In place gas cleaning of diffusion elements

Abstract

Cleaning of multi-pore diffusion elements in place with cleaning gases while submerged in liquid media by applying said cleaning gases intermittently or continuously to said diffusion elements between predetermined limits of operating pressure and flow through flow regulation means and plenums for the respective diffusion elements.

What is claimed is: 
     
       1. In a process of treating a liquid medium by passing treating gas through multi-pore diffusion elements submerged in the medium, wherein foulants in the medium or treating gas have tendency to form a deposits in the elements or at their surfaces and to cause a progressive increase in the dynamic wet pressure and/or mean bubble release pressure of the diffusion elements relative to a previous base condition of said elements, and in which the elements are cleaned in place by passing a cleaning gas through them alone or in admixture with treating gas, said cleaning gas having a different composition than said treating gas and being capable of reducing the dynamic wet pressure and/or the mean bubble release pressure of fouled diffusion elements, the improvement which comprises: conducting said gas cleaning with sufficient frequency, including continuously, and with sufficient amount(s) of cleaning gas for restricting any potential or actual increase in the dynamic wet pressure level of the elements, above a previous base condition of said dynamic wet pressure, to about 25 inches or less of water gauge .[.of.]. .Iadd.at .Iaddend.2 SCFM per square foot of active gas discharge surface of the elements, and/or restricting any potential or actual increase in the mean bubble release pressure level of the elements, above a previous base condition of said mean bubble release pressure, to about 25 inches of water gauge or less;   simultaneously applying said cleaning gas to one or more groups of at least about 10 diffusion elements per group by feeding the cleaning gas to the elements through a gas distribution network having a submerged portion including a plurality of flow regulating means, a plurality of plenums and a plurality of diffusion elements, said flow regulating means being connected with said plenums to deliver gas to said plenums, said flow regulating means being sized or adjusted to deliver said gas at a substantially similar rate to each of said plenums, each of said plenums being connected directly or indirectly with one or more of said diffusion elements whereby said diffusion of said diffusion elements whereby said diffusion element or elements connected to one of said plenums can be supplied with gas at a substantially similar rate as compared to the rate at which gas is supplied to the diffusion elements connected to the other plenum or plenums; and   discharging treating gas along or in admixture with cleaning gas, through the diffusion elements for an extended cycle of operation constituting at least about 30 total days of continuous or intermittent passage of treatment gas through the elements;   whereby fouling is restrained during said cycle by applying the cleaning gas with sufficient frequency and in sufficient amounts to maintain the dynamic wet pressure and/or mean bubble release pressure at or below the aforesaid levels.   
     
     
       2. In a process of treating a liquid medium by passing treating gas through multi-pore diffusion elements submerged in the medium, wherein foulants in the medium or treating gas tend to form deposits in the elements or at their surfaces causing a progressive increase in the dynamic wet pressure and/or the mean bubble release pressure of the diffusion elements relative to a previous base condition of said elements, and in which the elements are cleaned in place by intermittently passing a cleaning gas through them, alone or in admixture with treating gas, said cleaning gas having a different composition than said treating gas and being capable of reducing the dynamic wet pressure and/or the mean bubble release pressure of fouled diffusion elements, the improvement which comprises: initiating cleaning with said cleaning gas when the dynamic wet pressure level of the elements has increased to a level above their base condition by an amount which is equal or equivalent to about 25 inches of water gauge or less at 2 SCFM per square foot of active gas discharge surface, or when the mean bubble release pressure of the diffusion elements has increased to a level above their base condition by about 25 inches of water gauge or less;   simultaneously applying said cleaning gas to one or more groups of at least about 10 diffusion elements per group by feeding the cleaning gas to the elements through a gas distribution network having a submerged portion including a plurality of flow regulating means, a plurality of plenums and a plurality of diffusion elements, said flow regulating means being connected with said plenums to deliver gas to said plenums, said flow regulating means being sized or adjusted to deliver said gas at a substantially similar rate to each of said plenums, each said plenums being connected directly or indirectly with one or more of said diffusion elements whereby said diffusion element or elements connected to one of said plenums can be supplied with gas at a substantially similar rate as compared to the rate at which gas is supplied to the diffusion elements connected to the .[.outer.]. .Iadd.other .Iaddend.plenum or plenums; and applying said cleaning gas in one or more periods of application and in a sufficient amount .[.of.]. for reducing said increase in dynamic wet pressure by at least about 0.3 times said increase in dynamic wet pressure or for reducing said increase in mean bubble release pressure by at least about 0.5 times said increase in means bubble release pressure.   
     
     
       3. The process according to claim 2 wherein said cleaning gas is HCl in a gas mixture in a mole fraction in the gas mixture at atmospheric pressure in the range from about 6.6×10 -3  to about 3.1×10 -2  and said period of application of said cleaning gas is from about 30 to about 160 minutes. 
     
     
       4. The process according to claim 3 wherein the cleaning gas is passed through said diffusion elements at a rate from about 6 to about 8 SCFM per square foot of active discharge area. 
     
     
       5. The process according to claim 2 wherein said cleaning is initiated when said increase in dynamic wet pressure of said elements above said base condition, or said increase in mean bubble release pressure of said elements above said base condition is at least about 0.5 inches of water gauge. 
     
     
       6. The process according to claim 2 wherein said cleaning is initiated when said increase in dynamic wet pressure of said elements above said base condition, or said increase in mean bubble release pressure of said elements above said base condition is at least about 0.7 inches of water gauge. 
     
     
       7. The process according to claim 2 wherein said cleaning is initiated when said increase in dynamic wet pressure of said elements above said base condition, or said increase in mean bubble release pressure of said elements above said base condition is from about 0.5 to about 15 inches of water gauge. 
     
     
       8. The process according to claim 2 wherein said cleaning is initiated when said increase in dynamic wet pressure of said elements above said base condition, or said increase in mean bubble release pressure of said elements above said base condition is from about 0.5 to about 7 inches of water gauge. 
     
     
       9. The process according to claim 2 wherein said cleaning is initiated when said increase in dynamic wet pressure of said elements above said base condition, or said increase in mean bubble release pressure of said elements above said base condition, is from about 0.7 to about 15 inches of water gauge. 
     
     
       10. The process according to claim 2 wherein said cleaning is initiated when said increase in dynamic wet pressure of said elements above said base condition, or said increase in mean bubble release pressure of said elements above said base condition is from about 0.7 to about 7 inches of water guage. 
     
     
       11. The process according to claim 2 wherein said cleaning is conducted until the dynamic wet pressure of said elements is reduced by at least about 0.5 of the increase in dynamic wet pressure or mean bubble release pressure above said base condition caused by said foulants. 
     
     
       12. The process according to claim 2 wherein said cleaning is conducted until the dynamic wet pressure or means bubble release pressure of said elements is reduced by at least about 0.8 of the increase in dynamic wet pressure or mean bubble release pressure above said base condition caused by said foulants. 
     
     
       13. The process according to claim 2 wherein said cleaning is conducted until the mean bubble release pressure of said elements is reduced by at least about 0.9 of the increase in bubble release pressure above said base condition. 
     
     
       14. The process according to claim 2 wherein during the cleaning period, a surface-active agent is applied to the interfaces between the diffusion elements and the medium in an amount sufficient to reduce the surface tension of the liquid medium. 
     
     
       15. The process according to claim 14 wherein at least about 60 percent of the cleaning gas is discharged from said diffusion elements while said surface-active agent is present at said interfaces between the diffusion elements and the liquid medium. 
     
     
       16. The process according to claim 14 wherein at least about 80 percent of the cleaning gas is discharged from said diffusion elements while said surface-active agent is present at said interfaces between the diffusion elements and the liquid medium. 
     
     
       17. The process according to claim 2 wherein at least about 60 percent of the cleaning gas is discharged from said diffusion elements while an enhanced pressure differential of at least about 2 inches of water is maintained across said diffusion elements. 
     
     
       18. The process according to claim 2 wherein at least about 80 percent of the cleaning gas is discharged from said diffusion elements while an enhanced pressure differential of at least about 2 inches of water is maintained across said diffusion elements. 
     
     
       19. The process according to claim 1 or 2 wherein said liquid medium is an aqueous waste liquor from domestic sewage, or industrial wastes. 
     
     
       20. The process according to claims 1 or 2 wherein the treating gas is an oxygen-containing gas. 
     
     
       21. The process according to claims 1 or 2 wherein the treating gas is air. 
     
     
       22. The process according to claims 1 or 2 wherein said diffusion elements each comprise a porous mass of finely-divided solid particles bonded together, and having a fine-bubble emitting gas discharge surface free of macro openings and capable of discharging gas in a substantially uniform manner. 
     
     
       23. The process according to claim 22 wherein said finely-divided particles are ceramic particles in said mass which are bonded together. 
     
     
       24. The process according to claim 22 wherein said diffusion elements have a bubble release pressure in the range of about 2 to about 20 inches of water gauge. 
     
     
       25. The process according to claim 22 wherein said diffusion elements have a bubble release pressure in the range of about 3 to about 15 inches of water gauge. 
     
     
       26. The process according to claim 22 wherein said diffusion elements have a bubble release pressure in the range of about 4 to about 10 inches of water gauge. 
     
     
       27. The process according to claims 1 or 2 wherein said diffusion elements comprise finely pored ceramic plates having an upper gas discharge surface with a bubble release pressure in the range of about 2 to about 20 inches of water, said elements including a central zone and an outward zone, said central zone having an enhanced volumetric compression ratio relative to said outward zone, said element having pore sizes in the range of about 60 to about 600 microns, and a coefficient of variation of the bubble release pressure of the gas discharge surface not greater than about 0.25 based on the values of bubble release pressure measurement of at least 5 equally spaced points along each of two perpendicular reference lines extending across said upper surface of the element and through the center thereof. 
     
     
       28. The process according to claim 27 wherein the area of the central zone comprises from about 10 to about 80 percent of the total area of the gas discharge surface. 
     
     
       29. The process according to claim 27 wherein the area of the central zone comprises from about 25 to about 70 percent of the total area of the gas discharge surface. 
     
     
       30. The process according to claim 27 wherein said bubble release pressure is in the range of from about 5 to about 10 inches of water gauge. 
     
     
       31. The process according to claim 27 wherein the volumetric compression ratio of the central zone has been enhanced, relative to the outward zone, by at least about 2 percent. 
     
     
       32. The process according to claim 27 wherein the volumetric compression ratio of the central zone has been enhanced, relative to the outward zone, by from about 3 to about 15 percent. 
     
     
       33. The process according to claims 1 or 2 wherein at least about 90 percent of the diffusion elements to which said cleaning gas is applied are able, when new or when thoroughly cleaned, to transmit gas at a flow rate within about 15 percent of the average flow rate of all such elements when tested at 2 inches of water gauge in a dry condition. 
     
     
       34. The process according to claims 1 or 2 wherein at least about 95 percent of the diffusion elements to which said cleaning gas is applied are able, when new or when thoroughly cleaned, to transmit gas at a flow rate within about 10 percent of the average flow rate of all such elements when tested at 2 inches of water gauge in a dry condition. 
     
     
       35. The process according to claims 1 or 2 wherein said diffusion elements comprise finely-pored ceramic plates having upper gas discharge surfaces with a bubble release pressure in the range from about 2 to about 20 inches of water, each of said elements including a central zone, an outward zone, a boundary zone, a peripheral zone and vertical cylindrical sides, said central zone having an enhanced volumetric compression ratio relative to said outward zone by from about 3 to about 15 percent, said boundary zone having an enhanced volumetric compression ratio relative to said outward zone by from about 10 to about 100 percent, and each of said elements having a coefficient of variation of the bubble release pressure of its upper gas discharge surface not greater than about 0.25 based on the values of bubble release pressure measurements of at least 5 equally spaced points along each of two perpendicular reference lines extending across said upper gas discharge surface of the element and through the center thereof. 
     
     
       36. The process according to claim 35 wherein the area of the central zone comprises from about 10 to about 80 percent of the total area of the gas discharge surface. 
     
     
       37. The process according to claim 35 wherein the area of the central zone comprises from about 25 to about 70 percent of the total area of the gas discharge surface. 
     
     
       38. The process according to claim 35 wherein the vertical cylindrical sides of said diffusion elements are semi-permeable and free of adherent material preventing bubble emission, the diffusion elements are supported by holders which at least partly enclose plenums and are secured to said holders by peripheral retaining means and sealed to said holders by non-adherent elastomeric seals at the upper edge of said vertical cylindrical sides. 
     
     
       39. The process according to claims 1 or 2 wherein said diffusion elements each have a gas discharge surface area of at least about 20 square inches. 
     
     
       40. The process according to claims 1 or 2 wherein said diffusion elements each have a gas discharge area of at least about 40 square inches. 
     
     
       41. The process according to claims 1 or 2 wherein said diffusion elements each have a gas discharge area of at least 40 square inches per inch of thickness of said element. 
     
     
       42. The process according to claim 1 or 2 wherein said cleaning gas dissolves, or poisons said foulant or material which attaches said foulant to said diffusion elements. 
     
     
       43. The process according to claims 1 or 2 wherein said cleaning gas is an oxidizing agent. 
     
     
       44. The process according to claims 1 or 2 wherein said foulants comprise living organisms and said cleaning gas is a poison for said organisms. 
     
     
       45. The process according to claim 1 or 2 wherein said cleaning gas is an organic compound having a solvent effect on said foulant or on material which attaches said foulant to said diffusion elements. 
     
     
       46. The process according to claims 1 or 2 wherein the cleaning gas contains at least one member selected from the group consisting of H 2  O 2 , CH 3  OH, SO 2 , CO 2 , Cl 2 , ClO 2 , HCl, NO x , O 3  and Br 2 . 
     
     
       47. The process according to claims 1 or 2 wherein said cleaning gas is soluble in water and forms an acidic solution when dissolved in water. 
     
     
       48. The process according to claim 47 wherein said cleaning gas is present in a concentration in said treating gas sufficiently high to form an aqueous acidic solution having a pH at equilibrium of about 3 or less. 
     
     
       49. The process according to claim 47 wherein said cleaning gas is present in a concentration in said treating gas sufficiently high to form an aqueous acidic solution having a pH at equilibrium of about 2 or less. 
     
     
       50. The process according to claim 47 wherein said cleaning gas is present in a concentration in said treating gas sufficiently high to form an aqueous acidic solution having a pH at equilibrium of about 1 or less. 
     
     
       51. The process according to claim 47 wherein said cleaning gas is present in a concentration in said treating gas sufficiently high to form an aqueous acidic solution having a pH at equilibrium of about 0.7 or less. 
     
     
       52. The process according to claim 47 wherein said cleaning gas is present in a concentration in said treating gas sufficiently high to form an aqueous acidic solution having a pH at equilibrium of about 0.5 or less. 
     
     
       53. The process according to claim 47 wherein said cleaning gas is present in a concentration in said treating gas sufficiently high to form an aqueous acidic solution having a pH at equilibrium of about 0.4 or less. 
     
     
       54. The process according to claim 4 wherein said cleaning gas contains HCl in a mole fraction at atmospheric pressure of at least about 4×10 -5 . 
     
     
       55. The process according to claim 47 wherein said cleaning gas contains HCl in a mole fraction at atmospheric pressure of at least about 7.5×10 -5 . 
     
     
       56. The process according to claim 47 wherein said cleaning gas contains HCl in a mole fraction at atmospheric pressure of at least about 4×10 -4 . 
     
     
       57. The process according to claim 47 wherein said cleaning gas contains HCl in a mole fraction at atmospheric pressure at least about 1.3×10 -3 . 
     
     
       58. The process according to claim 47 wherein said cleaning gas contains HCl in a mole fraction at atmospheric pressure in the range from about 6.6×10 -3  to about 3.1×10 -2 . 
     
     
       59. The process according to claim 47 wherein said cleaning gas contains HCl in a sufficiently high concentration to form an aqueous acidic solution having a pH at equilibrium of less than about 0.7. 
     
     
       60. The process according to claim 47 wherein said cleaning gas contains HCl is a sufficiently high concentration to form an aqueous acidic solution having a pH at equilibrium of less than about 0.4. 
     
     
       61. The process according to claims 1 or 2 wherein said cleaning gas being passed through said diffusion elements is admixed with water vapor. 
     
     
       62. The process according to claims 1 or 2 wherein said cleaning gas being passed through said diffusion elements is admixed with water vapor and treating gas, and the admixture is obtained by dissolving cleaning gas in water, and passing treating gas through the resulting solution of cleaning gas in water. 
     
     
       63. The process according to claims 1 or 2 wherein said cleaning is conducted or initiated to restrict .[.said.]. .Iadd.any potential or actual .Iaddend.increase in dynamic wet pressure of the elements above said base condition, or .[.said.]. .Iadd.any potential or actual .Iaddend.increase in mean bubble release pressure of the elements above said base condition to about 15 inches or less of water gauge. 
     
     
       64. The process according to claims 1 or 2 wherein said cleaning is conducted or initiated to restrict .[.said.]. .Iadd.any potential or actual .Iaddend.increase in dynamic wet pressure of said elements above said base condition, or .[.said.]. .Iadd.any potential or actual .Iaddend.increase in mean bubble release pressure of said elements above said base condition to about 7 inches or less of water gauge. 
     
     
       65. The process according to claims 1 or 2 wherein in a portion of said cycle or period of application of said cleaning gas, the dynamic wet pressure across said diffusion elements is increased. 
     
     
       66. The process according to claim 65 wherein the dynamic wet pressure is increased by at least about 1 inch of water gauge. 
     
     
       67. The process according to claim 65 wherein the dynamic wet pressure is increased by at least about 2 inches of water gauge. 
     
     
       68. The process according to claim 65 wherein the dynamic wet pressure is increased by decreasing the depth of submergence of the diffusion elements in the liquid medium. 
     
     
       69. The process according to claim 65 wherein the dynamic wet pressure is increased by increasing the total gas pressure of the gas applied to said elements. 
     
     
       70. The process according to claims 1 or 2 wherein in a portion of said cycle or period of application of said cleaning gas the dynamic wet pressure across said diffusion elements is maintained substantially constant. 
     
     
       71. The process according to claims 1 or 2 wherein in a portion of said cycle or period of application of said cleaning gas, the rate of decrease of the dynamic wet pressure across said diffusion elements is less than the decrease which would be attributable solely to application of said cleaning gas. 
     
     
       72. The process according to claims 1 or 2 wherein at least about 40 percent of the cleaning gas is discharged from said diffusion elements while an enhanced pressure differential of at least about 2 inches of water is maintained across said diffusion elements. 
     
     
       73. The process of claims 1 or 2 wherein the flow regulating means is mounted .[.with.]. .Iadd.within .Iaddend.the plenum. 
     
     
       74. The process of claim 73 wherein the flow regulating means responds exponentially to pressure. 
     
     
       75. The process of claim 73 wherein the flow regulating means includes means to restrict the flow to unidirectional flow. 
     
     
       76. In a process of treating a liquid medium by passing oxygen-containing gas through multi-pore diffusion elements submerged in the medium wherein foulants in the medium or oxygen-containing gas have a tendency to form deposits in the elements or at their surfaces and to cause a progressive increase in the dynamic wet pressure and/or means bubble release pressure of the diffusion elements relative to a previous base condition of said elements, a method of cleaning said elements in place comprising; positioning diffusion elements for submergence in said liquid medium having a bubble release pressure in the range of about 2 to about 20 inches of water gauge, at least about 90 percent of said diffusion elements when new or thoroughly cleaned being able to transmit gas at a flow rate within about 15 percent of the average flow rate of all such elements when tested in a dry condition at a pressure of 2 inches of water gauge, said elements having a coefficient of variation of bubble release pressure of the gas discharge surface not greater than about 0.25 based on values of bubble release pressure measurements for at least 5 equally spaced points along each of two perpendicular reference lines across the gas discharge surface and through the center thereof;   initiating cleaning when the dynamic wet pressure level or mean bubble release pressure of the diffusion elements has increased above said base condition by from 0.7 to about 25 inches of water gauge by simultaneously applying a mixture of HCl and said oxygen-containing gas to the diffusion elements, said HCl being present in a mole fraction of at least about 7.5×10 -5  and sufficiently high to effectuate cleaning of said diffusion elements, said mixture being applied through a gas distribution network having a submerged portion including a plurality of flow regulating means, a plurality of plenums, and a plurality of diffusion elements, said flow regulating means being connected to said plenums and adapted to deliver gas at a substantially similar rate to each of said plenums, each of said plenums being connected directly or indirectly with one or more of said diffusion elements whereby said diffusion element or elements connected to one of said plenums can be supplied with gas at a rate substantially similar to the rate of gas supplied to the diffusion elements connected to the other plenum or plenums; and   applying said mixture of HCl and oxygen-containing gas in one or more periods in an amount sufficient for reducing said increase in dynamic wet pressure by at least about 0.3 times said increase in dynamic wet pressure or for reducing said increase in means bubble release pressure by at least about 0.5 times said increase in mean bubble release pressure.   
     
     
       77. The process according to claim 2 wherein said diffusion elements are divided into two or more groups, said treating gas is applied to the diffusion elements in a portion of said groups at an enhanced rate and/or pressure differential, and cleaning is initiated by selectively admixing cleaning gas with the treating gas applied to said portion. 
     
     
       78. The process according to claim 77 wherein each of said groups constitutes the elements within an individual tank in a multi-tank plant. 
     
     
       79. The process according to claims 1 or 2 wherein during at least a portion of the cleaning an enhanced pressure differential of at least about 0.5 inches of water is applied across the diffusion elements. 
     
     
       80. The process according to claim 2 wherein an enhanced pressure differential is applied across said diffusion elements before cleaning is initiated. .Iadd. 
     
     
       81.  The process according to claim 1 wherein the base condition is that dynamic wet pressure or bubble release pressure exhibited by the elements when they (a) were manufactured or (b) were first put into operation in the treatment of a liquid medium containing at least one foulant. .Iaddend. .Iadd.82. The process according to claim 2 wherein the base condition is that dynamic wet pressure or bubble release pressure exhibited by the elements when they (a) were manufactured or (b) were first put into operation in the treatment of a liquid medium containing at least one foulant. .Iaddend. .Iadd.83. The process according to claim 1 wherein the base condition is that dynamic wet pressure or bubble release pressure exhibited by the elements when they have been thoroughly cleaned. .Iaddend. .Iadd.84. The process according to claim 2 wherein the base condition is that dynamic wet pressure or bubble release pressure exhibited by the elements when they have been thoroughly cleaned. .Iaddend. .Iadd.85. The process according to claim 2 wherein the base condition is that dynamic wet pressure or bubble release pressure exhibited by the elements when they (a) were manufactured, or (b) were first put into operation in the treatment of a liquid medium containing at least one foulant, or (c) have been thoroughly cleaned, and cleaning gas is applied in each of a plurality of separate gas cleaning operations for restricting any potential or actual increase in the dynamic wet pressure or bubble release pressure level of the elements above said base condition to about 25 inches or less of water gauge, and the same base condition is applicable to a plurality of said gas cleaning operations. .Iaddend. .Iadd.86. The process according to claim 1 wherein the base condition is expressed in terms of the dynamic wet pressure exhibited by the elements, and said cleaning gas is applied for restricting any potential or actual increase in the dynamic wet pressure level of the elements above said base condition to about 25 inches or less of water gauge. .Iaddend. .Iadd.87. The process according to claim 2 wherein the base condition is expressed in terms of the dynamic wet pressure exhibited by the elements, and said cleaning gas is applied for restricting any potential or actual increase in the dynamic wet pressure level of the elements above said base condition to about 25 inches or less of water gauge. .Iaddend. .Iadd.88. The process according to claim 1 wherein the base condition is expressed in terms of the bubble release pressure exhibited by the elements, and said cleaning gas is applied for restricting any potential or actual increase in the bubble release pressure level of the elements above said base condition to about 25 inches or less of water gauge. .Iaddend. .Iadd.89. The process according to claim 1 wherein the base condition is expressed in terms of the bubble release pressure exhibited by the elements, and said cleaning gas is applied for restricting any potential or actual increase in the bubble release pressure level of the elements above said base condition to about 25 inches or less of water gauge. .Iaddend. .Iadd.90. The process according to claim 1 wherein cleaning gas alone or in admixture with treating gas is discharged through diffusion elements having their own individual plenums which receive said treating gas and cleaning gas through their own individual flow regulating means. .Iaddend. .Iadd.91. The process according to claim 2 wherein cleaning gas alone or in admixture with treating gas is discharged through diffusion elements having their own individual plenums which receive said treating gas and cleaning gas through their own individual flow regulating means. 
     
     
        .Iaddend. .Iadd.92.  The process according to claim 1 wherein the treating gas includes an oxygen-containing treating gas and treating gas and cleaning gas are discharged simultaneously through a group or groups of at least about ten diffusion elements having their own individual plenums which receive said treating gas and cleaning gas through their own individual flow regulating means and through said network. .Iaddend. .Iadd.93. The process according to claim 2 wherein the treating gas includes an oxygen-containing treating gas and treating gas and cleaning gas are discharged simultaneously through a group or groups of at least about ten diffusion elements having their own individual plenums which receive said treating gas and cleaning gas through their own individual flow regulating means and through said network. .Iaddend. .Iadd.94. The process according to claim 1 wherein the base condition is that dynamic wet pressure exhibited by the elements when they (a) were manufactured, or (b) were first put into operation in the treatment of a liquid medium containing at least one foulant, or (c) have been thoroughly cleaned, the treating gas includes an oxygen-containing treating gas, and treating gas and cleaning gas are discharged simultaneously through a group or groups of at least about ten diffusion elements having their own individual plenums which receive said treating gas and cleaning gas through their own individual flow regulating means and through said network, said cleaning gas being applied continuously or being applied intermittently in each of a plurality of separate gas cleaning operations for restricting any potential or actual increase in the dynamic wet pressure level of the elements above said base condition to about 25 inches or less of water gauge. .Iaddend. .Iadd.95. The process according to claim 2 wherein the base condition is that dynamic wet pressure exhibited by the elements when they (a) were manufactured, or (b) were first put into operation in the treatment of a liquid medium containing at least one foulant, or (c) have been thoroughly cleaned, the treating gas includes an oxygen-containing treating gas, and treating gas and cleaning gas are discharged simultaneously through a group or groups of at least about ten diffusion elements having their own individual plenums which receive said treating gas and cleaning gas through their own individual flow regulating means and through said network, said cleaning gas being applied in each of a plurality of separate gas cleaning operations for restricting any potential or actual increase in the dynamic wet pressure level of the elements above said base condition to about 25 inches or less of water gauge, and the same base condition being applicable to a plurality of said gas cleaning operations. .Iaddend. .Iadd.96. The process according to claim 1 wherein the base condition is that bubble release pressure exhibited by the elements when they (a) were manufactured, or (b) were first put into operation in the treatment of a liquid medium containing at least one foulant, or (c) have been thoroughly cleaned, the treating gas includes air, and treating gas and cleaning gas are discharged simultaneously through a group or groups of at least about ten diffusion elements having their own individual plenums which receive said treating gas and cleaning gas through their own individual flow regulating means and through said network, said cleaning gas being applied continuously or being applied intermittently in each of a plurality of separate gas cleaning operations for restricting any potential or actual increase in the bubble release pressure level of the elements above said base condition to about 25 inches or less of water gauge. .Iaddend. .Iadd.97. The process according to claim 2 wherein the base condition is that bubble release pressure exhibited by the elements when they (a) were manufactured, or (b) were first put into operation in the treatment of a liquid medium containing at least one foulant, or (c) have been thoroughly cleaned, the treating gas includes air, and treating gas and cleaning gas are discharged simultaneously through a group or groups of at least about ten diffusion elements having their own individual plenums which receive said treating gas and cleaning gas through their own individual flow regulating means and through said network, said cleaning gas being applied in each of a plurality of separate gas cleaning operations for restricting any potential or actual increase in the bubble release pressure level of the elements above said base condition to about 25 inches or less of water gauge, and the same base condition being applicable to a plurality of said 
     
     
        gas cleaning operations. .Iaddend. .Iadd.98.  The process according to claim 2 wherein diffusion elements are fouled during the treating process with organic slimes which tend to foul the diffusion element gas discharge surfaces more severely at low treating gas flux rates than at high flux rate, and said elements are cleaned by delivering cleaning gas to said elements through their own individual plenums which receive said treating gas and cleaning gas through their own individual flow regulating means. .Iaddend. .Iadd.99. The process according to claim 98 wherein the treating process is a tank type wastewater aeration process, the treating gas is air and the cleaning gas is introduced into the treating gas. .Iaddend. .Iadd.100. The process according to claim 99 wherein the cleaning gas is HCl. .Iaddend. .Iadd.101. The process according to claim 81, 82, 83, 84, 85, 86, 87, 88, 89, 98, 99 or 100 wherein any potential or actual increase in the dynamic wet pressure level of the elements above said base condition is restricted to about 15 inches or less of water gauge. .Iaddend. .Iadd.102. The process according to claim 81, 82, 83, 84, 85, 86, 87, 88, 89, 98, 99 or 100 wherein any potential or actual increase in the dynamic wet pressure level of the elements above said base condition is restricted to about 7 inches or less of water gauge. .Iaddend. 
     
     
        .Iadd.   .  The process according to claim 1, 2, 81, 82, 83, 84, 85, 86, 87, 88, 89, 98, 99 or 100 wherein any potential or actual increase in the dynamic wet pressure level of the elements above said base condition is restricted to about 5 inches or less of water gauge. .Iaddend. .Iadd.104. The process according to claim 1, 2, 81, 82, 83, 84, 85, 86, 87, 88, 89, 98, 99 or 100 wherein any potential or actual increase in the dynamic wet pressure level of the elements above said base condition is restricted to 1.5 times said base condition. .Iaddend.