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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.

Disney World’s biogas facility: a model for converting food waste into energy

guardian

Guardian Sustainable Business, “Disney World’s biogas facility: a model for converting food waste into energy,” by Marc Gunther

 

“The circular economy at Disney World may not be as pretty as Cinderella’s Castle, but this process for turning organic waste into energy, which is known as anaerobic digestion, could turn out to be the best way to extract value from food scraps and treated sewage that would otherwise wind up in a landfill.”

 

Balancing Digester Diets

Biomass Mag logoBiomass Magazine,Balancing Digester Diets” by Kate Fletcher

 

About twice the size of JC-Biomethane is Harvest Power’s 50,000-ton food waste community digester near Orlando, Florida. Some waste from Disney World is brought to the facility, including the Ritz-Carlton and JW Marriott hotels at Grande Lakes Orlando.

After contaminant removal in Harvest Power Florida’s low solids AD process, “the mix tank is used to mix some of the fat, oils and grease, food waste and the other types of materials and blend them together so you don’t send slugs of one heavy material over another into the digesters,” says Brandon Moffatt, senior vice president of energy for Harvest Power. “It’s trying to have the right mix and always trying to keep that in balance, so we’re continuing to optimize our recipe and make sure we have balanced feeding to make sure the system is stable.”

Here’s how NYC is going to start turning its food scraps into power: Inside the brown energy movement

scienceline-300x59[1]“Here’s how NYC is going to start turning its food scraps into power,” by Nick Stockton 2013.12.20 http://scienceline.org/2013/12/heres-how-nyc-is-going-to-start-turning-its-food-scraps-into-power/

Power From Food Scraps

livingonearth[1]Living on Earth, “Power from Food Scraps,” Curwood – Podcast – 2013.09.20  http://www.loe.org/shows/segments.html?programID=13-P13-00038&segmentID=4

 

Harvest Energy Garden – Richmond, BC – GRAND OPENING

We had an excellent day celebrating the partnerships that make the Energy Garden in Richmond BC possible.

Slideshow:

Speakers:

  • Paul Sellew, founder and CEO Harvest Power
  • Honourable Kerry-Lynne Findlay, Canada’s Minister of National Revenue
  • Michael Weedon, CEO BC Bioenergy Network
  • Mayor Malcolm Brodie, Mayor of the City of Richmond and Chair of Metro’s Zero Waste Committee
  • Prof. Grossmann, CEO GICON

Energy Garden in Richmond, BC

Press:

 

  • Azom.com, “Harper Government Celebrates Official Opening of Harvest Power Energy Garden Site,” 2013.09.12
  • CBC OnlineMention of Kerry Lynne Findlay and contribution
  • Clean Energy Canad at TIDEScanada, “Peelings Power: The Energy Garden is Growing a Cleaner Future,” Michael Weedon 2013.09.12
  • CleanTechnica, “The Power of Rubbish: It’s Quite Good,” 2013.09.16
  • Electrical Business Magazine, “Harvest Power Energy Garden site marks official opening,” 2013.09.12
  • Richmond News, “One person’s trash is another one’s power: ‘Energy garden’ opens, organic recycling facility can power 900 homes a year,’” Philip Raphael 2013.09.12
  • Salon.com, “North America’s largest ‘commercial digester’ turns pizza crusts to energy,” Pete Danko 2013.09.12
  • Earth Techling, “In BC, Old Leftovers (And More) Become Power,” Pete Danko
  • SingTao, “美商列市設廠 廚餘轉化能源” 2013.09.13
  • Solid Waste & Recycling, “BC’s massive new AD facility feeds electricity grid,” SWR Staff 2013.09.12
  • Waste & Recycling News, “Harvest Power opens new anaerobic digester,” Jim Johnson 2013.09.12
  • Waste 360, “Harvest Power Starts Anaerobic Digestion Facility for Organic Waste,” Allan Gerlat 2013.09.12

Efforts grow in Mass. to turn food waste into energy

Boston-Globe[1]Boston Globe, “Efforts grow in Mass to turn food waste into energy; State’s landfill rules spur plans for plants,” David Abel 2013.07.22  http://www.bostonglobe.com/metro/2013/07/21/new-industry-emerges-state-prepares-ban-commercial-food-waste/5oQrf2eCYJE9CvgYNPqIfP/story.html