First, depending on the tank available, this will be the amount of biogas produced by the digester. The uses of this biogas could be cooking certain foods, heating a room, lighting or simply for projects or experiments at home. For the latter, a Bunsen lighter because it makes it possible to regulate in a simple way the gas flow and the air-biogas mixture.
he biodigester It needs to be built based on resource availability and not try to do it exactly with the materials I will mention below. Remember the “three Rs”; reduce Reuse and Recycle.
How to build a biodigester?
The materials and their description.
The reactor and the supply of materials.
- A tank or barrel with a capacity of between 120 and 220 liters. They are usually blue with a tight fitting lid.
- Sanitary cleaning cap (4 ”): It is a kind of adapter with a screw cap.
- Short tube segment (4 ”): goes through the opening and connects the“ adapted plug ”on the outside with the reduction inside the tank. It must be short enough to allow both the reducer and the adapter to pinch the top wall of the tank and thus allow better hold and better sealing. Sanitary flanges glued with silicone to the tank can also be used.
- PVC reduction from 4 “to 3”.
- Sanitary PVC pipe (3 ”): Reduction to 5cm before the bottom of the tank.
For the effluent outlet:
- Tank adapter (2 ”).
- PVC pipe (2 ”) for the effluent outlet pipe.
- 3 PVC elbows (2 ”).
- Reservoir adapter (1 ”) to connect the valve.
- PVC ball valve (1 ”) For the bottom outlet of the heaviest effluent.
For the biogas outlet (in order):
- Tank connector (1/2 ”).
- Threaded ball valve (1/2 ”).
- Adapter for hose.
To join the parts and seal:
- Welding (glue) for PVC.
- Clear and fungus resistant silicone sealant !: To seal around tank seals and prevent leaks.
(”) = Inches.
Two side holes and two holes are made in the top of the tank. One on the bottom side for the 1 inch valve; another in the middle part for the effluent outlet. In the cover, one will be for the entry of the material and the other for the exit of the biogas, always the diameter of the part which passes through it.
To store biogas a floating bell is used, very easy to build with two drums; a large one where the water goes and a slightly narrower one which is placed face down inside the previous one. The pipe which comes from the digester is introduced into the larger tank and bubbles so that the gas rises and is trapped in the smaller tank which has a valve for the gas outlet with a pipe and a water trap.
How to use a drum biodigester.
Remember that this biodigester is more than anything “experimental”. What I mean is that it serves as a testing and information gathering unit rather than a stable source of home use biogas. For the latter, a higher capacity biodigester is recommended.
In order to use the biodigester, its manufacturer must first install the fittings, pipes, the safety valve, the biogas tank and the burner, as well as check the connections in order to avoid gas leaks or the entry air in the appliance. Once these preparations have been resolved, it will be possible to continue filling it.
Usable organic material.
- Fresh manure or slurry from herbivorous or omnivorous animals (example: pigs).
- Kitchen waste and food scraps, (except citrus).
- Used cooking oil (only 5%).
- Leftover vegetables from the market square.
- Fresh cut grass – mixed with other materials.
- “Old” sawdust – mixed with other materials.
There are other subjects that I recommend against because they are more difficult to degrade or not suitable for a biodigester of these characteristics. In general should not be used Citrus fruit residues, seeds or whole grains, straw or grain stalks, wood chips, dry leaves, pruning scraps, droppings of carnivorous animals such as cats or dogs, and not human faeces. Bones, stones, glass, metal, plastic and rice husks are out of consideration for this use.
To allow rapid degradation, all materials to be used should be shredded, shredded or crushed as the case may be, in fragments not exceeding 1 mm for the softest and less than 5 mm for the most consistent. The smaller the better.
The charge will consist of a mixture of 20 to 25% organic matter and 80 to 75% water. Part of this water can be replaced by the treated liquid (effluent) which comes out of the biodigester, also called biol, and thus produce more biogas to the detriment of obtaining less fertilizer.
Retention time and daily load.
Depending on the ambient temperature, this will be the retention time of the materials added to the biodigester. The following table shows the retention time as a function of temperature.
An “air” space will be left in the 25% (1/4) biodigester in the biodigester-tank, so only 75% of its capacity will be used, what we will call working volume (VT). The outlet tube will be arranged as an overflow, so that there is always 1/4 of the capacity for the gas phase.
The mixing charge to be added daily is calculated as follows:
- VT = CTT x 0.75
- CD = VT / TR
- Vermont: working volume in liters.
- CTT: total tank capacity in liters.
- CD: daily mixing charge to be added.
- TR: retention time in days (see table).
Example: In hot weather, for a 120 liter biodigester, the working volume will be 90 liters (120 L x 0.75 = 90) and the daily mixing load will be 4.5 liters (90 L / 20 = 4.5 L).
The biodigester must first be filled (3/4) with the mixture of organic matter and water within a few days to avoid the release of excessive odors. After filling, no mixture will be added until the methane production has started well, then maintained for several days. After that, the load calculated for your specific biodigester will be added daily, always through the PVC cover on the top of the digester.
The biodigester outlet tube will be the overflow through which the liquid effluent or biol will exit each time the load is added to the device.
As for the amount of biogas to be produced, there is no “magic number” for all possible substrates. It’s best to research each material that you plan to use in an article or book.
Remember, these are just a few tips and you can experiment with varying retention times, organics, loading dilutions, or other things.
CAUTION: BIOGAS IS A FUEL. Take appropriate safety measures and consult a professional. The process will be faster if you live in a hot climate, as the temperature speeds up the process. However, never leave the biodigester in direct sunlight as this will create sudden changes in temperature every day.