Society generates a huge amount of waste per annum, much of which is domestic in origin although a large proportion is produced by industrial processes. Conventionally this waste is dealt with by landfilling at appropriate sites or incineration.

Whilst these known procedures are sufficient for conventional household and industrial waste material, that which contains potentially harmful agents require much more stringent disposal protocols due to the possibility of transmission of these harmful agents to humans and other animals, via either inhalation, ingestion or direct contact.

It is known that of the more than 1700 known microbial pathogens about 50% are zoonotic, that is to say they may be transmitted to humans from animals. As well as these microbial pathogens there are also a variety of chemical agents which are present in food of animal origin. Indeed, evidence shows that these biological and chemical agents may be subject to modification between the farm and the plate which may increase the potential risk to health.

Therefore, a specific problem exists in the disposal of animal carcasses and associated processing wastes, and there are many situations where such disposal becomes necessary. Such carcasses may be both mammalian and non- mammalian and furthermore may contain a number of biological and chemical

agents which may pose a specific health risk to humans. These risks depend upon a number of factors which may be summarised thus: The nature and amount of the agent present in the animal carcass ; The prospect of intra and interspecies transmision; The processing and disposal method used; and The potential human exposure as a consequence of the processing and disposal method utilised.

A relatively new and potentially deadly infectious agent which has been afforded a great deal of public attention since it was first diagnosed in 1986 is Bovine Spongiform Encephalopathy (BSE). BSE is the bovine specific form of a larger group of diseases which together are termed Transmissable Spongiform Encephalopathies (TSEs), and which include Scrapie, which affects sheep, and Creutzfeld-Jakob disease (CJD) which affects humans.

It is thought that BSE was caused by feeding cattle with contaminated animal feed and it has been postulated that infected tissue from cattle entered the food chain and caused incidents of new variant CJD (nvCJD) in humans.

To the 3 lSt July 1999, forty three cases of probable or definite nvCJD had been identified in the United Kingdom.

In order to contain the epidemic of BSE in the United Kingdom, the UK Government ordered the Feed Ban in 1988 to prevent potentially infectious material being incorporated into ruminant feed and also adopted a policy of slaughtering cattle which have the most risk of developing BSE. This included any animals which might have been exposed to contaminated feed.

As the average incubation period for BSE is of the order of five years this

represented a significant number of cattle. Any cattle suspected of infection were examined after slaughter to establish the infection rate and the carcasses disposed of by incineration. This represented a significant cost as the farmer had to be refunded for the value of the cattle and the examination and disposal costs had to be centrally funded (in principle 70% of the cost is met from an EU budget).

Furthermore, and as a consequence of the conclusions in 1996 of the Spongiform Encephalopathy Advisory Committee (SEAC), the UK Government established the Over Thirty Month Scheme (OTMS) which prohibits the sale of meat from cattle aged over 30 months at slaughter. The SEAC advised that the carcasses from these cattle are deboned in licensed plants and the trimmings classified as Specified Bovine Offal (SBO), which was banned in 1989. Whilst the deboning recommendation was considered to be impractical the meat from cattle falling within the scheme is prohibited from sale.

The current practice adopted in the UK for the disposal of OTMS carcasses is rendering at temperatures of at least 133°C and at pressures of 304 kPa (3 atm). This provides a substance known as meat and bone meal (MBM) which may be stored in sealed repositories prior to disposal. Initially, however, rendering plants in the United Kingdom did not meet these criteria and consequently the stored MBM was rendered to a lower standard.

At present, the recommended method of disposal of MBM is incineration and the number of suitable facilities is insufficient to dispose of all the MBM which has been generated. Indeed, official figures show that up to the week ending 26 tu September 1999 only 242842 cattle (~ 7%) had been incinerated, the rest being held in storage until such time as incineration is possible. This obviously provides a risk as unforeseen circumstances may cause the repositories to be breached and thereby cause exposure. Indeed there

have been reports that certain facilities have become feeding grounds for rodents which can provide a vector for reintroduction to the food chain or direct exposure to humans.

It has been found that the infectious agent which cause TSEs is a so- called prion (PrP, of which the BSE analogue is denoted PrPBSE). These prions are protein structures comprising polypeptide chains folded into domains within three-dimensional space. As with all proteins the peptide linkage bonding the individual constituent amino acids together consists of strong covalent bonds strengthened further by delocalised 7C-bonding. The folding pattern of the polypeptide chains is determined by relatively weak interactions, such as hydrogen bonds, Van der Waals forces and electrostatic interactions, between the constituents. Obviously, the activity of any protein is determined by both its absolute constituents and by its three-dimensional configuration (which is necessarily controlled by its constituents and their chemical environment).

The BSE prion has been found to be very resilient to conventional destruction methods compared to other proteins. It is relatively resistant to treatment with proteolytic enzymes and heat treatment and, as such, the specific mode of disposal and destruction/inactivation is of primary concern. It is known that hot alkaline hydrolysis, that is to say boiling the prion in a strongly alkaline solution, will break the prion into its individual amino acid constituents.

Accordingly, it is an object of the present invention to provide a method of disposal of waste materials which contain infectious agents and which addresses and obviates the problems discussed above. In particular it is an object to provide a method which will sustain large-scale disposal of OTMS MBM and other potentially TSE infected materials and which renders any TSE infectivity contained therein to be inactivated and/or immobilised.

According to the first aspect of the invention there is provided a method of waste disposal comprising mixing the waste with an alkaline substance and water, depositing the resulting mixture and leaving the so- deposited mixture to harden such that it forms a solid matrix.

Preferably the waste contains, or potentially contains, an infectious agent, and preferably at least 90% of any that which is present is rendered inactive. The solid matrix renders substantially all the infectious agent that is present immobile. Preferably the infectious agent comprises a protein, which preferably is a prion.

According to a second aspect of the invention there is provided a method of disposal of cattle carcasses comprising rendering the carcass to provide MBM, as herein before defined, mixing the MBM with an alkaline substance and water, allowing the constituents of the mixture to react, depositing the mixture, and allowing the so-reacted, so-deposited mixture to harden such that it forms a solid matrix.

Preferably the alkaline substance is cement and/or lime and may additionally comprise air pollution control residues. The water may be either potable water or a mixture of potable and contaminated waste water. The cement may be waste cement such as that not suitable for its intended use upon manufacture.

The alkalinity of the alkaline substance is such that when the substances are mixed the pH is over 10 and preferably greater than, or equal to, 12.

The amount of water added to the other constituents is preferably in the range from 20 to 50 % (w/w), more preferably 22 to 45 % (w/w), and more

preferably still in the range from 23 to 32 % (w/w). The ratio of the amounts of alkaline substance to waste in the mix (w/w) is preferably in the range from 1.5 to 0.7, more preferably 1.3 to 0.8 and more preferably still 1.0.

Preferably the reaction between the constituents is such that hydroxide ions cause proteins contained within the waste or MBM to denature thereby rendering them inactive. The proteins are preferably prions such as those responsible for TSEs, and the infectivity of the mixture is preferably reduced by at least 90%.

Preferably the air pollution control residues are at a temperature in excess of 50°C, and more preferably at a temperature in excess of 85°C upon mixing, the temperature increasing the reduction of TSE infectivity in the mixture.

Preferably the mixture is deposited in a site which is suitable for landfill such that any leachate produced within the site is contained therein, and preferably once the site has been filled, and the mixture is fully hardened, it is buried.

A method of waste disposal in accordance with the various aspects of the invention will now be described by way of example only with reference to the accompanying drawing.

Referring to Figure 1, there is shown a schematic diagram of the process used in the disposal of wastes, specifically MBM or the residues following incineration thereof.

The apparatus used comprises a store 1 to hold the MBM, one or more silos 2 which contain air pollution control residues (APCs), also known as flue gas treatment residues (FGTs), and/or cement and/or lime and a container 4

which contains water. This is all mixed together, in the required amounts, in the mixing unit 3. The MBM store 1 is a specially designed sealed unit in order that once the MBM has been delivered to the store 1 it is contained until such times as it is conveyed 5,5', in a sealed conveyor into the mixing unit 3.

The conveyors 6,6'and 7, for respectively conveying the APC/cement/lime constituent and the water, are conventional conveyors known in the art for such a purpose, such as screw conveyors for solid material and pumps and pipework for the liquid.

The flow of each constituent into the mixing unit 3 is metered such that the proportions of the mix can be controlled. Once mixed, the damp mix of small particles is conveyed 8 to a suitable landfill site by either a truck or by a conventional conveyor belt. The so-deposited mix solidifies within two or three days depending on the weather conditions and the amount of water added, thereby encapsulating the MBM within the matrix.

Whilst the addition of water is necessary to enable the matrix to solidify, it also renders MBM particles immobile with respect to wind action.

A potential vector for TSE infectivity is atmospheric wind which may transport potentially infected MBM particles from the site during the encapsulation process. However, the water content of the mixture is such that dust production is substantially prevented and therefore the material is contained on site.

The so-solidified matrix in which the MBM is encapsulated is known as a monofill cell. That is to say, a cell which is composed exclusively of the MBM cementised mix and no other waste product. Each cell may be of the order of 250000 m3.

In practice the relative amounts of alkaline substances (the sum of APCs, cement and lime added), MBM and water (either potable or potable and

contaminated waste waters) are such that the pH of the resultant mixture is highly alkaline. It has been found that it is preferable that equal amounts (w/w) of alkaline substances and MBM are co-mixed and that the total amount of water is of the order of 25 w/w %. However, more or less water may be added depending on the environmental conditions, such as the weather, or on the nature of the mixed substances. Indeed the ratio of alkaline substances to MBM may vary between 1.5 and 0.7, although, as stated above, equal amounts are preferable.

APCs are the highly alkaline residues from the scrubbing units of municipal waste incinerators. They contain large amounts of lime (CaO) which is readily hydrated to form so-called hydrated lime Ca (OH) 2, a base.

Once a solution of hydrated lime is left, it dries to form a solid, cement-like, structure.

The APCs may be mixed with cement and/or extra lime to provide the alkaline substrate. The cement may be waste cement, which is not suitable to perform its intended role, and this may aid the solidification of the monofill cell. The water used may be either potable water, or a mixture of potable and contaminated waste water. Thus, this method of disposal of MBM simultaneously provides for the disposal of other unwanted substances such as APCs, waste cement and contaminated waste water.

The MBM produced by rendering plants is a mixture of all of the tissue of a cow, thus hair, bone, hooves, meat and tallow (a mixture of the less fusible fats, comprising olein, palmatin and stearin) may all be present. The rendering process, as described above, is known to reduce TSE infectivity by greater than 99%, but this still provides for a small, yet significant, risk.

As previously stated, the MBM produced and placed into store prior to 3 December 1998 had a range of particle sizes, whereas that produced after

that date was required to be ground such that the particles exhibit a uniform size.

Some of the MBM particles may be coated in fats, which make it difficult to process as the hydrophobicity of the fat repels water from the particles. However, it has been found that exposure to hydroxide ions, produced from Ca (OH) 2, causes catalytic saponification of any fat and this enhances the permeability of the hydroxide ions through MBM particles. The variance within the size range of particles which comprise older MBM helps in the encapsulation process. The aggregate formed between the cementised alkaline material and the MBM is stronger with a wide range of particle sizes.

Whilst the environment in which hydroxide ions come into contact with the prion is not volatile enough to directly hydrolyse the polypeptide chains, and so break them into their constituent amino acids, it has been found that the alkalinity (pH) of the mix is such that hydroxide ions are able to denature the prions. That is to say, the forces which hold the secondary and tertiary structures of the prion together, that is those which fold the polypeptide chains and maintain their three-dimensional structure, and in so doing provide their activity, are amenable to hydroxide ion attack.

Thus, percolation of hydroxide ions through the matrix, and subsequent permeation through MBM particles allows the hydroxide ion to destroy the three-dimensional structure of the prion and consequently render it inactive.

In order to denature the prion, the pH of the mixture is greater than 10 and preferably 12 or over which, at pH 12, gives an effective hydroxide concentration of 10-2 M. Sufficient water has to be added to the mix to provide means for the so-formed hydroxide ions to percolate through the mixture and come into contact with the MBM particles.

The APCs which are delivered from incinerators are often delivered at elevated temperatures, which may be as high as 90°C. This elevated temperature increases the activity of the hydroxide ions and, consequently, increases the rate of prion denaturation. Furthermore, addition of water to APCs is an exothermic reaction and, therefore, heat is generated in situ of the matrix which increases the temperature therein and, consequently, the rate of denaturation.

The percolation of hydroxide ions throughout the mixture depends upon the amount of water present. As the monofill cell solidifies the percolation of hydroxide ions ceases although a limited amount may continue due to waters of hydration contained within the MBM particles. However, any non-denatured prion is encapsulated within the cell and is thus immobilised. It has been found that prions become tightly bound to particulate matter due to their biophysical properties, and in particular their hydrophobicity. Thus, within the monofill cell the prions will become bound to the MBM/APC matrix.

The landfill sites to be used as disposal sites for the monofill cells are such that they minimise the ingress of water, and moreover, minimise the egress of leachate from the site. Such sites are lined with basal clay which renders them suitable for such uses. Indeed, the sites are designed with leachate collection systems to minimise the leachate head acting on the base of the site, thereby minimising the passage of leachate through the containment barriers. Furthermore, prior to burial each monofill cell is capped with at least a 1 m layer of engineered, compacted clay. This reduces the likelihood of ingress of water into the cell. It has also been found that, at the elevated levels of alkalinity used, it is unlikely that the monofill cells are susceptible to microbial degradation.

As discussed above, prions become tightly bound to particulate matter and consequently are unlikely to partition into the liquid phase. If any such prion does partition into the liquid phase available evidence suggests that it would be retained in the upper layers of the basal clay which lines each site.

The efficacy of the disposal technique has been established and it is known that a range of materials are processable by this method. For instance, a number of potentially low-level contaminated waste may be so-processed.

Indeed, any permeable substance containing, or suspected of containing an infectious protein may be treated and disposed of according to the above- exemplified method. Whilst the accepted method for disposal of high-risk wastes is incineration, this method also provides means for disposing of the residues of this incineration process safely and securely.