The Solar CITIES IBC Tank Biodigestor just got simpler!

 Actually it has been this simple for over a year now, but we just got around to learning enough Blender to make these 3D images.

 Gosh it's easy to build a family scale biodigestor!  All you have to do is buy a used IBC Tote Tank (those 1000 liter/275 gallon practically indestructible HDPE tanks that they ship liquids in all over the world; on Craig's List they can be had for about $125!) and put a 2 inch pipe (usually in two sections) running from about a half meter above the tank to about 15 cm above the bottom for introducing the feedstock (ground up food waste in our case),  a 1 inch pipe running from the middle of the tank to a valved elbow for letting out the effluent/fertilizer, and a 1/2 tank fitting with elbowed valve to let out the biogas that results.

You don't have to alter the tank in any way; all the inputs and outputs can be made through the cover.

You may find it a bit difficult to find the bulkhead tank fittings/adaptors in some hardware shops, but check out aquarium, pool and boating shops, or look online and you should find them.

To get the gas and the effluent out you can use garden hoses or clear plastic tubing. No big deal there; this stuff isn't under much pressure at all.

The gas comes right out the top so you don't need any pipe there.  The fertilizer comes out of the "dead zone" in the middle of the tank (fats and oils, which contain lots of energy, tend to float, and proteins and carbs tend to sink, so the top and bottom of the tank are biologically acive) so you want that one inch pipe going down to the halfway mark.  The feeding pipe should reach to about 10 or 15 cm above the bottom of the tank to leave room for the rocks and active sludge without the food getting blocked on its way down and into the tank.

But that's about it.  You'll want the feeding pipe up top to be about a half meter above the tank so that enough of a head can build up to force the gas out and so that you can pour all your food waste in without overflowing since the effluent outlet is half the diameter of the input pipe.  As you add the ground up food waste slurry (we use an Insinkerator brand garbage disposal to make it) fertilizer fluid will be coming out of the 1 inch pipe, but it will come out slower. You could use a larger pipe, but sometimes they don't fit the lid easily.  We've found with the proper feeding regime the dimensions depicted here work fine!

 Make sure the lid has an intact rubber O-ring gasket; one of ours didn't and while it eventually worked out using silcone, you can experience troublesome leaks. Some used IBC vendors sell you lids without the rubber ring inside; check and see just to be sure!
Then just fill your tank with about 50 to 100 kg animal manure, top off with water, screw the lid on and wait for a couple of weeks. When you get your first flammable gas you are ready to start feeding ground up food scraps -- about a 25 liter buckt of mixed food waste and water each day!