4. Fit the small tank into the large tank.
A 1100l and a 750l tank were used to build a prototype at AIDG Guatemala: exterior diameters 120cm and
ARTI Bio-digester built by AIDG Guatemala
4.1 Mark where to cut off the top of the big tank
Cut the top of the big tank (the fermenter) so that the small tank (the gas holder) will fit inside it (upside
down.) The big tank is cut so that its opening is the size of the small tank.
Cut off the top of the small tank and place it, centered, on top of the big tank to mark its perimeter.
Remark the cutting line so that the hole is 3” wider than this. Ensure that the line is level.
• Mark the small tank circumference on a cord = ∏ x diameter (photo 3)
• Use the cord to mark the big tank so that the small one fits inside exactly (photo 2)
• Lay the cord in a circle. Check that distance to the tank edge is the same all round. Centre the small
tank on top of the big one; check it’s directly above the cord.