According to the text, methanogens grow anaerobically on cellulosic material to produce large amounts of methane along with CO2 and H2. They are found in the anaerobic sludge during sewage treatment and in the rumen of cattle where they help in the breakdown of cellulose. The excreta (dung) of cattle is rich in these bacteria, which are used to produce biogas (methane). Therefore, the statement that they grow aerobically is incorrect.