Cooking with Biogas from Home-Scale Biogas Digester

Home-Scale Biogas Production | Free Heat from Organic Waste

June 01, 2020
(last update June 01, 2020 )

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When I heard that you can make free gas from kitchen scraps for stoves, heaters, or even propane refrigerators, I knew I had to look in to it. And, I love it! Here is everything you need to know to get started making your own biogas.

Biogas (primarily methane) is produced by composting organic material in a low oxygen environment. Mix compost solids at a ratio of 30:1 carbon / nitrogen, with 1:1 water to solids in an airtight container with gas collector. Allow fermentation at 70F–105F for until gas production slows. Remove compost tea and repeat.

While the process is simple, there are quite a few details to get right. In this article, I’ll go through each part in detail, to help you get started building a home-scale biogas generator.

How to Make a Biogas Digester

Firstly, if you are serious about making biogas, I highly recommend picking up a copy of The Biogas Handbook, which is the absolute best resource I’ve ever seen for a DIY biogas builder.

Biogas digesters come in two basic type: continuous and batch. Batch types are filled all at one time, left to cerement and produce gas, then emptied when production is finished. Continuous generators, on the other hand, once filled with a starting load can be added to little by little, and the finished product removed a little at a time as well.

Simple Barrel Batch Type Biogas Digesters

The simplest biogas digesters can be made from large plastic barrels, plastic totes, or large plastic bladders. Most metals corrode when in contact with the compost tea, so steel barrels would need to be lined with plastic in order to use.

At the top of the digester, fit a small pipe and valve for extracting the biogas. For a batch type biogas digester, you fill it once, then let it generate for a few weeks before emptying it out. For this, type a typical barrel lid will do just fine for loading and unloading. Although, you have the option of installing a larger diameter spigot to extract the finished liquid, so there is no need to tip over the barrel or siphon it out. The compost tea produced by a correctly functioning and loaded biogas digester should be a thin liquid, even if solids were added initially.

Working Barrel Biogas Digesters Example

Continuous Production Biogas Digester

The drawing below shows an example underground vault biogas digester designed for continuous production. Here the biggest difference from a batch type system is the addition of loading and unloading ports.

Loading ports can be a pipe or chute that enters the fermentation chamber from the bottom, and angles up to prevent compost liquid from spilling out the top. This port must be connected in the bottom of the chamber to prevent biogas from escaping, but should be lightly sealed when not in operation to prevent excess oxygen from entering the system and disrupting gas production.

The exit port is constructed similarly to the loading ports, connecting at the bottom of the biogas digester. But, can either be angled up to the right height to allow overflow to happen when the chamber is full. Or it can just be a valve that is manually opened for a bit when the chamber contents need to be partially drained.

Biogas Digester Design

Biogas Storage

Biogas can be stored directly the tank that produced it, or relieved out to an external tank. Storing the gas externally allows you to more easily regulate the pressure of the gas for a consistent flow, and to save space in your digester are. If you are purifying the gas before use, then external storage also makes a lot of sense.

Internal Gas Storage

With internal storage designs, like the diagram above, the gas will pressurize the digester. For a closed batch design, this is just fine, so long as you left enough free space initially. For continuous designs, the extra pressure will serve to push the compost tea up the feeding tubes and lower the level of water in the tank, building up gas bubble at the highest point of the tank. For this reason, continuous biogas digesters should have feed tubes that extend higher than the level of water in the tank.

External Gas Storage

Two popular biogas storage designs are the inner tube design and the inverted barrel design.

The inner tube storage method is simply any bike tire or rubber bladder inserted along the tube that extract biogas for use. This bladder will expand and fill up as the gas is produced, and helps keep a more consistent gas pressure which is convenient for use in stoves and other burners.

The second method is the inverted barrel method, which is simple a barrel turned upside down in a container (usually another slightly bigger barrel) of water or oil. Here the gas line from the digester is run under water, and allowed to bubble up and in to the inverted barrel.

Over time a pocket of gas forms under there, floating it up and allowing the weight of the barrel to pressurize the gas. Biogas can be extracted for use by running a second line down, around, and up to the top of the inside of the barrel. Or, hole can be drilled in the upper part of the upside down barrel to install a collection port.

A secondary benefit of the barrel storage method is that is also performs a level of purification.

Biogas Purification

Biogas is naturally a mixture of a number of different compounds, the most abundant being methane, carbon dioxide (CO2), and water vapor. There is also a small quantity of hydrogen sulfide, with has the distinct smell of rotten eggs.

In The Biogas Handbook, the author doesn’t recommend any form of purification or scrubbing for most uses, except for potentially long term storage or for use in internal combustion engines, where you want as pure of methane as possible.

If you are worried about the smell of biogas, which is very similar to that of commercially produced natural gas, then you can filter it out a couple of ways. The easiest is to bubble it through water. Adding sodium hydroxide (aka drain cleaner, NaOH) to the water will increase the efficiency of the process. You can also pass the biogas through a box of iron rust, which reacts with the hydrogen sulfide and removes it.

Removing carbon dioxide from biogas increases its potency, making it more energy dense for efficient long term storage or for use by internal combustion engines. One way is to bubble the solution through a solution of calcium hydroxide (CaOH). While any base would do the trick, such as the sodium hydroxide mentioned above, calcium hydroxide is nice because it can be partially rejuvenated through exposure to oxygen, allowing it to last much longer.

Temperature Control

Bacteria that produce biogas can live in a wide range of temperatures, from freezing all the way up to 140F. However, the optimum temperature range is from 70F – 105F. Below about 70F, the bacteria produce substantially less biogas, which is one of the main reasons your septic isn’t producing significant amounts of biogas. Any higher than about 105F and thermophilic bacteria start to take over, which increases the amount of smelly sulfur dioxide produced, and will slow down production of the temperature goes below 105F later.

During the summer in most climates a biogas digester should run quite well. A digester that runs hot can be kept in the shade or painted white, while a digester than runs cold can be painted black and placed in the sun.

If you plan to run the digester in the winter, it will most likely need insulation or external heat to keep running. While the bacteria do produce some of their own heat, they will need some assistance in all but the most mild or tropical climates.

Buried or underground digesters will need insulation to keep them isolated from ground temperature, which is too cold to allow for sufficient biogas production.

Safety

Biogas in it’s natural form is not explosive, but if it is allowed to mix with oxygen then it forms a flammable mixture. The Biogas Handbook recommends the following precautions:

  • Keep biogas digesters at least slightly pressurized, so oxygen can’t get it
  • Digesters should be operated in a well ventilated area
  • Flame arrestors, such as a water bubbler should be installed in supply lines between burners and storage tanks.

Seeding and Initial Startup

All biogas digesters are powered by bacteria, and need to be inoculated with proper bacteria in order to function well. Wet manure that has never dried out carries the proper organisms. Adding any amount will get the process started, but the more you add, the faster it will go.

Continuous biogas generators will continue to function after the initial seeding, so long as they are kept in operation, and not allowed to dry out.

Batch digesters are typically seeded with a portion of the compost tea extracted from the previous run, which is rich with the proper bateria.

Of course, if you plan to run your digest primarily on manure, then no special care needs to be taken.

What Can I Put In a Biogas Digester

Any organic matter will decompose and produce methane in a biogas digester. The most popular and productive materials are manure and plant vegetation.

Anything that you would normally throw in the compost pile will work with a biogas digester.

Like a typical hot compost pile, biogas digesters run most efficiently, producing as much gas as possible, when there is a good mixture of carbon and nitrogen. This can be accomplished by including a good mixture of “browns” and “greens”, while keeping an eye on how well the digester is doing. But, manures generally do pretty well on their own without the need to get a mixture just right.

If you are the measuring type, then pick up a copy of The Biogas Handbook, which has large tables showing the carbon nitrogen ratio of most common composting additives.

How Much Biogas Do You Get?

Some commonly accepted production values:

  • Manure produces 4 – 9.6 cubic feet / pound
  • Wood produces about 8 cubic feet / pound
  • Straw produces about 3.2 – 4.8 cubic feet / pound
  • Vegetables produce about 8 – 12.8 cubic feet / pound
  • Grass clippings produce about 4.8 cubic feet / pound

How much biogas you get depends on what material you put in there, and the conditions of the digester. Good temperatures and proper inoculation (seeding) insure optimum production.

Biogas produces about 654 Btu of heat per cubic foot of gas

The small example biogas digester in the video above produces enough methane to power a stove top burner for 45 min a day in a single 55 gallon size drum. This production comes from a single charge of 1/3rd manure and 1/3rd water, lasting for about 3 weeks each charge.

What Runs on Biogas

Any appliance which runs on propane, butane, or natural gas can be converted to run on biogas:

  • Space heaters
  • Water heaters
  • Gas ranges / ovens
  • Propane refrigerators
  • Lights / lanterns
  • Internal combustion engines
  • Clothes drier

Biogas is less energy per volume than other types of gas. So, while it will burn just fine in a propane or natural gas appliance without modification, it will not produce as much heat as before.

The adventurous can fix this deficiency by drilling the orfice (hole) of the burner by a set amount, allowing more gas to flow in. In the The Biogas Handbook, the author provides extensive instruction on how to calculate how much bigger the hole needs to be to operate correctly.

How Much Biogas Do You Need?

Here is a table of the estimated amount of power, in Btus, used by common gas appliances. Using the figure of 654 btu / cubic foot, I’ve also calculated the rough volume of gas that would be used per hour, in a appliance modified to run off biogas.

Estimated power usage of common gas appliances
Appliance Btu / hr cu ft / hr
Water heaters (6 gpm) 225 0.344037
Gas range burner 9 0.013761
Gas oven per cubic foot 8 0.012232
Propane refrigerator per cubic foot 2.3 0.003517
Lanterns per mantle 1.5 0.002294
Internal combustion engines per hp 10.7 0.016361
Clothes drier 22 0.033639

Ratings for space heaters vary quite a bit, but can be divided by 654 to get the per hour figure in terms of cubic feet of biogas.

Generating Biogas from Human Waste

Biogas can be generated from human waste, it is in fact a pretty productive producer. But there are a number of considerations to take in to account when doing this.

For those of us hoping to run their whole home on human waste, that unfortunately isn’t realistic because the amount of waste produced per person is actually quite low compared to the typical person’s gas requirement. Humanure alone won’t make enough gas.

Humane waste compost can be mixed in with other organic sources to increase production, but care should be taken to prevent disease. While human waste is very commonly used as a fertilizer around the world, sure you are taking proper precautions appropriate for the regulations in your jurisdiction.

What Exactly is Biogas?

Biogas is a natural mixture of primarily methane with carbon dioxide and water vapor, that is produced by bacteria that thrive in low oxygen (anaerobic) environments.

The process of producing biogas is very similar to regular composting, except that it is usually done in liquid form rather than a solid pile, and the fermentation process must happen in an air tight container to both keep oxygen out and store the biogas produced. But, both result in a wonderful organic fertilizer at the end.

Agricultural areas all over the world have been producing biogas for some time. German farmers in particular has been very active building industrial scale biogas plants to reuse manure, corn, and food waste to produce electricity. German biogas plants produced 2,291 megawatts of electricity in 2010. The developing world, small sized biogas generators have become very popular, providing propane like cooking gas for use in farmer’s homes.

How do you make biogas?

Biogas is produced by fermenting organic material in a low oxygen environment, usually an airtight barrel or cistern. Mix organic material with water to form a wet slurry of about 90% total water by weight. Ferment in a container at 70F – 105F for about 2 – 4 weeks, collecting gas as it develops.

How long does it take to produce biogas?

Biogas production typically takes 5 – 14 days in a efficiently functioning low temperature biogas generator. Temperatures below 70F or above 140F, improper inoculation, or poor / contaminated feed stock all can significantly increase production time to months.

Can you make biogas from human waste?

Human waste will produce biogas at the rate of about 8.8 cubic foot of biogas per pound of solid waste. The quantity of waste an adult produces is typically not enough to meet their gas needs, and so would need to be supplemented with other sources.


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