Sewage treatment and other waste material treatment apparatus and method are well known. In general, the apparatus comprises a pasteurisation tank for pasteurising the material to kill or neutralise pathogens and other hazardous substances in the waste material. The pasteurised material is then fed from the pasteuriser to a cooler for cooling the pasteurised material to a desired predetermined feed temperature at which the pasteurised material is to be fed to further processing apparatus, for example, to a digester or the like. Typically, in the case of sewage and other similar type waste materials, it is necessary to raise the temperature of the material to a pasteurisation temperature of the order of 70°C.

However, it is important that the temperature in the digester should not exceed 60°C, and preferably, should lie in the range of 35°C to 60°C, otherwise, significant damage may occur to the microbial population within the digester, which would thus disrupt the digestion process. Typically, the temperature in the digester should be about 47°C. Thus, in general, the treatment time for treating sewage and other waste materials tends to be relatively long and furthermore, known processes tend to be energy intensive. The cooling time for cooling the pasteurised material to the desired digester feed temperature can be relatively long, and the cooling process can be relatively energy wasteful. This is undesirable. Additionally, digesters operate more efficiently with a continuous feed of sewage sludge, while pasteurisation is inherently a batch process. This is a further problem which arises in the treatment of sewage sludge, and indeed, in the treatment of other materials which require a pasteurisation step and a continuous process step.

Furthermore, in other processes where pasteurisation is a required step, and where the pasteurised material is required at a temperature lower than the pasteurisation temperature, similar problems arise in that a cooling step is required after the pasteurisation process, and this, can be time consuming, and also energy intensive and inefficient.

There is therefore a need for a pasteurisation method and apparatus which overcomes these problems.

The present invention is directed towards providing such a pasteurisation method and apparatus, and the invention is also directed towards providing a treatment apparatus for material which includes a pasteurisation method and apparatus, and a subsequent processing step.

According to the invention there is provided apparatus for pasteurising a flowable material wherein the apparatus comprises a first pasteurisation tank, and a second pasteurisation tank, a charging means and a discharge means being provided to and from each of the pasteurisation tanks for independently charging and discharging the flowable material to and from the respective pasteurisation tanks, the respective charging and discharge means being selectively operable so that when the pasteurisation process has been completed in one of the first and second pasteurisation tanks, the other of the first and second pasteurisation tanks has been charged with material to be pasteurised, a heat exchange means being associated with each pasteurisation tank, and a communicating means for communicating the respective heat exchange means, the communicating means being selectively operable for communicating the respective heat exchange means of the first and second pasteurisation tanks for transferring heat between the pasteurised material in the one of the first and second tanks and the material to be pasteurised in the other of the first and second tanks for heating the said material to be pasteurised in the said other of the first and second tanks to an intermediate temperature prior to pasteurisation, and for cooling the pasteurised material prior to discharge from the said one of the first and second pasteurisation tanks.

In one embodiment of the invention the communicating means is selectively operable for isolating the respective heat exchange means of the first and second pasteurisation tanks, when the temperature has equalised in the respective heat exchange means.

In another embodiment of the invention the respective charging and discharge means are controlled so that the time for discharging pasteurised material and subsequently charging each of the first and second pasteurisation tanks is similar to the time for pasteurising material in each of the first and second pasteurisation tanks.

In a further embodiment of the invention the volume of the respective first and second pasteurisation tanks are matched to the respective charging and discharge means so that the time for discharging pasteurised material and subsequently charging each of the first and second pasteurisation tanks is similar to the time for pasteurising material in each of the first and second pasteurisation tanks.

Advantageously, the volume of the respective first and second heat exchange tanks are similar.

Preferably, the charging rates of the respective charging means of the first and second pasteurisation tanks are similar to each other, and the discharge rates of the respective discharge means of the first and second pasteurisation tanks are similar to each other.

Ideally, the charging and discharge means of the respective first and second pasteurisation tanks are operable so that one of the first and second pasteurisation tanks is being discharged of pasteurised material and subsequently charged with material to be pasteurised while material in the other of the first and second pasteurisation tanks is being pasteurised.

Advantageously, the time for pasteurising material in each of the first and second pasteurisation tanks includes the time for raising the material to be pasteurised from the intermediate temperature to the desired pasteurisation temperature and the dwell

time of the material being pasteurised at the pasteurisation temperature in the respective first and second pasteurisation tanks until pasteurisation has been completed.

In one embodiment of the invention the heat exchange means of each first and second pasteurisation tank is adapted for independently receiving heat from an external heat source for raising the temperature of the material to be pasteurised from the intermediate temperature to the pasteurisation temperature.

In another embodiment of the invention the heat exchange means of each first and second pasteurisation tank is adapted for independently receiving a cooiing medium from a cold source for reducing the temperature of the pasteurised material to a desired temperature prior to discharge, in the event that the temperature to which the pasteurised material has been reduced as a result of heat exchange between the heat exchange means of the respective first and second pasteurisation tanks is above a desired predetermined temperature.

In one embodiment of the invention each heat exchange means comprises a primary heat exchanger. Preferably, each primary heat exchanger comprises a primary heat exchange jacket for accommodating a heat transfer medium, the primary heat exchange jackets extending around the corresponding first and second pasteurisation tanks. Advantageously, each primary heat exchange jacket defines a corresponding vertically extending axis.

In another embodiment of the invention each heat exchange means comprises a secondary heat exchanger. Preferably, each secondary heat exchanger accommodates the heat transfer medium, the respective secondary heat exchangers being located in the respective first and second pasteurisation tanks, and being in communication with the corresponding primary heat exchanger for circulation of the heat transfer medium between the primary and secondary heat exchangers of each first and second pasteurisation tank. Advantageously, each secondary heat exchanger defines a central axially extending bore for accommodating the material

therethrough, and each secondary heat exchanger is located co-axialiy within and spaced apart from the corresponding primary heat exchanger.

Ideally, each secondary heat exchanger is located in the corresponding one of the first and second pasteurisation tanks for facilitating circulation of the material in the corresponding one of the first and second pasteurisation tanks, the circulation of the material being axially through the secondary heat exchanger and return flow of the material being between the primary and secondary heat exchangers.

In one embodiment of the invention respective circulating means are provided for circulating the material in the respective first and second pasteurisation tanks.

Advantageously, each circulating means is located within the corresponding secondary heat exchanger for causing axial flow of the material through the secondary heat exchangers.

In one embodiment of the invention each one of the first and second pasteurisation tanks is provided by a vertically extending tank.

In another embodiment of the invention each charging means is arranged for charging the corresponding one of the first and second pasteurisation tanks under gravity. Alternatively or additionally, the respective charging means comprise a pumping means for selectively pumping the material into the respective first and second pasteurisation tanks.

In a further embodiment of the invention each discharging means is arranged for discharging the corresponding one of the first and second pasteurisation tanks under gravity for ensuring emptying of the tanks.

In one embodiment of the invention the apparatus is adapted for pasteurising sewage material. In another embodiment of the invention the apparatus is adapted for pasteurising sewage material in sludge form.

The invention also provides treatment apparatus for treating flowable material, the treatment apparatus comprising pasteurisation apparatus according to the invention for pasteurising the material and reducing the temperature of the pasteurised material to a predetermined temperature, and a downstream processing apparatus for receiving the pasteurised material at the predetermined temperature from the pasteurisation apparatus, and for further processing the pasteurised material.

In one embodiment of the invention the downstream processing apparatus is a digester for further treating the material.

The invention further provides a method for pasteurising material, the method comprising the steps of charging one of first and second pasteurisation tanks with material to be pasteurised while material is being pasteurised in the other of the first and second pasteurisation tanks, and on completion of the pasteurisation of the material in the said other of the first and second pasteurisation tanks causing heat exchange between the pasteurised material and the material to be pasteurised for cooiing the pasteurised material prior to discharging thereof from the said other of the first and second pasteurisation tanks, and for heating the material to be pasteurised to an intermediate temperature in the said one of the first and second pasteurisation tanks.

In one embodiment of the invention the method further comprises the step of discharging material which has been pasteurised from each one of the first and second pasteurisation tanks and then charging the tank with material to be pasteurised, the time for discharging the pasteurised material from the respective tanks and for charging the respective tanks with material to be pasteurised is substantially similar to the time for pasteurising the material in the respective tanks.

In another embodiment of the invention the time for pasteurising the material in the respective tanks includes the time for raising the temperature of the material from the intermediate temperature to the desired pasteurisation temperature, and the dwell time of the material in the respective first and second pasteurisation tanks during pasteurisation.

Preferably, pasteurised material in one of the first and second pasteurisation tanks is being discharged from the said one of the first and second pasteurisation tanks and the said one of the first and second pasteurisation tanks is subsequently charged with material to be pasteurised while material is being pasteurised in the other one of the first and second pasteurisation tanks.

In a further embodiment of the invention the method further comprises the step of heating the material to be pasteurised in each one of the first and second pasteurisation tanks from the intermediate temperature to the desired pasteurisation temperature.

In a still further embodiment of the invention the method further comprises the step of further cooling the pasteurised material in each one of the first and second pasteurisation tanks after heat exchange has been completed between the respective pasteurised material and the material to be pasteurised in the event that the temperature of the pasteurised material has not been reduced to a desired predetermined temperature.

In one embodiment of the invention the material is charged into each one of the first and second pasteurisation tanks under gravity. Alternatively, the material is selectively charged into each one of the first and second pasteurisation tanks by a pumping means.

In another embodiment of the invention the pasteurised material is discharged from each one of the first and second pasteurisation tanks under gravity.

In one embodiment of the invention the method is for pasteurising waste material.

In another embodiment of the invention the method is for pasteurising sewage material.

In a further embodiment of the invention the method is for pasteurising sewage material in sludge form.

In one embodiment of the invention the pasteurised material at the desired predetermined temperature is fed to downstream processing apparatus for further processing.

In another embodiment of the invention the pasteurised material at the desired predetermined temperature is fed to a digester.

In one embodiment of the invention the material is charged into each one of the first and second pasteurisation tanks at an incoming temperature in the range 2°C to 10°C.

In another embodiment of the invention the material is charged into each one of the first and second pasteurisation tanks at an incoming temperature in the range 3°C to 6°C.

In a further embodiment of the invention the material is charged into each one of the first and second pasteurisation tanks at an incoming temperature in the range 4°C to 6°C.

In one embodiment of the invention the intermediate temperature is in the range of 30°C to 40°C.

In another embodiment of the invention the intermediate temperature is in the range of 31 °C to 36°C.

In a further embodiment of the invention the intermediate temperature is approximately33°C.

In one embodiment of the invention the desired predetermined temperature at which the pasteurised material is discharged from the respective first and second pasteurisation tanks is in the range of 35°C to 60°C.

In another embodiment of the invention the desired predetermined temperature at which the pasteurised material is discharged from the respective first and second pasteurisation tanks is in the range of 44°C to 51 °C.

In a still further embodiment of the invention the desired predetermined temperature at which the pasteurised material is discharged from the respective first and second pasteurisation tanks is in the range of approximately 47°C + 3°C.

The advantages of the invention are many. By virtue of the fact that the batches of material are pasteurised alternately in the respective first and second pasteurisation tanks and in parallel with each other significant savings in pasteurisation time are achieved, and furthermore, the pasteurising apparatus lends itself for use in continuous treatment apparatus and processes, in particular, in the treatment of sewage sludge. Additionally, the apparatus and the method for pasteurisation are relatively simple and can be impiemented with the minimum of equipment, and thus can be operated with minimum energy requirements, and thus, the apparatus is efficient to install and operate. Additionally, by virtue of the fact that heat is transferred from a pasteurised batch of material in one of the tanks to an incoming batch of sewage to be pasteurised in the other of the tanks, the cold incoming material cools the pasteurised material to a desired predetermined feed temperature for the next process to which the pasteurised material is to be transferred, and raises the temperature of the incoming batch of material to an intermediate temperature prior to pasteurisation. This also leads to significant energy savings. A further advantage of the invention is achieved by arranging the pasteurisation cycle in the respective tanks so that the pasteurisation time of the material from the time the material to be pasteurised has been brought to the intermediate temperature until pasteurisation has been completed is similar to the charging and discharging time of the respective tanks, this significantly reduces the cycle time.

A further advantage of the invention is that it provides a pasteurisation apparatus and method which provides a stand alone pasteurisation apparatus which is capable of operating as a pasteurisation apparatus only, or as a pasteurisation apparatus in conjunction with other material treatment apparatus. Additionally, the pasteurisation apparatus can be readily easily retrofitted into other treatment apparatus.

Furthermore, by virtue of the fact that two pasteurisation tanks are provided, even when one of the tanks is out of service, the other tank can continue to operate, thus avoiding complete shutdown of a process apparatus in which the pasteurisation apparatus is installed.

The invention will be more clearly understood from the following description of a preferred embodiment thereof which is given by way of example only with reference to the accompanying drawings, in which: Fig. 1 is a schematic representation of apparatus according to the invention for treating sewage sludge, Fig. 2 is a transverse cross-sectional elevational view of a portion of the apparatus of Fig. 1, Fig. 3 is a transverse cross-sectional plan view of a portion of the apparatus illustrated in Fig. 2 on the line III-III of Fig. 2, Fig. 4 is a circuit diagram of part of the apparatus of Fig. 1, Fig. 5 is a timing diagram illustrating the operation of the apparatus of Fig. 1.

Referring to the drawings there is illustrated a treatment apparatus according to the invention indicated generally by the reference numeral 1 for treating sewage sludge. The treatment apparatus 1 comprises a pasteurisation apparatus 2 also according to the invention for initially pasteurising batches of the sewage sludge and for reducing the temperature of the batches of pasteurised sludge to a predetermined desired temperature which is suitable for feeding the pasteurised sewage sludge to a

digester 5. The desired predetermined temperature depends on the digester, and the quality of insulation of the digester, but typically, lies in the range 35°C to 60°C but in general, is in the range of 47°C + 3°C. A digester feed tank 7 is located intermediate the pasteurisation apparatus 2 and the digester 5 for receiving the pasteurised sewage sludge for subsequent delivery to the digester 5, as will be described below.

The pasteurisation apparatus 2 comprises a pair of identical pasteurisation tanks, namely, a first pasteurisation tank 8 and a second pasteurisation tank 9 for alternately pasteurising batches of the sewage sludge. Since the two tanks 8 and 9 are identical, the first pasteurisation tank 8 will be described in detail, and components in the second pasteurisation tank 9 which are similar to those of the first pasteurisation tank 8 are identified by the same reference numerals. The first pasteurisation tank 8 is of rectangular cross-section in plan view, and comprises a base 15 of sheet stainless steel material and four side walls 16, also of sheet stainless steel material extending vertically upwardly from the base 15 and defining a central axis 17. A heat insulating material 18 is provided around the outer surface of the side walls 16 and beneath the base 15 for insulating the first pasteurisation tank 8. An outer shell 19 of fibreglass or other suitable material extends around and beneath the insulating material 18.

A heat exchange means which comprises a primary heat exchanger 20 and a secondary heat exchanger 29 are located in each of the first and second pasteurisation tanks 8 and 9 for facilitating heat transfer between a batch of pasteurised sewage sludge in one of the first and second pasteurisation tanks 8 and 9, and a batch of sludge to be pasteurised in the other of the first and second pasteurisation tanks 8 and 9 for cooling the pasteurised sewage sludge prior to discharge to the digester feed tank 7 and for heating the sewage sludge to be pasteurised to an intermediate temperature of approximately or just below 33°C prior to being pasteurised. The primary heat exchanger 20 is formed by a four sided inner shell 22 of sheet stainless steel material located within each of the first and second pasteurisation tanks 8 and 9 adjacent the corresponding side walls 16 and is seam welded around the top edge 24 and the bottom edge 25 to the side walls 16. The inner shell 22 is spaced apart from the side walls 16 and defines with the side walls

16 a hollow interior region 27 for accommodating a heat transfer medium for facilitating heat transfer between the sewage sludge in the respective first and second pasteurisation tanks 8 and 9. In this embodiment of the invention the heat transfer medium is water.

The secondary exchanger 29 of each first and second pasteurisation tank 8 and 9 is located co-axially within the corresponding pasteurisation tank. Each secondary heat exchanger 29 is of cylindrical construction and comprises a pair of c-axial and spaced apart cylindrical shells 30 which are seam welded at their top and bottom edges 31 and 32, respectively. The primary and secondary heat exchangers 20 and 29 of each pasteurisation tank 8 and 9 are connected in parallel by radially extending connecting pipes 34, but if desired may be connected in series.

A communicating means for selectively communicating the primary and secondary heat exchangers 20 and 29, respectively, of the respective first and second pasteurisation tanks 8 and 9 comprises a communicating circuit 35 for circulating the heat transfer water between the primary and secondary heat exchangers 20 and 29 of the respective first and second pasteurisation tank 8 and 9. A pair of solenoid operated isolating valves 36 and a circulating pump 37 are located in the communicating circuit 35 for facilitating selective circulation of the heat transfer water between the primary and secondary heat exchangers 20 and 29 of the respective first and second pasteurisation tanks 8 and 9 for selectively transferring heat between pasteurised sewage sludge in one of the first and second pasteurisation tanks 8 and 9 and sewage sludge to be pasteurised in the other of the tanks 8 and 9, as will be described below.

Each first and second pasteurisation tank 8 and 9 defines an upwardly directed open 39 which is closed by a removable closure plate 40.

A circulating means comprising a low speed circulating impeller 42 is located in each first and second pasteurising tank 8 and 9 within the corresponding secondary heat exchangers 29 for circulating the sewage sludge in the first and second pasteurisation tanks 8 and 9 in the direction of the arrows A. Each impeller 42 is

carried on a drive shaft 43, which is driven by a motor 44 for urging the sewage upwardly within the secondary heat exchangers 29 so that the sewage circulates within the respective first and second pasteurisation tanks 8 and 9 in the direction of the arrows A, namely, upwardly through the secondary heat exchangers 29, over the top edges 31 of the secondary heat exchangers 29, and downwardly between the primary and secondary heat exchangers 20 and 29. In order to facilitate circulation of the sewage in the respective first and second pasteurisation tanks 8 and 9 while the respective tanks 8 and 9 are being charged or discharged and the level of sewage in the tanks has not reached the top edge 31 of the secondary heat exchanger 29, an elongated communicating slot 45 extends downwardly from the top edge 31 of the respective secondary heat exchangers 29 for a distance of approximately three quarters of the height of the secondary heat exchangers 29 for accommodating circulation within the partly full tanks 8 and 9. Needless to say the shells 30 of the secondary heat exchangers 29 are seam welded along the slot 45. Typically, the slot 45 is sufficiently narrow and typically is of width of the order of 100mm to avoid short circuiting during circulation of the sewage in the respective tanks 8 and 9 when the tanks 8 and 9 are filled above the level of the top edge 31 of the secondary heat exchangers 29. Although the motors 44 are illustrated unmounted, and the drive shafts 43 to the respective impellers 42 are illustrated as passing through the closure plates 40, it will be appreciated by those skilled in the art that suitable mounting arrangements will be provided for the motors 44, and likewise suitable arrangements for extending the drive shafts 43 into the first and second pasteurisation tanks 8 and 9 will be provided.

Sewage sludge to be treated is fed under gravity to the apparatus 1 through a sludge supply pipe 51. Batches of sewage sludge from the supply pipe 51 are selectively and alternately fed to the respective first and second pasteurisation tanks 8 and 9 through respective charging means, namely, identical electrically or pneumatically operated charging valves 52 and 53. Down feeder pipes 54 from the respective charging valves 52 and 53 feed the sludge into the respective first and second pasteurisation tanks 8 and 9 through the corresponding closure plates 40, although, needless to say, the sludge may be fed into the first and second pasteurisation tanks 8 and 9 at any other suitable location.

Batches of pasteurised sewage are selectively and alternately fed under gravity from the first and second pasteurisation tanks 8 and 9 through respective discharge means, namely, electrically or pneumatically operated discharge valves 55 and 56 from outlets 58 in the side walls 16 of the first and second pasteurisation tanks 8 and 9 adjacent the respective bases 15 thereof. Downwardly extending feed pipes 59 from the discharge valves 55 and 56 discharge the pasteurised sewage from the respective first and second pasteurisation tanks 8 and 9 into the digester feed tank 7 under gravity. The digester feed tank 7 acts as a buffer tank for receiving the pasteurised sewage at a rapid rate under gravity from the respective first and second pasteurisation tanks 8 and 9 for subsequent feeding at a continuous constant relatively slow rate into the digester 5. A pump 60 pumps the pasteurised sewage sludge through a feed pipe 61 from the digester feed tank 7 to the digester 5 at the constant slow rate. Such digesters 5 will be well known to those skilled in the art, and typically, the sewage sludge is pumped from the digester feed tank 7 to the digester 5 at a rate of four cubic meters per hour.

Referring now in particular to Fig. 4, a boiler 62 is provided for heating the heat transfer water for circulating through the primary and secondary heat exchangers 20 and 29 of the respective first and second pasteurisation tanks 8 and 9 for raising the temperature of the sewage in the respective tanks 8 and 9 from the intermediate temperature of approximately 33°C to a pasteurisation temperature of approximately 70°C for facilitating pasteurisation of the batches of sewage in the respective first and second tanks 8 and 9. A chiller unit 63 chills the heat transfer water in the primary and secondary heat exchangers 20 and 29 for cooiing the pasteurised sewage sludge in the respective first and second pasteurisation tanks 8 and 9 prior to discharge therefrom. Circuits 64 and 65 connect the boiler 62 and the chiller unit 63, respectively, to a bank of solenoid operated valves 66 for selectively providing hot or chilled heat transfer water to the respective primary and secondary heat exchangers 20 and 29 of the respective first and second pasteurisation tanks 8 and 9. Circuits 67,68 and 69 through banks of solenoid operated valves 70 and 71 connect the bank of solenoid controlled valves 66 to the communicating circuit 35 of the respective primary and secondary heat exchangers 20 and 29. The banks of

solenoid operated valves 70 and 71 are selectively operable for selectively connecting the primary and secondary heat exchangers 20 and 29 of either of the first and second pasteurisation tanks 8 and 9 to either the boiler 62 or the chiller unit 63 through the bank of solenoid operated valves 66. A circulating pump 72 in the circuit 67 circulates the heat exchange medium between the boiler 62 on the one hand or the chiller unit 63 on the other hand and the primary and secondary heat exchangers 20 and 29 of either the first or second pasteurisation tanks 8 or 9.

A control means comprising a control circuit 80 controls the operation of the apparatus 1 and the pasteurisation apparatus 2. The control circuit 80 comprises a microprocessor 81 which under the control of suitable software operates the isolating valves 36 and the circulating pump 37 for selectively communicating the primary and secondary heat exchangers 20 and 29 of the first and second pasteurisation tanks 8 and 9. The microprocessor 81 also controls the operation of the circulating pump 72 and the banks of solenoid operated valves 66,70 and 71 for selectively connecting the primary and secondary heat exchangers 20 and 29 of either one of the first and second pasteurisation tanks 8 and 9 to either the boiler 62 or the chiller unit 63. The microprocessor 81 also controls the operation of the charging valves 52 and 53 and the discharge valves 55 and 56 for respectively charging and discharging sewage sludge to and from the first and second pasteurisation tanks 8 and 9.

Temperature sensors 84 and 85 located in the first and second pasteurisation tanks 8 and 9, respectively monitor the temperature in the respective tanks 8 and 9, and are read by the microprocessor 81 which controls the various valves and pumps in response to the monitored temperatures as will be described below.

The operation of the apparatus 1 in accordance with the method according to the invention will now be described with reference to the timing diagram of Fig. 5. Fig. 5 illustrates four graphs, namely, HTW1, SS1, HTW2 and SS2. Graph HTW1 is a plot of the temperature of the heat transfer water against time in the primary and secondary heat exchangers 20 and 29 of the first pasteurisation tank 8. Graph SS1 is a plot of the temperature of the sewage sludge against time in the first pasteurisation tank 8. Graphs HTW2 and SS2 are plots of temperature against time

of the heat transfer water and the sewage sludge of the second pasteurisation tank 9. It should be noted that the graphs are not to temperature or time scale, they are provided for the purpose of illustrating the operation of the apparatus 1 only.

In general, the sewage sludge is supplied to the pasteurisation apparatus 2 through the supply pipe 51 at an incoming temperature of approximately 4°C. For convenience a pasteurisation cycle will be described from time T1 in the diagram. At time T1 a batch of sewage to be pasteurised has already been admitted into the first pasteurisation tank 8 and the temperature of the sewage in the first tank 8 has been raised to an intermediate temperature of approximately 33°C by heat exchange with a previous batch of pasteurised sewage in the second tank 9 as will be described below. At time T1 under the control of the control circuit 80 the isolating valves 36 in the communicating circuit 35 are operated into the isolating mode for isolating the primary and secondary heat exchangers 20 and 29 of the respective first and second tanks 8 and 9 from each other, and the circulating pump 37 is deactivated. The valve banks 60,70 and 71 and the circulating pump 72 are operated for circulating heated water at approximately 90°C from the boiler 62 through the primary and secondary heat exchangers 20 and 29 of the first tank 8 for raising the temperature of the sewage of the first tank 8 from the intermediate temperature of approximately 33°C to the desired pasteurisation temperature of 70°C. When the temperature of the sewage in the first tank 8 has been raised to 70°C, time T2 has been reached. For the period from time T2 to time T4 the temperature of the sewage in the first tank 8 is maintained at the pasteurisation temperature 70°C by controlling the circulating pump 72 for controlling the rate of circulation of the heat transfer water, and allowing the temperature of the heat transfer water to drop to approximately 70°C. The dwell time between time T2 and time T4 during which pasteurisation of the sewage in the first tank 1 is carried out is approximately thirty minutes, but may vary.

At time T4 pasteurisation of the sewage in the first tank has been completed and between time T3 and T4 as will be described below a new batch of sewage to be pasteurised has been admitted into the second tank 9 which has cooled the temperature of the heat exchange water in the primary and secondary heat exchangers 20 and 29 in the second tank 9 to approximately 4°C as will also be

described below. Accordingly, at time T4 the circulating pump 72 is deactivated and the valve banks 66,70 and 71 are operated for isolating the primary and secondary heat exchangers 20 and 29 of the respective first and second tanks 8 and 9 from the boiler 62 and the chilling unit 63. The isolating valves 36 and the circulating pump 37 are activated for circulating and mixing the heat exchange water in the primary and secondary heat exchangers 20 and 29 of the first and second tanks 8 and 9 so that the heat exchange water which is at 70°C in the primary and secondary heat exchangers 20 and 29 of the first tank 8 is mixed with the heat exchanged water at approximately 4°C in the primary and secondary heat exchangers 20 and 29 of the second tank 9 to thereby equalise the temperature of the mixed heat exchange water at approximately 33°C. This has the effect of reducing the temperature of the pasteurised sewage in the first tank 8 to the desired predetermined feed temperature of approximately 47°C + 3°C and raising the temperature of the sewage admitted into the second tank 9 to the intermediate temperature of approximately 33°C or just below 33°C. At this stage time T5 has been reached. If as a result of the temperature of the pasteurised sewage in the first tank 8 which is being monitored by the microprocessor 81 not approaching the desired predetermined temperature of 47°C + 3°C shortly before the time T5 is reached, the isolating valves 36 are closed and the circulating pump 37 deactivated for isolating the primary and secondary heat exchangers 20 and 29 of the respective first and second tanks 8 and 9, and the valve banks 66,70 and 71 are operated for connecting the primary and secondary heat exchangers of the first tank 8 with the chilling unit 63, and the circulating pump 72 is also activated for circulating chilled water from the chilling unit 63 through the primary and secondary heat exchangers 20 and 29 of the first tank 8 for reducing the temperature of the pasteurised sewage in the first tank 8 to approximately 47°C by time T5.

At time T5 the valve banks 66,70 and 71 are operated for isolating the primary and secondary heat exchangers 20 and 29 of the first tank 8 from the chiller 63, and the circulating 72 is deactivated. Additionally, on time T5 having been reached if the isolating valves 36 have not already been closed and the circulating pump 37 deactivated the valves 36 are closed and the pump 37 is deactivated. At time T5 the pasteurised sewage in the first tank 8 is ready for discharge and the discharge valve

55 is opened for discharging the first tank 8. Discharge of the first tanks 8 continues until time T7 at which stage the pasteurised sewage from the first tank 8 has been fully discharged.

At time T7 the discharge valve 55 is closed and the charging valve 52 of the first tank 8 is opened for charging the first tank 8 with the next batch of sewage. At time T8 the first tank 8 is fully charged with the next batch of sewage and the charging valve 52 is closed. Between time T7 and time T8 while the first tank 8 is being charged with the next batch of sewage, the sewage being admitted being at 4°C approximately cools the heat transfer water in the primary and secondary heat exchangers 20 and 29 of the first tank 8 which is now isolated from the primary and heat exchangers 20 and 29 of the second tank 9 to approximately 4°C by the time time T8 has been reached.

At time T8 pasteurisation of a batch of sewage in the second tank 9 will have been completed, and the temperature of the heat transfer water in the primary and secondary heat exchangers 20 and 29 of the second tank 9 will be at approximately 70°C, while the temperature of the heat exchange water in the primary and secondary heat exchangers 20 and 29 of the first tank 8 is at 4°C. Thus, at time T8 the isolating valves 36 of the communicating circuit 35 are opened and the circulating pump 37 is activated for circulating and mixing the hot and cold heat transfer water in the respective primary and secondary heat exchangers 20 and 29 of the first and second tanks 8 and 9. This equalises the temperature of the heat exchange water in the primary and secondary heat exchangers 20 and 29 of the first and second tanks 8 and 9 at approximately 33°C. Thus, the heat exchange water at 33°C raises the temperature of the batch of sewage to be pasteurised in the first tank 8 to the intermediate temperature of approximately 33°C or just below 33°C, and one cycle has been completed and the cycle is back at time T1. At time T1 the temperature of the pasteurised batch in the second tank 8 has been reduced approximately to the desired predetermined feed temperature of 47°C + 3°C.

Turning now to the second tank 9, at time T1 as already discussed the temperature of the pasteurised batch of sewage has been reduced to the desired predetermined

feed temperature of 47°C + 3°C. Thus, at time T1 the pasteurised sewage in the second tank 9 is ready for discharge and the discharge valve 56 is opened for discharging the sewage therefrom. Discharge of the sewage from the second tank 9 is completed by time T3, at which stage the discharge valve 56 is closed and the charging valve 53 is opened for charging the second tank 9 with the next batch of sewage to be pasteurised. The incoming sewage at 4°C causes the heat exchange water in the primary and secondary heat exchangers 20 and 29 of the second tank 9 to be cooled to 4°C while the sewage is being admitted into the second tank 9. At time T4 the second tank 9 is fully charged and thus the charging valve 53 is closed.

At that stage the temperature of the heat exchange water in the primary and secondary heat exchangers 20 and 29 of the second tank 9 is at approximately 4°C while the temperature of the heat exchange water in the primary and secondary heat exchangers 20 and 29 of the first tank 8 is at approximately 70°C and pasteurisation as already described has been completed in the first tank 8. As already described the isolating valves 36 in the communicating circuit 35 are opened and the pump 37 activated for equalising the temperature of the heat exchange water in the primary and secondary heat exchangers 20 and 29 of the respective first and second tanks 8 and 9. At time T5 the temperature of the sewage in the second tank 9 has been raised to the intermediate temperature of approximately 33°C or just below 33°C.

From time T5 to time T1 the operation of the second tank 9 and the operation of the valves 66,70 and 71 and the circulating pump 72 with respect to the second tank 9 is similar to the operation of these components for the first tank 8 from time T1 to time T5. Similarly, the operation of the second tank 9 and the valve bank 66,70 and 71 and the circulating pump 72 with respect to the second tank 9 during time T1 to T5 is similar to the operation of the first tank 8 and the valves 66,70 and 71 and the circulating pump 72 with respect to the first tank 8 during the time period T5 to T1.

During the entire cycle the impellers 42 are operated for circulating the sewage in the respective first and second tanks 8 and 9, with the exception of the when the tanks 8 and 9 are almost empty during discharging, until the level of sludge in the tanks during charging reaches the slot 5 in the secondary heat exchangers 29.

Although not described and illustrated, it will be appreciated that level sensors for monitoring the level of the sewage in the respective first and second tanks 8 and 9 will be provided, and these will be monitored by the microprocessor 81. Temperature sensors will also be provided in the primary and secondary heat exchangers 20 and 29 of the first and second tanks 8 and 9 for monitoring the temperature of the heat exchange water therein, and these temperature sensors will also be monitored by the microprocessor 81. Accordingly, the microprocessor 81 during each cycle controls the various valves and pumps in response to the monitored levels and temperatures. A level sensor is also provided in the digester which is read by the microprocessor 81, and the microprocessor 81 controls the feed pump 60 for feeding the pasteurised sewage from the digester feed tank 7 to the digester 5 in response to the level sensor.

It can be seen from the timing diagram of Fig. 5 that the pasteurisation time from the time the temperature of the sewage is at the intermediate temperature at time T1 in the case of the first tank 8 to the time T4 in the case of the first tank 8 is similar to the charging and discharging time of the second tank 9 from time T1 to T4, and vice versa. This is achieved by selecting the charging and discharge valves 52 and 53, and 55 and 56, respectively, so that the charge and discharge rates are appropriately matched to the capacities of the first and second pasteurisation tanks 8 and 9.

Typical times of the steps in the pasteurisation cycle of the pasteurisation apparatus 2 are set out in Table 1.

While the apparatus and method for pasteurising material has been described for pasteurising sewage sludge prior to digestion, it will be appreciated that the method and apparatus may be used for pasteurising any material, whether sewage sludge or otherwise, or indeed, waste material or otherwise in a batch process where it is desired to provide the pasteurised material at a temperature below that of the pasteurisation temperature. U r i , T1 T2} } 45 minutes T2-T3} T3-T4 30 minutes T4-T5 60 minutes T5-T6} } 45 minutes T6-T7} T7-T8 30 minutes T8-T1 60 minutes

TABLE 1 While the heat exchange means have been described as comprising primary and secondary heat exchangers, any other suitable heat exchange means may be provided, and in certain cases, it is envisaged that the secondary heat exchange means may be dispensed with.

While the impellers have been described as circulating the sewage in the respective first and second pasteurisation tanks 8 and 9 in the direction of the arrows A, the impellers 42 could equally well be arranged for circulating the sewage in the reverse direction.

The pasteurisation tanks may be of any desired capacity to meet the pasteurisation throughput requirement, however, in particular, it is envisaged that the pasteurisation apparatus is suitable for pasteurising materials and in particular sewage sludge at rates in the range of 2 tons per hour to 8 per hour and in which cases the capacities of each of the first and second tanks would range between 5 cubic meters and 20 cubic meters.

While the charging means have been described as being provided by electrically or pneumatically operated valves, any other suitable valves may be used. It will also be appreciated that instead of the sewage to be pasteurised being fed under gravity into

the first and second pasteurisation tanks, the sewage to be pasteurised may be pumped into the first and second pasteurisation tanks by any suitable pump means, for example, by an electrically powered pump, a pneumatically powered pump or the like.

While the heat source for raising the temperature of the heat transfer water to a temperature suitable for raising the sewage to be pasteurised to the desired pasteurisation temperature has been described as being a boiler, any other suitable heat source besides a boiler may be used, for example, a heat store or the like.

While specific temperatures have been described for the desired pasteurisation temperature, the desired predetermined feed temperature and the intermediate temperature of the sewage sludge, it will be readily apparent to those skilled in the art that the apparatus and method may be operated with other temperatures.

Similarly, it will be appreciated that other temperatures of the heat transfer water may be used besides those described, and indeed, the temperatures of the heat transfer water, will be appropriate to the desired temperatures of the sewage sludge or other material being pasteurised. It will also be appreciated that while the heat transfer medium has been described as heat transfer water, any other suitable transfer medium may be used. Additionally, while the sewage sludge has been described as being heated to an intermediate temperature of approximately 33°C.

This will depend on the temperature of the heat transfer water after equalisation of the water temperatures in the primary and secondary heat exchangers of the first and second pasteurisation tanks. Furthermore, in practice it is envisaged that a temperature difference will normally exist between the equalisation temperature of the heat transfer water and the intermediate temperature of the sewage sludge. The intermediate temperature of the sewage would normally be a few degrees centigrade below the equalisation temperature of the heat transfer water.

Additionally, it will be appreciated that the times for each step from time T1 to T8 may vary significantly depending on the material being pasteurised.