Abstract:
An apparatus and method for treating organic waste sludge such as sewage sludge is disclosed wherein the sludge is first dewatered, moved to a day hopper for storage, and then successively passed through first and second reactors. As the sludge is passed through the first reactor, in a continuous fashion, the sludge and acid are thoroughly mixed and has the pH thereof substantially lowered due to the addition of acid in the first reactor. The sludge is then moved through the second reactor where the sludge is subjected to a base material to substantially raise the pH thereof. The treated sludge is then pumped from the second reactor to a pugmill and then to a dryer which dries the material. The dried material is then suitable for use as a fertilizer.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a continuation-in-part application of Petitioners&#39; earlier application Ser. No. 08/926,109 filed Sep. 9, 1997, entitled SEWAGE SLUDGE TREATMENT, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to an apparatus and method for treating organic waste sludge and, more particularly, to an apparatus and method for treating sewage sludge. 
     2. Description of the Related Art 
     The efficient and commercially effective utilization of organic waste sludge such as sewage sludge is important for society, particularly with the increase of population and demands which are made on land. At present, sewage sludge is dried in large bed pans, and the dried product is used as a compost high in organics. Although such composts are useful in enhancing the retention and storage of water in soil, the composts are low in inorganics and, hence, have limited fertilizer properties. The drying process is environmentally undesirable in that offensive odors are produced. Further, the prior art methods of drying the sludges are capital-intensive. 
     Processes have been suggested for treating sewage sludge to sterilize and disinfect the same. For example, South African Patent No. 89/6160 discloses such a method which involves treating sewage sludge with anhydrous ammonia gas to increase the pH of the sludge to at least 10, followed by using sufficient inorganic acid to neutralize, or substantially neutralize, the sewage sludge/ammonia admixture. The resulting product is a liquid which is said to be useful as a fertilizer. It is further suggested that the liquid can be dried, e.g., by evaporation. 
     A prior art process for treating organic material such as sewage sludge for use as a fertilizer is disclosed in U.S. Pat. No. 5,393,317, and it is believed that the process described in the &#39;317 patent is less than desirable due to the time required for treating the same inasmuch as in certain steps thereof, the mixture is allowed to stand for at least 20 minutes. Another prior art process for treating organic material such as sewage sludge is disclosed in U.S. Pat. No. 4,743,287. It is also believed that the &#39;287 process is less than desirable, since water must be added to the mixture of organic material and major elements to produce a moisture content of 12%–30% by weight. Still another process is disclosed in U.S. Pat. No. 5,443,613. It is also believed that the process of the &#39;613 patent is less than desirable, since water must be added to an acidified suspension. It is believed that those processes requiring an addition of water to the product increase the amount of drying required and, hence, causes an increase in drying time. 
     SUMMARY OF THE INVENTION 
     According to the present invention, an apparatus and method is disclosed for treating organic waste sludge such as sewage sludge with the sewage sludge being able to be used as a fertilizer. The sewage sludge which is treated by the method of this invention may be raw sewage sludge, activated sewage sludge, or aerobically or anaerobically digested sludge. The sewage sludge is first dewatered by a mechanical dewatering apparatus of conventional design. The sewage sludge is mechanically dewatered so that the sewage sludge is comprised of a predetermined percentage of dry solids. The dewatered sewage sludge is then moved into a day hopper for storage until the sewage sludge is to be processed. The day hopper has a live bottom therein so that the material therein may be constantly circulated or agitated to prevent the same from coagulating or caking at the bottom thereof. The dewatered sewage sludge or sludge cake is then delivered to a reactor pump which passes the sludge cake to a first non-pressurized acid reactor or mixer wherein the sludge cake is mixed with and treated with an acid to substantially lower the pH thereof. As the sludge cake is passed through the first acid reactor, in a continuous fashion, the sludge cake is subjected to a thorough mixing action. The sludge cake is then passed, in a continuous fashion, from the first acid reactor by a screw conveyor or the like to a second reactor wherein the sludge cake is mixed with and subjected to a base material to substantially raise the pH thereof. The treated sludge cake is then pumped from the second reactor to a pugmill and then to a dryer such as a rotary dryer or pulse combustion dryer. 
     The dried product from the dryer is supplied to a cyclone separator, with the finished product resembling a fine powder or granular material depending on the dryer being utilized. The treatment of the sludge cake with an acid, then a base, and then drying the same results in a bacteria-free, pathogen-free product which has very little, if any, offensive odor, with the finished product being suitable for use as a fertilizer. 
     The invention disclosed in the co-pending application has been proven to work at a site in Payson, Ariz. In the apparatus of the co-pending application, the acid reactor is a closed, pressurized reactor. It has been discovered that the pressures in the acid reactor may result in leaks around the bearing areas. Further, since the acid reactor in the co-pending application is closed, it is impossible to observe the mixing action within the acid reactor or mixer. In some cases, it has been found that the mixing action in the acid reactor of the co-pending application is not as thorough as is desired. The instant application provides at least a pair of elongated rotatable shafts having mixing paddles mounted thereon which extend transversely from the shafts and which are either right-hand or left-hand paddles which ensure that the acid is properly mixed with the dewatered sludge passing through the acid reactor. 
     It is therefore a principal object of the invention to provide an improved apparatus and process for treating sewage sludge. 
     Still another object of the invention is to provide a process for treating sewage sludge wherein the final product is substantially bacteria-free, pathogen-free and which may be used as a fertilizer. 
     Still another object of the invention is to provide a continuous process, as opposed to a batch process, for treating sewage sludge. 
     Yet another object of the invention is to provide an improved acid reactor or mixer which thoroughly mixes acid with the dewatered sludge. 
     Still another object of the invention is to provide a non-pressurized acid reactor or mixer having improved mixing capability. 
     Still another object of the invention is to provide a process for treating sewage sludge wherein the sewage sludge is converted from sludge cake to a finished product in a short period of time. 
     Still another object of the invention is to provide a process for treating sewage sludge which substantially reduces, if not eliminates, offensive odors normally associated with sewage sludge processing. 
     Still another object of the invention is to provide a process for treating sewage sludge which can be computer-controlled. 
     These and other objects will be apparent to those skilled in the art. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view of the apparatus of this invention; 
         FIG. 2  is a schematic of the apparatus and process; 
         FIG. 3  is an enlarged schematic of a portion of  FIG. 2 ; 
         FIG. 4  is an enlarged schematic of a portion of  FIG. 2 ; 
         FIG. 5  is a partial schematic of the apparatus of this invention; 
         FIG. 6  is a top view of the improved acid reactor or mixer of this invention; 
         FIG. 7  is a partial end view of the mixer of  FIG. 6 ; 
         FIG. 8  is a side elevational view of the mixer of  FIG. 6 ; and 
         FIG. 9  is a top view of the mixer of  FIG. 6  depicting its relationship with other components of the apparatus of this invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     One apparatus which is used to practice the process of this invention is illustrated in the drawings in  FIGS. 1–4  while  FIGS. 5–9  illustrated an improved acid reactor and associated structure. Although the drawings illustrate the preferred embodiment of the apparatus which is used for practicing the preferred embodiment, certain changes may be made therein without departing from the spirit of the invention. 
     Referring to  FIGS. 1–4 , the sewage sludge which is treated by the method of this invention may be raw sewage sludge, activated sewage sludge, or aerobically or anaerobically digested sludge. The sewage sludge is mechanically dewatered through the use of a belt-press dewatering apparatus of conventional design. Preferably, the sewage sludge is dewatered to a solid content of 18%–22%, with a 20% solid content being preferred. The dewatered sewage sludge is commonly referred to as sludge cake, with the consistency of the same typically being coherent and non-flowable and, due to the moisture content, it will be wet. 
     The dewatered sewage sludge is delivered to the upper end of a day hopper  10  which has a conventional live bottom apparatus  11  at its lower end to circulate the materials in the day hopper to prevent the coagulation or caking of the same. Preferably, day hopper  10  includes a level sensor  12  which will control the delivery of the sludge cake thereto. The numeral  14  refers to a sludge cake pump which is in communication with the lower end of the day hopper  10  and which has a discharge line conduit  16  extending therefrom which includes a pressure sensor  18 . Discharge line  16  is in fluid communication with a reactor pump  20  which pumps the sludge cake to the inlet  22  of a reactor  24 . Reactor  24  includes a high shear paddle mixer assembly therein to subject the sludge cake to a desirable high shear mixing action. The discharge end of reactor  24  is connected to the intake end of a second reactor  26  by means of pipe  28 . Reactor  26  also includes a high shear paddle mixer assembly therein to subject the sludge cake to the desired high shear mixing action. 
     The intake end of reactor  24  is operatively connected to a source of inorganic acid material such as phosphoric acid, sulphuric acid or nitric acid. The nature of the acid will be selected according to the nature of the inorganic component desired in the final product. Acid storage tank  30  is connected to a supply pump  32  by line  34  having a stop valve  36  therein. Supply pump  32  is preferably connected to smaller acid tank  38  which in turn is connected to an electronic valve assembly  40  through pump  42  and check valve  44 . Valve  40  is connected to the intake end of the first reactor  24  by means of line  46 . 
     The numeral  48  refers to a storage tank for the base material such as ammonia, ammonium hydroxide, potassium hydroxide, sodium carbonate, sodium hydroxide, lime or magnesite. The preferred base is ammonia gas, preferably anhydrous ammonia gas. Tank  48  is connected to electrically operated valve assembly  50  by means of line  52  having stop valves  54  and  56  imposed therein. As seen in the drawings, line  52  also has a check valve  58  imposed therein. Valve assembly  50  is connected to the intake end of reactor  26  by means of line  59  having check valve  60  therein. 
     Line  62  extends from the discharge end of reactor  26  and includes a pressure sensor  64  therein. Line  62  extends to a three-port, two-way valve  66  having lines  68  and  70  extending therefrom. Line  70  extends to a backwash tank  72  including a level sensor  74 . Line  76  extends from backwash tank  72  to line  16 , where it is connected thereto at  78 . Stop valve  80  is preferably imposed in line  76 , as indicated. 
     Line  68  extends to a dryer assembly referred to generally by the reference numeral  82 , which is preferably of the pulse combustion type although a rotary dryer will also work in a satisfactory manner. Pulse combustion dryers are described in U.S. Pat. Nos. 4,708,159; 4,819,873; 4,838,784; 4,941,820; and 4,992,039. The pulse combustion dryer utilized in this invention utilizes high sound levels to enhance the drying process. In the pulse combustion dryer of this invention, the treated sludge cake is subjected to a high temperature pulsing high sound level airstream which atomizes the material. It is believed that pulse combustion drying enhances the sterilization of the product due to the fact that any pathogens remaining in the product will be atomized and destroyed. 
     The sludge cake preferably passes through the pulse combustion dryer in approximately fifteen to twenty seconds with the temperature therein being approximately 235°–265° F. Dryer  82  includes a pulse combustion burner  84  having a supply of fuel being delivered thereto by means of line  86 . Combustion air is supplied to the pulse combustion burner  84  by means of the combustion air fan  88  which is in communication with a source of air  90 . Combustion air fan  88  has a conduit or the like  92  extending therefrom which delivers combustion air to the pulse combustion burner  84  by means of conduit  94  and which delivers diluent air to the upper end of the dryer chamber  96  by means of the conduit  98 . Conduits  94  and  98  are provided with dampers  100  and  102 , respectively. 
     The dried product is discharged from the dryer chamber  96  by means of the discharge conduit  104  and is preferably supplied to a cyclone separator  106  by means of conduit  108 . Air from the separator  106  is delivered to a scrubber assembly  110  by means of air line  112 . The finished product is collected at the lower end of the cyclone separator which is preferably provided with a rotating valve  112 . Washwater is provided to the scrubber  110  by means of washwater line  116  being in communication with line  118 . Line  118  is also in communication with the backwash tank  72 , as indicated. The numeral  120  refers to a slurry line which extends from the lower end of the scrubber assembly  110 . 
     Preferably, the finished product is taken from the cyclone separator  106  and is delivered to a module referred to generally by the reference numeral  121 , where the finished product is compacted, screened, ground, weighed, etc. If desired, the module  121  may be connected to the dust suction inlet of the scrubber  110  by means of conduit  122 . 
     When it is desired to process or treat the sludge cake in the day hopper  10 , the pumps  14 ,  20 ,  32  and  42  are activated. At the same time, the pulse combustion dryer assembly is activated, as will be the cyclone separator, scrubber, etc. Initially, as the dryer is heating to its desired temperature, the sludge cake will be passed through the reactors  24  and  26  and will be recirculated through the backwash tank by means of lines  62 ,  70  and the line  76 . After the dryer has sufficiently heated, line  70  will normally not be utilized. As previously stated, the sludge cake is subjected to high shear agitation or mixing in reactor  24  and in reactor  26 . The acid is mixed with the sludge cake in reactor  24  so that the pH of the sludge cake is substantially lowered to approximately 0.5 to 1.5, with the preferred pH being approximately 1.0. As the treated sludge cake subsequently passes through the reactor  26 , sufficient base material is mixed therewith to substantially raise the pH thereof. The pH of the sludge cake is preferably raised to approximately 4.3 to 5.5 in the second reactor  26 , with the preferred pH being 4.5. The reaction of the base material with the sludge cake in reactor  26  will cause the temperature of the same to be raised to approximately 85°–95° C. Preferably, it should take approximately 1.5 minutes for the sludge cake to pass through reactor  24  and will take approximately 1.5 minutes to pass through reactor  26 . The retention time in the reactors will depend upon the volume of material being treated and the particular acids and bases being used. 
     As stated, the inorganic acid will typically be phosphoric acid, sulphuric acid or nitric acid. The nature of the acid will be selected according to the nature of the inorganic component desired in the final product. The amount of inorganic acid used will also depend on the quantity of inorganic components required in the powdered or granular final product. Heat is generated from the contact between the acid and the sludge cake, although further heat may be added if required. Heat has the effect of killing the pathogens in the sludge cake. Acid treatment has the effect of hydrolyzing organic material in the sludge and reducing the viscosity of the sludge cake, making it flowable. As also stated, the base which is used and the concentration thereof will be determined by the inorganic component required in the final product. 
     A very important part of the invention of  FIGS. 1–4  is the use of the pulse combustion dryer. In such a dryer, sound pressures and heat are generated in a combustion chamber and are used to dry the product. The sound pressures and heat are generally passed into the drying chamber and the treated sludge is then introduced into the path of the sound pressures and heat in the drying chamber. Sound pressures as high as 180 dBA and combustion temperatures in the amount of 235°–265° F. are produced in the dryer. The sound pressures disperse the sludge or slurry into droplets which are dried by the heated air. A fine powder is produced, as previously described. When fine powders are not desired, a conventional rotary dryer is the preferred drying apparatus. 
       FIGS. 5–9  illustrate the improved acid reactor and related mechanisms or structures of this invention which ensure that the dewatered sludge will be properly mixed with the acid while passing through the acid reactor. As seen in  FIG. 5 , the dewatered sludge is transferred from the main hopper  200 , through line  202  to line  204  which is in communication with the acid being discharged from an acid pump. 
     The improved acid mixer or reactor of this invention is designated by the reference numeral  206 . Acid reactor  206  is comprised of an elongated body or housing  208  having opposite ends  210  and  212 . Body  208  is a non-pressurized body having an open upper end  209  which is selectively covered with an anti-splash cover. An electric gear motor  214  or the like is mounted on end  210  of body  208  and has a rotatable drive shaft  216  extending therefrom which drives gears  218  and  220  which drive square shafts  222  and  224  in the same direction, respectively. Shafts  222  and  224  are suitably rotatably mounted in body  208  ( FIG. 7 ). Although a pair of shafts  222  and  224  are illustrated, two or more shafts may be utilized in the acid reactor  206 . One end of shaft  222  preferably has a screw conveyor  226  mounted thereon which extends from end  212  through a tube  228  having a discharge pipe  230  extending therefore. Pipe  230  is suitably connected to the intake side of a peristaltic metering pump  232  by a pipe or conduit  234 . 
     A plurality of spaced-apart paddles  236  are preferably mounted on shaft  222  along the length thereof while a plurality of spaced-apart paddles  238  are preferably mounted on shaft  224  along the length thereof. Any desired number of paddles and the spacing thereof may be utilized. As seen in  FIG. 6 , the paddles  236  are longitudinally offset with respect to paddles  238  to prevent interference therebetween while providing enhanced mixing action. It is preferred that the paddles  236  on shaft  222  be left-handed while the paddles  238  on shaft  224  be right-handed. The paddles may be interchanged between shafts left-hand for right-hand to create intermittent reverse flow and obtain optimum mixing. Each of the paddles  236  comprises a square hub  240  which receives shaft  222  and a paddle arm  242  extending transversely therefrom. Each of the paddles  238  comprises a square hub  244  which receives shaft  224  and a paddle arm  246  extending transversely therefrom. 
     The sludge is introduced into one end of body  208  by means of a feed pipe  248  provided on line  204 . An acid injector  250  extends into the feed pipe  248  and is in communication with a source of acid. As the sludge and acid passes through the reactor  208  in a non-pressurized state, the mixing paddles  236  and  238  thoroughly mix the acid and sludge. The intermittent reverse flow described above ensures optimum mixing. The fact that the reactor  206  is not pressurized eliminates the problem of leaking bearings or the like. Further, the fact that the body  208  has an open top covered with a removable cover or splash shield enables the operator to view the mixing action within the body. 
     As seen in  FIG. 5 , the acidized sludge flows from reactor  206  to the pump  232  which then pumps the acidized sludge to the second reactor  242  wherein ammonia is metered thereinto by a suitable metering device. The treated sludge is delivered to a pugmill  254  and then to a rotary dryer for drying. After drying, the product is handled as described hereinbefore. 
     Thus it can be seen that the invention accomplishes at least all of its stated objectives.