NOVELTY - The system comprises a digester (32) containing a single sludge treatment zone including an upper portion and a lower portion, a mixer for generating large mixing bubbles within the lower portion of the sludge treatment zone, a unit for detecting the quantity of stored biogas available for mixing sludge (50), and an auxiliary tank (56) of compressed anaerobic gas connected by a normally closed valve to a supply line. The mixer comprises: a source that supplies anaerobic gas and comprises stored biogas generated by sludge within the digester; and a gas accumulating apparatus.    USE - The system is useful for mixing sludge from waste liquid treatment (claimed) such as sewage and industrial waste water treatment in a digester for anaerobic decomposition of the sludge to produce biogas containing methane and carbon dioxide.    ADVANTAGE - The system is capable of effectively mixing the sludge without requiring the complexity of successively mixing a set of sectors in the digester.    DETAILED DESCRIPTION - The system comprises a digester (32) containing a single sludge treatment zone including an upper portion and a lower portion, a mixer for generating large mixing bubbles within the lower portion of the sludge treatment zone, a unit for detecting the quantity of stored biogas available for mixing sludge (50), and an auxiliary tank (56) of compressed anaerobic gas connected by a normally closed valve to a supply line. The mixer comprises: a source that supplies anaerobic gas and comprises stored biogas generated by sludge within the digester; a gas accumulating apparatus arranged in the lower portion of the digester; a supply line for transporting the anaerobic gas from the source of anaerobic gas to the gas accumulating apparatus; and a valve arranged within the supply line. The valve controllably supplies pulses of the anaerobic gas from the supply line to the gas accumulating apparatus. The gas accumulating apparatus comprises a bubble forming plate (28) and a pivoting bucket assembly. The normally closed valve is opened when insufficient stored biogas is detected for mixing the sludge. The source supplying anaerobic gas further comprises the compressed anaerobic gas in the auxiliary tank. An INDEPENDENT CLAIM is included for a mixer for mixing sludge in a waste liquid treatment system digester.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of a closed digester.    Bubble forming plate (28)    Digester (32)    Sludge (50)    Auxiliary gas pressure tank (56)    Regulator. (70)