Abstract:
A waste treatment system comprises a first heat exchanger positioned before one or more reactors. The reactors discharge treated sludge to a second heat exchanger. The first and second heat exchangers share a heat transfer medium wherein the heat from the treated sludge is transferred to the incoming sludge. The treated sludge is cooled to reach an optimal temperature range before entry into a digester. The system also is adapted to allow a pipe cleaning device or “pig” to clean the heat exchangers by being forced through the heat exchangers by pressurized and pasteurized wastewater.

Description:
FIELD OF THE INVENTION 
     The present invention relates to sludge treating systems and more particularly to a predigestion pathogen reduction system. 
     BACKGROUND OF THE INVENTION 
     It is common practice to treat sludge for pathogens by heating the sludge and holding the heated sludge in one or more reactors for a certain time period. Typically incoming sludge is heated to approximately 70° C. and held in one or more reactors for approximately one hour. After being held in the reactor or reactors, the sludge is cooled and directed into a digester where the sludge is held for a selected time period. Further, in treating sludge for pathogens, it is known to heat the incoming sludge and cool the treated sludge by a network of heat exchangers. Essentially the heat exchangers remove heat from the treated sludge and add heat to the incoming sludge. 
     Cooling the treated sludge to a selected temperature is quite important. This is because the treated sludge is directed to a digester that is held at a certain temperature and if the treated sludge is of a temperature that substantially departs from the design temperature of the digester, then this can substantially and adversely affect the digestion process. Usually, in the case of a mesophillic digester, the sludge held therein is maintained at a temperature of approximately 35° C. To avoid adversely impacting the sludge held in the digester, it is common practice to attempt to cool the treated sludge leaving the reactors to a target temperature range of approximately 34°-38° C. 
     In most pathogen reduction systems, it is virtually impossible to precisely control the temperature of the treated sludge being directed to the digester. In some cases, the heat exchanger network utilized relies on sludge-to-sludge heat exchangers, that is, the treated sludge leaving the reactor or reactors is circulated through a heat exchanger through which the incoming sludge passes. Thus the degree of heat transfer that takes place is dependent in large part on the flow rates of the incoming sludge and the treated sludge. Suffice it to say that there is very little practical opportunity to precisely control the temperature of the treated sludge, especially in cold climates. As a result, many pathogen reduction systems produce a treated sludge that, although subject to passing through one or more heat exchangers, fails to meet the target temperature range prior to introduction into a digester. Indeed, the problem is significant because of the lack of control of the sludge and/or heat transfer medium passing through the heat exchangers and the fact that the systems are designed for constant flow rates for all seasons of the year. Thus while it may be easy to meet the targeted temperature range in summer months for a given system, it may be quite difficult to meet the targeted temperature range using that same system during winter months. 
     Furthermore, in sludge treatment systems the heat exchangers tend to accumulate a build up of material such as grease foulings and other deposits within their inner walls as a result of the continuous passing of sludge through the heat exchangers. Eventually the heat exchangers must be cleaned. Typically this requires that the sludge treatment process be halted in order that the individual heat exchangers can be cleaned. The cleaning process for heat exchangers is time consuming and laborious. Not only is it costly in terms of labor to clean heat exchangers, but the fact that the sludge treatment process has to be shut down results in an even greater cost. 
     SUMMARY OF THE INVENTION 
     The present invention entails a system and method for reducing pathogens in sludge prior to the sludge reaching a digester. The method entails directing incoming sludge through a first heat exchanger which heats the sludge prior to the sludge being directed into one or more reactors where the sludge is held at an elevated temperature for a selected time period. After the selected time period has elapsed, the treated sludge is then directed from the reactor or reactors to and through a second heat exchanger (which may include a number of sections) that effectively cools the sludge before the sludge is introduced into a digester. In one embodiment of the present invention, there is provided a closed loop conduit that channels a heat transfer medium through the first and second heat exchangers. The heat transfer medium effectively removes heat from the treated sludge and adds heat to the incoming sludge. In order to more precisely control the temperature of the treated sludge, a variable speed pump is associated with the closed loop heat transfer medium for pumping the heat transfer medium through the first and second heat exchangers. By controlling and varying the flow rate of the heat transfer medium, it is appreciated that the temperature of the treated sludge exiting the second heat exchanger and directed to the digester can be more precisely controlled to meet a target temperature range. 
     Further, additional temperature control can be realized by employing other heat exchangers within the pathogen reduction system. For example, the treated sludge may be directed through another cooling heat exchanger where the cooling medium is treated wastewater. In order to vary the heat transfer in this case, a variable speed pump can be utilized to pump the treated wastewater through the cooling heat exchanger so as to remove heat from the treated sludge. A programmable controller may be operatively connected to the variable speed pump for varying the flow rate of the treated wastewater through the heat exchanger so as to control the temperature of the treated sludge prior to it entering into the digester. In one embodiment of the present invention, a single programmable controller can be utilized to control a series of heat exchangers so as to optimize the amount of heat transferred to the incoming sludge while at the same time adjusting the temperature of the treated sludge such that it meets the temperature target range established for the treated sludge before it enters the digester. 
     Another aspect of the present invention entails a method and system for cleaning the heat exchangers. In this regard, a fluid such as treated wastewater is pasteurized and held within a supply tank. A pump is utilized to pump this pasteurized fluid to a point where it contacts a pipe cleaning device and then the pressurized fluid is used to drive or move the pipe cleaning device through one or more heat exchangers associated with the pathogen reduction system. As the pipe-cleaning device is driven through the heat exchanger, it along with the pressurized fluid cleans deposits such as grease foulings from the interior of the heat exchanger. 
     Other objects and advantages of the present invention will become apparent and obvious from a study of the following description and the accompanying drawings, which are merely illustrative of such invention. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a schematic illustration of the predigestion pathogen reduction system of the present invention. 
     FIG. 2 is a schematic illustration depicting a portion of the automatic heat exchanger cleaning system. 
     FIG. 3 is a cross sectional view illustrating a portion of the heat exchange network used in the pathogen reduction system. 
     FIG. 4 is a schematic illustration showing the mechanism utilized for automatically cleaning the heat exchangers. 
     FIG. 5 is a flow chart that illustrates the control steps employed by a controller in controlling the flow of a heat transfer medium through selected heat exchangers employed within the pathogen reduction system. 
     FIG. 6 is a schematic illustration of the heat exchangers and the controller for controlling the flow of heat transfer mediums through the respective heat exchangers. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Turning now to the drawings, FIG. 1 shows a pathogen reduction system  10 , which is designed to treat sludge and to remove or reduce pathogens, such as bacteria, viruses, etc., from the sludge. Generally, the sludge being treated by the pathogen reduction system  10  constitutes either primary or secondary sludge or both. Those skilled in the art will appreciate that primary sludge is typically separated from influent wastewater during the course of primary treatment. Secondary sludge, on the other hand, is separated from the wastewater during the course of secondary treatment. 
     In conventional sludge treatment, both the primary and secondary sludge is combined, and after being combined, is directed to a digester such as anaerobic or aerobic digester. Once in the digester, the sludge is held for an extended time, sometimes on the order of approximately 30 days, and at a selected temperature level, which is typically about 34°-38° C. During the process, the digester performs two important functions. First, the digester reduces the number of pathogens such as bacteria, viruses, etc. Secondly, the digester removes or at least substantially reduces, volatile solids found in the sludge. 
     In the sludge treatment process of the present invention, pathogen reduction is separated from the treatment for volatile solids. In fact, pathogen reduction occurs outside of the digester. In the preferred process illustrated and discussed herein, the pathogen reduction system  10  reduces the pathogens in the sludge prior to the sludge being admitted to the digester  104 . But after the pathogens have been reduced in the sludge, the sludge is then directed to a digester  104  which treats the sludge over a period of approximately 20-22 days so as to remove volatile solids from the sludge. 
     Turning to the pathogen reduction system of the present invention and with reference to FIG. 1, it is appreciated that sludge is directed into and held within a sludge holding tank  12 . The sludge directed into the holding tank  12  may be primary sludge or a biological sludge, which is sometimes refereed to as waste activated sludge, or a combination of both. Extending from the sludge holding tank  12  is an incoming sludge line  16 . Disposed in the incoming sludge line  16  just down from the holding tank  12  is a sludge macerator  15  that chops and grinds the incoming sludge into a fine consistency. Sludge from the macerator  15  is directed to a variable speed pump  18  and disposed downstream of the pump is a flow meter  20 . A control loop indicated generally by the numeral  22  basically controls and varies the flow of sludge from the holding tank  12  through the incoming sludge line  16 . As a part of the control loop, there is provided a level indicator LIC that senses the level of sludge in the holding tank  12 . Further, a pressure sensor P is connected in the incoming sludge line  16  between the variable speed pump  18  and the flow meter  20 . The control loop  22  is tied to the level indicator LIC, pressure sensor P, flow meter  20  and the variable speed pump  18 . Accordingly, the speed of the variable speed pump  18  is controlled and adjusted, in part at least, in accordance with the level of sludge in the holding tank  12 . 
     Disposed downstream of the flow meter  20  is a first heat exchanger indicated generally by the numeral  28 . The first heat exchanger is comprised of the incoming sludge line  16  and a closed loop conduit  90  that channels a heat transfer medium such as water through the first heat exchanger in a heat exchange relationship with the incoming sludge. As will be appreciated from subsequent portions of this disclosure, the heat utilized for this purpose is transferred to the heat transfer medium from the treated sludge in a heat exchanger relationship within heat exchangers  86  and  88 . 
     Disposed on opposite sides of the first heat exchanger is a pair of temperature indicators TI. The temperature indicators function to sense the temperature of the incoming sludge on opposite sides of the first heat exchanger  28 . 
     The incoming sludge line  16  leading from the first heat exchanger  28  is directed through a control valve  32  into another heat exchanger, indicated generally by the numeral  34  sometimes referred to as a heating heat exchanger. Heat exchanger  34  is operatively connected to a boiler  36 . Hot water from the boiler is directed through lines  38  and  40  via a pump  42  into and through the heat exchanger  34 . Operatively associated with the heating heat exchanger  34  is a temperature controller TIC that is coupled to a control valve  44  disposed in line  40 . Temperature controller TIC senses the temperature of the incoming sludge on the outlet side of the heat exchanger  34  and in response to the sensed temperature controls the flow rate of hot water from the boiler  36  to the heat exchanger  34 . In the case of a preferred design, the temperature of sludge exiting the heat exchanger  34  should be approximately 70° C. Consequently if the temperature falls below or above that targeted temperature, the controller TIC acts to increase or decrease the flow rate of hot water to the heat exchanger  34  so as to meet that target temperature. 
     From the heating heat exchanger  34 , the incoming sludge line  16  leads to a series of reactor inlet lines  48 ,  50  and  52 . Each of these inlet lines includes a control valve. Further the inlet lines  48 ,  50  and  52  lead to a series of three reactors  56 ,  58  and  60 . Incoming sludge, after it has been heated to the target temperature, is directed into each of these reactors. During the course of treatment, one of the reactors will be filling, while another one of the reactors will be discharging, and while the third reactor will simply be in a holding state. The reactors accumulatively act to hold the heated incoming sludge for a selected time period, for example one-hour, and to hold the sludge at an elevated temperature so as to reduce the pathogen concentrations within the sludge. For a more complete understanding of the function and role of the reactors  56 ,  58  and  60 , one is referred to the disclosure found in U.S. patent application Ser. No. 08/966,728, filed Nov. 10, 1997, now U.S. Pat. No. 6,117,203, and entitled Pathogen Reduction System Used in Treating Wastewater, the disclosure of which is expressly incorporated herein by reference. 
     The reactors are connected to a series of outlet lines  62 ,  64  and  66 . Each outlet line includes a control valve. As seen in FIG. 1, the sludge leaving the reactors, referred to as treated sludge, is directed through the respective outlet lines into a treated sludge line  70 . Once the treated sludge is directed into the treated sludge line  70 , the treated sludge is directed through a variable speed pump  72  and a flow meter  74 . A control loop is provided for controlling the flow of treated sludge through line  70 . In particular, the control loop is connected to the variable speed pump  72  and flow meter  74 . The flow rate of sludge through line  70  is controlled such that the flow rates of the sludge in the incoming line  16  and the treated sludge line  70  are generally equal. Thus from a control point of view, the variable speed pumps  72  and  18  are linked such that together the sludge flow rates in lines  16  and  70  are generally equal. However, the control loop would control the flow in each of these lines in accordance with the level indicator LIC associated with the holding tank  12 . 
     Downstream from the flow meter  74  there is provided a cross line  80  that extends between the treated sludge line  70  and the incoming sludge line  16 . A control valve is positioned in that line. This permits the operator in certain selected cases to return a batch volume of inadequately treated sludge back to the hot water heat exchanger  34 . This permits previously treated sludge to be retreated in the reactors  56 ,  58  and  60 . 
     The treated sludge line  70  extends from the flow meter  74  to a control valve  84 . Treated sludge passing through the control valve  84  is then routed through a second heat exchanger that in the case of the present embodiment includes two heat exchanger sections  86  and  88 . It will be appreciated that the second heat exchanger could simply be comprised of a single heat exchanger or multi-sections. In any event, there is provided a closed loop conduit  90  that extends through the first heat exchanger  28  and through the heat exchangers sections  86  and  88 . As will be discussed subsequently herein, the closed loop conduit extends around the incoming sludge line  16  within the first heat exchanger  28  and around the treated sludge line  70  passing through the heat exchanger sections  86  and  88 . Forming a part of the closed loop conduit  90  is a variable speed pump  92  and a flow meter  94 . Contained within the closed loop conduit  90  is a heat exchange medium such as water. The variable speed pump  92  pumps the heat exchange medium through the closed loop conduit  90  in a direction that is counter to both the flow of incoming sludge within line  16  and the flow of treated sludge in line  70 . To control the rate of heat transfer taking place within heat exchangers  28 ,  86  and  88 , the variable speed pump  92  is coupled to a controller  96 . Controller  96 , as illustrated in FIG. 6, effectively controls the speed of the variable speed pump  92  so as to optimize the heat transfer to the incoming sludge and yet maintain the temperature of the treated sludge at or within a target temperature range before it is introduced into the digester  104 , or at least reduce the temperature of the treated sludge to some degree so that further cooling is minimal. Subsequent portions of this disclosure will discuss this control feature in more detail. 
     Disposed on opposite sides of the heat exchanger sections  86  and  88  is a pair of temperature indicators TI. Branching from the treated sludge line  70  downstream of the heat exchanger section  88  is a secondary sludge outline  100 . A control valve  102  is disposed in the outlet line  100  and as seen in FIG. 1 the outlet line  100  leads to the digester  104 . 
     Disposed about the sludge treatment line  7 O downstream of the second heat exchanger (heat exchange sections  86  and  88 ) is a cooling heat exchanger indicated generally by the numeral  106 . If the cooling requirement of the treated sludge cannot be met by heat exchanger sections  86  and  88 , then the cooling heat exchanger  106  further acts to cool the treated sludge. This heat exchanger further acts to cool the treated sludge. In this case, lines  108  and  110  connect to the cooling heat exchanger  106  and one of those lines is connected to a source of water or treated wastewater. A pump  112  is communicatively connected to line  108  and functions to pump treated wastewater into and through the cooling heat exchanger  106 . Consequently, treated wastewater is directed through the cooling heat exchanger in a heat exchange relationship with the treated sludge so as to further cool the treated sludge. A pair of temperature indicators TI are disposed on opposite sides of the cooling heat exchanger  106 . 
     Downstream from the cooling heat exchanger  106  there is provided a primary sludge outlet line  114 , that includes a control valve  116 , that branches off from the treated sludge line  70 . Line  114  joins line  100  and consequently functions to direct sludge from the cooling heat exchanger  106  to the digester  104 . 
     Digester  104  is designed to perform a mesophilic digestion process and as such, the treated sludge will remain in the digester approximately 20-22 days and during this time period the sludge held therein will be maintained at a temperature of approximately 35° C. 
     Digested sludge is directed from the digester  104  through a digester outlet line  120  that branches into lines  122  and  124 . To transfer the digested sludge to a holding tank  136  there is provided a variable speed pump  126  that directs the sludge from the digester  104  through line  120  and through a flow meter  128 . To control the speed of the variable speed pump  126  and the flow rate of sludge through line  120 , there is provided another control loop  130 . This control loop basically ties the variable speed pump  126  with a level indicator LIC, a pressure gauge or sensor P and the flow meter  128 . Thus the flow rate of sludge through line  120  to the holding tank  136  is a function of the level of sludge within the digester  104  and the pressure and flow rate sensed by the pressure gauge  134  and the flow meter  128 . 
     As noted above, in the embodiment illustrated herein, the digester  104  is designed to perform a mesophilic digestion processes. However, the pathogen reduction system  10  of the present invention is provided with the capability of optionally directing the treated sludge to a thermophilic digester  140 . As illustrated in FIG. 1, there is provided a pair of lines  142  and  144  that extend from the sludge treatment line  70 , between heat exchanger sections  86  and  88 , to the thermophilic digester  140 . Here the temperature of the treated sludge would be maintained at approximately 55° C. for a period of approximately 10-12 days. Because of the higher temperature, it is appreciated that the treated sludge does not have to be cool to the degree required when operating the mesophilic digester  104 . 
     In a preferred embodiment, the respective heat exchangers  28 ,  34 ,  86 ,  88 , and  106  are designed such that the flow of sludge is always counter to the flow of the heat transfer medium or other fluid that is flowing in heat exchange relationship with the sludge. For example, as illustrated in FIG. 1, sludge flows from left to right through the heat exchanger  28  while the heat transfer medium flowing in line  90  flows right to left through the same heat exchanger. Generally, the sludge line passing through the respective heat exchangers is disposed in a serpentine configuration in that it zigzags back and forth through the heat exchanger. 
     FIG. 3 illustrates a portion of heat exchanger  28 . Note that the sludge line  16  enters the inlet side of the heat exchanger  28  and joins with a sludge segment  150 . The sludge segment extends a selected length and connects with an elbow  152  that connects with a succeeding sludge segment  150 . Thus, the heat exchanger  28  includes a series of stacked sludge segments  150  that are joined together by a series of elbows  152 . Thus, it is appreciated that the incoming sludge is channeled back and forth through the respected sludge segments  150 . 
     Disposed around each sludge segment  150  is an outer conduit  154 . The outer conduit  154  is designed to receive and channel the heat transfer medium being pumped through line  90 . In order to direct the heat transfer medium from one outer conduit to another outer conduit, about the end portions of respective pairs of outer conduits there is provided a connector  156 . The connector  156  allows the heat transfer medium to move from one outer conduit to another. Thus as illustrated in FIG. 3, the heat transfer medium, which would comprise water or other efficient heat transfer fluids, completely surrounds the sludge passing within the sludge segments  150 . 
     Details of the other heat exchangers are not shown herein but they also would assume the same type of configuration illustrated in FIG.  3 . In fact, for efficiency, heat exchanger  28  along with heat exchanger sections  86  and  88  may be consolidated in a single bank such that the respective heat exchangers would lie in close side-by-side relationship. 
     As the sludge moves through the respective heat exchangers on a continuing basis, the internal lines or pipes that carry the sludge tend to accumulate deposits such as grease foulings and the like. In order to clean the respective heat exchangers such that the pathogen reduction system of the present invention will operate efficiently, it is necessary to shut down the sludge treating process. The present invention entails an automatic cleaning system for the heat exchangers and this system is indicated generally by the numeral  200 . While the automatic sludge cleaning system is shown in FIG. 1, FIG. 2 is a schematic illustration of the same system shown separated from the pathogen reduction system detailed in FIG.  1 . To facilitate an appreciation for the automatic heat exchanger cleaning system  200 , reference will be made to FIG. 2 
     Basically, the automatic heat exchanger cleaning system  200  of the present invention entails utilizing a pipe cleaning device, sometime referred to as a pig  202  and actually driving the pig  202  through the respective heat exchangers by utilizing pressurized and pasteurized treated wastewater. With particular reference to FIG. 2, incoming treated wastewater is directed though line  208  into a holding tank  204 . Disposed on the outlet side of the holding tank  204  is a pump  210  that leads to both a recirculating line  212  and a delivery line  224 . By selectively opening and closing valves disposed in both the recirculating line  212  and the delivery line  224 , the treated wastewater held within the holding tank  204  can be selectively directed to either line. In one mode of operation, valve  232  in delivery line  224  is closed while control valve  234  is open. This enables pump  210  to circulate the wastewater in holding tank  204  through a heat exchanger  214 . The treated wastewater is heated by a hot water boiler  36 . Hot water is pumped by pump  220  through lines  216  and  218  back and forth between the boiler  36  and the heat exchanger  214 . In order to pasteurize the treated wastewater within holding tank  204 , it is contemplated that the wastewater would be heated to approximately 70° C. As suggested by FIG. 1, there is a temperature controller TIC associated with the holding tank  204  and that in conjunction with a control loop is operative to vary the flow rate of hot water from the boiler  36  through the heat exchanger  214  so as to maintain the temperature within the holding tank  204  at a temperature that is sufficient to ensure that the wastewater is pasteurized. 
     In the embodiment illustrated herein, the delivery line  224  leads to a pair of pig pickup stations indicated generally by the numeral  206 . Essentially, the pig pickup stations  206  act to receive a pig and to hold the same therein until a system of pressurized water is directed to the respective pig pick up stations  206 . Once the treated wastewater is pumped into delivery line  224 , the pressurized water enters the respective pig pickup stations  206  and causes the pig  202  contained therein to be directed from the pig pickup stations  206  through various heat exchangers in the system. In the embodiment illustrated in FIG. 2, the delivery line  224  leads to both the incoming sludge line  16  and the treated line  70 . As illustrated in FIG. 2, once the pasteurized treated wastewater reaches the upper pickup station  206 , that is the pickup station that communicates with line  16 , the pig  202  within the pickup station is forced through the incoming sludge line  16 . Because the pig  202  is a compressible cleaning device, the pressurized wastewater causes the pig to compress as it leaves the pickup station  206  and enters the incoming sludge line  16 . Thus it is seen that the pig  202  travels through the sludge line found in heat exchanger  34  as well as the sludge line or sludge segments  150  found in heat exchanger  28 . Thus the pressurized wastewater causes the pig to move through the various serpentine segments of each heat exchanger and to clean the same in the process. By selectively controlling various valves in the system, the pig  202  is directed from the system to where it is caught by a combination drain and grate  230 . Likewise, in FIG. 2, along the lower branch illustrated, the pig  202  is forced to compress and move through the heat exchanger sections  86  and  88  as well as the final cooling heat exchanger  106 . Again, the pig  202  exits the system and is caught by a combination drain and grate  230 . 
     FIG. 4 is another schematic illustration that shows the automatic heat exchanger cleaning system of the present invention. Note that the treated wastewater is pumped by pump  210  into the side of the pig pickup station  206 . The pig pickup station  206  includes a receptor  210  that includes an inlet opening  212 . Receptor  210  in this embodiment is generally tapered inwardly from the inlet  212 . The inlet  212  is normally closed by a closure plate  214 . Closure plate  214  can be secured to the inlet through a flange and bolt assembly construction. Note that the closure plate  214  includes a tee  214   a  that projects into the receptor  210 . The base of the tee functions to engage the pig  202  and to basically confine the pig  202  within the receptor until pressurized wastewater is directed into the receptor  210 . 
     It is appreciated from FIG. 4, that the treated wastewater under pressure enters the side of the receptor  210 . Once the treated wastewater under pressure enters the receptor, the pig  202  will be driven through the connecting line and any number of control valves to one or more of the heat exchangers. In FIG. 4, a portion of a heat exchanger is shown schematically. Note that the pig will be driven through the internal pipe or conduit within the heat exchanger, that is the internal pipe or conduit that carries the sludge. As discussed before, the pig  202  can be driven through one or more heat exchangers before the pig  202  exits the cleaning system. As illustrated in FIG. 4, by selectively actuating various control valves, the pig  202  can be directed from the system into a combination drain and grate  230 . The cleaning operation is accomplished by the combined effects of the pig  202  and the hot pressurized wastewater. Since the pig  202  is both compressible and aggressive, it itself tends to conform to the shape of the sludge line and as it is driven through the sludge line it will clean and break away grease foulings and other deposits. Once these deposits have been broken away, the pressurized wastewater will clean and rinse the deposits from the sludge line. 
     As is appreciated from the foregoing disclosures, the series of heat exchangers employed within the pathogen reduction system  10  function to heat the incoming sludge and at the satile time cool the treated sludge to meet a target temperature range for the digester  104 . Essentially it is important to heat the incoming sludge to approximately 70° C. before the sludge reaches the reactors  56 ,  58  and  60 . Thus the treated sludge exiting the reactors will have a temperature of approximately 70° C. or slightly less. This treated sludge will eventually be directed into the digester  104 . Digester  104  in this design functions as a mesophilic digester and consequently for optimum performance the temperature of the sludge held therein is approximately 35°. Thus it is important from a digestion point of view, to make sure that the treated sludge entering the digester  104  falls within a target temperature range, in this case approximately 34-38° C. Consequently, in most cases it will be required that the treated sludge be cooled in order to meet this target temperature range. 
     The present invention entails a control system for controlling the temperature of the incoming sludge as well as the treated sludge. Primarily, the system aims to control the temperature of the treated sludge reaching the digester to the target temperature range of 34-38° C. while at the same time transferring as much heat as possible from the treated sludge to the incoming sludge. 
     The controller, indicated by the numeral  96 , is integrated into the pathogen reduction system as schematically shown in FIG.  6 . Controller  96  is programmable and is operatively connected to a number of components that make up the pathogen reduction system. As illustrated in FIG. 6, the controller  96  is connected to the variable speed pumps  42 ,  92 , and  112 . Thus the controller  96  is operative to control the speeds of these respective pumps independently of each other. In addition, there are a series of temperature indicators TI disposed throughout the system at strategic locations. These temperature indicators TI are also connected to the controller. As noted above, the function of the controller  96  is to receive the temperature data from the respective temperature indicators TI and to control the speed of the various pumps so as to assure that the sludge entering the digester  104  meets the target temperature range while at the same time efficiently transferring heat from the treated sludge to the incoming sludge. 
     In a majority of the cases, it is contemplated that the temperature of the sludge entering the digester  104  would be greater than the target temperature range assuming the absence of any heat exchangers performing a cooling operation. Therefore, in most cases the treated sludge leaving the reactors  56 ,  58  and  60  will require cooling. Since it is important to achieve optimum heat transfer to the incoming sludge, it is appropriate to designate, in this particular embodiment, heat exchanger sections  86  and  88  as the priority cooling exchangers. If sufficient cooling cannot be achieved by exchanger sections  86  and  88 , then the treated wastewater exchanger  106  can be employed. It should also be appreciated that the controller  96  may very well function to employ both the heat exchanger sections  86  and  88  along with the treated wastewater exchanger  106  to effectuate cooling. 
     FIG. 5 is a flow chart showing the basic programming functions for the controller  96  with regard to controlling the cooling and heat transfer effectuated by exchangers  28 ,  86  and  88 . After the target temperature range has been established, the temperature of the sludge is measured by the temperature indicator TI just prior to the sludge entering the digester. If the measured temperature is greater than the target temperature range, then the controller  96  acts to increase the speed of pump  92  so as to increase the flow rate of the heat transfer medium through the closed conduit  90 . This will increase the rate of heat transfer from the treated sludge to the incoming sludge and in that process, the temperature of the treated sludge will be reduced further. Thus, the flow rate of the heat transfer medium is continued to be increased until the target temperature range of 34-38° C. is reached. In practice, there is a limit to the flow rate of the heat transfer medium. The mass flow rate of the heat transfer medium should not exceed the mass flow rate of the sludge. 
     On the other hand, if the measured temperature of the sludge just prior to reaching the digester is not greater than the target temperature range, then the program determines if the measured temperature is less than the target temperature range. If no, the program simply bypasses the remaining functions and returns to measuring the temperature of the treated sludge just prior to entry into the digester. If the measured temperature is less than the target temperature range, then the controller  96  functions to decrease the speed of the variable speed pump  92  so as to decrease the flow rate of the heat transfer medium through the closed conduit  90 . This function is effectively repeated until the temperature of the sludge entering the digester  104  meets the target temperature range. 
     The above discussion deals with controlling one group of heat exchangers within the total pathogen reduction system. However, it is appreciated, that the same programmable controller  96  may be used to control all of the heat exchangers within the pathogen reduction system by controlling the various heat transfer mediums and fluids that are used to transfer heat. In short, the programmable controller can be programmed to assure that the boiler  36  in combination with the heat transfer to the incoming sludge from the treated sludge is sufficient to assure that the sludge entering the reactors is at a selected temperature and at the same time assure that the temperature of the sludge reaching the digester  104  is within the target of temperature range. 
     The present invention may, of course, be carried out in other specific ways than those herein set forth without departing from the spirit and the essential characteristics of the invention The present embodiments are therefore to be construed in all aspects as illustrative and not restrictive and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.