Methane Digesters 101

Part 1

flameWith gas and fuel oil prices on the rise, many people are tapping into the alternative energy market. Quite a few farmers and municipalities are turning sewage, manure, or garbage into fuel. What sort of fuel is it and what is the process that produces it? Well, the best answer is, its perfectly natural. When you bury garbage in a landfill, bacterial action decomposes and forms volatile gases. Over time those gases leak into the atmosphere and contribte to the greenhouse effect. Landfills in particular are being challenged to become emission free, by trapping and burning the "landfill gas". Landfill gas, or biogas, as it is now called, is composed of approsimately 65% methane, 30% carbon dioxide, and 5% other minor gases.

The process by which biogas is produced is anaerobic digestion; meaning without oxygen. This is not the same process as "composting" vegetation in your backyard. Composting requires aerating the mixture by turning; thereby exposing it to air (oxygen). The byproduct of this reaction is soil and predominantly carbon dioxide gas. Anaerboic digestion requires that the reaction take place in a container that is sealed or isolated from the outside air. The product is a large amount of methane, a little carbon dioxide, and a high grade fertilizer.


A simple digester is just a big balloon full of crap! Once placed in a sealed container, oxygen loving bacteria within the orginal product begins to break down the manure into simpler compounds; these are typically ammonia compounds. At this point, a fair amount of CO2 is given off. Once all of the oxygen is used up, other critters, ammonia loving bacteria, take over. The ammonia loving bacteria work to liquefy the material. Eventually, fodder for the a ammonia loving bacteria is used up and the next process begins; the production of methane. For this process to begin and proceed in a timely manner, the reaction requires two things, a neutral or slightly basic pH and an optimum temperature. A well buffered digester will be at around 8 to 8.5 on the pH scale. A digester is pretty much self buffering once it is started, but you can speed things up by starting it off on the right foot. The optimum temperature is 95 degrees F to 110 degrees F. While you will produce gas at lower temperatures, it will take a much longer period of time. Landfills don't begin producing methane gas until many years after they are sealed up.

There are different types of digesters; the two major types being batch and continuous feed. Each is exactly as its name denotes. The simplest is the batch digester and it can literally be as simple as putting some cow manure in a barrel and sealing it up. However, without adding some heat, there will be very little perceptible activity. Add a little heat in the form of a drum heater, as shown in the picture above, and voila - instant fuel. A little tubing and a few valves, and you have a simple digester. We have heard of running a small gasoline engine from a barrel typ type batch digester. However, it is very limited in the amount of methane it can produce. Typical life of a batch digester is around 45 to 60 days. At that point, you would have to dump the fully digested material and refill the container with fresh manure.

The most practical type of digester is the continuous feed digester, since it will provide a continuous output of gas instead of a large volume all at once. A small quantity of manure is fed into one end of the digester at regular intervals. With each feeding, the previous "slugs" are forced along the lenght of the digester until it comes to the outlet at the other end; sort of like a big instestine!

TAP Point:

When you plan for your digester, plan for a constant supply of manure and a way of disposing of the effluent. The continuous feed digester requires regular feeding and will produce lots of high quality fertilzer and lots of gas!

Most of the examples we see today are on the large corporate farms with sophisticated controls and people to tend the daily needs of the digester. What about the small farm? Well, a digester can be designed to be as large or as small as you like. Design it to suit your needs and budget.

How can I use the methane that I produce?

Methane can be burned to provide both heat and cold. Used in an absorption chiller, the heat produced by burning methane can be converted to cooling for buildings or processes. This is actually a very efficient system since for every BTU produced, you get a BTU back in cooling. No so with electric production. If you choose to produce electricity, the system efficiency is very low. This is mainly due to the fact that internal combustion engines are exremely inefficient, only about 25% so. Diesel engines are higher, around 35%. However, if you opt for a conventional engine generator set, then you are going to end up with a converted gasoline engine.

Using the gas directly for water heating is another very efficient use of the fuel. New hot water tanks are around 95% efficient. If you start off with a small digester, then plan for several independent usage methods. That way if demand is low, you can use all of the gas that you produce. If you have to store it by compression, then you will use a lot of teh gas you produce just to compress the rest. This will discussed in more detail later.

Another way of utilizing the fuel you make is to heat water to make steam and drive a turbine connected to a generator. Most back yard mechanics are not going to tackle this and industrial units are hideously expensive. Still, there are some interesting new micro-turbines being marketed today that would be perfect for small installation.

Each of these alternatives requires a constant supply of gas, but will not be able to tolerate oversupply. Correct sizing of the components is critical to maintain operation and to get the most from your unit. It is impossbile to accurately determine the rate at which gas will be produced because of all the variables - manure content, temperature, amount of water. If you have control over at least some of these parameters, you should be able to predict the flow of gas within a reasonable tolerance. However, you need to plan for a reserve that can act as a flywheel or capacitor and provide even flow to your engine/generator.

continue to Part 2 >