By Alexandra Biogas is a mixture of different gases produced by the breakdown of organic matter in the absence of oxygen, called also anaerobic digestion. It is produced from biodegradable materials and more than half of the resulting gas is methane.
In order to create an anaerobic digestion we started our project with a tank made for the purpose of storing water (picture on the left). We tried to seal it by gluing it, but since it was initially designed for water we couldn’t seal it properly and gas was escaping through the lid. This is the only design for tanks that you can find to buy in Saint Vincent, so we decided to import the IBC tanks from Ariaponics Ltd. in Trinidad and Tobago.
IBC stands for Intermediate Bulk Container and is used for shipping different things, so you can find it everywhere around the world.
One of the advantages of building a biogas plant made out of IBC tanks is the fact that it’s an open source design made by Solar Cities, so you can find a lot of information on their website. It’s very easy to build. For the first model we build we used 2 IBC tanks for digesters, one underground concrete tank filled with water and a water tank with the top cut and rotated 180 degrees in the concrete tank to store the gas. And it works great. The only disadvantage is the labor needed in order to build the concrete tank. So we decided to stick to the design used by Solar Cities and use one IBC tank for the digester and 2 IBC tanks for the storage, as you can see in the picture.
For the digester it’s good to paint it in black so that the temperature keeps steady and for light not to go in, to not encourage the growth of algae. Algae produces oxygen which is not beneficial for anaerobic bacteria. We put a layer of gravel on the bottom of the tank in order to act as a growing place for bacteria (more surface area) and not to be flushed out with the effluent.
You need a 5 inch hole for the feeding pipe and a 3 inch hole for the fertilizer pipe. Than we used plumber’s silicone grease in between the uniseal and the tank to form a more reliable seal. To put the pipes in we used the blue soap, so we don’t add any chemicals in the tank which can alter the production of bacteria.
For the gas outlet we used the cap of the IBC and we just screw the 2 inch PVC threaded male adapter, than we used a 2 inch PVC pipe segment and a 2 inch PVC coupling for connecting - no. 9. Make sure you use Standard plumbers teflon tape (thread tape) for every threaded adapter that you use and glue all the segments of PVC with PVC glue between them.
For starting the digester we used cow manure – around 6 buckets of fresh cow manure and than we filled up the tank with water (chlorine free – rain water or we left the tap water for 2 days in order for the chlorine to evaporate) and sealed the lid of the IBC tank using multipurpose grease. Make sure that the lid is clean and grease up the threads of the cap, so when you screw, it becomes gas tight.
For the storage we used 2 IBC tanks. For the first one we cut the top of the tank and for the second one that will float up/down inside we rotated the tank 90 degrees with the narrower side facing the ground so it fits in, but before we made a 2 inch hole on the upper part and we used a bulkhead that fits a 2 inch pipe inside and than a coupling for connecting - no. 10. The suggestion from the Solar Cities was to cut also the second tank in order to fill it easier with water, but we decided to make a 5 inch hole at the bottom of the second tank in order to get filled with water and it’s working fine.
After this you connect the digester with the storage tank with a hose (we used 3/8 inch clear vinyl tubing) and the storage tank with the scrubber, which we attached to a wall close to the kitchen and from the scrubber you use another piece of hose to connect it to the stove. We used a scrubber for purifying the gas from Hydrogen Sulfide. For this we used a 2 inch pipe filled with steel wool. You can use any size of pipe. We choose this option because we could only find a reducer from a 2 inch pipe to a ½ inch.
This design is aesthetic, simple and easy to build. The system doesn’t require much space and is mobile. You should get approximately 2-3 hours of cooking on a single burner at a medium flame height.
More information on how to build a biogas plant on: www.solarcities.eu