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Written by Joanna Hoang   
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Wednesday, 23 February 2011

Waste Digesters
Energy is everywhere—it is stored in all matter, but needs to be converted into energy that humans can use. Likewise, all of the waste we produce is a source of energy. The law of conservation states that energy cannot be created AND that it cannot be destroyed—meaning our daily waste contains useful energy ☺ The task we are left with is simply to extract this useful energy. The solution: a waste digester that turns trash, such as food waste, and even human waste, into energy.

Waste Digesters in Action: China has already started producing energy from human byproducts effectively. And in agricultural regions all around the world, as well as many third world nations, waste digesters are already being used daily to harvest energy from waste products. Conceptually, this way of thinking should be embraced by industrial countries as well. Why waste a useful resource while we spend millions every year investing in fossil fuels and other dirty energy sources and processes that are harmful to our health and to the environment?

How it Works: A waste digester works by taking advantage of a basic fermentation process very similar to the fermentation process used to bake bread with yeast or ferment grapes to make wine! In each case a biological microorganism digests whatever we feed it and then it produces a waste product—gas. In bread this gas formation causes the dough to rise, while in a waste digester it forms useful combustible methane gas. Methane gas is a major component of natural gas and is an important energy source,1 used in many common day applications for fuels.

A variety of bacteria are necessary to produce a mix of methane and carbon dioxide gas. Below is a simple flow chart which shows the transformation of the organic products to the final gas products through the waste digester:

Methane Carbon Dioxide2


The Downside: Of course this solution is not without its drawbacks. Along with the methane gas, carbon dioxide and sulfides are produced as well. While methane gas is not the cleanest fuel solution out there, it is much cleaner than the petroleum based fuel we currently use in our everyday lives. Currently, research is under way to make these waste digesters cleaner and more efficient through bacteria and food variation. This kind of technology is not new but there is definitely room for improvement!

Experiment With Your Own Waste Digester

Interested in trying to build your own waste digester? Here are a some guidelines on how to make a basic waste digester at home! It is quite simple to build, just put some waste in a container with some bacteria and the bacteria will do the rest of the work for you ☺ However, safety is key because you will be working with flammable gases and bacteria—remember to wash your hands!

Materials:
  • -    Mylar balloon (those foil-like birthday balloons that you can find at local grocery markets)
  • -    2 liter bottle or any size plastic bottle (reuse an old soda bottle!)
  • -    Some tubing
  • -    Seedstock bacteria, which can be obtained from a local wastewater treatment plant. This is also a great way to learn how your wastewater is treated.
  • -    Some organic waste (simple food leftovers or fruit/vegetable peels, really any compostable organic waste)
  • -    Blender
  • -    Valve
Waste Digester 3


  1. Start off by drilling a hole through the cap of your 2 liter bottle, insert some tubing through the hole, then seal it with a sealant and insert the other end of that tube into the mylar balloon.
  2. When filling up the 2 liter bottle with the material from the waste water treatment plant, leave about a third of the top unfilled for headspace so that the gas has room to build up. A good general rule of thumb is to use about two grams of waste per liter, so you will use roughly four grams. Use a blender to mix some regular water to the waste water that you will put into you 2 liter bottle.
  3. You may want to add a valve between your 2 liter bottle and your mylar balloon in order to stop the gas flow from the main digester to its gas storage area.
  4. Seal the top of the bottle with the cap and wait a couple of days. Make sure to stir the bottle a couple of times a day.
  5. In a few days you should notice your mylar balloon inflating. That means gas is being produced!
Mylar Balloon
Photo Credit: Sarah Chih


As a part of a ten week project I built a digester with three other team members. We used a glass flask because we had an adequate pressure relief system. In the guidelines it was suggested to use plastic bottles in case the pressure was too great. A plastic bottle bursting is much safer than a glass bottle bursting. In fact, our first prototype shattered because of an inadequate pressure relief system. Luckily no one was hurt. The black bag on the right is similar to the mylar balloon and it is also the gas storage system. The glass cylinder filled with water acts as the pressure relief. If the gas could not get to the bag it would escape out through the water safely.

Waste Digesters
Photo Credit: Sarah Chih


We fed our digester a healthy balanced diet of pasta, cabbage, beef, and cucumbers. In the picture you can see the cabbage and pasta.

Waste Product Gas
Photo Credit: Joanna Hoang


After we obtained a good amount of gas, we held a demonstration. By connecting the Bunsen burner to the bag we were able to burn the gas for about twelve minutes, proving that there is definitely enough energy in waste products to be harnessed!

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_______________________________________________________________________________

1 http://www.epa.gov/methane/
2 http://en.wikipedia.org/wiki/File:Stages_of_anaerobic_digestion.JPG
3 http://www.adelaide.edu.au/biogas/fair/




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