GB2230004A

GB2230004A - Method for treating solid waste

Info

Publication number: GB2230004A
Application number: GB9008034A
Authority: GB
Inventors: Christopher Paul Richa Reynell
Original assignee: COOK ARTHUR ERNEST; PALLETT IVOR; RIDDELL KEITH JOHN
Current assignee: COOK ARTHUR ERNEST; PALLETT IVOR; RIDDELL KEITH JOHN
Priority date: 1989-04-08
Filing date: 1990-04-09
Publication date: 1990-10-10
Anticipated expiration: 2010-04-09
Also published as: GB9008034D0; GB2230004B

Abstract

A process for digesting a solid waste material comprises: (a) subjecting the decomposable fraction of a solid waste to anaerobic bacterial digestion, so as to produce a fluid fraction, a methane rich gas fraction and a solid fraction which is environmentally more acceptable than the feed solids; (b) subjecting a fluid waste material, which may be the fluid fraction from the solids digestion stage or a separate fluid waste material, to anaerobic bacterial digestion in a digestion stage separate from the solids digestion to produce a fluid fraction containing active anaerobic bacteria and a methane rich gas fraction; (c) feeding at least part of the fluid fraction from the fluid digestion stage to the input to the solid digestion stage; and (d) recovering at least part of the environmentally more acceptable solids fraction from the solids digestion stage. <IMAGE>

Description

TITLE: METHOD FOR TREATING WASTE The present invention relates to a method for treating waste, notably for treating fluid and solid wastes in two stages.

BACKGROUND TO THE INVENTION: Waste from many sources is produced in both solid and fluid forms and each form has hitherto required separate methods of treatment in order to render it suitable for discharge to the environment. Thus, many forms of fluid waste can be subjected to aerobic or anaerobic digestion to decompose the organic components thereof to produce an environmentally acceptable fluid which can be discharged, optionally after further treatment, into drains, streams, rivers etc, or can be used as a fluid fertiliser. This digestion may also produce a solid fraction which can find use as a soil conditioner. Many forms of such digestion process also generate methane gas which can be used to provide a useful source of energy.

For solid wastes, it is often necessary to subject the waste to a number of separation stages to remove glass, metals and non-digestible materials to leave a residue which can either be buried or which can be subjected to bacterial decomposition to yield a solid by-product which also can find use as a soil conditioner.

Various forms of each of the above types of process are known and have been used to treat municipal and other wastes. However, since each type of waste requires an essentially different type of treatment, such treatment has hitherto been carried out in separate processes and usually on separate sites under the control of different organisations.

Furthermore, when handling solid wastes, the waste is often disposed of by burying it in a land infill site without subjecting it beforehand to bacterial decomposition.

However, the waste material will undergo decomposition in the soil after it has been buried and this can lead to major health and safety problems due to the slow release of methane from the decomposing material. It has therefore been proposed to treat the solids with a bacterially active fluid under acidic conditions. These conditions minimise the formation of methane as the solids decompose and produce a digestable fluid effluent which can be recovered and subjected to a separate decomposition process under conditions which favour the formation of methane. In this way, the solids undergo decomposition within the soil, but the problem of methane generation in the soil is minimised.

The methane values in the fluid phase are recovered in the separate process stage under conditions where it can be collected and used without the safety and health hazards of methane generation within the soil. Such a two stage process requires that positive steps are taken to prevent methane generation during the solids decomposition stage.

Surprisingly, we have found that if an anaerobic fluid digestion process is operated in series with, but separate from, a solid waste anaerobic digestion process, with at least part of the digested fluid being re-cycled to provide at least part of the fluid input to the solids digestion process, the rate of digestion of the solid waste is greater than could have been predicted from the operation of either process in isolation. Furthermore, generation of methane in both the solids and fluid digestion process stages can be optimised to assist digestion of the organic components in solid and fluid wastes to yield products which are environmentally more acceptable than the intial waste materials and which can be discharged to the environment or can be converted into useful products.

SUMMARY OF THE INVENTION Accordingly, the present invention provides a process for dige@ti@g @ s@li@ @@@@@@@@@, which pr@cess comprises a. subjecting the decomposable fraction of a solid waste to anaerobic bacterial digestion, so as to produce a fluid fraction, a methane rich gas fraction and a solid fraction which is environmentally more acceptable than the feed solids; b. subjecting a fluid waste material, which may be the fluid fraction from the solids digestion stage or a separate fluid waste material, to anaerobic bacterial digestion in a digestion stage separate from the solids digestion to produce a fluid fraction containing active anaerobic bacteria and a methane rich gas fraction; c. feeding at least part of the fluid fraction from the fluid digestion stage to the input to the solid digestion stage; and d. recovering at least part of the environmentally more acceptable solids fraction from the solids digestion stage.

Preferably, the process of the invention is applied to the treatment of both solid and fluid waste materials with the fluid waste material being fed either to the input to the solids digestion stage or to the input to a separate fluid digestion stage. However, the invention can be applied to the treatment of only solid waste, with water or water containing nutrients for the bacteria (for example urea or nitrogenous fertilizers) being fed to the input to the solids digestion stage.

The fluid waste material for use in the fluid digestion stage is provided at least in part by the fluid fraction from the solids digestion stage, so that the fluid fraction recycles through the solids digestion stage, in which case make up water, optionally containing nutrients for the bacteria will usually be required. Where a separate fluid effluent or waste feed is to be treated, this can be an animal waste, for example the slurry from a pig, cattle or poultry house or from an abattoir; or a municipal domestic sewage, which may have been partially settled to give a sewage sludge and this can be fed either to the input to the solid and/or to the input to the fluids digestion stages.

The fluid digestion stage can be operated as a continuous or discontinuous operation in two or more digestion vessels operated in series or in parallel.

The solid waste to be treated in the process of the invention is preferably derived from domestic refuse and typically contains predominantly decomposable organic materials, but some, eg. up to 20% by weight, of nondecomposable solids may be present. This feed material can be obtained from municipal and other solid wastes by subjecting them to some pre-treatment to remove metallic and non-decomposable fractions, for example-using rotating drum sieves, ballistic or cyclone type separators as is known in the waste treatment technology, so as to leave a digestible fraction containing predominantly organic material. If desired, the paper fraction in such wastes can be removed for use in the preparation of pelleted or floc fuel for use on site to generate heat and/or power, or for sale as a solid fuel.

The solid waste digestion stage is typically carried out as a batch process, but may be carried out on a continuous basis if desired and in two or more solids digestion vessels operated in series or in parallel.

For convenience, the term tank will be used herein to denote the vessel(s) within which the fluid waste is to be digested and the term tower will be used to denote the vessel(s) in which the solid waste is to be digested. It will be appreciated that the vessels in each stage can take @ @a@ie@@ @@ forms and @@@@es and @@@, for ex@mple be located above or below ground. In a particularly preferred form of the invention, the two stages are carried out in a single vertical tower having an upper solids digestion zone and a lower fluids digestion zone, preferably the two zones are separated by a transverse partition which allows passage of fluid from the upper zone to the lower zone, but prevent the passage of solids.The fluid draining from the upper solids digestion zone is used to provide at least in part the fluid feed to the lower digestion zone. Fluid input, for example a fluid effluent, can be fed to either the upper or lower zones and at least part of the fluid from the lower zone is recycled to the input to the upper zone.

Where a batch solids digestion process is being used, the solid digestion towers or series of towers will be operated in parallel, with one or more towers being filled or emptied whilst other towers are in operation. Thus, for example with four towers, one will be loading solid waste, two will be in use and actively digesting the solid waste and the fourth will be discharging digested solids.

Where a continuous operation is used, two or more towers can be operated in parallel or series so that the waste is continuously fed to and discharged from the towers.

Typically, it will be desired that from 5 to 25% of the volume of each tower be exchanged per day.

In the process of the invention, fluid is recycled between the fluid and solids digestion stages. However, some fluid will have to be removed from the system to maintain the desired overall balance of materials. This fluid can be removed from the discharge from either or both of the fluid and solid digestion stages. Ideally, the two stages are operated so that the fluid discharge is acceptable for direct discharge to the environment, eg. into drains or other environmental water systems, or for use directly as a fluid fertiliser. However, it may be necessary to subject at least part of the fluid discharge to further treatment, for example using conventional water treatment methods, to render it fit for direct discharge to the environment.

The solid digestion is carried out until adequate digestion of the solids has been achieved to give a solids fraction which is environmentally more acceptable than the solid feed. It is preferred to operate the solids digestion stage so that the solids fraction is environmentally acceptable without further treatment, so that the solid discharge from the solids digestion stage can be used directly as a soil conditioning agent or can be pelleted for use as a solid fuel.

In some cases, the fluid and solid discharges from the digestion stages may contain proteinaceous materials which can be recovered for use in animal feeds.

As stated above, the digestion stages are carried out under conditions which favour the formation of a methane rich gas phase. The term methane rich is used herein and in the claims to denote that gas phase contains at least 30% by volume of methane. In the case of the solids digestion stage, the gas phase-will typically contain from 40 to 50% of methane;-and in the case of the liquid digestion stage from 60 to 80%. The conditions required to achieve a methane rich gas phase are that the mixture being digested have an overall pH value of at least 7, typically 7 to 9.5.

Such pH conditions can be achieved by the addition of a liquid or solid base or alkali to the digestion mixture.

Typically, ground chalk or lime can be added to the mixture, for example in admixture with the solid feed, or @@@@ous @@@@@@@ @@@@@ on c n @e added to the liquid phase.

The optimum amount of base or alkali can readily be established by simple trial and error having regard to the nature of the material to be digested. We have also found that the rate of feed of material to the digestion stage can often affect the pH of the mixture being digested.

Again, the optimum rate of addition of feed materials can readily be determined by simple trial and error. The amount of fluid phase in the solids digestion stage may affect the rate at which the solids can be digested and we have found it advantageous to maintain the solids to liquid ratio so that the organic loading rate is 7.5 to 12.5 kgs volatile solids/cubic metre digester volume/day. It is also preferred that the total liquids feed to solids feed to the digestion system weight ratio is in the range 2:1 to 6:1.

We have further found that it is desirable to pre-heat at least part of the solids fed to the solids digestion stage to a temperature above ambient, but below that at which thermal destruction of the desired bacteria occurs. By gently pre-heating the solids feed we have found that the time taken to produce an environmentally acceptable decomposed solid can be markedly reduced. Typically, the solids feed will be pre-heated to a temperature of from 25 to 600C.

DESCRIPTION OF THE DRAWINGS: To aid understanding of the invention, it will now be described with reference to two preferred forms thereof and with respect to the accompanying drawings in which Figure 1 is a block flow diagram of the pretreatment of the solid waste material for use in the method of the invention; Figure 2 is a diagrammatic flow diagram of the solid and fluid digestion stages of the form of the method of the invention where the solid and fluid digestion stages are carried out in separate vessels; and Figure 3 shows in diagrammatic vertical section a tower for use where the digestion stages are carried out in a single vessel having upper and lower digestion zones.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION: Solid refuse waste from a town or city typically contains about 40% w/w paper, cardboard and other light-weight combustible materials, about 20% w/w biodegradable organic kitchen, garden and vegetable materials, and about 40% w/w non-biodegradable materials, such as glass, metals, stones or plastics. Such refuse can be separated into its major constituents by various processes as is known in the waste treatment art. The invention can also be applied to solid wastes from other sources which contain biologically degradable material. For convenience, the invention will be described hereinafter in terms of the treatment of a municipal waste.

As shown in Figure 1, the solid waste material is initially sieved in a rotary drum type of sieve to produce a fines fraction which is predominantly the organic fraction of the waste. The material passing through the drum can be passed through a hammer mill or the like to break up large agglomerates of material and the material re-cycled or passed through a second drum sieve to remove further organic material.

The material from the drum sieve is then passed through one or more air classifiers or the like, which separate the lighter material, such as cardboard or paper, from the heavy particles of metal, stones, etc. The lighter fraction contains predominantly combustible material which can be burnt on site to generate heat and power, can be subjectded to drying and pelletieing @ta@@s as shown in Figure 1 to give a pelleted fuel for use or sale, or part can be comminuted, eg. in a shredder or the like, and returned for admixture with the organic fraction for treatment according to the method of the invention.

The heavy fraction is largely free from bio-degradable material and can be used in land infill with a reduced risk of methane generation due to decomposition of organic materials. Alternatively, some of the heavy fraction can be used as an inert filler in the organic fraction which is to be subjected to anaerobic digestion as described below.

The organic material is not suitable for direct discharge to the environment and is to be treated by the method of the invention in the solids digestion stage.

Municipal domestic sewage or non-toxic industrial fluid effluent provides the fluid waste material which is to be digested in the method of the invention. However, as indicated above, agricultural effluent such as an animal or poultry house effluent, or the fluid waste from animal slaughter can also be used to provide part or all of the fluid fed to the fluid digestion stage. If desired, at least part of the fluid can be supplied as water or aqueous solutions or slurries of nitrogenous nutrients for the bacteria in the solids digestion stage.

The raw sewage or fluid effluent can be subjected to a settlement pre-treatment to remove excess fluid, which is treated in a conventional sewage works, and provide a higher solids content, eg. upto 108 w/w, fluid feed for use in the method of the invention.

The fluid waste is subjected to anaerobic digestion in the presence of bacteria to provide an output which is biologically active, but in which much of the environmentally unacceptable material has been converted into acceptable material and methane gas. Typically, the fluid digestion stage will require the presence of a bacterial inoculum to initiate the digestion and this may be provided by the use of partially treated sewage sludge in the initial feed material. However, since the fluid digestion is preferably operated continuously and material is to be recycled within the process, it is usually only necessary to provide an inoculum during start up of the process or following a period of shut down, for example for repair or maintenance of a digester vessel.

The fluid digestion is carried out under anaerobic conditions and these can be obtained using a wide range of suitable digestion vessels, as is known in the art.

Various forms of vessel and digestion system are available commercially and may be used as such without significant modification in the method of the invention. Thus, the vessel can comprise a main tank body made from steel or other suitable sheet material, preferably having a rubber, plastic or glass coating to reduce corrosion and to aid cleaning of the interior of the tank when required. The fluid waste is fed under gravity or by a suitable pump into the tank, preferably into the upper levels thereof, eg.

into the top as shown in Figure 2. The fluid can be passed over a screen to remove excessively large solid detritus, but is not otherwise treated to remove any solids which may be present.

Th# 3 tank is preferably provided with heating/cooling elements therein to control the temperature within the tank during the digestion, since it will usually be desired to maintain the temperature within the tank within the range 25 to 800C. The heating/cooling elements can be fed with water, gas or other heat exchange medium which has been heated an6/or rov Is -efl:ir.d under the sllservision of conventional control systems and equipment.

It is also desired that the waste within the tank be subjected to agitation either on a continuous basis or at predetermined intervals. This can be achieved by the use of conventional mechanical agitators. However, a particularly preferred method of agitation is to pass part or all of the gas which is evolved during the digestion process through one or more spargers within the tank as shown in Figure 2.

The digestion generates carbon dioxide and methane gas in proportions which vary, as is known, with the pH under which the digestion is carried out. In the process of the invention, the proportion of methane in the gas phase is enhanced by the use of pH values of 7 or more in the digesting mixture. This can be achieved by the addition of lime or ammonia to the fluid digestion stage. However, the required conditions may also be achieved by the addition of a suitable alkali or base to the solids digestion stage to produce a fluid phase having the desired pH value.

The methane rich gas phase is preferably vented from the top of the tank and collected for use to provide the agitating gas as described above and/or to provide a source of heat and power for the process. For example, the gas can be burned to provide the heat to control the temperature within the tank and/or to pre-heat the solids waste materials.

The output from the fluid digestion stage is a fluid containing suspended solids and active bacteria, but which can have been digested to the point where it is acceptable for discharge to the environment. At least part of this output is to be used to provide the bacterially active fluid for use in the digestion of the solid waste as described below. However, the total output from the fluid digestion stage may be more than is required in the solids digestion stage. The excess fluid can be treated to separate any remaining solids therefrom and passed to a conventional water treatment process to render the fluid acceptable for use as a fluid fertiliser or for direct discharge into a drain or other environmental water system.

The separated solid fraction from the fluid digestion stage can be recovered for use as a soil conditioning agent or for pelleting into a solid fuel.

The solid organic waste material obtained as described above, is fed to the solids digestion stage which can be carried out in any suitable form of anaerobic digestion vessel or tower. Various forms of suitable digestion tower are available commercially and may be used as such without significant modification in the process of the invention.

Thus, the solid waste will usually be fed by auger, conveyor belt or other suitable means from a storage hopper or directly from the output of the separation process described above, to an inlet at the top of the digestion tower as shown in Figure 2. If desired, the material travelling on the conveyor or other feed means to the tower can be pre-heated by passing heated air over it or by passing hot air through a tumbling bed of the material in a rotary drum or the like. The optimum form and rate of heating will depend upon the bacteria used in the solids digestion stage and can readily be determined by simple trial and error 'efts. Typically, it will be desired to pre-heat the solids fed to the solids digestion stage to a temperature of from 20 to 800C, for example 25 to 500.

As indicated above, ground chalk, lime or other solid or liquid base or alkali is incorporated into the solids @ixtu@e @@ @@ digested. Typically, this will be by @dding the particulate chalk or lime to the solids fed to the preheating stage.

Typically, the bacteria for use in the method of the invention will be selected from one of more of Bacteroides succinogenes, Butyrivibrio fibrisolvens, Ruminococcus flavefaciens, Clostriduim thermocellum, Thermoanaerobium brockii, Clostriduim thermohydrosulphuricium, bacteroides amylpohilis, Succinivibrio dextrinosolvens, Selenomonas ruminantium, Streptococcus bovis, Veillonella alcolescens, Peptostreptococcus elsdenii, Bacteroides ruminicola, Methanobacterum ruminantium, Methanobacteruim thermoautothrophicum, Methanobacterium arbophilicum, Methanospirillum, Methanosarcina barkeri, Methanococcus, Methanothix, Desulphotomaculum,, Desulphovibrio.

As stated above, it is preferred to operate the solids digestion stage as a batch process with two or more towers operated in parallel and sequentially being charged and emptied. Typically, the towers will be charged with the desired amount of solid waste and digestion then initiated by the introduction of fluid from the output of the fluid digestion stage described above. This fluid is preferably introduced through one or more spargers located at the top of the digestion tower and/or onto one or more transverse distribution meshes or plates, for example those sold under the Trade Mark Flocore, to assist uniform distribution of the fluid across the tower.

The fluid percolates down the tower and covers the exposed surfaces of the solids within the tower, which are thus subjected to bacterial digestion by the bacteria within the fluid. It will be appreciated that the fluid need not flow down the contents of the tower under gravity but may be pumped to increase the flow rate through the tower. It is also within the scope of the present invention for the fluid to flow up the tower, if desired. For convenience, the invention will be described hereinafter in terms of simple gravity flow of the fluid down the tower.

It is preferred that the fluid is fed to the tower at such a rate that it forms a fluid coating upon the particles of solid within the tower and does not collect to any substantial extent as free bodies of static fluid within the tower. Typically, the fluid will be provided so as to at least saturate the solids in the digester vessel.

However, it may be possible to operate smaller digestion towers with the tower filled with the fluid having the solid particles suspended therein.

As indicated above, the solid waste can contain nondigestible solids and these act as fillers within the waste and may assist in the prevention of the formation of compacted and fluid impenetrable masses within the tower and will increase the effective surface area upon which the bacteria in the fluid phase can adhere, thus enhancing the bacterial action within the tower.

As with the fluid digestion stage, it will usually be desired to control the temperature within the solids digestion tower, for example by cooling or heating the tower walls, or by heating or cooling the fluid fed to the tower to within the range 25 to 800C. The digestion process will release methane and carbon dioxide gas and this is usually vented from the top of the digestion tower. This gas can be heated or cooled and recycled through the tower to regulate the temperature within the tower, and also to agitate the fluid film which covers the solid particles within the tower.

At least part of the fluid draining from the solids t cn r forrlri tart of the input to the fluid digestion stage. As stated above, since fluid, usually a fluid waste, is fed to the method of the invention from an external source, a corresponding volume of fluid must be discharged from some point in the process, for example either from the fluid digestion stage as described above and/or from the output from the solids digestion tower.

Where fluid is discharged from the process at the solids digestion tower, this can be subjected to other treatments as described above to render it fit for direct discharge into the environment. Alternatively, the fluid discharge from either stage of the process of the invention can be subjected to other treatments, for example to recover fertiliser values therein which are typically of a slow release form and do not give rise to the problems of nitrate run off encountered with conventional solid fertiliser compositions. The fluid compositions may also contain protein values which can be recovered for use in animal feedstuffs. Such other recovery processes can be of a conventional nature.

The solids digestion process is carried out until the solid waste has been digested to a sufficient extent. This will usually be achieved when the solid is acceptable for direct discharge or use in the environment. Surprisingly, we have found that this can be achieved in a shorter time than would be required if the solid waste were digested in the customary manner without the recycle of material between the fluid and solid digestion stages.

The solid discharge from the solids digestion tower can be recovered in any suitable manner. Thus, where the tower is operated on a continuous basis, digested solids can be removed, for example by auger, from the base of the tower as feed solids are fed to the top of the tower.

Alternatively, the tower can be drained of fluid and then the whole of the remaining solids can be discharged where the tower is operated on a batch process.

The solids discharged can be dried and processed as required having regard to the end use to which they are to be put. Typically, the solids will be dried and blended with fertilizer nutrients to provide a soil conditioning agent.

In the form of the method of the invention shown in Figure 3, the solids digestion tower is located above the fluids digestion tank so that the two stages are carried out in separate zones within the same overall vessel. It is preferred that the upper solids digestion zone be separated by a transverse partition from the lower fluid digestion zone. The partition can be apertured to enable fluids to drain directly from the upper zone into the lower zone.

However, it will usually be preferred that the partition be imperforate so that the flow of fluid into the lower zone can be regulated, for example by a simple overflow device or by means of a transfer pump.

The digestion of the solids is carried out essentially as described above with respect to Figure 2 to give a digested waste which is suitable for discharge at the foot of the upper zone, or which can be fed to the solids digestion stage of a system as shown in Figure 2 for further treatment. The fluid flowing down the upper zone can be water, nutrient solutions or slurries or a fluid waste.

However, at least part of the fluid in the upper zone is provided by recycle of fluid from the lower zone. As the fluid percolates down the upper zone, bacterial digestion of the solids occurs to provide a bacterially active fluid phase. Where the fluid feed contains a waste material, this too will undergo bacterial digestion.

The fluid collecting at #t he f@@t of he e upper zn##e #t' be fed to the lower zone, or part thereof recycled to the feed to the upper zone to provide part of the fluid feed to the upper zone. The fluid fed to the lower zone undergoes further bacterial digestion to provide a bacterially rich fluid which may be used as part of the feed to the upper zone and/or can be discharged from the system for further treatment as described above to recover useful values therefrom.

The process of the invention thus provides a method by which both solid and fluid wastes can be treated in a single process more rapidly than hitherto to provide a range of environmentally acceptable end products.

Claims

WHAT WE CLAIM IS:

1. A process for digesting a solid waste material, which process comprises: a. subjecting the decomposable fraction of a solid waste to anaerobic bacterial digestion, so as to produce a fluid fraction, a methane rich gas fraction and a solid fraction which is environmentally more acceptable than the feed solids; b. subjecting a fluid waste material, which may be the fluid fraction from the solids digestion stage or a separate fluid waste material, to anaerobic bacterial digestion in a digestion stage separate from the solids digestion to produce a fluid fraction containing active anaerobic bacteria and a methane rich gas fraction; c. feeding at least part of the fluid fraction from the fluid digestion stage to the input to the solid digestion stage; and d. recovering at least part of the environmentally more acceptable solids fraction from the solids digestion stage.

2. A process as claimed in claim 1 wherein an alkali or base is added to one or both of the solid and fluid digestion stages so as to maintain the overall pH of the mixture being digested within the range 7 to 9.5.

3. A method as claimed in either of claims 1 or 2 wherein the fluid waste material is provided at least in part by the fluid fraction from the solids digestion stage.

4. A method as claimed in any one of claims 1 to 3 wherein a fluid waste material from an external source provides at least part of the fluid fed to the method.

5. A method as claimed in any one of claims 1 to 4, wherein the solid and fluid digestion stages are carried out in zones within a single digestion vessel, with at least part of the fluid required for the solids digestion zone being provided by digested fluid from the fluids digestion zone.

6. A method as claimed in any one of the preceding claims wherein the solids fed to the solids digestion sage are preheated.

7. A method as claimed in any one of the preceding claims wherein the fluid phase in the fluid digestion stage is agitated by passing a gas through the fluid phase.

8. A method as claimed in any one of the preceding claims wherein the solids to liquid ratio in the mixture being digested so that the organic loading rate is 7.5 to 12.5 kgs volatile solids/cubic metre digester volüme/day.

9. A method as claimed in any one of the preceding claims wherein the total liquids feed to solids feed to the digestion system weight ratio is in the range 2:1 to 6:1.

10. A method for digesting solid and fluid waste materials substantially as hereinbefore described with respect to and as shown in Figures 1 and 2 of the accompanying drawings.

11. A method for digesting solid and fluid waste materials substantially as hereinbefore described with respect to and as shown in Figure 3 of the accompanying drawings.