Abstract:
This invention provides a waste-processing system capable of processing high-solids wastes such as manure. This invention provides a compact U-shaped digester that allows for recycling of activated sludge to improve the efficiency of the process. Efficiency is also improved through a sludge heating design that creates a current in the digester and efficiently heats the sludge. A composter is provided to further process the sludge through aerobic digestion to create usable fertilizer. Finally, one embodiment provides a turbine that is fueled by biological gases from the digester to generate heat and electricity to be used by the system.

Description:
RELATED APPLICATION 
     This application is a divisional of application Ser. No. 09/534,116 filed on Mar. 23, 2000, which issued as U.S. Pat. No. 6,451,589 on Sep. 17, 2002, which claims the priority of U.S. provisional patent application, Ser. No. 60/161,246, filed Oct. 25, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The invention relates to waste-processing systems for processing manure. 
     2. Background Prior Art 
     Many prior art waste-processing systems are designed for low-solids waste, such as municipal waste, that has a solids content of approximately one percent. High-solids wastes such as manure that have a solids content of approximately twelve percent either clog the system or are insufficiently processed. The processing of high-solids waste has typically been performed using a plug flow process that is characterized by a straight-through system. 
     Prior art waste-processing systems for either high- or low-solids waste use large amounts of purchased energy in the form of electricity or natural gas to generate heat and run pumps to process the wastes because these systems typically exhibit inefficient heating of the waste as it is processed. In addition, prior art waste-processing systems have the added problem of disposing of the products of their processing. It is anticipated that stricter environmental regulations will limit the amount of waste than can be applied to fields as fertilizer because of the phosphates and nitrogen content of the waste. As fields reach their limits, other fields must be found. As the amount of unfertilized land dwindles, either other outlets for waste must be found, or a disposal method that meets the stricter environmental regulations must be developed and used. 
     SUMMARY OF THE INVENTION 
     The apparatus and method embodying this invention provide a waste-processing system capable of processing high-solids wastes such as manure. Total process flows are controlled in substantially-closed systems to minimize end waste products and maximize energy efficiency. The apparatus and method embodying this invention provide a compact U-shaped digester that allows for recycling of activated sludge to improve the efficiency of the process. Efficiency is also improved through a sludge heating design that creates a current in the digester and efficiently heats the sludge. Resource use is optimized to preclude the need to purchase outside energy, and to minimize the outflow of water that is unusable without further processing. For example, sludge is dried using waste heat from the processes, rather than using heat generated with energy from outside sources. Finally, a composter is provided to further process the sludge through aerobic digestion to create usable fertilizer, thus minimizing the output of unusable waste products. 
     A digester for processing high-solids waste is provided comprising a mixing chamber, a clarifier, and a generally U-shaped digester. The mixing chamber is located adjacent the clarifier such that activated sludge may be recycled to the mixing chamber. A heater is provided in the digester such that thermal agitation of the waste causes controlled mixing of wastes in the digester. 
     In another embodiment, gas jets are provided in the digester such that agitation of the waste by the gas jets causes the waste to be mixed. 
     In another embodiment, a gas turbine is provided that is fueled by biological gases produced in the waste-processing system. 
     In another embodiment, a composting tank mounted in a water tank is provided to aerobically digest the waste. 
     In another embodiment, the composter is replaced with a solids dryer using waste heat to dry the sludge. 
     In another embodiment, a combination of a fluidizing bed dryer and an air/air heat exchanger replaces the solids dryer to dry the solids and recapture heat produced by the turbines that would otherwise be lost in the turbine exhaust. The heated air in the fluidizing bed dryer evaporates water carried in the effluent from the solids press. The latent heat of vaporization carried by the moisture in the air leaving the fluidizing bed dryer is substantially recaptured in the water condenser. 
     Other features and advantages of the invention are set forth in the following drawings, detailed description and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of a waste-processing system embodying the invention. 
         FIG. 2  is a partial cross-section elevational view of the digester of the waste-processing system shown in  FIG. 1 . 
         FIG. 3  is a cross-section elevational view of a wall between a mixing chamber and the digester and taken along the  3 — 3  line of  FIG. 1 . 
         FIG. 4  is a partial cross-section elevational view of a clarifier, taken along the  4 — 4  line of  FIG. 1 . 
         FIG. 5  is a perspective view of a composter of the waste-processing system shown in  FIG. 1 . 
         FIG. 6  is a cross-sectional view of the composter taken along the  6 — 6  line in  FIG. 5 . 
         FIG. 7  is a flowchart of the process employed in the waste-processing system shown in  FIG. 1 . 
         FIG. 8  is a view similar to  FIG. 7  and shows an alternative process of the invention. 
         FIG. 9  is a view similar to  FIGS. 7 and 8  and shows another alternative process of the invention. 
         FIG. 10  is a view similar to  FIGS. 7–9  and shows another alternative process of the invention. 
         FIG. 11  is an enlarged view of a portion of the waste processing system shown in  FIG. 1 . 
     
    
    
     Before one embodiment of the invention is explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or being carried out in various ways. Also, it is understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including” and “comprising” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A waste-processing system  10  embodying the invention is illustrated in  FIGS. 1–10 .  FIGS. 1–6  show the apparatus in which the process is conducted. The system  10  is described in terms of processing manure, but may also be used to process wood pulp, municipal wastes, or organic waste products in general. 
       FIG. 1  shows schematically the apparatus used to process high-solids farm waste. A digester enclosure  20  includes three major sections: a mixing chamber  30 , a digester  40 , and a clarifier  50 . The digester enclosure  20  is arranged such that a relatively large digester  40  may be built in relatively small space. 
       FIG. 2  illustrates the construction of an outside wall  54  of the digester enclosure  20 . The height of the outer wall  54  of the digester enclosure  20  is approximately 17 feet, with a liquid depth  58  in the digester enclosure  20  of approximately 14 feet. A footing  62  provides an interface between the wall  54  and the ground  66 , and supports the wall  54  and the edge  70  of the floor  74 . Both the footing  62  and the wall  54  are constructed of poured concrete. The wall  54  is approximately twelve inches thick at the lower end  78  of the wall  54 , and approximately eight inches thick at the upper end  82  of the wall. The floor  74  of the digester enclosure  20  is approximately four inches of concrete. Insulation  86  with a thickness of approximately four inches is arranged below the floor  74  and provides an interface between the floor  74  and the ground  66 . 
     The roof  90  of the digester enclosure  20  is located approximately 15 feet, 8 inches above the floor  74  of the digester enclosure  20 . The roof  90  is constructed of an approximately ten-inch thickness  98  of SPANCRETE concrete topped by a four-inch thickness of insulation  94 . 
     A bio gas storage chamber  102  is located above the roof  90 . The primary component of the chamber  102  is a liner  106  including an upper liner section  110  and a lower liner section  114 . The liner  106  is preferably constructed from high-density polyethylene (HDPE), but may be any other suitable material. The liner  106  is sealed around the edges  118  of the liner  106  by capturing the edges  118  beneath six-inch channel iron  122 , which is removably attached to the digester enclosure walls  54  using nuts  126  on a plurality of anchor bolts  130  embedded in the digester enclosure wall  54 . A ten-inch PVC pipe  134  is inserted around the periphery of the chamber  102  within the liner  106  to assist in maintaining the seal around the periphery of the liner  106 . The liner  106  is constructed such that it can flexibly fill with bio gas as the bio gas is produced in the digester  40 , and can be emptied of bio gas as is needed. The bio gas storage chamber  102  may be replaced by any other suitable gas storage system including a roofed storage system. 
     Returning to  FIG. 1 , the mixing chamber  30  has horizontal dimensions of approximately 36 feet by 15 feet. Arranged within the mixing chamber  30  is approximately 2000 feet of four-inch black heating pipe  142 , which is designed to carry hot water to heat sludge  144  within the mixing chamber  30 . An influent pipe  148  carries manure  336  into the mixing chamber  30 . Mixing within the mixing chamber  30  is provided by both a mixing nozzle  145  on the end of an activated sludge recirculation pipe  147  and by convective flow resulting from the heating of the manure  336  by the heating pipe  142 . A standard auger  146  used for removing solids from the mixing chamber  30  is arranged near the floor  150  of the mixing chamber  30  such that it can transport solids from the floor  150  of the mixing chamber  30  through the wall  154  of the mixing chamber  30  and to a collection device  158 . In another embodiment (not shown), solids may be removed from the mixing chamber  30  by any other suitable system, such as a sump pump. 
     As illustrated in  FIG. 3 , a cutout  160  formed in the wall  162  between the mixing chamber  30  and the digester  40  allows sludge to flow from the mixing chamber  30  into the digester  40 . In addition, removable panels  161  are positioned to block opening  163  in the wall  162 . Removable panels  161  may be removed as needed to allow greater flow from mixing chamber  30  to digester  40 , if desired. 
     Returning to  FIG. 1 , the digester  40  is a generally U-shaped tank with overall horizontal dimensions of approximately 100 feet long and 72 feet wide. A center wall  165  approximately 90 feet in length divides the digester  40  into the two legs  166 ,  170  of the U-shape. Thus each leg  166 ,  170  of the digester  40  is approximately 100 feet long and 36 feet wide. 
     The first leg  166  of the digester  40  includes approximately 800 feet of four-inch black heating pipe  174  through which heated water or gas can flow. The heating pipe  174  is arranged along the center wall  165 . The second leg  170  of the digester  40  includes approximately 200 feet of four-inch black heating pipe  178 , which is also arranged along the center wall  165 . In another embodiment illustrated in  FIG. 11 , the heating pipes  174 ,  178  may include jet nozzles  180  to dispense heated gas into the sludge  144 . 
     In addition to producing activated sludge  184 , the anaerobic digestion of the digester  40  also produces bio gas in the form of methane gas, which is collected in the bio gas storage chamber  102 . Any liquid that condenses within the chamber  102  is directed through the effluent pipe  196  (see  FIGS. 7–9 ) to the liquid storage lagoon  198  (see  FIGS. 7–9 ). After a storage time of approximately twelve hours, the collected bio gas is used to fuel an internal combustion engine  138  (see  FIG. 7 ) that, in combination with an electric generator, is used to produce electricity that is sold to a power utility  332  (see  FIG. 7 ). The cooling system of the internal combustion engine  138  also produces hot coolant that is used for heating in the mixing chamber  30  and for heating and agitation in the digester  40 . Hot water from the engine  138  passes through an air/water cooler  334  (see  FIG. 7 ) to reduce the temperature of the water from the approximately 180° F. temperature at the exit of the engine  138  to approximately 160° F. for use in the mixing chamber  30  and the digester  40 . 
     As shown in  FIG. 1 , the clarifier  50  is located adjacent the digester  40  beyond clarifier panels  182  and adjacent the mixing chamber  30 . The clarifier  50  has horizontal dimensions of approximately 36 feet by 21 feet, and is largely empty of any equipment or hardware, with the exception of an equipment room  183 . Turning to  FIG. 4 , the clarifier panels  182  are constructed from HDPE and form a partial barrier between the digester  40  and the clarifier  50 . The clarifier panels  182  cover the entire horizontal dimension across the clarifier  50  from center wall  165  to outer wall  54 . Separation panels  186  within the clarifier  50  serve to direct solids in a downward direction to the bottom  190  of the clarifier  50 , where the solids collect in a sump  194 . Sump pipe  198  leads to a standard solids press  214  (see  FIGS. 7–9 ), and to the activated sludge recirculation pipe  147  carrying activated sludge  184  to the mixing chamber  30  (see  FIG. 1 ). 
     As illustrated in  FIGS. 7–9 , liquid produced as a result of the operation of the solids press  214  is recycled to the mixing chamber  30  for further processing. 
     Returning to  FIG. 4 , liquids in the clarifier  50  decant through gap  202  and collect in a liquid sump  206 . A liquid effluent pipe  210  within the liquid sump  206  leads through a heat exchanger  340  (see  FIG. 7 ) and to a liquid storage lagoon  198  (see  FIG. 7 ). 
     A composter  220  as illustrated in more detail in  FIGS. 5 and 6  is located downstream of the solids press  214 . The primary components of the composter  220  include a water tank  224  and a composting barrel  228 . The water tank  224  is generally a rectangular parallelepiped with six-inch-thick walls  230  constructed from concrete. A four-inch layer of insulation  232  (not shown in  FIG. 6 ) covers the periphery of the walls  230 . A sump  236  is located in the floor  240  of the water tank  224 . Extending through the floor  240  of the water tank  224  is an air supply pipe  244 . A port  248  in the first wall  252  of the water tank  224  accommodates a sludge supply pipe  256  that connects the solids press  214  with the composter barrel  228 . A port  260  in the second wall  264  of the water tank  224  accommodates a composter solids exit pipe  268 . 
     The water level  272  of the water tank  224  may be varied to provide buoyant support to the composter barrel  228 ; the water level  272  as illustrated in  FIGS. 5 and 6  is representative of a typical level. The water  276  is typically at 140–160° F. A water inlet pipe  280  provides a flow of water  276  to the composter barrel  228  and the water tank  224 . The water  276  is supplied from the cooler  334  of engine  138 . 
     The composter barrel  228  defines an interior chamber  232 . A sludge supply auger  284  is located within the sludge supply pipe  256  and extends from within the sludge supply pipe  256  into chamber  232  of the barrel  228 . A composted solids exit auger  288  extends from within chamber  232  of barrel  228  into the composter solids exit pipe  268 . Each pipe  256 ,  268  is connected to the ends  292 ,  294  of the composter barrel  228  using a double rotating union seal with an internal air pressure/water drain (not shown). The pipes  256 ,  268  and augers  284 ,  288  are designed such that air that is necessary for drying the sludge and for aerobic digestion may pass through the composter barrel  228 . Air passes through solids exit pipe  268  and air inlet pipe  266 , into the composter barrel  228 , and out through air outlet pipe  258  and sludge supply pipe  256 . The air pipes  258 ,  266  extend vertically to keep their ends  270  above the activated sludge  184  in the composter barrel  228 . 
     The composter barrel  228  is generally cylindrical and approximately 100 feet long and 10 feet in diameter. A plurality of wear bars  296  is attached to the exterior circumference of the barrel  228 . Rubber tires  300  acting on the wear bars  296  serve to hold the composter barrel  228  in position. 
     As illustrated in  FIGS. 5 and 6 , a plurality of vanes  304  is attached to the barrel  228 . These vanes  304  extend between the third and fourth wear bars  308 ,  312 . The vanes  304  are generally parallel to the longitudinal axis of the composter barrel  228 . As shown in  FIG. 6 , to effect cooperation with the vanes  304 , the water inlet pipe  280  and the air inlet pipe  244  are laterally offset in opposite directions from the vertical centerline of the composter barrel  228 . As a result, when water  276  flows from the water inlet pipe  280 , the water  276  collects on the vanes  304  on a first side  316  of the composter barrel  228 , and when air  320  flows from the air inlet pipe  244 , air  320  collects under the vanes  304  on a second side  318  opposite the first side  316  of the composter barrel  228 . The lateral imbalance resulting from weight of water  276  on the first side  316  of the barrel  228  and the buoyancy of the air  320  on the second side of the barrel  228  causes the barrel  228  to rotate in a clockwise direction as viewed in  FIG. 6 . 
     The composter barrel  228  is slightly declined toward the exit end  294  of the composter barrel  228  to encourage the activated sludge  184  within the composter barrel  228  to move along the longitudinal axis of the composter barrel  228  toward the exit end  294 . As shown in  FIG. 6 , the composter barrel  228  also includes internal baffles  296  that serve to catch and turn the activated sludge  184  as the composter barrel  228  rotates. 
     As illustrated in  FIG. 1 , the composter solids exit pipe  268  connects to a standard bagging device  324  that places the composted solids into bags  328  for sale. 
     In operation of the waste-processing system  10 , as illustrated in  FIGS. 1 and 7 , unprocessed cow manure  336  from area farms and other sources is transported to the waste processing site and transferred to a heat exchanger  340  where, if necessary, the manure  336  is thawed using warm water from the clarifier  50  by way of liquid effluent pipe  210 . 
     Manure  336  is then transferred from the heat exchanger  340  to the mixing chamber  30  through influent pipe  148 , where the manure  336  is mixed with activated sludge  184  recycled from the clarifier  50  by way of activated sludge recirculation pipe  147  to become sludge  144 . The sludge  144  is heated to approximately 100–130° Fahrenheit by directing coolant at approximately 160° F. from the engine cooler  334  through the mixing chamber heating pipes  142 . In addition, solids such as grit fall to the bottom of the mixing chamber  30  under the influence of gravity and are removed using the mixing chamber auger  146 . The solids are then transferred to a disposal site. 
     After a stay of approximately one day in the mixing chamber  30 , the sludge  144  flows through cutout  160  and opening  163 , if not blocked, in the wall  162  and into the digester  40 , where anaerobic digestion takes place. The activated sludge  184  added to the manure  336  in the mixing chamber  30  serves to start the anaerobic digestion process. 
     The apparatus and method described herein employ modified plug flow or slurry flow to move the sludge, unlike the plug flow in prior art systems. The digester heating pipes  174 ,  178  locally heat the sludge  144  using hot water at approximately 160° F. from the cooler  334  of the engine  138 , causing the heated mixed sludge to rise under convective forces. The convection develops a current in the digester  40  that is uncharacteristic of prior art high-solids digesters. Sludge  144  is heated by the digester heating pipes  174 ,  178  near the digester center wall  165 , such that convective forces cause the heated sludge  144  to rise near the center wall  165 . At the same time, sludge  144  near the relatively cooler outer wall  54  falls under convective forces. As a result, the convective forces cause the sludge  144  to follow a circular flow path upward along the center wall  165  and downward along the outer wall  54 . At the same time, the sludge  144  flows along the first and second legs  166 ,  170  of the digester  50 , resulting in a combined corkscrew-like flow path for the sludge  144 . 
     In another embodiment (not shown), hot gas injection jets using heated gases from the output of the engine  138  replace the hot water digester heating pipes  174 ,  178  as a heating and current-generating source. The injection of hot gases circulates the sludge  144  through both natural and forced convection. A similar corkscrew-like flow path is developed in the digester  40 . 
     In the arrangement shown in  FIG. 1 , the U-shape of the digester  40  results in a long sludge flow path and thus a long residence time of approximately twenty days. As the sludge  144  flows through the digester  40 , anaerobic digestion processes the sludge  144  into activated sludge  184 . The anaerobic digestion process also reduces the phosphate content of the liquid effluent by approximately fifty percent, which is a key factor in meeting future environmental regulations. 
     From the digester  40  the activated sludge  184  flows into the clarifier  50 . The clarifier  50  uses gravity to separate the activated sludge  184  into liquid and solid portions. Under the influence of gravity and separation panels  186 , the liquid portion rises to the top of the mixture and is decanted through a gap  202  into a liquid sump  206 . It is later transferred to lagoon storage  198  through effluent pipe  210 . The liquid is then taken from the lagoon  198  for either treatment or use as fertilizer. 
     The solid portion of the activated sludge  184  settles to the bottom  190  of the clarifier  50  in sump  194 . From there, approximately ten to twenty-five percent of the activated sludge  184  is recycled to the mixing chamber  30  through activated sludge recirculation pipe  147  to mix with the incoming manure  336 , as described above. The remaining approximately seventy-five to ninety percent of the activated sludge  184  is removed from the clarifier  50  through sump pipe  198  and is transferred to the solids press  214  in which the moisture content of the activated sludge  184  is reduced to approximately sixty-five percent. 
     From the solids press  214 , the activated sludge  184  is transferred through sludge supply pipe  256  using sludge supply auger  284  to the interior chamber  232  of the composter barrel  228  where the activated sludge  184  is heated and agitated such that aerobic digestion transforms the activated sludge  184  into usable fertilizer. Outside bulking compost material can be added to the chamber  232  to make the fertilizer more suitable for later retail sale. As the composter barrel  228  turns, baffles  296  within the chamber  232  agitate and turn the sludge. This agitation also serves to aerate the sludge to enhance aerobic digestion. At the same time, the tank of water  224  in which the barrel  228  sits heats the barrel  228 . This heating also promotes aerobic digestion. 
     In the preferred embodiment, water  276  falling from the water inlet pipe  280  and air  320  rising from the air inlet pipe  244  collects on the vanes  304  and causes the composter barrel  228  to turn around its longitudinal axis. In other embodiments, direct motor or belt drives, or any other suitable drive mechanism may turn the composter barrel  228 . 
     As the activated sludge  184  turns over and undergoes aerobic digestion in the chamber  232 , it also travels longitudinally and eventually exits the composter barrel  228  through the composter solids exit pipe  268 , driven by the composter solids exit auger  288 . The processed sludge, which has become usable fertilizer at approximately forty-percent moisture, is transferred to a bagging device  324 . In the bagging device  324 , the processed sludge is bagged for sale as fertilizer. 
     In an alternative embodiment illustrated in  FIG. 8 , a turbine  139  replaces the internal combustion engine as described above. The turbine  139  is preferably an AlliedSystems TURBOGENERATOR turbine power system as distributed by Unicom Distributed Energy, but may be any other suitable turbine. The turbine  139  is fueled by the methane collected in the bio gas storage chamber  102 . The differences with the use of a turbine  139  from the previously-discussed process are outlined as follows. Instead of an engine cooler  334  producing heated coolant, the turbine  139  produces exhaust gases at approximately 455° F. The hot exhaust gases are used to heat water in a closed loop  335  through an air/water heat exchanger  337 . The heated water is then used for heating in the mixing chamber  30  and for heating and agitation in the digester  40 . This embodiment is used in conjunction with a composter (not shown) as described above. 
     As shown in  FIG. 8 , the composter is replaced with a solids dryer  218  in which hot exhaust from the turbine  139  is used to dry the sludge taken from the solids press  214 . From the solids dryer  218 , the activated sludge  184  is transferred to a bagging device  324 . In the bagging device  324 , the processed sludge is bagged for sale as fertilizer. 
     In another embodiment illustrated in  FIG. 9 , hot exhaust gases from the turbine  139  are used to heat methane from the bio gas storage chamber  102  to approximately 160° F. in an air/air heat exchanger  220 . The heated methane is then injected into the mixing chamber  30  and the digester  40  for heating and agitation. In this embodiment, it is possible to seal off the digester  40  from any air contamination because only methane is used for heating and agitation. The methane is then recaptured in the bio gas storage chamber for reuse. This embodiment is used in conjunction with a composter (not shown) as described above. 
     In the embodiment illustrated in  FIG. 9 , the composter is replaced with a solids dryer  218  in which hot exhaust from the turbine  139  is used to dry the sludge taken from the solids press  214 . Again, from the solids dryer  218 , the activated sludge  184  is transferred to a bagging device  324 . In the bagging device  324 , the processed sludge is bagged for sale as fertilizer. 
     In still another embodiment illustrated in  FIG. 10 , a fluidizing bed dryer  350  takes the place of the composter or solids dryer described in previous embodiments. Pressed bio solids at approximately 35 percent solids from the solids press  214  enter the fluidizing bed dryer  350  where the solids are fluidized using heated air in a closed-loop air system  354 . This fluidizing results in moisture from the bio solids being entrained in the heated air. The moisture-laden heated air passes through a water condenser  358  where water is removed from the heated air and circulated back to the heating pipe  142  in the mixing chamber  30  and to the heating pipe  174  in the digester  40 . Heat is provided to the closed-loop air system  354  through an air/air heat exchanger  362 . Hot exhaust gases from a series of turbines  139  provide heat to the air/air heat exchanger  362 . The exhaust gases then enter the water condenser  358  to remove combustion moisture from the turbine exhaust before the remaining gases are vented to the atmosphere. The water condenser  358 , in addition to recapturing water, also recaptures heat carried by the turbine exhaust and by the heated air in the closed-loop air system  354 . This recaptured heat is used to heat the water circulating in the closed-loop water heating system. 
     The combination of a fluidizing bed dryer  350  and an air/air heat exchanger  362  recaptures heat produced by the turbines  139  that would otherwise be lost in the turbine exhaust. The heated air in the fluidizing bed dryer  350  evaporates water carried in the effluent from the solids press. The latent heat of vaporization carried by the moisture in the air leaving the fluidizing bed dryer  350  is substantially recaptured in the water condenser  358 . The closed-loop air system  354  allows for air with reduced oxygen content to be used in the fluidizing bed dryer  350  to reduce the risk of fire associated with drying organic material. In addition, the closed-loop air system  354  allows for the addition of an auxiliary burner (not shown) if needed to process wetter material in the fluidizing bed dryer  350 . A variable speed fan (not shown) can be added to the closed-loop air system  354  after the water condenser  358  to pressurize the air for the fluidizing bed dryer  350 . 
     In the embodiment illustrated in  FIG. 10 , from the solids dryer  218 , the activated sludge  184  is transferred to the bagging device  324 . In the bagging device  324 , the processed sludge is bagged for sale as fertilizer. 
     In another embodiment (not shown), the composter is replaced with a solids dryer  218  in which hot exhaust from the internal combustion engine  138  is used to dry the sludge taken from the solids press  214 . Again, from the solids dryer  218 , the activated sludge  184  is transferred to a bagging device  324 . In the bagging device  324 , the processed sludge is bagged for sale as fertilizer. 
     Various features of the invention are set forth in the following claims.