SLUDGE" TECHNICAL FIELD

This invention concerns a method for the sterilization of sludge involving the use of microwave radiation.

Throughout this specification the term "sterilization" is intended to encompass both complete sterilization processes and partial sterilization processes such as "pasteurization".

Also, throughout this specification the term "sludge" has been used to refer not only to sewerage but also to any matter loosely falling within the broad definition of the word, that is a muddy or slushy sediment. It is intended that any paste, mixture, suspension, solution or other pumpable fluidic substance having a distributed water or other liquid content will be included.

BACKGROUND ART

Sterilization is commonly carried out by autoclaving, which is a process involving the use of high pressure steam. In this process the temperature is raised to say 125°C, over a period of time to achieve sterilization. This process is expensive in terms of energy input, and inefficient for use with sludges because the temperature increase is not uniform throughout the solid parts of the sludge, and the process is difficult to operate on a continuous basis. DISCLOSURE OF INVENTION

According to the invention there is provided a continuous method for the sterilization of sludge comprising the steps of: pumping the sludge continuously through a microwave transparent pipe; maintaining the sludge in the pipe under higher than atmospheric pressure; maintaining the pipe substantially full of the sludge; and irradiating the sludge in the pipe with microwave radiation.

The process may be used to sterilize substances whic occur in a sludge-like form. Alternatively a pre-processing step may optionally be employed to reduce the matter to be sterilized to a flowable sludge-like form. Such as by, for instance, grinding and mixing with water.

The process has been found to achieve sterilization at a faster rate than previous processes, and experimental work conducted using sewerage sludge has confirmed an increase of productivity in a continuous process in excess of 25%. This advantage arises from the synergistic effect of employing microwave radiation and enclosing the sludge within the microwave transparent pipe during processing. Containment within the pipe retains the moisture content in the sludge and ensures effective coupling with the microwave energy, which results in efficient and uniform transmission of the heat to the solids in the sludge.

By ensuring a greater than atmospheric pressure within the sludge in the pipe, it is possible to prevent the energy wastage which would occur if the water were allowed to boil taking up latent energy. Also it is possible to elevate the temperature above 100°C without loss of water, and this increases the rate of kill.

It is also possible to modify the process to allow the "flashing-o f" of steam to reduce the water content of the product. This is done, using the inbuilt energy of the system under pressure and at elevated temperature, by reducing the pressure to atmospheric level once sterilization has been achieved.

Alternatively, where a high water content is required in the product, temperature and pressure are reduced to ambient levels while the product is still contained.

BRIEF DESCRIPTION OF DRAWINGS

The invention will now be described by way of example only with reference to the accompanying drawing which illustrates the steps of a method according to an embodiment of the invention.

BEST MODE FOR CARRYING OUT INVENTION

Sewerage treatment plant 1 comprises a sludge inlet 2 leading to a pumping chamber 3 from which sludge is pumped into pipe 4. A section 5 of pipe 4 is transparent to microwave radiation. A sludge collection chamber 6 collects the sludge that has been pumped through the pipe prior to releasing it through outlet 7.

Sludge in the form of liquid and solids is pumped into pipe 4, together with a certain amount of vapour and air, by means of pump 8. A flow control valve 9 is biased shut and regulated by a spring member 10. Flow control valve 9 maintains pipe 4 full of sludge and prevents leakage of microwaves during startup. Pressure is initially developed and then maintained in the system by air pumped into collection chamber 6 through inlet 11. The sludge passing through the microwave transparent section 5 of the pipe 4 is continuously irradiated by microwaves indicated generally at 12, and heated.

The combined effects of control valve 9, sludge delivery pump 8 and air pressure introduced through inlet 11, maintains the sludge under pressure as it passes through the pipe. Maintaining the sludge under increased pressure permits higher temperatures to be reached in the pipe without vapourising the liquid in the sludge.

The sludge is heated by the microwaves until steady state optimum pressure and temperature conditions are reached. These conditions have been determined by experiment to be a minimum of 125° at 2.5 atmospheres pressure absolute for a minimum of 8 minutes processing time.

Sterilization is achieved by the combined effects of high temperature, high pressure, and the direct penetration of cellular structures by microwave radiation.

Processing conditions can be varied to suit sterilization requirements and may include higher temperature and pressure conditions or longer processing times in the microwave radiation.

An important feature of the system is that the back-pressure ensures that the pipe is kept substantially full of sludge to the exclusion of vapour and other gases. In this way the microwave energy heats the sludge, where the ecoli and other bacteria are living, rather than the gas mixture above the sludge,

which is comparatively sterile. This is an important improvement in relation to the alternative where the liquid/solid sludge is allowed to flow along the bottom of the pipe and a gaseous mixture to fill the remainder (in which case the microwave energy is not used efficiently to heat the sludge, but is largely wasted in vapourising water and superheating the steam produced) .

Pressure measuring devices 13 and 14 are incorporated into the processing system to measure the pressure in pipe 4 and collection chamber 6.

A pressure equalisation system including a pipe 15 through which only vapour passes, and a monitoring device prevents large disparities in pressure from building up on either side of the gate valve 9. An emergency pressure release valve 16 prevents excess pressure building up in collection chamber 6, and an air release valve 17 may be used to de-pressurise the chamber. Samples of treated sludge may be taken from the sample point 18 for quality control.

It should be appreciated that the invention may be performed in many other configurations of apparatus, for instance the transparent section 5 may be inclined or even vertical, in which case back pressure need not be applied to maintain the pipe sufficiently full of liquid and solid phase sludge throughout the irradiated- section.

INDUSTRIAL APPLICABILITY

It should be appreciated that the process has application, in addition to sewerage sludge, to a range of flowable substances which can be pumped extruded etc. These include meat, fish, chicken litter and pig droppings.

It should also be appreciated that the process may be employed where partial sterilization is required, for instance in the pasteurization of milk.