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A diffuser for dispersing gas into a body of liquid that may contain suspended solids. Under ordinary clogging conditions, the diffuser will remain operative over an indefinite period of time by reason of large gas outlet openings at the bottom of the gas plenum chamber. In addition, it can be emplo

A diffuser for dispersing gas into a body of liquid that may contain suspended solids. Under ordinary clogging conditions, the diffuser will remain operative over an indefinite period of time by reason of large gas outlet openings at the bottom of the gas plenum chamber. In addition, it can be employed for a good portion of that time to disperse a substantial flow of coarse bubbles and fine bubbles, in a desired combination, simultaneously from a single gas plenum. Careful selection of the permeability and area of a top porous wall of a gas plenum chamber, together with proper selection of the level, size and number of coarse bubble gas outlet openings in the side walls of the plenum chamber, achieve the second feature.

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1. A gas diffuser for diffusing gas into a body of receiving liquid in which the diffuser is submerged, which comprises a chamber that forms a gas plenum, said chamber including: (a) a top wall at least a substantial portion of which is formed of a porous material having a stabilized wet permeabi

1. A gas diffuser for diffusing gas into a body of receiving liquid in which the diffuser is submerged, which comprises a chamber that forms a gas plenum, said chamber including: (a) a top wall at least a substantial portion of which is formed of a porous material having a stabilized wet permeability of a magnitude to cause the production of fine gas bubbles when gas is introduced under a first predetermined pressure into said gas plenum chamber and exits through said porous top wall; (b) side walls defining, at a predetermined level below said top wall, a plurality of gas outlet openings of a first predetermined, fixed size to produce coarse gas bubbles when gas is introduced under a second, higher predetermined pressure into said gas plenum chamber and exits through said gas outlet openings, while continuing at all times to exit simultaneously through said porous top wall and through said gas outlet openings in said side walls, said gas outlet openings being unobstructed at all times except for any deposit that may be built up in said openings during use of said diffuser, said unobstructed gas outlet openings in the side walls of said plenum chamber being substantially equally spaced around the perimeter of the chamber; and (c) a gas inlet opening, whereby gas introduced under said first predetermined pressure into said gas inlet opening: will fill the upper portion of said gas plenum and exit through said porous top wall of the plenum chamber to form a rising central cylindrical column of densely packed fine gas bubbles in the body of receiving liquid in which the diffuser is submerged, and if the pressure at which the gas is maintained in said plenum is increased to said second predetermined figure, will fill said plenum chamber down to said predetermined level of said plurality of coarse bubble outlet openings and exit through said openings to form a rising outer, hollow, cylindrical sheath of coarse gas bubbles in the body of receiving liquid in which the diffuser is submerged, while said column of fine gas bubbles continues to be discharged through said porous top wall of the plenum chamber. 2. The gas diffuser of claim 1 in which said plenum chamber has a central opening adjacent the bottom of the chamber, and a gas inlet stem positioned within said chamber is in fluid communication with said central opening, with the upper end of said stem forming said gas inlet opening into said gas plenum. 3. The gas diffuser of claim 1 in which said gas plenum chamber includes a bottom wall spaced below said side walls of the chamber to form a plurality of horizontal slots that provide outlet openings between said bottom wall and said side walls large enough to avoid clogging during an indefinite period under ordinary conditions of use, said slots extending throughout a substantial portion of the perimeter of said side walls, whereby, if the porous top wall and all said gas outlet openings in said side walls become completely clogged, gas introduced into said plenum chamber at any pressure will fill the entire plenum and exit through said horizontal slots. 4. The gas diffuser of claim 1 which produces fine gas bubbles and coarse gas bubbles in predetermined proportions by gas flow rate, in which: (a) the portion of said top wall of the plenum chamber that is formed of said porous material has a stabilized wet permeability and a surface area of a magnitude to produce a first predetermined flow rate of fine gas bubbles when gas is introduced into said plenum chamber at the aforesaid first predetermined pressure, and a second predetermined flow rate of fine gas bubbles when gas is introduced at said second predetermined pressure; (b) said gas outlet openings of a first predetermined size in the plenum chamber side walls are provided in a number to produce a predetermined flow rate of coarse gas bubbles when gas is introduced into said plenum chamber at said second predetermined pressure, said second flow rate of fine gas bubbles and said flow rate of coarse gas bubbles having said predetermined proportions; and (c) said plurality of openings of a first predetermined size is located at a level that permits the quantity of pressurized gas in said plenum to increase, as the pressure at which the gas is introduced into the plenum is increased from said first to said second predetermined pressure, until it causes a pressure drop across said porous top wall of a magnitude to produce said predetermined proportion by flow rate of fine gas bubbles, while the pressure of the gas in said plenum chamber is maintained at the aforesaid second predetermined pressure to produce ssaid predetermined proportion by flow rate of coarse bubbles. 5. The gas diffuser of claim 1 in which said porous material of said top wall of said gas plenum chamber has a stabilized wet permeability of no more than about 35. 6. The gas diffuser of claim 1 in which said porous material of said top wall of said gas plenum chamber has a stabilized wet permeability of no more than about 5. 7. The gas diffuser of claim 1 in which said porous material of said top wall of said gas plenum chamber has a stabilized wet permeability of more than about 10 but less than about 35. 8. The gas diffuser of claim 1 in which said porous material of said top wall of said gas plenum chamber has a stabilized wet permeability of from about 5 to about 10. 9. The gas diffuser of claim 1 in which said porous material of said top wall of said gas plenum chamber has a stabilized wet permeability of about 7.5. 10. The gas diffuser of claim 1 in which said gas inlet opening is located at a level below said porous top wall and above said plurality of coarse bubble outlet openings. 11. The gas diffuser of claim 1 in which the upper surface of said porous portion of said plenum chamber top wall is constructed so as to be horizontal throughout its area when said diffuser is submerged in said body of receiving liquid. 12. The gas diffuser of claim 1 in which the porous portion of said plenum chamber top wall is inwardly spaced throughout its perimeter from the exterior surface of said side walls of the plenum chamber.