NOVELTY - A system for mixing sludge from waste liquid treatment comprises: a digester containing at least one sludge treatment zone having upper and lower portions; and a mixer for generating large mixing bubbles within the lower portion of sludge treatment zone. The mixer comprises at least one bubble forming plate (28) disposed in the lower portion of digester; at least one supply line (30) for transporting anaerobic gas from source of the gas to bubble forming plate; and a valve within the supply line to controllably supply pulses of gas from supply line to underside of bubble forming plate.    USE - As system for mixing sludge from waste liquid treatment (claimed).    ADVANTAGE - The improved system mixes the contents of anaerobic liquid waste treatment tanks by employing bubble mixing technology in an effective manner and thus provides large anaerobic bubbles that mix materials in all levels of the tank. The mixing bubbles are large enough to move sludge as they rise to the surface and generate a mixing current in the sludge. The mixing current mixes the sludge, and bacteria and other microorganisms to promote the bacteria and other microorganism's conversion of the pollutants contained in the sludge. Moreover, instead of employing the bubble mixer to mix sludge in the early stages of anaerobic decomposition, a mechanical mixer can be used to mix sludge until such time as sufficient biogas is produced by sludge to operate the bubble mixer, thus obviating the need for auxiliary gas pressure tank and associated valve altogether. Additionally, when anaerobic decomposition of sludge has progressed sufficiently that a quantity of biogas is produced, a portion of the biogas can be treated and used as fuel to power compressor in a manner well known to those in the alternative energy arts.    DETAILED DESCRIPTION - A system for mixing sludge from waste liquid treatment comprises: a digester containing at least one sludge treatment zone having an upper portion and a lower portion; and a mixer for generating large mixing bubbles within the lower portion of the sludge treatment zone. The mixer comprises a source of anaerobic gas; at least one bubble forming plate (28) disposed in the lower portion of the digester; at least one supply line (30) for transporting anaerobic gas from the source of anaerobic gas to the bubble forming plate; and a valve within the supply line. The valve controllably supplies pulses of the anaerobic gas from the supply line to the underside of the bubble forming plate. The system further comprises a unit for capturing and storing biogas obtained from the anaerobic decomposition of sludge in the digester; unit for detecting the quantity of stored biogas available for mixing the sludge; and an auxiliary tank of compressed anaerobic gas connected by a normally closed valve to the supply line, where when insufficient biogas is detected for mixing the sludge, the normally closed valve is opened and the source of anaerobic gas further comprises the tank. An INDEPENDENT CLAIM is included for a mixer for mixing sludge in a waste liquid treatment system digester having an upper portion and a lower portion comprising: at least one bubble forming plate disposed in the lower portion of the digester; unit for capturing and storing biogas emitted by decomposing sludge in the digester, such unit is connected to provide stored biogas to at least one supply line, where the supply line transports gas to the bubble forming plate; and a valve within the supply line, where the valve supplies pulses of gas from the supply line to the underside of the bubble forming plate.    DESCRIPTION OF DRAWING(S) - The figure shows a perspective view of a bubble forming plate.    Bubble forming plate assembly (14)    Bubble forming plate (28)    Gas distribution line (30)    Orifice (36)