Abstract:
Method and process for dramatically increasing mixing, energy and operating efficiency of wastewater treatment plants at reduced capital and operating cost per unit volume.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of Invention  
           [0002]    This invention pertains generally to anaerobic digestion and, more particularly, to an anaerobic digestion system and method in which the pH control, mixing, and blending of the biological phases of the anaerobic process in the digestion of biosolid feedstock is accomplished in a single tank or vessel utilizing vertical phase separation, thus eliminating the necessity of separate or additional tanks to provide and/or maintain these functions.  
           [0003]    2. Related Art  
           [0004]    Anaerobic digestion of biosolids for the reduction of liquid or liquefied wastes such as sewage, garbage, agricultural, and industrial bi-products into energy in the form of methane or natural gas, carbon dioxide, sulfur compounds, and sludge that is useful for fuel or fertilizer, is a multi-phase or stage process. Generally speaking there are two recognized principal phases or types of anaerobic bacteria commonly known as the acid formers and the methane fermenters. In addition there are a number of minor phases or biochemical reactions that take place in the transition between these two major phases, designated herein simply as the transitional phase. There is ample evidence today that some separation of these phases contributes to the digestion efficiency overall, Dague &amp; Kaiser U.S. Pat. No. 5,525,228, and it may be found advantageous to separate any number of these phases, Steiner U.S. Pat No. 5,630,942.  
           [0005]    However, historically and until relatively recent time anaerobic digestion took place in a single tank or vessel. Noted improvement in the anaerobic process has been demonstrated by separating the phases of digestion by using 2 to 4 tanks connected in series. Mixing of the liquor has been on a tank-wise basis in such a manner that the operation of each of the biochemical phases was completely intermixed in a single tank, and phase separation was accomplished by the use of multiple tanks. There are practitioners who believe that the total separation of the major phases is most efficient while other practitioners believe that a certain amount of feedback and blending is necessary in order to maintain the maximum bacterial growth rate and process stability. This process provides the ultimate flexibility to operate in either of these modes as well as any combination of the two. The varying conditions encountered in the anaerobic process during start-up, up-set, recovery, and normal variances in the constitution of the bio-feedstock dictate that the operation of such processes must have the ability to constantly separate, monitor and control the pH, feedback, temperature, and mixing of the liquor in the various phases of the digestion process. The invention described herein provides all of these features in the enclosure of a single tank or vessel.  
         OBJECTS AND SUMMARY OF THE INVENTION  
         [0006]    It is in general an object of the invention to provide an new and improved anaerobic digestion system and method.  
           [0007]    Another object of the invention is to provide an anaerobic digestion system and method of the above character which overcomes the limitations and disadvantages of systems and methods heretofore provided.  
           [0008]    These and other objects are achieved in accordance with the invention by providing an anaerobic digestion system and method in which the pH control, mixing, and blending of the biological phases of the anaerobic process in the digestion of biosolid feedstock is accomplished in a single tank or vessel utilizing vertical phase separation, thus eliminating the necessity of separate or additional tanks to provide and/or maintain these functions. The invention also provides the ability to control the pH of the liquor in each phase by a hydro mechanical means, while at the same time providing a mixing of the liquor within each phase and allows for feedback and blending of the liquor as may be required for optimum performance. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    [0009]FIG. 1 depicts in schematic form the preferred embodiment of the invention, showing one form of a digester tank, with a three phase version of the several possible digestion schemes, one version of an optional fixed or floating media scheme, one version of a horizontal mixing scheme using uplift reverse cyclonic horizontal mixing, a diffusion gas or chemical mixing pump scheme, a feedback loop, the preferred embodiment of the many styles of digester heating apparatus, major piping lines and control valves, influent pumping, and effluent ports.  
         [0010]    [0010]FIG. 2 depicts in vertical view, either upward or downward, a flow arrangement for the preferred upward reverse cyclonic horizontal mixing embodiment of this invention with a provision for a quiescent zone for greater bacterial colonization and growth that may be controlled and varied to meet the demands of the biosolid supply, with mixing lines numbered in correlation with those of FIG. 1 for a vertical top view mixing scheme.  
     
    
     DETAILED DESCRIPTION  
       [0011]    In the preferred embodiment of the invention as shown in FIG. 1 the liquefied biosolid influent enters the system at  1 , where it is pumped  2 , through heat exchanger  3 , which is supplied by heat loop  4  to  5 , and is discharged through nozzle  6 , in the dome  7 , of digester  8 , into scum layer  9 , creating the cyclonic mixing and breakup of that layer, 10  (FIG. 2), the biosolids settle into quiescent bacterial colonization zone  11 , created by the reverse cyclonic action  12 , at the top of mixing zone  1 ,  13 , aided by the optional fixed or level controlled floating plastic bacterial residence media  44 . In the preferred embodiment of the Invention, a multi-faceted floating media  14 ,  46 , and  47 , providing maximum bacterial residence area, is employed, that floats in colonization quiescent zone  11  and is restrained into position by plastic screen or netting  44 , and  45  in quiescent zones  46  and  47 .  
         [0012]    The primary biochemical action of the phase  1  quiescent  11 , and mixing  13 , zones is the conversion of biomass into carbon dioxide gas and water. The carbon dioxide gas bubbles up through the liquor aiding in the mixing and is collected in dome  7 , to be eventually drawn off at the raw gas outlet  15 , where it may be separated from the raw gas, bottled for sale, and/or used as a pH control substance and mixing enhancement through line  16  and control valves  17 ,  18 , and  19 . In the preferred embodiment raw gas separation, Miller U.S. Pat. No. 6,291,232 is used to separate the two major gas products methane and carbon dioxide for pH control, otherwise other chemical feed may be substituted at line  16 . Similarly in the preferred embodiment raw gas separation permits a certain amount of methane gas to be injected at  20 , in lieu of a base chemical. The use of methane gas in mixing greatly enhances the action of methane fermentation to the point that basic chemicals are not required in the pH control process. Methane injected at  20  is fed into the process through control valves  21 ,  22 , and  23  where it not only controls the pH and stability of the process but markedly improves mixing. Liquor drawn off at zone outlets  24 ,  25  and  26  is circulated by diffusion pumps  27 ,  28 , and  29  through uplift nozzles  30 ,  31 , and  32 , resulting In the flow patterns shown in FIG. 2. Valve arrangement  23 ,  33 , and  34 , provide the possibility to provide a bacteria feedback loop to the process as may be required. Control valves  36 ,  37 , and  38  permit sampling of the phases and emergency drains in the preferred embodiment which incorporates Vacuum Retort Anaerobic Digestion, Miller U.S. Pat. No. 6,291,232, in which water is evaporated off the digester and influent at dome  7 , and drawn off through outlet  15  as the major component of the raw gas vapor. In the non-preferred embodiments supernatant liquor is drawn off through valves  36 ,  37 , and  38  and outlet  39 .  
         [0013]    As the biosolid influent gradually works its way downward through the quiescent and mixing zones of Phase  1 ,  40 , Phase  2 ,  41 , and Phase  3 ,  42  it is slowly transformed into carbon dioxide gas, methane gas and a small amount of hydrogen sulfide gas and a fairly inert sludge that has released the greatest part of its water and in the preferred embodiment is completely pasteurized before being drawn off at outlet  43 .