Yube Boitumelo Siyangaphi, Joseph Kabimba and Kebonyemang Mashabe and I have built a three stage biodigestor at the worker's camp at CSU base camp, Selinda wildlife reserve, Botswana. It consists of a fiberglass-sealed 1000 liter horizontal digestor tank (primary digestor), a 200 liter used steel oil drum with a telescoping gas collector made from a second oil drum, chopped in two (with 5 cm cut out all around) and then brazed (torch welded) back together so it fits inside the first drum (5 leaks had to be patched with metal expoxy). The third collector was made out of two plastic drums (110 and 100 liter). All were filled with a mixture of sewage sludge pumped from the bottom of the septic tank and elephant dung (collected from the herds that walked through camp the other night).
The CSU base camp biodigestor was our proving ground where we showed that we could get flammable gas within two days using septic tank sludge and hippo-dung saturated Okavango Delta Lake Mud instead of the three weeks that animal dung (like the elephant dung we were trying) needs. In the CSU system we also pioneered the idea of connecting an Insinkerator Evolution 200 (donated by Emerson Electronics, which now grinds the staff kitchen waste) to an Admiral Macerating Toilet (which I bought using the Blackstone Ranch/Nat Geo Innovation Challenge Grant in Germany) connected to the staff toilets. This way the kitchen scraps provide the primary energy while the macerating toilet provides a continuous supply of bacteria and some residual energy, and pumping energy to get the slurry into the distant biodigestor. The digestor also was where we experimented with different gas collection systems, trying out in reality what we saw in a drawing, a diagram we saw in an old biogas manual from the 1980s that talked about making biogas digestors from old oil drums. So... we did it.