The sludge digesters have gas holder type floating covers. Fixed type covers are sometimes used but it has been found that if excessive sludge is withdrawn, the internal pressure becomes negative and air is drawn in to prevent roof collapse. Safety is a priority whichever design is chosen. Gas is the final product of the stabilisation of the organic material added to an anaerobic digester in the feed sludge. Gas handling systems are designed on the ability to handle periodic gas production at the upper end of this range.
Gas composition varies with sludge characteristics.
The table below lists typical concentrations of major gas components.