Abstract:
A diffused aeration system has an aeration tank which contains a plurality or peripheral coarse bubble jet diffusers and a plurality of fine bubble jet diffusers strategically located therebetween to provide maximum oxygen transfer efficiency and mixing with little or no increase in operating power costs.

Description:
RELATED APPLICATIONS 
     This application claims the priority benefits of U.S. Provisional Application No. 60/104,362, entitled “Diffused Aeration System”, and filed Oct. 15, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to aeration systems and, more particularly, systems for aerating effluent in wastewater treatment plants, or the like. 
     2. Description of the Prior Art 
     In wastewater treatment plants, it is known to aerate effluent or sludge as part of the wastewater purification process commonly known as the “activated sludge” process. This process typically includes a biological treatment system consisting of a contained volume of wastewater with an aerobic microorganism colony which thrives on biodegradation of the wastewater in an aerobic environment. In order to maintain the requisite aerobic environment, supplemental air must be introduced into the system to promote the dissolution of gaseous oxygen into the liquid phase. The introduction of supplemental air is commonly achieved through the use of either mechanical surface aerators or submerged diffused aerators. 
     The rate of dissolution of gaseous oxygen into the liquid phase is directly proportional to (1) the surface area of the gas bubbles produced by these aerators, wherein the oxygen molecule travels across this “boundary” from the gaseous phase within the bubble to the liquid phase without the bubble; and (2) the residence time, i.e., the length of time the gas bubble is exposed to the liquid. For example, several hundred smaller gas bubbles contain more surface area than one large bubble containing the same total volume of air, and hence, the smaller bubbles provide more efficient oxygen transfer. Due to friction losses, however, the energy required to produce fine bubbles increases as the size of the diffuser orifices decreases. As a result, fine bubble diffusers which provide more efficient oxygen transfer, have high operating power costs, while larger bubble jet diffusers, i.e., “coarse bubble jet diffusers”, which produce a larger bubble size with less oxygen transfer efficiency, have a lower operating power cost. 
     The coarse bubble diffuser offers the advantage of higher exit velocity which provides greater mixing activity within the aeration tank, resulting in increased residence time of the air in solution. This, in turn, provides an increase in the total oxygen transferred while increasing the oxygen transfer efficiency closer to that of a fine bubble system. Complete mixing of the microorganism colony within the aeration tank system is critical to ensure adequate waste treatment within the aeration tank. Inadequate mixing can result in portions of the microorganism colony to settle to the tank bottom thereby limiting exposure of the microorganisms to the wastewater. 
     It is an object of the present invention to provide a diffused aeration system with improved overall oxygen transfer efficiency with little or no concomitant increase in operating power cost. 
     It is a further object of the present invention to provide a diffused aeration system having improved oxygen transfer efficiency while providing improved mixing capacity. 
     SUMMARY OF THE INVENTION 
     The above objects of the invention are satisfied with an improved aeration system which, in accordance with the present invention, is comprised of a plurality of coarse bubble jet diffusers connected with an air supply and a plurality of fine bubble jet diffusers connected with an air supply and disposed between the coarse bubble jet diffusers. Exemplary of suitable coarse bubble jet diffusers are the duckbill check valves described in U.S. Pat. No. 4,607,663, incorporated herein by reference. Preferred are the rubber duckbill check valves commercially available as TIDEFLEX® valves from Red Valve Company, Inc. of Carnegie, Pa. Exemplary of suitable fine bubble jet diffusers are those commercially available as DOMEFLEX™ diffusers, also from Red Valve Company, Inc. 
     Each of the coarse bubble jet diffusers is connected with the air supply by at least one, preferably two, outer conduits. Likewise, each of the fine bubble jet diffusers is connected with the air supply by at least one inner conduit. In a preferred embodiment, the improved aeration system of the present invention comprises an aeration tank which contains a plurality of peripheral coarse bubble jet diffusers with a plurality of fine bubble jet diffusers located therebetween. The coarse bubble jet diffusers are connected with parallel, outer air distribution header pipes, and the fine bubble jet diffusers are connected with parallel inner air distribution header pipes. The inner air distribution header pipes are parallel to each other and to the outer header pipes. All header pipes are connected with an external air supply. 
     The coarse bubble jet diffusers are oriented to produce a high velocity air flow upward along opposing tank walls and across the tank surface such that the current from outer headers converges in the middle and is drawn back toward tank bottom due to the inductive downward flow streams created by the coarse bubble jet diffusers. The fine bubble jet diffusers are strategically located within the path of the inductive downward flow streams such that fine bubbles emitted by these diffusers travel in a flow path which is counter-current to the inductive downward stream created by the forced perimeter flow of the coarse bubbles. In operation, the diffused aeration system of the invention maximizes oxygen transfer efficiency, and maximizes mixing with little or no increase in operating power costs. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a section view of an aeration tank illustrating the flow patterns resulting from strategic placement of the coarse bubble jet diffusers and the fine bubble jet diffusers in accordance with the present invention. 
     FIG.  2 . is an isometric view of an aeration tank illustrating the placement of the coarse bubble diffuser headers and the fine bubble diffuser headers in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1 and 2 show an aeration tank  6  having a bottom  6   a  and opposing walls  10  and  10   a  which contain the improved diffused aeration system of the invention. The diffused aeration system generally includes a plurality of outer or peripheral coarse bubble jet diffusers  1  and  1   a  and a plurality of fine bubble jet diffusers  2 . The plurality of coarse bubble jet diffusers  1  and  1   a  are oriented linearly adjacent to opposing tank walls  10  and  10   a , respectively, adjacent the tank bottom  6   a . Outer air distribution header pipes  7  and  7   a , which are connected with each of the plurality of peripheral coarse bubble jet diffusers  1  and  1   a , respectively, are disposed adjacent the tank bottom  6   a , parallel to opposing tank walls  10  and  10   a , respectively. 
     Each of the outer header pipes  7  and  7   a  contain therein a plurality of discharge ports (not shown). The header pipes  7  and  7   a  are connected with an external air supply  9 . The plurality of coarse bubble jet diffusers  1  and  1   a  are oriented to produce high velocity current flow  3  and  3   a  upward along opposing tank walls  10  and  10   a , and across the surface of the tank  11  and  11   a . The currents of opposing headers  7  and  7   a  intersect at the point of convergence  12  and are drawn back to the tank bottom  6   a  via the inductive flow paths  4  and  4   a  created by the coarse bubble jet diffusers  1  and  1   a . Preferably, coarse bubble jet diffusers  1  and  1   a  are directed outwardly toward the tank walls  10  and  10   a  as shown in FIG.  1 . 
     A plurality of inner air distribution header pipes  8 , each having therein numerous discharge ports (not shown), are located near the center of the tank bottom  6   a , parallel to the outer header pipes  7  and  7   a  and disposed therebetween. Each of the inner air distribution header pipes  8  is connected with the fine bubble jet diffusers  2  and with the external air supply  9 . The fine bubble jet diffusers  2  are located within the inductive flow paths  4  and  4   a  such that the fine bubbles emitted therefrom travel in a counter-current air flow  5  which opposes the above-described inductive downward flow streams  4  and  4   a.    
     Based on the foregoing, it can be seen that the flow pattern produced by the improved diffused aeration system of the present invention maximizes oxygen transfer by increasing residence time beyond the capabilities of a fine bubble system while maintaining a power consumption comparable to a coarse bubble-only aeration system. Further it can be seen that the present invention provides improved mixing. The unique attributes of this system result from the use of fine bubble jet diffusers strategically located to produce a counter-current air flow path in a downward flow stream induced by a system of parallel, generally oppositely directed coarse bubble jet diffusers. 
     It should be understood that the present invention is not limited to activated sludge applications, but is useful in any application where it is desirable to inject and diffuse one process fluid (i.e., liquid, gas solid particulate, etc.) into another process fluid for the purpose of aeration, diffusion, agitation or mixing. 
     The invention has been described with reference to the preferred embodiment, which is merely illustrative of the present invention, and not restrictive thereof. Obvious modifications and/or alterations of the invention may be made without departing from the spirit and scope thereof.