Sewage sludge consists principally of microorganisms and organic materials and is a by­product of the aerobic treatment of sewage. Sewage sludge can be disposed of on land, dried, put in landfill, composted, or incinerated, but one of the most efficient methods of disposal is to anaerobically digest the sewage sludge. The advantage of anaerobic digestion is that it reduces the volume of sludge, produces biogas and as it is enclosed less smell is associated with the process. There are a number of anaerobic digester designs but the simplest design is a large, sealed vessel of 200,000-400,000 l which is maintained at 30-37°C for 30 to 60 days (Fig. 5.5). The sewage sludge is broken down by anaerobic microorganisms producing a gas which is predominantly methane (biogas) and also contains carbon dioxide and hydrogen. A digester can produce gas at a rate of

1.0 m3/m3/day that represents between 4.2 and 10.4 GJ. The biogas collected has a reduced calorific value compared with 100% methane as it contains carbon dioxide.