacteria, found on every inch of the globe, to produce renewable energy in an effective and low-cost manner.

Food Waste and Carbon Footprints

We thought we’d heard it all until we read these:

  • Cheese brine being used in Wisconsin for snow and ice management?
  • Tomato skins being explored as car parts?!

Elizabeth Matsangou of The New Economy explores the intersection of food waste and a low-carbon economy.

Following the Flow of Urban Organics

Upstream Emphasis: Setting the Table for Food Waste Diversion

With society’s strong appetite for addressing the issue of food waste, this article focuses on defining the players in urban organic waste streams, and the drivers that lead to organic waste policies and practices. It assumes that all upstream efforts have been made to follow the EPA’s organic waste hierarchy by reducing organic waste at the source and directing any remaining edible food towards human consumption.

Downstream Technologies: Anaerobic Digestion and Composting

Also relevant is a general understanding of downstream processing technologies. Anaerobic digestion has two primary flavors: “high solids” which accommodates “stackable” feedstocks (think yard trimmings laced with pasta); “low solids” digestion accommodates “pumpable” feedstocks (think slurries of food scraps mixed with fats, oils and grease (FOG), wastewater treatment plants, or manures). Composting, the notable aerob ic cousin of digestion, converts a range of yard trimmings and food scraps into nutrient-rich soil products. Given this context, let’s eat.

Organic Waste Generators

Fork it over: know thy audience. Wherever there are people, there is organic waste. When thinking about the flow of these wastes, it’s helpful to define the waste generator and characterize the waste composition. Waste generators are typically grouped as follows:

  • Residential: Homes and properties generating yard trimmings and kitchen scraps.
  • Commercial: Grocery stores, restaurants, hotels and other business generating larger volumes of pre- and post-consumer food scraps as well as FOG.
  • Institutional, Commercial, and Industrial (ICI): Concentrated populations such as schools, prisons, campuses, and hospitals; or food-related industries such as food processors, breweries, and dairies, all generating large volumes of organic waste.
  • Other: Multi-family dwellings (housing with four or more units) and events (festivals, conferences) have their own unique characteristics.

Key Characteristics of Organic Waste

Now that you know the audience, it’s time to recognize what they eat. In terms of the composition of wastes generated, not all rinds are created – or collected – equally. The following four drivers shape local policies and practices.

  • Energetic densities of food waste: The same foods that make humans fat help anaerobic bacteria fart and burp. Those farts and burps, known as biogas, can make electricity, pipeline grade natural gas, or vehicle fuel. If you want to optimize biogas generation, be sure to get the donuts to digestion and leave the leafy greens for other technologies.
  • Volumes of food waste: Large volumes of waste generated in a few locations are typically easier to manage than lots of smaller generators. But it’s a balance: you don’t necessarily want to get all of your eggs from one basket.
  • Geographic clusters: Dovetailing with volumes, every hauler knows that a collection route in a same region is ideal because it efficiently shares the fixed collection costs associated with starting up a collection truck and driving it around town.
  • Capture rate versus contamination rate: This can be tricky. The generator and the processor typically do a dance. The processor’s ideal “clean” load of feedstock leads to lower processing costs (removing and disposing of contaminants), increased safety (associated with removing said contaminants from equipment), and optimal product quality (improved biogas yields and fertilizer products). However, the generator might value the ability to include contaminants. This balance is typically struck via acceptance specifications and tip fees.

With this background, you will better understand the organic waste conversations in your community.


Editor and Author’s Note: Content from this post originally appeared in the February 2018 issue of BioEnergy Insight as a contributed article from Harvest.