Bio Digester

When we hear the word biodigestors, we think of messy manure to produce some gas for cooking. I have built a simple one using 2x1000L water tanks. The feedstock for this biodigestor is old, thick cooking oil and rotting fruits. The calorific content is higher than that of manure, roughly 40 times higher. I used the principles described by
 As the tanks are above ground, the quantity of gas produced is dependent on the ambient temperature. If larger volume tanks were buried in the ground, we would have a more stable and steady supply of gas. Once a week, I add 2 litre of feed with 20L of water. The daily quantity of gas produced, depends on the weather. On cloudy and cold days there is no gas produced. If the sun is shining, the unit produces about 15min of gas per day. I use it to boil water for tea or heat up food. To cook longer food items, I accumulate gas for 2-3 days or otherwise I use the solar oven or woodgas stove. The unit will produce negligible gas over winter months.


The cap is fitted with a 600mm long, 50mm PVC pipe. It has to be airtight.   


Drill a 20mm hole to insert an elbow with locknut on the inside. This is for extracting the gas. I added a T piece to store the excess gas in another tank. 


A mixture of about 40kg fresh cow/horse manure is put into the tank and add about 800 litre of water. Wait until the gas starts to produce. In JHB, it did not produce gas over the winter months.
The outlet pipe is a 20mm hose, which overflows if the
pressure exceeds about 4kpa. As the pressure increases,
the slurry rises up the pipe. I use this in the garden as compost. The white tank in the background, stores the excess gas.
I used and old paint tin for the base of the burner. (Woodgas stove in the background).
The jet hole had to be increased to 1.5mm, due to the lower pressure.
 A 50mm PVC pipe section, filled with steel wool and blocked off on the ends, acts as a "catalytic converter" to protect the burner.
 A clear blue flame.
 To extract the gas from the enclosed tank, I added another tank cut in half and fill this with water which then displaces the gas in the enclosed tank. Both tanks are connected at the bottom with 20mm hose. As the pressure increases, the water moves from the enclosed tank to the 2nd tank and vice versa as the pressure reduces.
Albert and his biogas digestor.
 To store the gas, we used a 910mmWx1500mmHx150micron LDPE bag donated by Gundle plastics. The volume is about 250L, which is enough gas to burn for around 1.5hr. We used 50mm PVC pipe and wrapped the bag around the pipe and tied it with bicycle tube. Albert puts a plank with some bricks to create pressure for the gas to exit.
CLICK HERE (size:1.4MB) for various research articles that I downloaded from the web.