Abstract:
An apparatus for digesting waste comprises a tank, a tube, a propeller, and a trough. The tube is disposed within the tank. The propeller is disposed within the tube and configured to propel a waste material upward through the tube from a lower portion of the tank towards an upper portion of the tank. The waste material comprises heavy solids that fall to the lower portion of the tank and light solids that rise to a waste material top level located near a top of the tank. The trough is configured to collect the light solids that rise to the waste material top level. A top portion of the tube is disposed far enough below the waste material top level such that the heavy solids fall to the lower portion of the tank after being propelled up the tube rather than being propelled to at or above the waste material top level and collected by the trough.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 12/259,544, entitled “PORTABLE HIGH RATE ANAEROBIC DIGESTER APPARATUS AND METHOD OF OPERATION,” filed on Oct. 28, 2008, which is hereby incorporated by reference in its entirety. 
     
    
     BACKGROUND 
       [0002]    Farms, vegetable processing plants, meat processing plants, creameries and cheese processing plants, and restaurants have continually struggled with the disposal of organic waste. For example, pig farms produce tremendous amounts of pig manure. This manure has become the object of public controversy because of environmental concerns such as run-off pollution and odor. 
         [0003]    Traditionally, organic waste is disposed of by using it as fertilizer in a farming operation. However, environmental regulations now restrict the time of the year and weather conditions during which manure, especially liquefied manure, can be spread; and, in some cases, require the manure to be plowed in immediately which is not always possible. This requires the farmer to invest in costly manure storage units or to pay to have the manure removed. Regulations also restricts the amount of nutrients, particularly phosphorus, that can be applied in a given area of land, requiring farmers to either purchase more land or to pay other land owners to spread the excess manure. 
         [0004]    Anaerobic digestion is another method of disposing of manure containing high concentrations of organic matter. Through anaerobic digestion, large quantities of organic matter are broken down by bacteria that convert the organic matter into biogas. Anaerobic digestion is particularly suitable for disposing of liquefied manure containing high concentrations of organics. In addition, operators can also use or sell the biogas that is produced. In digested manure, nutrients are broken-down. Consequently, plants can absorb digested manure faster, thereby requiring less land to spread a given amount of manure. 
         [0005]    Traditional anaerobic digesters suffer from plugging where heavy and light solids that precipitate from the organic waste coalesce into obstructions that prevent the flow of the effluent produced during digestion. The plugging occurs frequently and becomes a nuisance for the operator. Consequently, the operator (farmer, owner, etc.) devotes a considerable amount of time towards maintaining the anaerobic digester. Plugging of the digester interrupts the steady flow of the digestion process disturbing and slowing down the bacterial activity. Additionally, unplugging a digester risks the removal of the mass of bacteria that digest the organic material. If the bacteria population is diminished, it will take days before the bacteria repopulates to an optimal level. Complicating matters, increasing the rate of anaerobic digestion increases the amount of organic waste processed, which increases the frequency and severity of plugging. 
         [0006]    Furthermore, traditional anaerobic digesters are typically semi-permanent installations and cannot be moved easily. To move a digester, a team would need to prepare a new site, disassemble the digester, and move the individual pieces to the new site. Then, an engineering team would reassemble the digester piece-by-piece. Consequently, it is expensive to move a digester. Thus, operations that are interested in using a digester only for a short time will be dissuaded from doing so. 
         [0007]    For example, a crop processing plant produces a tremendous amount of waste during the harvest; hence, the need for a digester is great. However, during the growing season the need for digesters is diminished. Despite the periodic need, the crop processing plant does not invest in a permanent installation because of the capital requirements. 
         [0008]    Therefore, there is a need for a high rate anaerobic digester that is portable, automated, and plugs infrequently. 
       SUMMARY 
       [0009]    In a representative embodiment, an apparatus for digesting waste comprises a tank, a tube, a propeller, and a trough. The tube is disposed within the tank. The propeller is disposed within the tube and configured to propel a waste material upward through the tube from a lower portion of the tank towards an upper portion of the tank. The waste material comprises heavy solids that fall to the lower portion of the tank and light solids that rise to a waste material top level located near a top of the tank. The trough is configured to collect the light solids that rise to the waste material top level. A top portion of the tube is disposed far enough below the waste material top level such that the heavy solids fall to the lower portion of the tank after being propelled up the tube rather than being propelled to at or above the waste material top level and collected by the trough. 
         [0010]    In another embodiment, A digesting system comprises a plurality of digester devices coupled together. Each of the digester devices comprises a tank, a tube, a propeller, a trough, and a chassis. The tube is disposed within the tank. The propeller is disposed within the tube and configured to propel a waste material upward through the tube from a lower portion of the tank towards an upper portion of the tank. The waste material comprises heavy solids that fall to the lower portion of the tank and light solids that rise to a waste material top level located near a top of the tank. The trough is configured to collect the light solids that rise to the waste material top level. The tank is mounted to the chassis. A top portion of the tube is disposed far enough below the waste material top level such that the heavy solids fall to the lower portion of the tank after being propelled up the tube rather than being propelled to at or above the waste material top level and collected by the trough. 
         [0011]    In yet another embodiment, a method of digesting waste comprises controlling a propeller within a tube disposed within a tank to propel a waste material upwards through the tube from a lower portion of the tank towards an upper portion of the tank. The waste material comprises heavy solids that fall to the lower portion of the tank and light solids that rise to a waste material top level located near a top of the tank. The method further comprises collecting the light solids that rise to the waste material top level in a trough. A top portion of the tube is disposed far enough below the waste material top such that the heavy solids fall to the lower portion of the tank after being propelled up the tube rather than being propelled to at or above the waste material top level and collected in the trough. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a representative block diagram of a digester system in accordance with a representative embodiment. 
           [0013]      FIG. 2  is a side view of a digester apparatus in the digester system of  FIG. 1  in accordance with a representative embodiment 
           [0014]      FIG. 3  is a flowchart of operations performed by a digester apparatus in accordance with a representative embodiment. 
           [0015]      FIG. 4  is a front view of a digester apparatus mounted on a chassis in accordance with a representative embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    A portable high rate anaerobic digester apparatus and method of operating are described. In the following description, for purposes of explanation, numerous specific details are set forth to provide a thorough understanding of exemplary embodiments of the invention. It will be evident, however, to one skilled in the art that the invention may be practiced without these specific details. The same number is used in the figures to represent the same element. The figures are not to scale. In other instances, well-known structures and devices are shown in simplified form to facilitate description of the representative embodiments. 
       Structure of the Digester Apparatus 
       [0017]    Referring to  FIG. 1 , a front view of a digester apparatus  100  in accordance with a representative embodiment is shown. The digester apparatus  100  includes a digester tank  101 . The digester tank  101  includes a fluid inlet  102 , a heavy solids outlet  103 , a drainage trough  104 , a vertical pump tube  105 , a maintenance porthole  109 , a transfer outlet  107 , and a biogas outlet  140 . The digester tank  101  is made out of steel; however, stainless steel, polyethylene, concrete, or other materials commonly used in vat construction can be used. The maintenance porthole  109  is used to access the digester tank  101  for maintenance purposes. The transfer outlet  107  is used to drain the digester tank  101  or transfer partially digested waste to another tank. The digester tank  101  also includes sampling ports and additional maintenance ports. 
         [0018]    In a representative embodiment, the digester tank  101  is approximately 40-50 feet tall and 12-14 feet wide. The digester tank  101  holds 30,000-40,000 gallons. The ratio of the side walls to the diameter of the digester tank  101  is preferably 2.5-4.5. The bottom of the digester tank  101  has a conical shape. 
         [0019]    The digester tank  101  is heated by a hot-water jacketed cone at the bottom of the digester tank  101  (not shown) supplied by a boiler (not shown). Placing heating elements outside of the tank eliminates internal obstructions which promote plugging. Alternatively, the digester tank  101  is heated by resistance-type heating elements wrapped around the body of the digester tank  101 . Heating can also be accomplished with fluid-based heat exchangers, circulating glycol or water heated by a waste gas burner, disposed as coils inside the digester tank  101 . 
         [0020]    The digester tank  101  is insulated (not shown). The insulation can be a insulating blanket, for example, made of fiberglass. Alternatively, the digester tank  101  is double-walled and the cavity between the walls is filled with a polyurethane or isocyanate-type expanding foam. 
         [0021]    During operation, the digester tank  101  is filled with an organic waste effluent such as manure. Bacteria, either added to the waste or naturally occurring in the waste, convert the manure into a number of products including biogas, water, light solids (specific gravity less than 1), and heavy solids (specific gravity greater than 1). The biogas degasses from the waste. The biogas produced is primarily methane (along with carbon dioxide and hydrogen sulphide) although designer bacteria can be introduced that produce other hydrocarbons. The light solids are typically animal bedding such as straw, sawdust, and other undigested animal food particles. The light solids rise to the top of the waste and heavy solids fall to the bottom of the waste. 
         [0022]    The digester tank  101  is air-tight. The biogas exits the digester tank  101  through a biogas outlet  140 . The biogas outlet  140  leads to a gas processing apparatus (not shown) that prevents backflow and regulates pressure. Biogas needs to be treated to remove water and other non-methane gases before it can be piped into the gas lines. The gas is then either used in powering the digester apparatus  100 , piped to other on-site operations, or refined and distributed. For example, the digester apparatus  100  can be connected to a gas pipeline network. 
         [0023]    The fluid inlet  102  is located on the side wall of the digester tank  101  underneath a digester tank waste material top level  190 . The fluid inlet  102  is connected to a pump  111  and a heater  115 . In a representative embodiment, waste material is pre-heated by heater  115  and pumped by pump  111  into the digester tank  101 . The heater  115  may be after the pump  111  or may be a combination of both before and after the pump  111 . In one embodiment, waste material is pumped into the digester tank  101  at a constant rate. Alternatively, waste material is pumped into the digester tank  101  on demand. The amount of waste pumped into the digester is measurable and controllable in order to control the hydraulic retention time. 
         [0024]    The heavy solids outlet  103  is located at the bottom of the digester tank  101 . The heavy solids outlet  103  is connected to the bottom of a waste column  125 . The waste column  125  is a vertical cylinder such as a pipe. The waste column  125  is arranged as a double valve chamber where a first waste valve  121  separates the heavy solids outlet  103  and the waste column  125 , and a second waste valve  127  separates the waste column  125  from a first lagoon outlet  129 . The first lagoon outlet  129  is connected to a hose or pipe that empties into a lagoon or holding pond. Alternatively, the first lagoon outlet  129  can lead to a holding tank which is emptied periodically. The waste column  125  should be at least a height  193  that is one gas trap submersion height  195  above the digester tank waste material top level  190  so that heavy solids do not spill out of the waste column  125 . The first waste valve  121  and the second waste valve  127  usually remain closed. Alternatively, the waste column  125 , the heavy solids outlet  103 , and/or the first lagoon outlet  129  have augers which can be used to move the heavy solids. 
         [0025]    When enough heavy solids have collected at the bottom of the digester tank  101 , the first waste valve  121  opens while the second waste valve  127  is closed. Hydraulic pressure forces the heavy solids into the waste column  125 . The first waste valve  121  closes and the heavy solids are now trapped in the waste column  125 . After the waste column  125  is full, the second waste valve  127  is opened while the first waste valve  121  remains closed. The heavy solids are forced out of the first lagoon outlet  129  and into a holding pond. 
         [0026]    Referring now to  FIG. 2 , a side view of the digester apparatus  100  of  FIG. 1  in accordance with a representative embodiment is shown. The drainage trough  104  is located near the top of the digester tank  101  so that the top of the drainage trough  104  corresponds with the digester tank waste material top level  190 . The drainage trough  104  is horizontal and offset from the center of the digester tank  101  so that a main drive shaft  151  can pass through the centerline of the digester tank  101 . The drainage trough  104  is U-shaped. Hence, when light solids rise to the top of the waste, which corresponds to the digester tank waste material top level  190 , the light solids fall into the drainage trough  104 . Likewise, when the level of the waste rises past the digester tank waste material top level  190 , the effluent at the top of the digested waste drains into the drainage trough  104 . The portion of the drainage trough  104  that extends beyond the digester tank  101  is air-tight so that the biogas cannot escape. 
         [0027]    Referring again to  FIG. 1 , an auger  160  runs across or near the center of the drainage trough  104 . The auger  160  is driven by an auger motor  165 . The auger  160  usually runs at a constant speed; however, the auger can be programmed to run periodically. When light solids fall into the drainage trough  104 , the turning auger  160  pushes the light solids to the end of the drainage trough  104 . Additional augers can be placed throughout the digester apparatus  100  wherever plugging is problematic. 
         [0028]    At the end of the drainage trough  104  the light solids and effluent of digested waste fall into a drainage tube  131 . The drainage tube  131  is connected to gas trap tank  130  where the light solids and effluent of digested waste are stored temporarily. The discharge end of the drainage tube  131  is submerged one gas trap height  195  into the liquid of the gas trap tank  130  in order to create a desirable operating pressure in the digester tank  101 . The effluent of the gas trap tank  130  is drained off the top of the gas trap tank  130  through a second lagoon outlet  137 . The second lagoon outlet  137  is connected to a hose or pipe that empties into a lagoon or holding pond. Alternatively, the second lagoon outlet  137  is connected to a holding tank that is emptied periodically. The drainage tube  131  can also have its own auger which prevents plugging. 
         [0029]    The gas trap tank  130  is equipped with a mixer  133 . The mixer  133  is driven by a mixer motor  135 . The mixer  133  is used to agitate the light solids and effluent of digested waste whenever more waste is pumped into the digester tank  101 . 
         [0030]    The digester tank  101  includes an agitation and skimming system in order to promote digestion and prevent clogging. The vertical pump tube  105  is a tube of steel or stainless steel positioned vertically in the center of the digester tank  101  by fins (not shown) that are welded to the inside of the digester tank  101 . Hence, organic waste and biomass can flow on the outside and inside of the vertical pump tube  105 . 
         [0031]    A propeller  157  is located inside of the vertical pump tube  105 . The propeller  157  is connected to the main drive shaft  151  driven by a main motor  150 . When propeller  157  spins, the waste material is pumped upward through the vertical pump tube  105  and allowed to return downward around the outer walls of the digester tank  101 . Hence, a convection-like current is created. The recycling movement of the organic waste maintains the heavier solids in suspension and improves bacteria contact with the organic waste material. Additionally, the propeller  157  can be designed to pulverize and mix the organic waste material as it passes through the vertical pump tube  105 . 
         [0032]    A skimmer motor  158  drives a skimmer drive shaft  152 . The skimmer drive shaft  152  and the main drive shaft  151  enter the digester tank  101  through a sealed coupling  153 . The skimmer drive shaft  152  is concentric with the main drive shaft  151 . The main drive shaft  151  typically spins much faster than the skimmer drive shaft  152 . The sealed coupling  153  also prevents biogas from escaping the digester tank  101 . The skimmer motor  158  and the main motor  150  can be run independently and at different, varying speeds. Alternatively, the main drive shaft  151  and the skimmer drive shaft  152  can be driven by the same motor; and relative angular velocity can be modified using a differential. 
         [0033]    The skimmer drive shaft  152  drives a skimmer  155 . The skimmer  155  is an arm or arms that skim the light solids off the top of the waste and into the drainage trough  104 . In a representative embodiment, the arms have brushes attached so that the skimmer  155  can reach below the digester tank waste material top level  190  while the skimmer  155  rotates. However, the skimmer  155  is adjustable and can be positioned at various levels relative to the digester tank waste material top level  190  and the top of the drainage trough  104 . Alternatively, the skimmer  155  can have a plain face or have a profile like a rake. The skimmer can be controlled to operate in either clockwise, counterclockwise, or alternating rotational direction to improve the removal of the lighter solids. 
         [0034]    The digester apparatus  100  is automatically controlled by a controller  180 . The controller  180  includes a programmable logic controller (PLC), motor contacts, motor controllers, relays, sensor inputs, outputs, and a human machine interface. The PLC operates the first waste valve  121 , the second waste valve  127 , the heater  115 , the pump  111 , the main motor  150 , the skimmer motor  158 , the auger motor  165 , and the mixer motor  135 . The PLC also controls the hot-water jacketed cone used to heat the digester tank  101 . The PLC also monitors the state of the digester apparatus  100  using temperature sensors, pressure sensors, encoders, and proximity sensors. In particular, the digester apparatus  100  includes a variety of sensors that test the waste itself. Consequently, the controller can determine when the waste has produced the optimum yield of biogas, for example, by determining the temperature, pH, specific gravity, ratio of substrates, and hydraulic flowrate of the organic waste and digested effluent, or the amount of gas produced. A user can also override the automatic controls. The user can also remotely monitor and control the system. Alternatively, the controller  180  can be any computer or circuit. 
         [0035]    Referring to  FIG. 3 , a front view of a digester apparatus  300  mounted on a chassis  310  in accordance with a representative embodiment is shown. The digester apparatus  300  is mounted on the steel chassis  310  so that the digester apparatus  300  can be easily constructed at a factory and moved to a operations site. Hence, the digester apparatus  300  is packaged as a self-contained module or modules. Smaller versions of the digester apparatus  300  mounted on the chassis  310  can be moved by a crane and a flatbed truck using lifting points  330 . Smaller versions of the digester apparatus  300  can be moved by a forklift and a trailer using skid holes  320 . 
         [0036]    Likewise, digester apparatus  300  can be easily broken into modules and split between a number of chassis. In a representative embodiment, a digester tank assembly comprising a digester tank and a waste column is 50 feet tall, including legs, and 14 feet by 14 feet wide. A separate module comprising a gas trap tank, a controller, pumps, heaters, and boilers is mounted on the steel chassis  310 . The digester tank assembly and module are shipped to a site on separate flatbed trailers. The digester tank assembly is removed and positioned upright. The module is placed in proximity of the digester tank assembly. The installation team simply connects power, control wiring, and piping for the digester tank assembly and the module. 
         [0037]    Hence, the digester apparatus  300  can be easily moved from site to site with minimal effort and without the support of an engineering team. The portability of digester apparatus  300  makes it easy for a factory or farm to rent a digester apparatus based on periodic need rather than invest in a permanent installation. Farmers, food processing plants etc. could also purchase the packaged digester through financing, using the digester as collateral. The digester would not become part of the real estate; it would be financed either through a bank or leasing company as a separate piece of equipment. 
         [0038]    Additionally, a plurality of self-contained modules can be ganged together where a large amount of waste needs to be processed. A master self-contained module can direct the activity of the plurality of self-contained modules. For example, the master self-contained module can control the overall waste intake and direct which slave module receives new waste in order to prevent a bacteria mass from dying off. 
         [0039]    In another embodiment, the gang of self-contained modules can also be placed in series where the output of one module is the input to the next module. Each self-contained module has a different kind of bacteria that performs a specific function, thereby increasing the efficiency and yield of the gang of modules. For example, bacteria X in module X works particularly well when the waste it digests is in a state Y. Bacteria W in module W produces a significant amount of biogas and a waste product in state Y. Hence the output of module W is fed into module X thereby increasing efficiency and yield of the overall system. Partially digested waste can be transferred out of a digester tank through a transfer outlet. Alternatively, waste captured by a gas trap tank can be used as input for another module. 
         [0040]    More particularly, separate self-contained modules can be devoted towards specific anaerobic digestion stages, namely: hydrolysis, acidogenesis and methanogenesis. In another embodiment, a first self-contained module specializes in hydrolysis and acidogenesis. A second self-contained module takes the output of the first self-contained module and specializes in the methanogenesis stage. 
       Operation and Control of the Digester Apparatus 
       [0041]    Referring to  FIG. 4 , a flowchart of the operation of a digester apparatus in accordance with a representative embodiment is shown. The digester apparatus is automatically controlled by a controller. The controller performs the operations based on the ability of the digester apparatus to accept more waste, and in no particular sequence. In a representative embodiment, the waste is dairy cow manure. However, any organic waste can serve as feedstock for the digester such as farm animal manure, waste products from vegetable processing plants, meat processing plants, creameries and cheese processing plants, restaurant cooking oil, grease waste, etc. 
         [0042]    In a check fluid level operation  410 , the controller determines if the digester tank is full by reading a fluid proximity sensor located in the digester tank. Alternatively, the fluid level in the tank can be determined using a float and switch. If the digester tank is not full, the controller tries to fill the digester tank with more waste. 
         [0043]    In a fill digester tank operation  415 , the controller first determines if there is feed waste to fill the digester tank with. If there is waste available, the controller activates the heater. Once the feed waste is at an acceptable temperature, the controller activates the pump. The digester apparatus can operate at either mesophilic (95-98° F.) or thermophilic (140-145° temperature range. The pump moves the heated waste into the digester tank. The controller also takes into account that the gas trap tank must be charged with effluent (raw organic waste). The controller also activates the mixer in the gas trap whenever the tank is charged with organic waste in order to assimilate overflow from the digester tank caused by additional filling. 
         [0044]    After the digester tank is filled, the heater and pump continue to supply heated waste to the digester tank at a controlled feed rate. Feed rates can vary in order to obtain a hydraulic retention time from 5 to 20 days based on the quality of the waste and the mass of bacteria available to digest. The controller can dynamically alter the retention time based on how quickly the waste is digested. 
         [0045]    The amount organic waste pumped into the digester relates to the digester retention time: (Digester Volume)/(Volume/Day)=Retention Time in Days. As raw manure is pumped into the digester, the waste level will rise and the digested effluent will flow into the drainage trough thereby maintaining the desired operating level. A float, proximity switch, ultrasonic sensor, or other kind of fluid level switch can sense an over fill situation that indicates a plug. 
         [0046]    As the waste digests, heavy solids that have a specific gravity higher than water fall out of suspension and accumulate in the cone at the bottom of the digester tank. After a sufficient amount of heavy solids have accumulated, in a heavy solids purge operation  420 , the controller moves the heavy solids to the waste column. The heavy solids purge operation  420  is designed to retain as much active bacteria as possible, thereby reducing the hydraulic retention time. As described above, the waste column is arranged as a double valve chamber. 
         [0047]    During normal operation, the first waste valve and the second waste valve of the waste column remain closed. When controller determines that enough heavy solids have accumulated, through time and measurement, the controller opens the first waste valve. The accumulated heavy solids are forced up into the waste column through the hydraulic force of the waste material in the digester tank. The controller closes the first waste valve after the fluid levels in the waste column and digester tank are in hydraulic equilibrium. It may be desirable to leave some heavy solids in the bottom of the digester tank in order to maintain a seal that will prevent effluent from leaving the digester tank. Alternatively, the controller may use a number of iterations to fill the waste column. 
         [0048]    After the waste column is full, the controller opens the second waste valve. The captured heavy solids are forced out of a first lagoon outlet and into a lagoon or holding pond. Alternatively, the first lagoon outlet can lead to a holding tank which is emptied periodically. Afterward, the controller closes the second waste valve. 
         [0049]    In a recycle operation  425 , the controller activates the propeller. The controller runs the main motor constantly or periodically. When the propeller spins, the organic waste material is pumped upward through the vertical pump tube and allowed to return downward around the outer walls of the digester tank. The controller also varies the pump speed to obtain the optimum internal hydraulic velocity in order to maximize the amount of solids in suspension. The recycling movement of the organic waste maintains the heavier solids in suspension and improves bacteria contact with the organic waste material. Lighter, fibrous material floats to the top of the digester tank. 
         [0050]    In another embodiment, the propeller can be programmed to create different suspension profiles. For example, the blades of the propeller can be programmed to act similarly to the agitator of a washing machine. Hence, various agitation cycles can be created to handle different types and qualities of waste. 
         [0051]    In a skim operation  430 , the controller activates the skimmer and the auger. The accumulation of the lighter, fibrous material is removed by a skimmer with two rotating sweep arms. The skimmer moves over the top of and against the floating solids thereby pushing the solids into the discharge trough. The skimmer can be rotated at various speeds and in different directions. In a representative embodiment, the skimmer is rotated in a first direction alternating with a second direction. In another embodiment, the skimmer is rotated for an extend period of time in one direction and periodically reversed to knock accumulated waste off of the skimmer. In another embodiment, the skimmer is programmed to flick accumulated waste off of the skimmer by quickly running the skimmer blades back and forth over the edge of the drainage trough. 
         [0052]    In the skim operation  430 , the controller also activates the auger motor. Whenever the skimmer is running, the auger should also run in order to prevent plugging of the discharge trough. As the solids enter the discharge trough, the auger pulls the solids towards the discharge tube where the solids fall into the gas trap tank. The auger can be reversed to un-jam plugs. 
         [0053]    Advantageously, the check fluid level operation  410 , the fill digester tank operation  415 , the heavy solids purge operation  420 , the recycle operation  425 , and the skim operation  430  work together to increase the rate of anaerobic digestion and keep the digester apparatus unplugged. Additionally, the digester apparatus processes organic waste automatically at a high rate and plugs infrequently. Additionally, the digester apparatus can be easily manufactured at a factory and then be moved easily from site to site. 
         [0054]    The foregoing description of representative embodiments has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the present invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the present invention. The embodiments were chosen and described in order to explain the principles of the present invention and its practical application to enable one skilled in the art to utilize the present invention in various embodiments and with various modifications as are suited to the particular use contemplated.