Abstract:
An anaerobic digester for use in wastewater treatment systems having a fixed cover including a central tower for controlling gas and foam. A constant operating level is maintained in the digester by utilization of an overflow weir located in the tower. The fixed cover having an under flat surface that is in direct contact with contents of the digester. The tower has a number of spray nozzles that operably direct movement of foam toward discharge locations.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention is directed to methods and facilities for stabilizing and reducing solids content of excess activated sludge, foam, scum and the like that results from a biological wastewater treatment process or the like. More specifically, the invention is directed to a constant level anaerobic digester adapted to provide better handling of foam and gas by directing same into a central tower or elevated region that is equipped with gas venting and a weir overflow system to control sludge at a fixed level in the digester and in contact with most of the roof of the digester, while also providing piping and nozzle structure to control foam.  
         [0002]     A functioning society produces an enormous amount of raw liquid sewage in the form of wastewater, which must be rendered innocuous prior to release into the environment. While some of this wastewater is treated chemically, the vast majority is treated using microorganisms that form a biomass that essentially eats, modifies or removes contaminants such as organic material, phosphorus and nitrates. Such processes especially convert organic materials into water, carbon dioxide and/or methane depending on the particular process being used. Such processes produce an excess of biomass or activated sludge because the microorganisms grow and multiply as they eat. Furthermore, stages of waste water treatment systems, such as primary clarifiers may produce scum which are removed from the clarified water. In most processes of this type, the excess activated sludge and scum are collected and must be treated in some manner. At some locations, these undesirable products may be sterilized, dried and used as fertilizer or transported to a landfill. Usually such processes are expensive and may result in substantial environmental problems and complaints from persons living in close proximity to the facility. Such process may also violate local or federal sludge disposal laws or guidelines.  
         [0003]     Therefore, in some instances it is highly desirable to reduce the solids in the collected side streams by converting a portion of the solids to other substances such as gas that can be collected and beneficially used, especially methane gas, or that can be released, such as carbon dioxide. Such a process reduces the solids in the discharge components that must be later treated. A preferred system for such solids reduction is an anaerobic digester which utilizes microorganisms that live under anaerobic conditions and which modify much of the non-biomass organic material that remains in any side stream separated from the water treatment process and effectively eats or modifies other microorganisms in the sludge. Therefore, the streams are collected and placed into an anaerobic digester for stabilization and solids reduction. In such an anaerobic digester, the sludge is mixed and heated to maintain optimum conditions for certain organisms in the biomass to utilize other portions of the sludge as food so that the eaten organic material is stabilized.  
         [0004]     The digestion process yields a number of by-products including methane gas released by biological activity of the organisms. Further, the mixing of the sludge along with the activity of the organisms has the potential to produce foam and scum, which rise to the surface of the digester contents creating a foam and scum layer. In many installations, scum from other areas of a facility may also be added to the contents of the digester. While some of the added scum is absorbed by the other contents or reduced in quantity by the microorganisms, some is not, thereby adding to the light material rising to the top of the digester. Controlling and removing the gas, especially methane gas, along with the scum and foam layer is essential in the proper operation of the digester. It is beneficial for the methane gas to be captured to prevent release into the environment and for preserving the methane to be used as a heat-producing gas at the facility. Therefore, the methane gas is collected from the top of the digester.  
         [0005]     Most digesters of this type are relatively large and it is difficult to control the foam layer over a large surface. The collection of gas and foam can produce significant problems. In particular, the gas is normally withdrawn from digesters through ducting by downstream compression, which draws the methane and other gases out of the digester head space. If the foam gets out of control at the top of the digester, it can rise to the level of the gas take off and be drawn into the gas compressor system. Since such compressors are not designed for liquids or solids, foam drawn into such a compressor can seriously damage or destroy the machine. Therefore, it is desirable to control the foam layer by maintaining the top of the foam layer well below the gas take off during normal operation and by providing additional foam control apparatuses and methods of operation to control the foam layer, when needed.  
         [0006]     Anaerobic digesters have long been employed for wastewater treatment processes. Designs for conventional shape digesters employ either fixed or floating dome covers that do not come into contact with the liquid. Fixed cover systems effectively control foam from escaping from the vessel; however, they do not limit foam generation within the vessel. Floating covers, typically used to provide gas storage within the vessel, also do not limit foam generation but have an added disadvantage of allowing foam to escape the vessel at the perimeter edge seals. Such covers are less effective in deterring foam formation, and do not promote re-entrainment of foam into the sludge to promote digestion while collecting methane gas in the uppermost part of the cover via a gas collection system. Such collection systems can lead to entrainment of foam into gas collection lines that are intended to capture only gas, thus potentially damaging compressors or other equipment.  
         [0007]     Other prior art teaches an egg shaped vessel with a central dome that was developed to promote full mixing of the contents. However, such designs are relatively expensive and the design itself does not effectively resolve the problems of handling foam and collecting gas without foam getting into gas lines.  
       SUMMARY OF THE INVENTION  
       [0008]     The present invention overcomes the problems previously described by providing a conventionally shaped anaerobic digester for processing sludge wherein the digester has a cover that is stationary and in direct contact with the contents contained within the digester over a large majority of an upper surface of the contents. The cover includes a slightly-sloped surface to provide a natural biased flow toward a central tower that extends above the contents of the digester for collection of gas, discharge of sludge and control of scum and foam. Sludge is maintained at a constant level in the digester and in contact with the entire under surface of the cover. Sludge is continuously fed into the digester and overflows a first sludge level control weir in the tower into a discharge with some entrained foam. A second weir is provided with a collection pan and drain which can be selectively flushed to help dispose of a scum and foam layer, when excess scum and foam collects. A set or series of spray nozzles is provided to direct and control foam located within the tower. In particular, water spray nozzles are provided which can be selectively utilized to direct the foam in the tower over the discharge weir. A gas collection dome located at the top of the tower collects the gas that naturally rises to the top of the tower. The gas is withdrawn through piping joined to the dome at a level above the sludge overflow weir and above the foam layer. In this manner, foam is minimized and separated from the gas thereby minimizing the likelihood that foam will get into gas lines and cause damage to the compressor or other downstream equipment.  
       OBJECTS AND ADVANTAGES OF THE INVENTION  
       [0009]     The principal objects of the present invention include: providing an anaerobic digester with a cover that is in direct contact with the fluid contents of the digester over substantially all of the under surface of the cover leaving only a small area of the upper surface that is not in direct contact with the cover and having a central tower for collecting and controlling foam and gas produced in the digester; providing such a digester with an expansive cover and relatively small central dome area for directing foam and gas to a relatively smaller collection area wherein the foam can be effectively controlled; providing such a digester having a constant level; providing such a digester having a relatively universal and unrestrictive design that accommodates or can be applied in numerous applications; providing such a digester which can direct scum and foam via spray nozzles to a foam collection area in the tower to direct and control the collected scum and foam; providing such a digester having such a central tower to control scum and foam and gas and provide other benefits; to provide such a digester utilizing a weir located in the tower to control the level of sludge in the digester; to provide such a digester having a collection pan with a controllable drain to allow removal of excess scum and foam from the tower; to provide such a digester wherein the pan and drain act in consort with the spray nozzles which are selectively controllable by an operator to urge excess foam toward the pan and drain such that the foam is controlled or exits the digester tower without being entrained in gas exiting the digester tower; providing such a digester having a method for capturing, collecting and separating foam and gas; providing such a digester that maintains a generally constant sludge level therein while continuously flowing sludge into and out of the digester; providing such a digester having a collection area for the purpose of diverting foam away from the gas collection area to prevent foam from being entrained in gas lines; providing such a digester having such a cover that directly engages sludge throughout the upper surface of the digester contents except in the tower for the purpose of limiting open area that may collect foam in the top of the digester; providing a method of operating such a digester utilizing a level control weir located within the top portion of the digester to control contents level; providing a method of operating such a digester utilizing a flush option so that an operator can selectively dispose of excess accumulated scum and foam quickly, if needed; and providing such a digester and associated methods which are relatively easy to use, inexpensive to produce and particularly well-suited for the intended usage thereof.  
         [0010]     Other objects and advantages of this invention will become apparent from the following description taken in conjunction with the accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of this invention.  
         [0011]     The drawings constitute a part of this specification and include exemplary embodiments of the present invention and illustrate various objects and features thereof. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  is a partially schematic and cross-sectional view of an anaerobic digester in accordance with the invention having a cover with a central tower.  
         [0013]      FIG. 2  is an enlarged, fragmentary and partially exploded perspective view of the anaerobic digester cover central tower that is shown in  FIG. 1  with portions broken away to illustrate detail thereof.  
         [0014]      FIG. 3  is an enlarged and fragmentary side elevational view of the central tower of  FIG. 2  with portions broken away to illustrate detail.  
         [0015]      FIG. 4  is an enlarged and fragmentary cross-sectional view of the central tower, taken along  4 - 4  of  FIG. 3 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]     As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which may be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure.  
         [0017]     The reference numeral  1  generally identifies an anaerobic digester in accordance with the present invention for use in conjunction with one or more sludge producing facilities such as waste water treatment plants which may be located in close proximity to or remote from the digester  1 . The digester  1  has a non-moveable or fixed-position digester cover  2  and a digester tank, reservoir or basin  3 . The digester  1  is designed to maintain a constant contents level under normal operating conditions.  
         [0018]     The illustrated basin  3  has a sloped bottom  6  and a cylindrical, upright sidewall  7  that forms a liquid containing enclosure suitable for receiving sludge for processing. A preferred material of construction for the basin  3  is concrete. The basin  3  has a relatively large opening  9  at an upper end  10  thereof that is enclosed by the cover  2 . The basin  3  is normally formed partially buried in ground. The opening  9  at the basin upper end  10  can vary in size according to the size requirements of the digester  1 , but is comparatively wide often having a diameter that is substantially greater than the height of the basin  3 . For example, a digester of the type illustrated may have a basin opening  9  with a diameter of 65 feet with a sidewall height of 30 feet.  
         [0019]     The fixed cover  2  extends over or spans the entire basin upper end  10  and can be sized and shaped to be used with basins of varying shapes and sizes. The cover  2  has a liquid engaging portion  12  and a central tower  14 , as best shown in  FIG. 1 . The cover  2  is attached and sealed about a periphery  15  thereof to the basin top end  10  so as to form a liquid and gas tight containment. The cover  2  is fixed in position relative to the basin  3  and is of a rigid design so as not to move substantially during operation. The liquid engaging portion  12  includes spans  20  that extend from the side  7  radially to the tower  14 . The spans  20  have lower beams  22 , upper beams  23 , struts  24  and cross braces  25  which are assembled to provide a strong fixed position and generally rigid roof across the basin  3 . The underside of the cover  2  is constructed with an impermeable and continuous liquid and gas containing surface  30  to prevent gas and/or liquid from escaping except directly beneath the tower  14 . The underside surface  30  is joined to the basin wall  7  around a periphery of the cover  2 . A junction between the cover  2  and basin wall  7  is liquid and gas tight.  
         [0020]     Although the cover described in the illustrated embodiment is of such a design as to be constructed of principally metal components, it is foreseen that in accordance with the invention that the cover could be constructed of other materials. In particular, it is foreseen that the cover could be of concrete construction and formed with or joined with the walls of the basin.  
         [0021]     The illustrated tower  14  is centrally located and this location is preferred; however, the tower  14  may be asymmetrically located in some installations. The surface  30  in conjunction with the basin  3  urges lighter components of digester  1  contents to remain in a liquid phase  33  for digestion or to provide a naturally-biased flow of excess and lighter components toward the tower  14 . The surface  30  may be substantially horizontal or slightly sloped toward the tower  14  to urge gas toward the tower  14 . With references to  FIG. 3 , the tower  14  is disposed or located substantially or mainly above the surface  30  and receives excess liquid sludge  35 , light components  36  (including foam scum and the like) and gas  37 . The illustrated tower  14  is generally cylindrical in shape; however, other shapes are foreseen to be useable in accordance with the invention. The tower  14  has a horizontal cross-section that is substantially smaller than the area of the cover lower surface  30 .  
         [0022]     In the illustrated embodiment, the tower has a horizontal diameter of about 13.5 feet and a height of about 8 feet. The tower  14  has an cylindrical upright sidewall  40  and a flat top  41  forming a liquid and gas containment except as noted below where penetrated by piping or the like.  
         [0023]     Located along and joined to the tower side wall  40  is a pan or trough  44  which is best seen in  FIG. 2 . The trough  44  has a bottom wall  45  and a trough side wall  46  that has end segments  49  and  50  and a curved outer segment  51  that is generally uniformly spaced from and follows the curvature of the tower side wall  40 . A portion of the trough side wall  46  that corresponds to the trough  44  has an elongated upper edge  53  that is horizontal and operably functions as a first flow control and sludge level control weir. In the illustrated embodiment, the weir upper edge  53  is spaced about 3 feet from the tower top wall  41 . The trough wall  46  of the illustrated embodiment is about 1.5 feet high.  
         [0024]     Located at one end of the trough  44  and extending between the tower side wall  40  and the side wall  46  is an end wall  60  that has a height that is the same as the trough wall  46  and end segment  49 . The end wall  60  has a horizontal upper edge  62 .  
         [0025]     The trough walls  50 ,  51  and  60  in conjunction with the tower wall  40  form a receptacle  64  which is flow connected through an opening  65  to a pipe  66 . The upper edges of the walls  50  and part of  51  forming the receptacle  64  are approximately located twelve inches below the edge  53  of the remainder of the wall  51  that functions as the normal overflow weir. Flow into the pipe  66  is controlled by a valve  63  which is opened to especially remove scum or foam  36  collected in the tower  14 .  
         [0026]     The wall  60  wall segment  51  and wall  49  cooperative with the tower wall  40  to form a main sludge overflow collection receptacle  67  with sludge  35  normally overflowing the weir formed by the wall tops of the walls  46 ,  60  and  49 , and especially the edge  53  so as to be collected in the receptacle  67  during normal operation. The arrow identified by the letter A indicates such flow.  
         [0027]     The trough bottom wall  45  is penetrated by a stand pipe  68  that has an upper opening  69  that is located above the weir forming upper edge  53 . The stand pipe  68  functions to provide emergency overflow in the situation where other overflow paths become blocked. The stand pipe  68  joins with the overflow pipe  66  at a junction  70  and there is joined to a discharge pipe  72 . The discharge pipe  72  operably flow connects to a sludge storage facility  71  that can be a tank, multiple tanks or other structure suitable for storing sludge and receiving overflow of sludge from the discharge pipe  72  as well as special discharge of scum, foam and sludge through the pipe  66  by operation of the valve  63 . A trap  74  is also provided in the discharge pipe  72 .  
         [0028]     As can be best seen in  FIG. 2 , a series of three gravity discharge pipes  77 ,  78  and  79  also open into the trough  44  and have respective valves  80 ,  81  and  82 , as well as respective lower inlets  84 ,  85  and  86  that open into lower portions of the digester basin  3  (for example at 3, 10 and 16 feet below the tower  14 ). The discharge pipes  77 ,  78  and  79  may be selectively used to discharge sludge from lower levels of the digester  1  by bypassing the overflow weir  53  in the tower  14  by opening one or more of the valves  80 ,  81  or  82  respectively. The invert elevations of these lines at the connection to receptacle  67  is approximately six inches below the elevation of weir edge  53 .  
         [0029]     Located at spaced locations within the tower  14  are a series of spraying nozzles  88 . Each of the nozzles  88  is joined to a water line  89  and the spray flow through each nozzle  88  is controlled by an independent valve  90 .  
         [0030]     The trough bottom wall  45  is also penetrated by an opening  92  of a second discharge pipe  93  that operably joins with the sludge storage facility  71 . Under normal operating conditions, the excess sludge flow in the digester  1  which is approximately equal to the incoming sludge flow overflows the weir  53 , enters the receptacle  67  and exits the digester  1  through the opening  92  so as to enter the pipe  93 .  
         [0031]     Penetrating the tower top wall  41  is a gas collection dome  98  that is flow joined to a gas discharge pipe  99 . The gas discharge pipe  99  joins with a compressor  101  and collection facility schematically indicated by the box with the reference numeral  100 . The compressor  101  draws gas from the dome  98 .  
         [0032]     An access manhole  102  also penetrates the tower top wall  41 . A safety pressure-vacuum release valve  103  also is located on the tower  14  and communicates with the interior thereof.  
         [0033]     A sample port  105  is provided in the cover  2 . A recirculation suction line or pipe  109  is flow linked to a sludge recirculation pump  110  for recirculating sludge to a multiapertured discharge pipe  108  in the bottom of the digester  1 . Normally, the basin  3  is maintained in the mesophilic range of from about 35° C. to 55° C. Mixing of the digester contents  33  can be by various conventional means and is preferably provided by gas driven eductor tube mixers (not shown).  
         [0034]     In use, at least partly dewatered sludge containing high concentration of biomass or microorganisms, some other organic compounds and certain other components is transferred into the basin  3  and preferably maintained at an elevated temperature in the mesophilic range which is conducive to digestion of at least a portion of the sludge by certain of the microorganisms therein.  
         [0035]     The digester  1  is filled with liquid sludge  33  from a source so that the sludge  33  contacts or engages the entire under surface  30  of the basin cover  2  which is fixed in position and which does not raise or lower substantially during operation of the digester  1 . The sludge  33  continuously enters the tower  14  and overflows the weir formed by the trough front wall upper edge  53  and top edges of side walls  49  and  60 . During normal operation, the sludge  33  thereafter flows into the trough  45 . In particular, excess sludge in an amount approximately equal to the incoming sludge normally continuously exits the discharge pipe  93 . If the discharge pipe  93  gets blocked or flow is too great, the sludge level  33  rises slightly and overflows into the standpipe  68 .  
         [0036]     When foam  36  collects in the tower  14  and is not satisfactorily entrained into the discharge pipe  93  with the sludge  33 , water sprays can be selectively activated by an operator from nozzles  88  so as to drive the foam  36  toward the collection trough  67 . If this still fails to remove sufficient foam  36 , the valve  63  may be opened to encourage discharge therethrough of sludge  33  along with a flushing action with respect to the sludge  33 , as the height of the receptacle  64  is below the normal level of the sludge  33  in the tower  14 . This flushing can be accompanied with water spray from the nozzles  88 .  
         [0037]     Gas  37  (mostly methane) generated by the digester  1  collects in the top of the tower  14  and in the dome  98  from which the gas  37  is drawn through gas discharge pipe  99  by a compressor  101  and delivered to the storage facility  100 . By effective use of spray and operation of discharge pipes  99 , foam  36  can be separated from gas  37 , so that the gas  37  can be withdrawn essentially free of entrained foam  36 . The pipes  77 ,  78  and  79  can be used to gravity recycle sludge, as required by operation.  
         [0038]     It is to be understood that while certain forms of the present invention have been illustrated and described herein, it is not to be limited to the specific forms or arrangement of parts described and shown.