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.

Description

This application is continuation-in-part of Ser. No. 191,974, filed on Sept. 29, 1980, now abandoned.

FIELD OF THE INVENTION

This present invention relates to the treatment of liquid media containing organic and/or inorganic foulants. More particularly the invention relates to the cleaning of multipore diffusion elements while submerged in liquid media including such foulants.

BACKGROUND OF THE INVENTION

The aeration of waste liquid media, including for example domestic sewage and industrial waste waters, is an old art. The activated sludge process, which includes aeration of liquors containing domestic sewage, has been in continuous use for about sixty years.

The liquid media treated in such aeration processes very commonly contain organic and/or inorganic foulants such as for example relatively insoluble salts which are responsible for the hardness of the water, and living and non-living organic residues which contribute tot he formation of scales and slimes. Upon aeration of these media with submerged aeration devices, there is a tendency for the foulants to progressively foul such devices at the point of release of oxygen-containing gas into the liquid media, closing up or otherwise modifying the opening through which the oxygen-containing gas is released into the media with various undesirable results.

Such fouling can impair the uniformity of gas distribution from aeration devices, especially when such devices are of the area discharge type, such as for example the flat porous ceramic plates which were used to discharge air into sewage liquors in early activated sludge plants. Also, fouling can in certain circumstances increase the pressure differential required to drive oxygen-containing gas through the aeration devices at a given flow rate, thus either reducing the flow of oxygen available, and therefore the oxygen transfer rate of the aeration system, and/or increasing the amount of power consumed in maintaining the desired rate of flow, thus substantially increasing the energy requirements and cost of the process.

Since these fouling phenomena are often progressive in nature they can eventually lead to a complete or near complete disablement of the aerating devices if permitted to continue long enough. However, a long-standing recognition of the intolerable circumstances that can result from failure of a sewage treatment plant has provided considerable encouragement for persons skilled in the art to explore remedial measures.

The fouling problem has been discussed and confronted in various ways with varying degrees of success for many years. Literature references discussing the problem and proposed solutions were available in the 1930s. These and subsequent publications demonstrate the severity of the problem and the elusiveness of any truly satisfactory solution. At a very early stage it was recognized that the removal of diffusion elements from an aeration tank for cleaning was both inconvenient and relatively expensive in view of the labor costs and the loss of use of the facility. Accordingly various attempts were made to develop satisfactory processes for cleaning the aeration devices in place, i.e. without removal from the aeration tanks and, wherever, possible, without draining the liquid media from the tanks.

One of the techniques tried was injection of chlorine gas into the aeration system in admixture with air while the aeration system was in operation. A measure of success was obtained in that there was reduction of flow resistance and apparently some prolongation of the life of the elements. However, such techniques were only sporadically successful.

For example, R. B. Jackson reported in his article "Maintaining Open Diffuser Plates With Chlorine", Water Works & Sewerage, Sept. 1942, pages 380-382, that the application of chlorine, whenever required, was effective in maintaining operation for a period of time, following which it again became necessary to drain the aeration tank and clean the decommissioned diffusion elements with liquid cleaners including acids. But Jackson was only one of a number of individuals who experimented with in place cleaning with gaseous cleaning agents in a variety of plants. See for example W. M. Franklin, "Purging Diffuser Plates With Chlorine", Water works & Sewerage, June 1939, pages 232-233; "Manual of Practice No. 5", Federation of Sewage and Industrial Wastes Associations, Champaign, Ill., 1952, pages 60-61; And U.S. Pat. No. 2,686,138 to Klein.

However, despite the early attempts at perfecting this technique, it has not been widely regarded as generally acceptable heretofore for large sewage treatment plants with multi-pore diffusion elements.

It is of interest to note that sewage treatment plant designers are generally familiar with the tubing-type diffusers for the sewage treatment ponds or lagoons used by small communities. Such systems usually employ rows of small diameter plastic tubing resting on or suspended above the bottom of a lagoon or basin and having small holes or slits formed in the tubing at relatively widely spaced intervals along the length of the tubing. For example, one commercially available type of tubing-type diffuser marketed by Lagoon Aeration Corporation under the trademark LASAIRE® is weighted tubing having an inside diameter of approximately one-half inch with a small bore on the order of 0.012" in diameter about every four inches along the crown of the tubing. Another commercially available form of tubing type diffuser employs slits instead of bores. Still another type employs rigid plastic tubing having small porous ceramic inserts cemented into the tubing wall instead of the bores or slits previously mentioned. Sanitary engineers are, of course, aware of the successful cleaning of such tubing type diffusers by the addition of a cleaning gas such as hydrogen chloride to the oxygen-containing gas, which mixture is forced through the bores, slits or small ceramic inserts, while the latter are in place submerged in the liquid media, to remove incrustations of organic and/or inorganic foulants.

Notwithstanding the apparent success of in place gas cleaning of tubing-type diffusers and the long-standing knowledge of and early attempts at in place gas cleaning of the multi-pore area release diffusion elements customarily employed in the tank-type aeration facilities generally used by large cities and counties, gas cleaning in place has not been generally adopted for such facilities. Considering the long-standing nature of the fouling problem and the fact that the technology relating to in place gas cleaning of tubing type diffusers has been readily available to sewage plant designers for years, it might seem reasonable to assume that in place gas claning would have long ago become the technique of choice for the tank-type aeration facilities equipped with multi-pore diffusion elements. That is has not become a commonly used method bears silent but effective witness to the fact that a practical, economical and dependable technique for in place gas cleaning of multipore diffusion elements in tank-type aeration facilities was not obvious to plant designers and operators of ordinary skill in the art.

Further evidence of such non-obviousness is provided by the willingness of facility operators to indulge in such inconvenient, time consuming and expensive measures as removing the unit from service, draining the tank, doing preliminary cleaning of the tank and of the foulded diffusion elements with fire hoses and the like, removing literally tons of elements from the tanks transporting them to a cleaning facility, subjecting them to acid and/or caustic solution cleaning, drying the elements, refiring them at elevated temperatures, replacing the rather substantial number of elements which are inevitably destroyed by cracking or warping in the refiring process, transporting the elements back to the plant, reinstalling them with removal of damaged gasket material from the holders, installation of new gaskets, retightening and torqueing of the means for holding the diffusion elements in their holders, refilling the tank and returning the facility to operation.

Additional evidence has been provided by a study entitled "survey and Evaluation of Fine Bubble Dome Diffuser Aeration Equipment", by Daniel H. Houck and Arthur G. Boon, completed Sept. 15, 1980, in fulfillment of a grant from the Association of Metropolitan Sewerage Agencies and the British Water Research Centre under the partial sponsorship of the U.S. Environmental Protection Agency. While making an indepth review of the designs, operating procedures, preformance and maintenance procedures of U.S. and overseas activated sludge plants equipped with fine bubble diffusers, the investigators surveted fouling problems and cleaning methods. None of the plants which required periodic cleaning employed in-place gas cleaning. Among the cleaning methods used for ceramic diffusion elements were refiring, acid washing combined with clean water-and steam-cleaning, ultrasonic cleaning, hand brushing and others. The study provided detailed information and observations cleaninginsolublized insolubilized inorganic salts are deposited in the gas discharge passages and/or at their outlets during discharge of the treatment gas.

Gas

For purposes of the present invention a gas is a gaseous material or mixture, which may be or include true gases or vapors or mixtures thereof, and which may also include entrained liquids or solids in the form of fine or substantially colloidal droplets or particles, which material or mixture is in the gaseous state to a sufficient extent under conditions of use for forming bubbles when discharged through a multi-pore diffusion element into liquid media.

Liquid Medium or Media

A "liquid medium" is any material (including single materials or mixtures) which is liquid to a sufficient extent to form gas bubbles therein when gas is discharged therein through a multi-pore diffusion element. The liquid material may for example include one or more organic liquids, or one or more inorganic liquids, or any combination of organic and/or inorganic liquids in admixture (including miscible and immiscible liquids) and may also include other liquid, gas and solid materials which do not deprive the medium of the above-described liquidity under the conditions prevailing during discharge of the treatment gas therein. Also, at least one of the components of the medium, which may be one or more foulant(s) and/or one of the other components of such medium, is subject to change in a predetermined manner in response to discharge of a treatment gas therein. A preferred category of such media is aqueous waste liquors, such as for example, the mixed liquors treated in the activated sludge process, refinery and brewery waste liquors, paper mill white water, and the like.

Mean Bubble Release Pressure

The arithmetic average of a statistically significant number of bubble release pressure measurements at randomly or systematically established locations on any surface of a diffusion element.

Multi-Pore Diffusion Element

In general, such an element is an area release diffuser having an active gas discharge surface of substantial are e.g. usually at least about 20 square inches, more preferably at least about 30 square inches, and still more preferably at least about 40 square inches, having a multiplicity of fine pores closely spaced with respect to one another and distributed in a random or ordered manner throughout said active gas discharge surface. However, it should be understood, for reasons apparent from the above discussion of theoretical considerations, that only a portion of the pores will transmit gas under a given set of operating conditions. On the other hand, there will normally be a plurality of active pores per square inch of active gas discharge area under the design operating conditions of the element, when in its new or as manufactured condition in clear water.

Such elements may include certain optional, additional characteristics or properties, certain of which are preferred embodiments. For example such elements include pores comprising gas discharge passages. Said passages may be essentially linear but are usually tortuous. Although discrete passages are contemplated, elements with substantial numbers or even major proportions of interconnecting passages are more commonly encountered. Irrespective of their degree of linearity, tortuousness, intercommunication or lack thereof, the respective gas discharge passages have discrete or shared inlets at the gas influent surfaces of the elements and outlets at the gas discharge surfaces of the elements. The closely spaced nature of the passages in the preferred elements is indicated in part by the fact that the average lateral spacing between pores is smaller than the average thickness of the elements and is therefore smaller than the average length of the gas discharge passages. The reference to spacing is intended to refer to the spacing between all pores in the diffusion element not merely those which happen to be active at a given set of operating conditions.

It is also preferred that the pores have a pore size which is in the range of about 60 to about 600 and more preferable about 90 to about 400 and most preferably about 120 to about 300 microns, as computed in applying the mean bubble release pressure of the element to the equation D=30γ/P, shown in ASTM E-128, wherein D=maximum pore diameter, γ=surface tension of the test liquid in dynes/cm, and p=pressure in mm of Mercury.

The elements may be members of any desired shape in plan view and vertical cross section and, as viewed in vertical cross section, include a substantially horizontal portion having a ratio of at least about 4:1, more preferably at least about 6:1 and still more preferably at least about 8:1 between the maximum horizontal dimension of the element and the area-weighted average thickness of the element.

Said horizontal portion preferably also is characterized by having a ratio of active gas discharge area to average thickness, weighed on an area basis, of at least about 10, more preferably at least about 20 and still more preferably at least about 40 square inches of area per inch of thickness.

Said horizontal portion may and preferably does include, beneath in active gas discharge surface, zones of enhanced volumetric compression ratio. For this purpose the said horizontal portion may be regarded as including various zone (defined herein) referred to as the central, outward, boundary and peripheral zones, which may or may not have visible characteristics, but will usually be associated with depressions at the central zone and/or angled surface(s) at the boundary zone. For example, it is contemplated that the central zone would preferably have an apparent volumetric compression ratio which is enhanced relative to that of the outward zone by at least about 2%, preferably about 2 to about 20%, and more preferably about 3 to about 15%. Where a boundary zone is provided, preferred applications of the invention will involve enhancement of the apparent volumetric compression ratio of the boundary zone, relative to the outward zone, by at least about 10%, more particularly about 10 to about 35%, and preferably about 35 to about 100%.

The diffusion elements may include a wide variety of particulate (including fibrous)materials of both organic and inorganic character, but preferably have a modulus in compression of at least about 0.2×10⁵ psi, more particularly about 0.2×10⁵ to about 4×10⁵ psi in applications involving softer particulate materials, and preferably about 4×10⁵ to about 6×10⁶ psi when working with the harder inorganic materials.

Outward Zone

The "outward zone" includes all or a substantial portion of the body of the diffusion element beneath the total active gas discharge surface other than the "central zone". As compared to the central zone the outward zone lies further outward from the center of the element than the central zone. Other than the central zone, the outward zone may or may not be the sole additional zone in the element.

Oxygen-containing Gas

An "oxygen-containing gas" includes pure oxygen and any gas (as defined herein) that includes oxygen in appreciable quantities and is useful as a treatment gas.

Peripheral Zone

The "peripheral zone", if such is present, constitutes a portion of the volume of the diffusion element at or along the edge of the active gas discharge surface of the element which normally constitutes the outermost edge or periphery of the element. The peripheral zone, whether annular or non-circular, is one in which the element has been treated by pressing, including a combination of pressing with other techniques, to develop a zone having lesser permeability (including no permeability), greater density or lesser height than all or a portion of the outward zone. An element with a peripheral zone may or may not include a boundary zone.

Pre-cleaning Operating Head

The total amount of hydrostatic head of any liquid medium, expressed in inches of water gauge, and applied to the effluent surfaces of diffusion elements in a given plant during a period prior to the commencement of cleaning, including either the time weighted average of the liquid head during the last ninety (90) days of operation prior to cleaning, or more preferably the last head applied in normal operation prior to cleaning, or still more preferably the larger of said average and said last head.

SCFM

SCFM means rate of gas flowing in terms of cubic feet per minute corrected to a temperature of 20°C, an absolute pressure of 760 mm of mercury and a relative humidity of 36%.

Specific Permeability

The term "specific permeability" describes the overall rate of passage of gas through a dry diffusion element, and for purposes of the present invention is expressed in standard cubic feet per minute per square foot of area per inch of thickness at a driving pressure of 2 inches in water gauge under standard conditions of temperature, pressure and relative humidity (20°C, 760 mm Hg. and 36%, respectively). The specific permeability is calculated from the equation G=Q(T/A), wherein G equals specific permeability, Q equals flow in standard cubic feet per minute, t equals thickness of the element in inches and A equals the mean effective gas flow area in ft.² through the element normal to the direction of flow. If the gas discharge surface of the diffusion element overlies portions of the element which are of varying thickness, the average thickness is used, the thickness being weighted on the basis of area.

System Back Pressure

This is a measure of the overall resistance to flow of an entire gas distribution network or of at least a portion of such a network at a given rate of flow through the diffusion elements. The resisitance resistance in question includes the frictional and surface tension resistance attributable to conduits, to the diffusion elements and other elements of the network, to fouling, whenever such is present and to the hydrostatic head. It is assumed that the network or portion thereof which is under consideration comprises a plurality of pneumatically interconnected diffusion elements. These are elements which share a source of treating gas, and which thus contribute along with conduits and other components of the network to a combined resistance to flow which must be overcome by that source. By way of illustration and not limitation, activated sludge plants are known in which the design system back pressure is about 3 to about 20, more typically about 4 to about 15, and frequently about 6 to about 10 psi measured at the compressor or blower outlets.

Tank-type Process

For purposes of the present disclosure, a tank type process, whether for aeration or other purposes, is a process of discharging treating gas into a liquid medium at a rate of at least about 2, more preferably at least about 4, and still more preferably at least about 6 SCFM per 1000 cubic feet of liquid medium, a rate which is far in excess of those employed in the sewage treatment lagoons or ponds in which tubing type diffusers are installed. Alternately, or in combination with the foregoing, a tank-type process may be characterized by an average liquid medium retention time of less than about 48 hours, more commonly less than 24 hours, and quite frequently less than 12 hours, considerably less than the retention times of the aforementioned lagoons or ponds.

Treatment Gas

This is a gas, as defined, herein, which includes one or more components that is able to effect a predetermined desired change in at least one component of a liquid medium, and is less aggressive towards foulant deposities than a cleaning gas, as defined herein.

Vertical

The term "vertical", as applied to a surface of a diffusion element, includes truly vertical and near vertical, e.g. within about 20° of vertical.

Claims

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 at 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 other 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 H₂ O₂, CH₃ OH, SO₂, CO₂, Cl₂, ClO₂, hcl, NO_x, O₃ and Br₂.

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 any potential or actual increase in dynamic wet pressure of the elements above said base condition, or said any potential or actual 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 any potential or actual increase in dynamic wet pressure of said elements above said base condition, or said any potential or actual 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 within 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.

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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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.

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. 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. 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. 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. 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. 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. 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. 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. 100. The process according to claim 99 wherein the cleaning gas is hcl. 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. 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.

. 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. 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.