Abstract:
The present invention provides a system that utilizes microwave heating to sterilize contaminated effluent. The system sterilizes the contaminated effluent by heating it to a minimum temperature for a minimum duration under pressure, and then discharging it to a sanitary drain as sterilized effluent. The process maximizes energy efficiency by using a heat exchanger to raise the temperature of the contaminated effluent with heat drawn from the sterilized effluent of the previous batch. All solids and particles originally present in the contaminated effluent pass through the system and are discharged as part of the sterilized effluent.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates to the sterilization of waste. In particular, this invention relates to a system for sterilizing contaminated effluent by heating it using microwave energy to a minimum temperature for minimum duration at a controlled pressure.  
       BACKGROUND OF THE INVENTION  
       [0002]     There has long been and continues to be a very significant need for effective means of destroying, sterilizing or otherwise neutralizing waste from a variety of industrial or institutional sources. In many instances, it is impractical or In the case of aqueous liquid waste where flow is more or less constant, it is undesirable and impractical to move these waste materials off-site for treatment. While there has and continues to be a number of means to heat waste water to the point of sterilization, most involve the use of steam heating and consume large amounts of energy to achieve these ends.  
         [0003]     For instance, scientific study of new species of pathogens requires source control of these pathogens after study in laboratories to prevent their spread into the surrounding environment. As biological safety levels are refined, there becomes an increasing need to process wastewater from all streams leaving the laboratories. Such sources include showers, toilet flushes, sink drains, laboratory sinks, laboratory equipment, and autoclaves. Present systems are unable to process liquid and solid waste without screening out the solid material.  
         [0004]     Recent published scientific work indicates that organisms suspended in water have a reduced tolerance to heat after exposure to microwave radiation.  
         [0005]     While microwave treatment has been proposed as a method of dealing with such waste, systems have not become widely available which are secure against leakage, environmentally acceptable and economically reasonable. For example, the systems must be sealed against leakage at all steps of the process to prevent not only against leaking of pathogenic containing liquids but also of microwaves. The systems must also be cost and energy efficient. Thus it is necessary to apply an appropriate amount of microwave energy, for a suitable time period, for the amount of waste being treated.  
         [0006]     Accordingly, there is a need for a system able to process liquid and solid waste to sterilize the waste. There is a further need for a system able to process liquid and solid waste that is energy efficient.  
       SUMMARY OF THE INVENTION  
       [0007]     The present invention provides a system that utilizes microwave heating, and resistance immersion heating coils, to sterilize biologically contaminated primarily liquid waste (“effluent”), which is essentially institutional sewage and wastewater from laboratory and greenhouse facilities (“contaminated effluent”). The sewage is initially ground in grinder pumps into particles less than a quarter inch in diameter. The process sterilizes the contaminated effluent by heating it to a minimum temperature for a minimum duration at a controlled pressure, and then discharges it to a sanitary drain as “sterilized effluent”. The process maximizes energy efficiency by using a heat exchanger to raise the temperature of the contaminated effluent with heat drawn from the sterilized effluent of the previous batch. The heat exchanger is designed to accommodate the residual particles in the ground sewage waste stream. All solids and particles originally present in the contaminated effluent pass through the system and are discharged as part of the sterilized effluent. The process is controlled and monitored by a programmable logic controller (PLC) and an array of instrumentation.  
         [0008]     The preferred embodiment is accomplished by providing a system for A system for sterilizing contaminated liquid and solid effluent comprising an inlet for receiving the contaminated effluent, pumping means to move the contaminated effluent and sterilized effluent within the system, a pressurized heating vessel for sterilizing the contaminated effluent, comprising heating means for heating the contaminated effluent to a minimum temperature for a minimum duration, the heating means comprising a plurality of magnetron assemblies for directing microwave energy into contaminated effluent in the vessel, a heat exchanger for raising the temperature of the contaminated effluent with heat drawn from the sterilized effluent comprising a first side and a second side, where heat is exchanged as the contaminated effluent moves through the first side and the sterilized effluent simultaneously moves counter-current through the second side and no cross-contamination occurs between the contaminated effluent and the sterilized effluent during the heat exchange, pressurizing means to maintain a pressure in both the pressurized heating vessel and the second side of the heat exchanger that prevents boiling of the contaminated effluent and the sterilized effluent at the minimum temperature, and an outlet for discharging the sterilized effluent. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]     In drawings which illustrate by way of example only a preferred embodiment of the invention,  
         [0010]      FIG. 1  is a schematic flow diagram of the system;  
         [0011]      FIG. 2  includes views of a general assembly drawing of an embodiment of the system;  
         [0012]      FIG. 3  includes views of a processing tank assembly drawing of an embodiment of the system;  
         [0013]      FIG. 4  includes views of an auxiliary skid of an embodiment of the system; and,  
         [0014]      FIG. 5  includes views of a heat exchanger assembly drawing of an embodiment of the system. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]     The present invention provides for the treatment of all waste including solid waste present in ordinary sewage. The sewage is ground by grinder pumps into particles less than a quarter inch in diameter. The ground sewage passes through a heat exchanger designed to accommodate this particle size in a waste stream. One particular use of the invention is in treating laboratory sewage where the effluent must be sterilized.  
         [0016]     According to the preferred embodiment shown in  FIGS. 1-5 , the system contains five main groups of equipment: a processing tank subsystem, an auxiliary skid (which includes a heat exchanger  300 , a discharge tank  200 , an air compressor, instruments and pumps), the electrical/support group, a sump tank group, and a main air compressor and filters. These groups are all linked through a distributed control system, embodied in a programmable logic controller (PLC), that coordinates the functions of all components, to ensure the safe and efficient treatment of the contaminated effluent.  
         [0017]     The processing tank subsystem includes a processing tank  100 , used to sterilize the contaminated effluent, a recirculation pump  130 , used to keep an even temperature in the processing tank  100 , a booster pump  140 , used to boost the contaminated effluent pressure into the processing tank  100 , fourteen (14) magnetron assemblies  110  and six (6) resistance heaters  120 , and an air conditioning unit  150 . The subsystem also includes all relevant support equipment: thermocouple temperature sensors, flow switches, humidity and temperature sensors, a pressure transducer, a pressure gauge, a radar level sensor  135 , microwave detectors  137 , resistance heaters  120 , band heaters  122 , air operated valves, isolation valves, non-return valves, a flow meter, a pressure release valve, drain valves, a solenoid valve and a pressure sensor.  
         [0018]     The auxiliary skid includes a discharge tank  200 , a heat exchanger  300 , pumps and all support equipment, instruments, and an air compressor. On its way to the processing tank  100 , the contaminated effluent from the sump tank  20  flows through the heat exchanger&#39;s first side, namely tube side  310 , where it gains heat from sterilized effluent flowing from the discharge tank  200  through the heat exchanger&#39;s second side, namely shell side  320 . After the contaminated effluent has been processed in the processing tank  100 , it flows to the discharge tank  200  located on this skid. The sterilized effluent remains there until it is pumped through the heat exchanger  300  to pre-heat the next incoming batch, losing heat to below the temperature required for discharge to the sanitary drain  450 . The auxiliary skid also contains two pumps, a transfer pump  190  and a discharge pump  210  which are used in transferring the sterilize effluent through the system and all associated piping and valves, as well as mixing/dilution pump  460 , required to mix cold tap water with the warm sterilized effluent, in case the exit temperature is above limit. The auxiliary skid also includes all relevant support equipment: thermocouple temperature sensors, pressure transducers, pressure gauges, a radar level sensor  136 , air operated valves, isolation valves, non-return valves, pressure release valves, a proportional valve  134 , pressure regulators, a back flow preventer, a globe valve, a drain valve, a static water mixer  340 , instruments, and an air compressor. Compressed air is provided to pressurize the processing tank  100  and the discharge tank  200 , as well as for the pressure test required for the heat exchanger  300 .  
         [0019]     The sump tank group includes the main in-ground sump tank  20 , two grinder pumps  40  and  50  and radar level sensor  30 . Contaminated effluent is collected and stored in the sump tank  20  prior to processing. Radar level sensor  30  is used by the PLC to calculate the fill level of the sump tank  20 . At the beginning of a processing cycle, one of the two submersible grinder pumps  40  or  50  is used to grind and pump contaminated effluent. The sump tank subsystem also includes all relevant support equipment, namely isolation valves and non-return valves.  
         [0020]     The electrical/support equipment group contains all electrical/control equipment, including the power supplies for the magnetrons, the PLC control system, the interface, all power and control wiring, and the electrical and control cabinet.  
         [0021]     The system of the preferred embodiment also includes isolation valves, air operated valves and various filters (coalescing, particulate, exhaust HEPA). All actuated and proportional valves in the system fail automatically to the normally closed (N/C) position.  
         [0022]     In the preferred embodiment, the contaminated effluent is sterilized by heating it to a minimum 285 degrees Fahrenheit (141 degrees Celsius) for at least 30 seconds. This is achieved in the processing tank  100  through the use of microwave heating and resistance immersion heating coils  120 . In the preferred embodiment, fourteen 2,450 MHz magnetron assemblies  110  are used to produce the microwaves. As this temperature is above the boiling point of water at normal atmospheric pressure, a pressure of 60 PSI is maintained to prevent boiling in the processing tank  100 , discharge tank  200  and the shell side  320  of the heat exchanger  300 . The required system pressure is provided through a combination of vapour pressure from the effluent and the application of regulated compressed air.  
         [0023]     Contaminated effluent is collected and stored in a sump tank  20 . A radar level sensor  30  located in the sump tank  20  monitors the level of effluent in the sump tank  20 . The system will not begin processing a batch unless there is sufficient effluent in the sump tank  20 . The system leaves a minimum of 24 inches of effluent in the sump tank  20  at all times; this is the minimum required effluent to ensure proper pump cooling.  
         [0024]     In the preferred embodiment, the system has two normal operating cycles:  
         [0025]     1. Start-Up/Loading/Discharge—contaminated effluent is transferred from the sump tank  20 , through the heat exchanger&#39;s tube side  310 , into the processing tank  100 , with sterilized effluent coming from the discharge tank  200 , moving counter-current through the heat exchanger&#39;s shell side  320  at the same time as the contaminated effluent.  
         [0026]     2. Sterilizing/Unloading—contaminated effluent is heated and sterilized in the processing tank  100  and then transferred to the discharge tank  200 .  
         [0027]     Apart from the initial start-up of the system or after a maintenance routine, the daily start-up condition will have a sterilized effluent load in the discharge tank  200 . The last load of each day is normally left in the discharge tank  200  to be transferred through the heat exchanger  300  concurrently to the loading of the first batch of contaminated effluent the following day.  
         [0028]     Cold, contaminated effluent from the sump tank  20  is loaded through the tube side of the heat exchanger  300 , while at the same time the hot, sterilized effluent from the discharge tank  200  is pumped through the shell side  320  of the heat exchanger  300 . The heat exchanger  300  has a dual purpose: it saves energy by pre-heating the contaminated effluent as it is loaded into the processing tank  100 , and it cools the sterilized effluent. If temperature of sterilized effluent, sensed by temperature sensor  330 , is still above the maximum allowed temperature (140 degrees Fahrenheit), it is diluted with cool tap water pumped through a static water mixer  340 , with the help of the mixing/diluting pump  460 .  
         [0029]     The control system will balance flow across the heat exchanger  300  by monitoring the rate of change in tank levels in the processing tank  100  and discharge tank  200 , and controls flow volume via the variable frequency drives on the effluent grinder pumps  40  and  50  and the discharge pump  210 .  
         [0030]     The control system includes safety interlocks, both mechanical and electrical, that prevent the process from continuing if the key subsystems or equipment are not functioning within acceptable parameters. As will be appreciated by the skilled worker, the use of such safety interlocks is well known in the art. An example of a mechanical interlock would be the door interlocks  350  on the processing tank comprising trip switches. An example of an electrical interlock would be a pressure measurement by a PLC that leads to the opening of a safety valve.  
         [0031]     The control system employs cycles that control various functions. Each cycle executes a number of steps in a specific sequence to attain a state in which the machine can load, process and then discharge the effluent. The control system uses a number of operating parameters based on information collected throughout the system, such as level, temperature and pressure readings. These values are used to control the motors, pumps, valves, etc.  
         [0032]     Various terms used to describe the operation of the control system are defined below:  
         [0033]     A cycle is a set of instructions or steps that are followed in sequence by the PLC. Each cycle controls a different function within the operation of the machine. There are two primary operating cycles following the system start-up, i.e.: Loading/Discharge and Sterilization/Unloading. Each cycle is limited by a number of conditions or interlocks and will only run if these conditions are met.  
         [0034]     A step refers to the specific instruction within a cycle. Only one step can be active while a cycle is active.  
         [0035]     An interlock is a condition that must be met before a cycle is permitted to start or must be maintained while the cycle is operating. If one or more interlocks are not met while the cycle is operating, the cycle will either pause and wait for instructions from the operator, or will carry out recovery steps. Each cycle will have a set of specific interlocks. Any given interlock may be used by more than one cycle. In the preferred embodiment, system interlocks, which are a number of conditions grouped to form a single main interlock, are included to simplify troubleshooting.  
         [0036]     An initial condition is a condition that must be met to initiate a cycle. This may include an operator initiation request. The initial conditions of some cycles do not include any operator interaction.  
         [0037]     Recovery refers to how a cycle will resume operating if it has paused. A cycle will pause if one of the steps is unsuccessful, the cycle is aborted by the operator, or if an interlock is not met. Most cycles will allow the operator to resume at the step at which it was paused, or to reset the cycle back to an idle state. Other cycles will automatically execute a number of steps and then return to an idle state.  
         [0038]     A cycle stop can be selected by the operator during any step in the operation of a cycle. The operator should only stop a cycle if it is necessary to adjust equipment before resuming the cycle.  
         [0039]     There are two start-up situations:  
         [0040]     1. At the beginning of regular daily operation, when the discharge tank  200  is full. A number of basic system checks (interlocks, faults, alarms) are performed automatically by PLC to ensure the process is ready to commence.  
         [0041]     2. After a complete system shutdown (power off), when the discharge tank  200  is empty. A more thorough diagnostic check of all pumps and valves takes place. This situation generally occurs after maintenance on the unit, or after an emergency shutdown, that requires the discharge tank  200  to be emptied. If all equipment tests are completed without alarm, and there is enough effluent in the sump tank  20  to begin processing, the system can commence the Loading/Discharge Cycle.  
         [0042]     Prior to starting the system, the operator conducts a visual inspection of the equipment and operating room to ensure there are no leaks. Once the operator manually starts the system, PLC takes over all operations. The PLC checks the fill status of the processing tank  100  and discharge tank  200 . The radar level sensor  30  in the sump tank  20  is checked to make sure there is enough effluent for processing. Valve  91  between the sump tank  20  and the processing tank  100  is opened. Grinder pumps,  40  and  50 , (alternating on daily loads) start. Booster pump  140  starts and valve  92  opens. Valves  101  and  102  open. Discharge pump  210  also starts. This creates flow on both the tube and shell sides of the heat exchanger  300 . Valve  93  opens as required to regulate the pressure in the processing tank  100 . Valve  103  opens if pressure in heat exchanger  300 , monitored by pressure transducer  305 , reaches a maximum set limit. The discharge pressure from the discharge tank  200  via the heat exchanger  300  is controlled using proportional valve  134 . This and the speed of grinder pumps  40  or  50  and booster pump  140  are adjusted by the PLC to ensure equal flow on both sides of heat exchanger  300  and that the temperature of fluid discharge at the sanitary drain  450  does not exceed 140 degrees Fahrenheit. If temperature of sterilized effluent, sensed by temperature sensor  330 , reaches a set maximum limit, the mixing/diluting pump  460  starts, mixing cold tap water in the effluent through the static water mixer  340 . Pressure in discharge tank  200  is maintained at 65 PSI through air operated valve  206 . Discharge pump  210  stops when sterilized effluent has been completely pumped out of discharge tank  200 , and then all air operated valves between the discharge tank  200  and the sanitary drain  450  are closed. Grinder pumps  40  or  50  and booster pump  140  stop when the processing tank  100  reaches operating capacity. Valves  91 ,  92  and  93  close. This cycle is completed and will return to idle status. The discharge tank  200  is empty and the processing tank  100  is full. The Sterilizing/Unloading cycle will start automatically when this step is reached.  
         [0043]     The Sterilizing/Unloading cycle sterilizes the contaminated effluent in the processing tank  100  by heating, using microwave generators and resistance heaters. The effluent is heated to minimum 285 degrees Fahrenheit and held for at least 30 seconds to ensure complete sterilization. Following sterilization, the effluent is pumped to the discharge tank  200 . The cycle steps are as follows: the PLC confirms the fill status of the processing tank  100 . The processing tank  100  is pressurized by opening air operated valve  104 , with initial pressure set at 30 PSI. Resistance heaters  122  and band heaters  122  are turned on. Power supplies are turned on energizing their respective magnetrons. Recirculation pump  130  is turned on to circulate the fluid in the processing tank  100 , creating a uniform heating of the contaminated effluent. The pressure in processing tank  100  nominally rises to 60 PSI during the heating process due to vapour pressure. The PLC monitors the pressure in processing tank  100  with a pressure transducer. Valves  103  and  104  are opened and closed as required, to regulate the pressure in the system. If the pressure rises above 60 PSI, the PLC operates air valve  103  to dump pressure back to the in-ground sump tank  20 . At 70 PSI pressure, an alarm is issued and the resistance heaters  122  and the band heaters  122  are turned off. If pressure reaches 85 PSI, the pressure relief valve  45  dumps pressure back to the in-ground sump tank  20 . During this step the Heat Exchanger Pressure Test Cycle is performed. When all thermocouple temperature sensors connected to the processing tank  100  nominally reach 285 degrees Fahrenheit, the cycle timer is started. If any thermocouple reading drops below this temperature, the cycle timer is reset and is started again when all readings have returned above 285 degrees Fahrenheit. The controller stays at this step until the cycle timer stays above this temperature for 30 seconds. When that occurs, the process cycle is completed and all power supplies and heaters are turned off. Recirculation pump  130  is also shut down. Valve  103  is closed. Sterilized effluent is transferred to the discharge tank  200  by opening valve  105 . Valve  109  is also opened to allow a balance of pressure between the processing tank  100  and discharge tank  200 . Transfer pump  190  is started to pump sterilized effluent from the processing tank  100  to the discharge tank  200 . This step continues until the processing tank  100  registers as empty using the radar level sensor  135 . Transfer pump  190  is stopped, and valves  105  and  109  between the processing tank  100  and discharge tank  200  are closed. Valve  104  controlling the compressed air intake of the processing tank  100  is also closed. The Sterilizing/Unloading Cycle is completed successfully and will return to idle status. The Loading/Discharge Cycle will start automatically when this step is reached, unless the operator has manually stopped the system, in which case the system pauses and awaits further command.  
         [0044]     The Heat Exchanger Pressure Test is run during the Sterilizing/unloading Cycle. The test ensures that there is no leak between the hot &amp; sterilized shell side  320  and the cold &amp; contaminated tube side  310  of the heat exchanger  300 . This is done by closing all valves leading into the heat exchanger  300 , pressurizing the shell side  320 , and then keeping track of the pressure on both sides of the heat exchanger  300 . If there is any change of pressure, a leak is detected and an alarm is set. The cycle steps in the heat exchanger pressure test are as follows: all valves leading to/coming from heat exchanger  300  are closed. The shell side  320  of the heat exchanger  300  is pressurized to 90 PSI by opening valve  108 . When 90 PSI is reached, valve  108  is closed. Pressure status timer is set for 30 minutes. During this time the controller keeps track of the pressure readings at the tube side  310  and the shell side  320  of the heat exchanger  300 . If pressure readings at the tube side  310  and the shell side  320  remain the same for the duration of the test, the pressure test is successful and the cycle moves to the next step. If the reading at the shell side  320  is falling and reading at the tube side  310  remains constant, there is a valve leak on the discharge side. A minor alarm is set specifying that valve maintenance is necessary, and the cycle continues to the next step. If the reading at the shell side  320  is falling and the reading at the tube side  310  is rising, there is a leak through the heat exchanger  300  and containment has been lost. A major alarm is set, and the system performs an emergency shutdown. If the Heat Exchanger Pressure Test Cycle is completed successfully, the system will return to idle status.  
         [0045]     If the Heat Exchanger Pressure Cycle is manually stopped by the operator, or any interlock condition is not met, the operator has the option to manually resume operation of the system. If the operator manually resumes operation of the system, the Heat Exchanger Pressure Test Cycle returns to the idle step until the initial conditions of the cycle are met. Note that a successful Heat Exchanger Pressure Test must be performed every cycle before the Processing Cycle can be run. If the cycle was cancelled before the test was completed successfully, the test must be run again before the system can continue processing the effluent.  
         [0046]     If a major alarm has been triggered, such as a tube side  310  leak, the system immediately performs an emergency shutdown. An emergency shutdown describes any stoppage of equipment and processing that is not part of a normal cycle process. This includes stopping a cycle before completion and shutting the whole system down because of a fault or alarm. An emergency shutdown can be initiated by an operator or by the control program.  
         [0047]     Various embodiments of the present invention having been thus described in detail by way of example, it will be apparent to those skilled in the art that variations and modifications may be made without departing from the invention. The invention includes all such variations and modifications as fall within the scope of the appended claims.