Abstract:
In a submerged membrane bioreactor (MBR or SMU), diffusers are used to eject air bubbles, primarily to facilitate the movement of mixed liquor past the membrane surfaces, cleaning and scouring the surfaces of the membranes with the mechanical energy of the combined air/liquid/solids movement. Oxygen transfer from the bubbles into the mixed liquor is often a secondary goal, for supplying oxygen to biological processes. The invention involves the use of a specific bubble diffuser, oriented at a selected upward angle, to enable a single style diffuser to accomplish simultaneous generation of mid-size and fine bubble aeration, with adjustment of the tilt angle varying the percentages of mid-size and fine bubble aeration. The angle selection, along with the number of diffusers and air volume selected, allows achievement of target liquid movement through the MBR.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This application claims benefit of provisional patent application Ser. No. 61/887,262, filed Oct. 4, 2013. 
         [0002]    This invention concerns wastewater treatment, particularly membrane separators or MBRs (membrane bioreactors) submerged in mixed liquor to remove permeate and concentrate the mixed liquor. 
         [0003]    More specifically, the invention is concerned with the use of a single type of air diffuser for MBRs and optionally for other wastewater treatment applications, with adjustment to provide coarse, mid-size or fine bubbles and to arrive at a targeted mix of such bubbles appropriate for the particular application. 
         [0004]    Air diffusers have been used in wastewater treatment for two primary purposes: supplying oxygen to the biological processes, and supplying mixing energy to basins. When used for supplying oxygen, fine bubble diffusers are usually used. The fine bubbles have a greater ability to supply oxygen because of their higher ratio of bubble area to gas volume, as well as their slower rise velocity in liquids. The slow rise velocity, however, renders the fine bubbles less efficient at mixing. More energy is expended for fine bubbles to provide a given mixing energy as compared to larger bubbles. 
         [0005]    When used for supplying mixing energy, coarse or mid-size bubble diffusers are typically chosen. The coarse bubbles are more efficient at mixing (electric cost per unit volume of basin) due to their higher rise velocity and lower pressure drop across the diffuser. 
         [0006]    When used in a submerged membrane bioreactor (MBR, sometimes referred to as submerged membrane unit, SMU), diffusers are used primarily for facilitating the movement of mixed liquor past the membrane surfaces (a pumping/mixing action), while simultaneously cleaning and scouring the surfaces of the membranes with the mechanical energy of the combined air, liquid and solids movement. Oxygen transfer from the bubbles is a secondary goal, and in some applications, it can be a detriment to the overall treatment system design. 
         [0007]    It would be desirable if a single configuration of diffuser could be used for different applications, whether to maximize the supplying of oxygen to mixed liquor or to maximize mixing and scouring action. Further, it would be desirable to have diffusers that could be adjusted to suit these different objectives. 
       SUMMARY OF THE INVENTION 
       [0008]    The invention provides these benefits, with a novel arrangement of air diffusers. 
         [0009]    As noted above, diffusers are used in MBRs to eject air bubbles, primarily to facilitate the movement of mixed liquor past the membrane surfaces, cleaning and scouring the surfaces of the membranes with the mechanical energy of the combined air/liquid/solids movement. Different diffusers can be selected. Oxygen transfer from the bubbles into the mixed liquor is usually secondary. The invention involves the use of a specific configuration of flat panel air diffuser, positioned with its narrow dimension tilted upwardly so that an array of the diffusers resembles louvers. Using fine-bubble air diffusers, this causes bubbles from the lower areas of the diffuser to coalesce together to form mid-size or coarse bubbles, while bubbles from the upper areas of the diffuser remain fine. The upward angle can be adjustable, to enable a single style diffuser to be used for different applications and to accomplish simultaneous generation of mid-size and fine bubble aeration. The diffusers can be rotationally adjustable so as to enable adjustment of the percentage of mid-size and fine bubble aeration, to achieve a target liquid movement through the SMU. If needed to achieve desired biological oxygen demand, the angle can be lowered to produce a higher proportion of fine bubbles. 
         [0010]    In addition, the SMU diffusers preferably are non-clogging and self-cleaning. 
         [0011]    A primary object of the invention is to provide arrays of air diffusers in sewage treatment facilities, especially MBRs (SMUs), in a way that enables one style of diffuser to be used for different purposes. The diffusers can generate fine and mid-size to coarse bubbles simultaneously, with the proportion selectively adjustable. These and other objects, advantages and features of the invention will be apparent from the following description of a preferred embodiment, considered along with the accompanying drawings. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a schematic elevation view showing an arrangement of air diffusers according to the invention. 
           [0013]      FIG. 2  is a perspective view showing a type of air diffuser that can be used with the invention. 
           [0014]      FIG. 3  is a schematic view similar to  FIG. 1 , showing the air diffusers oriented at a different angle. 
           [0015]      FIG. 4  is a schematic elevation view in cross section, showing an array of diffusers at a selected angle, positioned within a submerged membrane unit, shown at the bottom of a basin. 
           [0016]      FIG. 5  is a perspective view showing a bank of membrane cassettes and a bottom frame retaining air diffusers below the membranes and configured according to the invention. 
           [0017]      FIG. 6  is an enlarged detail view showing the bottom portion of  FIG. 5 . 
           [0018]      FIG. 7  is a perspective view showing an array of air diffusers angled as in an embodiment of the invention. 
           [0019]      FIG. 8  is an elevation view showing the diffusers, without the retaining frame. 
           [0020]      FIG. 9  is a plan view showing the diffusers and retention frame. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0021]      FIG. 1  illustrates the principle of the invention. Air diffusers are shown at  10 , within a wastewater treatment basin, particularly a submerged membrane unit. The diffusers may be, for example, of the type sold under the name AEROSTRIP by Aquaconsult Anlagenbau GmbH of Austria, shown in  FIG. 2 . These diffusers are fine bubble diffusers, formed in an elongated rectangular shape with a metal frame and a polyurethane bladder air diffuser surface with a multiplicity of small air orifices. When pressurized with air, as via the compression fitting  12  shown in  FIG. 2 , the diffuser surface or bladder  14  expands outwardly and releases air through the orifices. When air pressure is shut off, the orifices actually function to block wastewater or mixed liquor from backflowing into the diffuser. The small holes will substantially close in the absence of internal diffuser pressure. The orifices can be produced by puncturing needles that are pushed through the bladder. 
         [0022]    As an example, a diffuser used with the invention can have specifications as follows:
       Nominal pore size: 0.1 mm-2.0 mm   Pressure drop: 1 kPa-20 kPa   Flux range: 0-300 Nm 3 /hm 2          
 
         [0026]    The invention is not limited to the described form of diffuser. Other diffusers having a width w ( FIG. 2 ) presenting a surface at least a few inches wide can be used. The diffuser  10 , particularly of the type shown in  FIG. 2 , are ordinarily installed flat, horizontally down to a surface such as the floor of a basin. The air compression fitting  12  is thus shown in  FIG. 2  on the top surface of the diffuser. However, for the invention the diffusers are not horizontal, and the air fitting could be on the back side of the diffuser. Such a diffuser under typical design conditions of air flow and pressure will produce bubbles of roughly about 1 mm or somewhat larger, which can be considered fine bubbles. 
         [0027]    In  FIG. 1  the diffusers are shown inclined upwardly, i.e. the width of each diffuser is at an inclined angle to the floor of the tank. As an example, this angle φ can be about 80°. Preferably they are mounted above the tank floor, which is indicated in  FIG. 1  by the dashed line  16 . The effect of the upwardly angled diffusers  10  is to release fine air bubbles at a series of different heights along the inclined width of each diffuser. The bubbles released lowest tend to gather with other bubbles as they rise, coalescing into larger bubbles. The higher the location where the bubbles are originally released by the diffuser, the more they tend to remain as fine bubbles. The slightly billowed shape of the inflated diffuser face  14  can contribute to this effect, in that the lowermost released bubbles will tend to slide along the surface more than bubbles released near the upper end, where the angle of the bladder surface is at a lower angle (less vertically inclined), as can be seen in  FIG. 1 . Bubble travel along the surface promotes coalescing. 
         [0028]    The result is that these inclined diffusers release a mix of air bubbles, fine bubbles for better delivery of oxygen to the mixed liquor, these bubbles being slower rising, and coarser bubbles for faster rising, less oxygenation, and better for generating water convection and air scouring, as when needed for membrane bioreactors. The coarser bubbles will typically be mid-size bubbles (less than ¼ inch), although coarse bubbles can be generated in high MLSS or along a longer slope. The angle of the diffuser can be set as desired, depending on the proportion of fine and coarser bubbles desired for the particular application. 
         [0029]    Water convection, as induced by the rising bubbles and particularly the coarser bubbles, is indicated by arrows  18  in 
         [0030]      FIG. 1 . The diffusers are spaced above the floor to better promote water flow up between the diffusers, as induced by the rising bubbles. 
         [0031]    The angle of inclination at which the diffusers are set can be vertical, i.e. 90° to the basin floor  16 , or can be a lower angle than shown in  FIG. 1 , or it could even be an angle beyond 90°, such as 105°, 110° or 120°, thus to further increase the proportion of coarser bubbles (and the coarseness of bubbles), by allowing more bubbles to gather along the surface  14  and coalesce. The range of angles could be from about 0° to about 180°. For benefits of the invention the range will usually be about 60° to about 120°, although an adjustable array of diffusers can be adjustable to horizontal (or nearly horizontal) for a mode wherein only fine bubbles are to be generated.  FIG. 3  illustrates an inclination beyond 90°, and indicates a large proportion of coarse bubbles rising from those diffusers. In this inclination many of the orifices, i.e. those near the bottom end, and potentially all orifices, will be oriented at a downward inclination. 
         [0032]      FIG. 4  is a schematic view in sectional elevation, indicating a tank or basin  20  having a series of flat plate membranes or membrane cartridges  22  positioned in plane-vertical configuration, as in an MBR. The cartridges actually are in cassettes or membrane assemblies, each holding many parallel membranes. The MBR contains wastewater, i.e. mixed liquor indicated at  24 . Below the membrane cartridges is a row or array of air diffusers  10 , shown at an inclined angle as discussed above. These diffusers are positioned above the bottom  16  of the basin as discussed above, to promote water convection, again indicated by arrows. 
         [0033]    In  FIG. 4  the diffusers  10  are schematically shown with pivot axes  26  at their lower ends. This may be desirable in some installations in order to allow angular adjustment after installation, to “tune” the operation of a basin process, achieving a desired balance of fine and coarser aeration bubbles for a particular process application. Similarly, a process might be modified within an MLSS tank, whether an MBR tank or another process, where oxygen requirements might be increased or decreased for the modified process, or coarser bubble generation might be needed to induce liquid flow, as in a highly thickened MLSS. An actuator is indicated at  28  (dashed line), mechanically connected to the ends of all diffusers in the array, to effectuate such dynamic angular adjustment. 
         [0034]      FIG. 5  shows a bank of membrane assemblies or cassettes of flat-plate membranes  30 , each holding flat-plate membrane cartridges. The unit  30  can be one of several or many within an MBR tank or basin. At the bottom is shown a frame  32  that retains a series of air diffusers  10 , upwardly angled as discussed above, spaced a prescribed, effective distance below the membranes. In an MBR the air bubbles from the diffusers are important for mixing and scouring, inducing water convection and keeping the membrane surfaces free of fouling. Aeration of the mixed liquor can also be important in many MBR applications. 
         [0035]      FIGS. 6-9  show the air diffusers. In  FIG. 6  the lower end of the unit  30  is shown in more detail, revealing the portion of the frame retaining the diffusers  10  in the desired angled positions.  FIG. 7  also shows the diffusers  10 , retained by end members  34  of the frame.  FIG. 8  shows the angled diffusers themselves, spaced apart as desired for the volume and distribution of bubbles desired, and without the frame.  FIG. 9  is a plan view of the diffusers  10  retained by the frame in members  34 . 
         [0036]      FIG. 8  shows the air compression fittings  12  on the diffusers. With the diffusers tilted upwardly pursuant to the invention, these compression air fittings could be on the back sides if desired. Although the array of diffusers is shown such the long dimension of each diffuser would be parallel to the planes of the membrane cartridges, this relationship could be perpendicular if desired. 
         [0037]    Several tests were conducted with the invention in clean water, comparing bubble rise velocity, which is a function of bubble size, to diffuser inclination angle, and also comparing bubble rise velocity to air flow rate for a constant diffuser angle. The tables below show the results. 
         [0000]    
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 BUBBLE RISE VELOCITY WITH DIFFERENT ANGLES 
               
               
                 AIR FLOW = 89 scfm 
               
             
          
           
               
                   
                 Inclination Angle 
                 Avg. Rise Velocity 
               
               
                   
                   
               
               
                   
                  80° 
                 43.84 cm/s 
               
               
                   
                 105° 
                 45.24 cm/s 
               
               
                   
                   
               
             
          
         
       
     
         [0000]    
       
         
               
             
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 BUBBLE RISE VELOCITY VS. AIR FLOW 
               
               
                 INCLINATION ANGLE = 80° 
               
             
          
           
               
                   
                 Air Flow Rate 
                 Avg. Rise Velocity 
               
               
                   
                   
               
               
                   
                  76 scfm 
                 42.58 cm/s 
               
               
                   
                 100 scfm 
                 44.10 cm/s 
               
               
                   
                   
               
             
          
         
       
     
         [0038]    Table 1 shows that an inclination at an angle of 80°, i.e. 80° tilt from horizontal, produced an average bubble rise velocity of 43.84 cm/s. This is approximately the angle shown in  FIG. 1 . The average rise velocity was calculated using twelve different points within a membrane tank, or MBR, bubble velocities being measured approximately two inches above the array of diffusers. Air flow was at 89 scfm. As the table shows, this is compared to an inclination angle of 105° (beyond vertical, generally as shown in  FIG. 3 ), where average bubble rise velocity increased to 45.24 cm/s. The air flow rate was the same. The result shows that further bubble coalescing occurs at 105° as compared to 80° since, as discussed above, the fine bubbles evolving from the pores of the diffusing units are close together for a greater time and distance and tend to flow along the surface of the diffuser for a greater distance, thus promoting additional coalescing and coarse bubbles and a greater population of coarser bubbles. 
         [0039]    Table 2 compares bubble rise velocity to air flow, for a constant inclination angle of 80°. The table shows that, at 76 scfm flow rate, average rise velocity was 42.58 cm/s. This is consistent with Table 1, where the first test was also at the 80° angle and average rise velocity was 43.84 cm/s, somewhat higher. Table 2 shows that when the flow rate was increased to 100 scfm (at the same 80° tilt angle), rise velocity increased to 44.10 cm/s. 
         [0040]    Importantly, the two tables also show that an appreciably lower air flow rate can be employed to obtain bubbles that will produce a desired rise velocity to promote scouring flow. At only 89 scfm, but a 105° diffuser angle, average rise velocity was higher than at 100 scfm, and at 80° angle. This demonstrates the invention can save considerable energy by allowing reduced air flow when relatively coarse bubbles are required for scouring. 
         [0041]    In the claims the terms “fine”, “mid-size” and “coarse” are used relative to one another, not to be limiting except where a size is stated. Also, “frame” is to be considered any mechanical arrangement providing for pivoting of the diffusers. It should also be understood that the diffusers could be pivotable downwardly (below the plane of the pivot axes) rather than upwardly, and the invention is to be interpreted broadly in that sense. 
         [0042]    The above described preferred embodiments are intended to illustrate the principles of the invention, but not to limit its scope. Other embodiments and variations to these preferred embodiments will be apparent to those skilled in the art and may be made without departing from the scope of the invention as defined in the following claims.