Anaerobic Digestion

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Anaerobic Digestion is simply when bacteria break down organic substances without oxygen present. It is similar to composting in that bacteria break down the organic material, only with composting, it is in the presence of oxygen (aerobic). When we compost our raw glycerin, if we use too much on the pile, it will turn the pile to slime, starving it for oxygen, sending it Anaerobic, causing it to stink to high heaven. The solution is to go Anaerobic from the start.

 

There are four stages to anaerobic digestion.

  1. Hydrolysis
  2. Acidogenesis
  3. Acetogenesis
  4. Methanogenesis

Each of these stages calls for different bacteria, which feed on different materials, giving off different products. The products given off by the bacteria are either CO2 or methane, or are food for one of the other stages. Some material like cellulose will not break down in any stage and are passed through unchanged. 

The first three stages break down the organic material into successively smaller organic molecules, including Carbon Dioxide. The fourth stage is the production of methane from the small organic molecules produced in the first three stages.

It turns out that methanol skips the first three steps and goes straight into methanogenesis, producing methane without producing carbon dioxide. This is a huge benefit for anaerobic digesters, as it increases the concentration of methane in the digesters' output gas stream. This increases the BTU energy content of the biogas.

In addition to methanol, our byproduct also contains free fatty acids (FFA). These skip the first stage in the process, generating less CO2 than would be generated by fats and oils.

The bacteria are pH sensitive. Left over catalyst can be a problem for anaerobic digesters if introduced in high concentrations. However, when introduced slowly at the head end of a large digester, the concentrations have no detrimental effect on the digestion process. The FFA and glycerin are eventually broken down into biogas and the salts are bound up with the high-powered fertilizer discharged from the digester.

Some owners of digesters have acquired a taste for our raw byproduct. They have found that it will boost methane production measurably in their large digesters. The people with the power to say yes are easier to find and approach for farm-based digesters. Municipal digesters are more common, but are often inaccessible to the average home brewer.

Farm based digesters are often found on dairy farms and swine farms to deal with the manure. Once they find out that methane production is boosted by adding glycerin, they will usually take all you can give them.

Wastewater treatment plants will sometimes accept raw glycerin as digester food. They do NOT want you pouring glycerin down the drain as it can overwhelm other areas the treatment plant. If they will accept it, you will have to bring it to them by the drum to be loaded directly into their digester, bypassing earlier stages that are sensitive to oils and fats.

One area of possible experimentation is the personal digester. You could build a digester designed to consume your raw byproduct as well as kitchen wastes. Searching on the Internet will reveal a number of digester designs made from 55 gallon drums.

I hope to build a drum based anaerobic digester in the near future. I hope to use the biogas to heat water for processing biodiesel. When I have something to share, I'll post it here.

One of my earliest ideas on anaerobic digestion of biodiesel waste glycerin was to put it in a kiddie swimming pool, add water, and a gallon or so of pond water. Then urinate in the thing.
Theoretically you will need about a six pack of beer to produce enough urine. The urine is to add nitrogen to the glycerin and methanol carbon. Cover with black plastic to soak up heat and keep out rainwater.  It most likely will stink at near skunk strength, so this is a back 40 experiment, not a back yard experiment.

Related Links:

Anaerobic Digester Project
Composter Connection on Anaerobic Digesters
The design and theory of a Basic Anaerobic Digester

Oregon.gov Biogas Technology
Biorealis - Understanding the Digester
Farm Power Anerobic Digester
Introduction to Biogas and Anaerobic Digestion

 

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