Here is a drawing of the above-ground version of the Park Spark in Cambridge, MA. The tank on the left is the digester, it is designed to be anaerobic (without oxygen). This tank collects dog waste in the bottom of the tank and the methane that comes from the dog waste in the top of the tank. As people put dog waste into this tank it fills up to the level of the outlet pipe (the big pipe which connects the two tanks) and then overflows into the tank on the right (overflow tank), thus keeping the digester tank at a constant level. The small pipe coming off the digester (left tank) is the pipe that brings the methane to the lamppost where it is burned.