After the primary treatment of sewage water, primary effluent is passed into large aeration tanks, where it is constantly agitated mechanically and air is pumped into it. This allows vigorous growth of useful aerobic microbes into flocs (masses of bacteria associated with fungal filaments to form mesh like structures). While growing, these microbes consume the major part of the organic matter in the effluent. This significantly reduces the BOD or Biochemical Oxygen Demand of the effluent. BOD refers to the amount of oxygen that would be consumed if all the organic matter in one litre of water is oxidised by bacteria. The sewage water is treated till the BOD is reduced. The effluent is then passed into a settling tank, where the bacterial flocs are allowed to sediment. This sediment is called activated sludge. A small part of activated sludge is pumped back into the aeration tank to serve as the inoculum. The remaining major part of the sludge is pumped into large tanks called anaerobic sludge digesters. Here, other kind of bacteria which grow anaerobically, digest the bacteria and fungi in the sludge. During this digestion, bacteria produce a mixture of gases such as methane, hydrogen sulphide and carbon dioxide.