Abstract: A digester tank assembly for use in digester systems for anaerobic digestion of waste water sludge comprising a tank having a fixed exterior cover, and a diaphragm member mounted in the tank dividing the tank into upper and lower compartments, said diaphragm member being positioned above the normal sludge level and being movable relative to the sludge level to provide for regulation of gas pressure on the interior of the digestion tank and to allow for substantial variations in gas storage volume at generally uniform pressures. The diaphragm member is connected to a sliding stack that is guided with respect to the fixed cover so that the stack will be allowed to move with the diaphragm as the diaphragm position varies in accordance with the gas storage volume. The diaphragm carries a flowable ballast material that will shift as the diaphragm raises and lowers and which can be varied in volume to maintain the desired pressure on the inside of the digester tank.