TECHNICAL FIELD This invention relates to a new and improved diffused gas system for treating aqueous media with fine bubbles of air or other gas, and to the method of conducting such treatment.

BACKGROUND ART For many years, it has been known that oxygenation results in the biological and chemical breakdown of organic contaminants in effluent or wastewater. The surface of a body of liquid provides some oxygen uptake or absorption, because of its exposure to the atmosphere. However, when prior art diffused gas systems are used in order to accelerate the oxygenation, they require the use of additional devices to promote proper mixing. It is well- known to be important for cost reasons, to minimize the time and energy required for such treatment. The unique design of the diffuser of this invention enables it to efficiently achieve both of these requirements from a single power source.

Similar considerations apply to the newer area of commerce, often referred to as aquaculture or"fish farming". The medium in which fish will thrive must be rich in oxygen and well mixed. Supplying adequate amounts of oxygen, and dispersing it uniformly throughout the tank is of primary importance to the success of such enterprises. The diffuser of this invention accomplishes both functions economically.

In the treatment of effluent, there have been a number of methods used in the prior art to expose a greater surface area of effluent for contact with the atmosphere, including devices such as fountains to spray the liquid into the air. Others, such as the Bearden patent, U. S. 3,852,384 have devised submerged vertical columns with means to intermix liquid and air passing therethrough. Kober, U. S. patent 3,133,878 discloses an arrangement to create a circular flow of the liquid and treating gas. Like many others, both of these patents employ a perforated pipe to dispense the gas.

The Morgan patent, U. S. 3,232,866 is of interest because it relates to the spacing of diffusers in the container of liquid being treated. Morgan discloses a critical relationship between oxygen uptake and the arrangement of the diffuser heads. The Goudy et. aL patent, U. S. 4,597,530 discusses several prior art patents in the field and covers a diffuser in the shape of a disc which is designed in such a manner as to prevent clogging of its orifices.

Some prior art employs coarse aerators or"spargers"which have the advantage of moving the liquid upward, or rolling it, as a result of relatively large bubbles of air boiling to the surface. Current diffusers which produce fine bubbles, on the other hand, are less effective in mixing or rolling the liquid as the bubbles rise to the surface, but are more effective than the coarse diffusers in their aeration effectiveness, because of the greater surface area of the fine bubbles exposed to the liquid. The design of the present invention is effective both in mixing and aeration.

Some diffusers of the prior art employ a membrane through which air is passed to produce fine bubbles, while others employ a porous stone. They are of a simpler construction than that of the present invention, and like

conventional coarse air diffusers, they merely inject air into the liquid which forms large bubbles without enhancing oxygenation. The membranes of the prior art, however, are so flexible and not self supporting, that they require mounting on a kind of mandrel or internal support. Moreover, they are so easily distended when subjected to air pressure, that air is not emitted from their entire circumference. For example a tubular shaped diffuse emits air mostly from its upper surface.

SUMMARY OF THE INVENTION The present invention consists of a flexible microporous continuous tubular membrane arranged in the form of a spiral or grid configuration.

When connected to a source of air, the membrane provides fine bubbles which are emitted around its entire circumference, even at the bottom.

Because the resistance of the membrane is low, this is accomplished with less than two (2) inches of water column pressure difference between the top and bottom of the membrane. In both the spiral and grid configurations, substantial openings must be provided between adjacent elements of the tubular membrane. The fine bubbles generated by the membrane entrain the liquid being treated to move it through such openings. This is a critical feature of the structure of the present invention, resulting in the liquid being rolled over or inverted on a horizontal axis, like the coarse bubble diffusers, but, because of the fine bubbles emitted, it is far superior to them in oxygenation effectiveness. It has been observed, in the spiral diffuser of this invention, that the volume of liquid being raised by the fine bubbles, is also rotated around a vertical axis as it rises. This further contributes to mixing and oxygen absorption, due to increased retention time of the fine bubbles

emitted. The continuous turnover of the liquid from top to bottom, increases the natural effectiveness of this invention. As is well known, there is a natural cleansing process which occurs twice annually in lakes, rivers and streams. The present invention mimics that natural inversion, with the advantage that it can be repeated as often as needed.

The efficiency of the present invention is so substantial that the energy requirements are dramatically reduced by permitting the use of a much smaller motor to drive the blower. This is confirmed by the much higher Standard Oxygen Transfer Efficiency levels of the present invention as compared to the prior art, resulting in 25% to 45% reductions in the horsepower requirement and corresponding reductions in the cost of both energy and equipment. This accompanies the greatly enhanced natural process of mixing, fine bubble aeration, and surface absorption of oxygen.

The open structural design also minimizes fouling from debris in the liquid being treated, an advantage especially important in wastewater applications.

Preliminary testing indicates than a 22 inch fine air grid has more than four times the rolling movement or inversion of the liquid being treated as compared to all known prior art fine air diffusers, when the same amount of air flow is used. Such tests also show a better total liquid movement as compared to coarse air diffusers when the same amount of air flow is used. This is because the present design provides at least four times the active membrane surface area, and emits"fine point"bubbles in that area having a vertical interface about ten times as large as most conventional diffusers. The"vertical interface"refers to the vertical edges of exposed membrane.

As for effectiveness, independent testing has shown that the diffusers of present invention have superior oxygen uptake performance which is more than twice as great as the best conventional aerator previously employed, a membrane tube. No comparison was made with rubber dome or ceramic diffusers of the prior art, which are known to be even less effective than such membrane tubes.

Oxygen transfer, also called Standard Aerator Efficiency (SAE) is measured by using a test procedure called the"clean water non-steady state" test, and results are given as the number of pounds of oxygen per hour per horsepower (OJHr./HP). All test results were calculated using the standard ASCE (American Society of Civil Engineers) method, i. e. the accepted non- linear regression method.

Typical diffusers of the prior art have SAE values of 2 to 7 pounds of 02/Hr./HP, when the air flow rate ranges from 1 to 10 Standard Cubic Feet of Air per Minute (SCFM). Many prior art diffusers have a very low oxygen transfer rate when the air flow rate is greater than 2 or 3 SCFM, and some cannot be operated above 5 SCFM. The diffusers of the present invention, however, have a SAE values even at 10 SCFM, ranging from nearly 5 to 10 pounds 02/Hr./HP.

The independent comparative tests also show loss of air pressure or "Head Loss" (HL) values of diffusers of the present invention are much lower and far superior to the prior art. Prior art head losses range from 6"to 35"of water column pressure over a range of 1 to 10 SCFM per diffuser. Most diffusers'head loss is so inefficient at 4-5 SCFM that they cannot be used.

The diffusers of the present invention have a low head loss even at 10 SCFM ranging from 2.5 to 27 inches..

In addition, such tests demonstrate the Standard Oxygen Transfer Efficiency (SOTE) percent per foot of submergence is often one and one- half to two times better than prior art diffusers. Typical diffuser SOTE values range from 0.5 to 2.2% over a range of 1 to 10 SCFM per diffuser.

Most diffusers are very inefficient at greater than 4 to 5 SCFM, with some being inoperable at levels above 5 SCFM. The diffusers of the present invention have SOTE's of 2.4% to 2.9% even when operated as high as 10 SCFM.

An ideal spiral structure according to this invention uses a buoyant manifold frame which allows the spiral to be anchored in a generally horizontal position at any desired depth in the liquid being treated. By locating the diffuser near the bottom of the liquid, more bottom cleaning and detoxification is achieved, due to the boiling or rolling action described above. The manifold for the present invention furnishes air to both ends of the membrane, providing a more balanced distribution of air throughout the membrane than if it were connected to one end only.

The ideal grid structure, the embodiment preferred for use in large aquaculture tanks, has a different structure because it is designed to rest on the tank bottoms. However the principles employed are the same. In the grid shown in the drawings and described below, the several lengths of tubing membrane are not disposed in a horizontal plane. They are arranged in a way which produces the required openings between adjacent lengths of tubular

membrane. Like the spiral design, membranes in the grid are furnished with air at each end in order to balance the air flow more evenly among them.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a top elevational view of the spiral diffuser of the present invention ; Figure 2 is a side elevation of said spiral diffuser ; Figure 3 is a perspective view, showing a pair of said spiral diffusers anchored in a body of liquid; Figure 4 is a top elevational view of the grid diffuser of the present invention ; Figure 5 is a side elevation of said grid diffuser ; and Figure 6 is an end elevation thereof.

BEST MODE FOR CARRYING OUT THE INVENTION Figure 1 depicts one embodiment of the present invention, wherein a microporous tubular membrane 11 is arranged in a spiral configuration. The spiral diffuser is shown generally at 7 in Figure 1 of the drawings. As shown in Figure 2, said diffuser consists of tubular membrane 11 interwoven around vanes 12, of which three are shown to be extending radially from the center of the structure, in such a way as to create large openings between the coils of said membrane 11. Tubular membrane 11 is self supporting in the sense that it does not require any internal structure to maintain its integrity.

It is not measurably distended by the pressure of air passing through it, by the depth at which it is operated, nor by any combination of these two factors. It

has been found that tubular membrane 11 should preferably have a maximum internal diameter of about 1 inch and a maximum outer diameter of about 1 1/2 inches, and a pore size in the range of 50-500 microns, preferably at the low end of such range. The minimum optimum membrane diameters should be about 3/8"ID and about 1/2"OD, and the optimum pore size should be about 50 to 100 microns. Pores smaller than about 50p will produce the fine bubbles desired under this invention, but they are less suitable because of the greater air pressure and resulting higher operating cost. One preferred membrane is that which is made according to the method disclosed in U. S. Patent # 4, 958, 770.

As shown in Fig. 3, spiral diffuser 7, when in use, is generally horizontally disposed, but lies in multiple planes because of the interweaving around vanes 12. One spiral diffuser, found to be very effective, uses about 17 feet of such tubing, and has an overall diameter of about 22 inches. A critical element of the present invention is that there be ample openings between the proximate arcs of tubing 11 of spiral diffuser 7, and between proximate lengths of tubing 11 in the grid diffuser 8. Such openings should be at least as large as the outer diameter of tubing 11, to provide optimum performance in agitating or rolling the liquid being treated, in aerating the same and in raising debris from the lower levels of said liquid.

Manifold 14 of spiral diffuser 7 is connected to pump 30 by means of non-porous tubing 20, as shown in Fig. 3. It will be obvious that a similar hook-up can be made for grid diffuser 8. Cylinder 15 and pipe 14 in spiral diffuser 7 comprise an air distribution manifold which provides buoyancy to the diffuser. Figure 1 shows diffuser 7 as having three vanes 12 extending

radially from cylinder 15, but a greater number could be employed without departing from this invention. Pipe 14 is attached at both of its ends to a source of air, namely at the bottom of cylinder 15 and at the end of its radius.

This arrangement helps to equalize the air pressure throughout the length of tubular membrane 11. As best seen in Fig 3, anchor lines 31 position one or more spiral diffusers 7 at the desired depth and location in a pond of liquid to be treated, where hose 20 connects the manifold with pump 30.

One skilled in the art would recognize that cylinder 15 can be located elsewhere, or even eliminated from the diffuser, depending on the buoyancy desired. Feeding air to both ends of pipe 14 must still be arranged, however, because the tubular membrane causes so little back pressure that the most remote portions are otherwise nearly devoid of air. Vanes 12, shown in figures 1 and 2, consist of polyvinyl chloride, but any other plastic can be substituted as long as it has requisite structural characteristics and is inert with respect to the liquid being treated. It is also possible to employ stainless steel in place of the PVC, and spiral diffusers of the present invention have been built using stainless wire to form the vanes.

Grid diffuser 8 has similar performance characteristics to spiral diffuser 7. However, it has a different configuration to adapt it for use in the large tanks or pools used in aquaculture. In that use, it is desirable for the diffuser to be positioned on the tank bottom, which requires that it be provided with supporting legs or feet Spiral diffuser 7, on the other hand is better suited for treatment of wastewater, where it is usually most effective when positioned above the bottom where it will be less subject to fouling by sediment usually found at that medium.

Grid diffuser 8 is provided with multiple lengths of porous tubing 11, three lengths being shown in Figures 4,5, and 6, each of which is attached at each end to pipe 14. Figure 4 shows pipe 14 forming a rectangular shaped manifold, although one skilled in the art would appreciate that a square configuration may desirable for certain uses. It also shows the three lengths of porous tubing 11 being parallel to one another, parallel to the major axis of the rectangular manifold, and appears to show them to be of equal length and equidistant from one another. Diffuser 8 is depicted in its operative position in Figures 5 and 6, supported by a pair of equilateral trapezoidal legs 24, one of which is mounted on each end of weight 25. Weight 25 preferably has a cylindrical shape and extends between legs 24 with its major axis perpendicular thereto and parallel to the base of legs 24.

The lengths of porous tubing 11 in grid diffuser 8 extend over weight 25 between the two ends of manifold 14, as best seen in Figures 5 and 6. Said manifold is shown in Figure 6 to be mounted at a different level on each of the legs 24, so that its minor axis is not parallel to the base of either of legs 24. The result of this design is that the lengths of porous tubing 11 are not of equal length, and are not equidistant from one another at all points, when passed over cylindrical weight 25. They are equidistant where attached to manifold 14, and at the portion which passes over weight 25, as best seen in Fig 5. Such an arrangement is necessary in order to help equalize air pressure throughout the manifold, with the elevated length of tubing 11 receiving more air merely because of its higher elevation. The structure also has the benefit of enlarging the openings between adjacent lengths of tubular membrane, whereby the spacing between them is greater than if they

were completely parallel. Preferably such openings are at least as large as the outer diameter of said tubing.

INDUSTRIAL APPLICABILITY The product and process of this invention may be used for the low cost and effective treatment of aqueous media with gas in all forms of commerce.