NOVELTY - An anaerobic digester comprising a sludge receiving basin adapted to receive sludge in it; a cover (2) weir positioned within the tower (14) to provide sludge level control by allowing an overflow and discharge of sludge raising through the tower; and a gas dome located in the tower above the weir for collecting and discharging gas, is new.    USE - The digester is for use in treatment of sludge containing microorganism. It is also used for stabilizing and reducing solids content of excess activated sludge, foam, and scum that results from a biological wastewater treatment process.    ADVANTAGE - The digester provides design that is inexpensive and the design does resolve the problems of handling foam and collecting gas without foam getting into gas lines, overflow weir and above the foam layer. It allows foam to be minimized and separated from the gas thus minimizing the likelihood that foam will get into gas lines and cause damage to the compressor or other downstream equipment.    DETAILED DESCRIPTION - An anaerobic digester comprising a sludge receiving basin adapted to receive sludge in it; a cover positioned over basin and having an under surface adapted to be in direct contact with respect to sludge contents within the digester; a tower mounted so as to extend above the cover and flow communicate with the contents in the digester; a weir positioned within the tower to provide sludge level control by allowing an overflow and discharge of sludge raising through the tower; and a gas dome located in the tower above the weir for collecting and discharging gas.    An INDEPENDENT CLAIM is included for biologically treating sludge in a digester while controlling sludge level and separating foam from gas produced in the digester comprising:    (a) providing an anaerobic digester having a cover and a tower extending above the cover and flow communicating with the digester;    (b) flowing sludge into the digester and filing the digester such that the sludge contacts an under surface of the cover and rises in the tower;    (c) providing a level control weir in the tower such that sludge and foam overflows the weir so as to maintain a constant height of sludge relative to the digester during normal operation;    (d) providing a gas collection dome in the tower spaced above the weir; and    (e) collecting gas in the dome and discharging collected gas from the tower.    DESCRIPTION OF DRAWING(S) - The figure shows an enlarged and fragmentary side elevational view of the central tower of an aerobic digester for use in sludge treatment (with portions broken away to illustrate detail).    Cover (2)    Engaging portion (12)    Tower (14)    Light component (36)    Sidewall (40)