Sludge contains pathogenic bacteria, viruses and protozoa together with other parasites which can present a potential health risk to humans, animals and. plants, particularly if that sludge is used agriculturally. In order to reduce that risk by reducing the level of such pathogens etc. down to a safe level it has been proposed that a pasteurisation step should be included as a pre-treatment to the normal sewage treatment process, that is before the step of mesophilic anaerobic digestion (MAD treatment). In the Department of the Environment (UK) code of practice for Agricultural use of sewage sludge it is proposed that to achieve satisfactory pasteurisation the sludge should be maintained at a minimum temperature of 70°C for at least 30 minutes. A similar guideline exists in the US, US code of practice US 503 Regulations.

Systems have been developed to introduce a pasteurisation step to the treatment process. In one system vast quantities of air are introduced into a vat of sludge in order to increase its temperature, however this produced instabilities and high operating costs. In a further system steam at high pressure is introduced into pressurised vessels containing sludge at 180°C and 10 bar. However, steam generation is a potentially hazardous process involving both high pressures and high temperatures and in the UK steam generation is currently regulated by"Pressure Equipment Regulations 1999"Sl 1999/2001. The regulations cover the basic concepts of safe and operating design of compressed air systems whereby skilled operators are required to constantly monitor such systems, thereby increasing the cost of sewage treatment.

It is an object of the present invention to provide a method and apparatus for sludge pasteurisation which overcomes or alleviates the drawback of the known methods.

In accordance with one aspect of the present invention there is provided a method for the treatment of sludge comprising the step of heating the sludge to a temperature suitable for sludge pasteurisation by direct, in-line injection of low pressure steam into the flowing sludge. This has the advantage that the use of low-pressure steam to heat the sludge via direct injection provides a system which is simple and reliable to operate with a rapid transfer of energy into the sludge. The low pressure steam does not require operator monitoring for its production and application and is therefore able to operate with the minimum of intervention, thereby allowing this system to be used on smaller digestion sites (sewage treatment sites) which operate unmanned for significant periods. Also costs are reduced.

In a preferred embodiment the low-pressure steam injected into the sludge is open to atmospheric pressure.

Preferably the steam is injected into the flowing sludge and in counterflow to the sludge. This enables efficient mixing of the steam and sludge for both even transfer and good energy transfer.

Preferably the temperature of the steam heated sludge is automatically monitored and if the temperature is below a defined minimum the sludge is returned for further heating by the steam. This ensures that a minimum temperature for the pasteurisation, (pathogen kill) is achieved without the need for operator intervention allowing the process to be unmanned.

The method may further comprise the step of batch storage of the steam heated sludge for a predetermined period of time at temperature to achieve pasteurisation. This ensures that the sludge is maintained at temperature for an appropriate length of time in order to achieve pasteurisation of the sludge.

The method may further comprise the step of automatically monitoring the temperature of the heated sludge at the end of the batch storage time and if the temperature is below a predetermined minimum returning the sludge for further steam heating. This ensures that the minimum time at a predetermined temperature is maintained in order to ensure pasteurisation and if the temperature of the sludge drops below the minimum required during that storage, the sludge is automatically returned for further heating. This has the additional advantage that if the minimum temperature and time for pasteurisation is maintained, adequate pathogen kill is achieved, eliminating the need for testing for pathogens, which is both time consuming and expensive.

The method may comprise the additional step of cooling the sludge after holding to bring the temperature of the sludge down to that required for the next stage of treatment.

An intrinsic benefit of the in-line steam injection is that the process becomes instantly auditable with all sludge passing forward and being readily temperature measured, controlled and logged. Preferably, the method comprises the step of modulating the steam input based on the initial sludge temperature. This ensures that all sludge passing the heating point is at a temperature suitable for pasteurisation and is also energy efficient. The method further having the advantage that all sludge passing a monitoring point which is not at the required temperature is returned to the process inlet for reheating.

In a preferred embodiment the steam is set to heat the sludge to 74°C, this allows for some hysteresis in the control loop and sludge cooling within the batch storage stage. Preferably the sludge is returned for re- heating if the temperature thereof is less than 70°C. More preferably the sludge is stored for at least 30 minutes.

In accordance with a second aspect of the present invention there is provided an apparatus for the treatment of sludge comprising a heating column through which sludge to be pasteurised may flow, the column having a steam input for the injection of tpw. pressure steam into the sludge in counter flow to the throughput of sludge. This has the advantage that the sludge is heated as it progresses through the column with an efficient energy transfer.

The apparatus may also comprise at least one holding tank in which the steam heated sludge is retained for a defined period of time and at a temperature suitable for pasteurisation. This has the advantage that the sludge is maintained for an appropriate period of time to achieve pasteurisation.

The holding tank may be insulated. This helps maintain the sludge at the required temperature during the required holding time. Preferably, there are at least three sequentially operating holding tanks. This enables a continuous feed with the tanks in a sequential fill, hold and emptying phase.

The apparatus may comprise at least, one sensor which monitors the temperature of the sludge. This ensures that the sludge at a particular stage of the process is at the required temperature. In a preferred embodiment the apparatus comprises at least one return means for enabling the sludge to be returned for further heating if the sludge is not at the required temperature at a defined stage of the process.

The apparatus may include cooling means for reducing the temperature of the sludge to a temperature suitable for its further processing. This enables the apparatus to be readily retrofit to an existing sewage (digestive) treatment system. Sludge cooled to 35°C is suitable for MAD treatment.

In a preferred embodiment the apparatus comprises a steam generator for producing said low-pressure steam, the generator being fuelled by bio gas recovered from the sludge. This provides an energy efficient means for pasteurising the sludge.

By way of example only a specific embodiment of the invention will now be described with reference to the accompanying drawings, in which:- Fig. 1 is a schematic view of an apparatus for treating sludge constructed in accordance with one embodiment of the present invention; Fig. 2 is a table showing effect of steam heating on sludge composition; and Fig. 3 illustrates operational plots for a two-holding system constructed in accordance with the invention.

Referring to Fig. 1 the apparatus for treating sludge comprises a steam heated column 2 into which raw sludge 4 is fed via a one-way valve 6. The raw sludge 4 enters at the top of the column 2 via a sensor 8 which is connected to processing means (not illustrated) controlled by software.

The sensor 8 measures the temperature of the inflowing sludge and also its rate of flow. The use of these measurements is described further hereinunder.

A steam generator (not illustrated) comprising duel fuel burners and boiler feed water treatment plants capable of utilising final effluent injects steam at the base 10 of the column 2. The steam temperature and rate of injection is controlled by the processor based on the sludge conditions measured by sensor 8 and the steam parameters are adapted to heat the sludge flowing down through the column 2 to 74°C.

The heated sludge is discharged at the base 12 of the column 2 where its temperature is measured by sensor 14. If the temperature of the sludge is less than 70°C the sludge is returned by control valve 16,18 to the start of the process for re-heating. Pump 20 delivers sludge to holding thanks 22,24,26.

If the temperature of the sludge is at or above 70°C the sludge is feed to the insulated holding tanks 22,24,26 which are filled in sequence by control valve 28,30,32. The discrete holding tanks 22, 24,26 enable the batch storage process to become essentially a continuous production operation, in that the tanks controlled by their respective valves 28,30,32 sequentially in"fill","hold"and"discharge", with normally one tank in each stage at any moment. The residence capacity of each tank is set to at least 45 minutes, ensuring a minimum residence of 30 minutes whilst providing further flexibility in the operation of the process. The charging and discharging of the tanks via their discharge control valves 34,36,38 are controlled by the processor.

Sludge exiting the tanks 22,24,26 is passed via pump 40 to a heat recover system or heat exchanger 44, whilst sensor 42 measures the temperature of the sludge. If the measured'temperature is less than 70°C the sludge is returned to the start of the process for reheating. In the heat exchanger 44 the sludge passes through a pipe about which water is passed to cool the sludge to a temperature of around 35°C which is suitable for the next and standard stage of treatment of mesophilic anaerobic digestion (MAD). The now heated water from the heat exchanger can be fed to the steam generator and utilised for producing steam, thereby reducing the energy of requirements of the generator.

As mentioned previously the steam generator is provided with duel fuel burners and one of these is run via the collected bio gas. The amount of auxiliary fuel used by the boiler of the generator will be dependent on sludge availability, sludge and atmospheric temperatures and the size of the gas holder.

Steam is an efficient, in the order of 95% means of heat transfer into sludge. The steam is injected at essentially atmospheric pressure and as such is readily controllable, however the direct injection of steam has the disadvantage that the condensate dilutes the sludge (see Fig. 2). Assuming steam is the : le sole source of energy input into the raw sludge, the condensate will result in a maximum 10 to 12% dilution. For example, this would reduce a 6% DS (dry solid) sludge feed to 5.3% DS, assuming an initial sludge temperature of approximately 15°C. Therefore to maintain a digester feed of 6°C, the sludge is thickened for the present process to approximately 6.7 to 7% pre-pasteurisation. With current operational practices this dilution is considered acceptable or readily recoverable by additional thickening.

It has been demonstrated during tests of the present system that providing the feed sludge is of at least 6% DS, the process is likely to be self sufficient in bio gas without relying on heat recovery, and with a 4% DS heat recovery is necessary.

Minimal of odorous off-gases are produced from the heating of the sludge are evolved from the column and by displacement from the filing of the batch holding tanks. This odorous air is treated with a 2 stage chemical scrubber. The first stage provides chemical oxidation for the neutralisation of such compounds as hydrogen sulphide, organic sulphurs and organic amines, while the second stage is a chemical adsorbent for the removal of non-oxidisable odorous such as ammonia.

Sludge sampling points are provided on the cold, heated and pasteurised sludge sample lines. These are provided as part of good design practice as off-line sampling is not required or anticipated as part of normal operation.. As part of the present process verification, however, quantification of the following four key pathogen indicators has been undertaken: Faecal coliforms, total coliforms, faecal streptococci, and Salmonella.

It has been determined that across the pasteurisation process, and within the limitations of the pathogen content of the raw sludge and the confidence level of the procedures used, the pathogen numbers are reduced to or below detectable levels.

Fig. 3 shows operational plots for a ; 2 tank holding tank process.

(The hold event has been manually reduced for clarity on these plots). Plot A shows the sludge temperature and flow into the heating column and the temperature exiting the column. Plot B shows the control of the heating system as vessel 1 fills followed by vessel 2. A period in the middle of the heating cycle is an idle period with sludge being recirculated without being fed forward into the holding vessels. This is a consequence of a 2 tank system. Plot C shows the exit sludge temperature confirming that all sludge is above 70°C during discharge. This demonstrates that the process can be readily controlled and monitored to ensure the required time and temperature residences together with the production of a Class A sludge.

Although a minimum sludge temperature of 70°C for a minimum of 30 minutes has been described it is to be understood that the present system is readily adapted to operate at a variety of temperatures and holding times and with any number of holding tanks. For example a sludge temperature of 55°C with a minimum hold time of 4 hours has produced an effective sludge pasteurisation with acceptable pathogen reduction. Although it has been described that sludge with a measured temperature of less than 70°C is returned to the start of the process for further heating, it is however possible for some sludge with a temperature of less than 70°C to pass for storage in that mixing within the batch tanks will buffer such small fluctuations.