GB1587718A

GB1587718A - Treatment of aqueous sludges

Info

Publication number: GB1587718A
Application number: GB29785/76A
Authority: GB
Original assignee: WELSH NAT WATER DEV
Current assignee: WELSH NAT WATER DEV
Priority date: 1976-07-16
Filing date: 1976-07-16
Publication date: 1981-04-08
Also published as: FR2358363A1; NL7707907A; FR2358363B3; DE2731918A1; CA1085976A; JPS54109243A; AU513489B2; ZA774162B; SE7708254L; AU2698477A; NZ184618A

Description

(54) TREATMENT OF AQUEOUS SLUDGES (71) We, WELSH NATIONAL WATER DEVELOPMENT AUTHORITY, a British body corporate established under the Water Act, 1973, of Cambrian Way, Brecon, Powys, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention is concerned with the treatment of aqueous sludges, particularly but not exclusively sewage sludges.

In conventional sewage treatment, raw sewage is subjected to sedimentation and the sedimented sludge (called a primary sludge) is separated from the liquor. The liquor is then subjected to a biological oxidation treatment such as, for example, in a percolating filter plant, a high rate percolating filter plant, an activated sludge plant or by contact stabilisation. The treated liquor is subjected to sedimentation and the sludge formed is called a secondary sludge.

Usually, a mixture is then formed of primary and secondary sludge, and the mixture is either subjected to anaerobic digestion or chemical treatment or is dumped (for example at sea). Final treated sludges are sometimes left in open drying beds or spread on agricultural land.

Attention has been focused recently on the possibility of modifying this conventional process by subjecting the mixture of primary and secondary sludges to aerobic digestion.

Among the advantages of such a procedure is the possibility of the sludge mixture becoming heated (by the exothermic fermentation therein) to fairly high temperatures (up to 600C) under the fermentation conditions. Heating of sludge to such temperatures kills many of the normal pathogens, such as salmonella, present therein and usually the heated sludge can be safely disposed of on agricultural land without substantial risk of spreading infection.

Another advantage of aerobic digestion is that it generally takes a shorter time than anaerobic digestion.

In the proposals so far made for effecting aerobic digestion of sludges, air or pure oxygen is pumped into the sludge and the sludge is vigorously agitated to disperse the oxygen therein. The sludge is maintained in a closed vessel, and the oxygen atmosphere in the vessel above the sludge may be recirculated through the sludge. In most of the prior proposals, the sludge is agitated by means of a paddle stirrer, and the oxygen introduced into the sludge through a submerged disperser, of which several types have been used.

Whilst it is possible using these prior proposals, to effect aerobic digestion, none of the proposals is entirely satisfactory.

Those necessitating a closed vessel involve the use of expensive equipment, and the use of paddle stirrers and submerged oxygen dispersers is not a very efficient way of obtaining thorough and intimate sludge! oxygen contact throughout the sludge mass being treated. It will be appreciated that full scale equipment may well involve a vessel of 250,000 gallon capacity.

We have now devised certain improvements in the process of aerobic digestion of sludge, whereby many of the disadvantages of prior proposals can be avoided or mitigated.

According to the present invention there is provided a continuous autothermic process for digesting an aerobically digestible sludge at an elevated temperature to produce a pathogen-free stable sludge, which comprises: a) forming a mass of aerobically digestible sludge in a vessel, the sludge containing thermophilic micro-organisms for aerobic digestion of the sludge, which organisms are active in the range 450 to 650C; b) continuously supplying oxygen or an oxygen-containing gas to the sludge for the aerobic digestion thereof whilst continuously agitating the sludge to thoroughly distribute the oxygen or oxygen-containing gas throughout the sludge in the vessel, wherein the sludge is agitated by directing at least one jet of sludge under pressure into the sludge from at least one nozzle disposed in the vessel below the surface of the sludge; c) periodically removing pathogen-free digested sludge from the vessel at a rate such that the residence time of sludge within the vessel is from 5 to 7 days; d) periodically supplying to the vessel fresh sludge to be treated to maintain the level of sludge in the vessel; e) the temperature of the sludge in the vessel being in the range from 45O to 650C, the arrangement being such that the aerobic digestion process continuously occurring in the vessel is autothermic in that it gives out sufficient heat to maintain the sludge continuously within the said temperature range without the external supply of any extra heat, fresh sludge in step (d) being supplied at ambient temperature.

The invention also provides apparatus for the continuous autothermic digestion of an aerobically digestible sludge at an elevated temperature to produce a pathogen-free stable sludge, which comprises: a vessel for holding a mass of aerobically digestible sludge for containing thermophilic micro-organisms for aerobic digestion of the sludge, which organisms are active in the range 45C to 650C; means for continuously supplying oxygen to the sludge for the aerobic digestion thereof; at least one nozzle disposed in the vessel below the surface of the sludge whereby the sludge is continuously agitated to thoroughly distribute the oxygen throughout the sludge in the vessel; means for periodically removing pathogenfree digested sludge from the vessel at a rate such that the residence time of sludge within the vessel is from 5 to 7 days; means for periodically supplying to the vessel fresh sludge to be treated to maintain the level of sludge in the vessel; wherein the apparatus is so arranged that, in use, the temperature of the sludge in the vessel is in the range of from 45O to 65 C.

and the aerobic digestion continuously occurring in the vessel is autothermic in that it gives out sufficient heat to maintain the sludge continuously within the said temperature range without the external supply of any extra heat, fresh sludge being supplied at ambient temperature.

We have found that, in the process of the invention, by suitable control and positioning of one or more nozzles, improved agitation of the sludge, and hence improved dispersion of oxygen throughout the sludge mass, can be obtained than with the prior used paddle stirrers and like devices.

In one arrangement, the or each nozzle is supplied with sludge under pressure by a pump located interiorly of the vessel and submerged in the mass of sludge, the pump drawing sludge feed from the surrounding mass of sludge in the vessel. Alternatively, the or each nozzle can be supplied with sludge under pressure by a pump located exteriorly of the vessel, the pump being fed with sludge from the mass of sludge in the vessel. Supply conduits are arranged to carry the pressurised sludge from the exterior pump to the nozzles in the vessel below the sludge surface therein. For example, a pump mounted externally of the vessel can have a supply line from the pump which enters a side or bottom wall of the vessel below the sludge surface. The sludge feed to the exterior pump will normally be from the sludge in the vessel, but fresh sludge to be treated may be introduced into the vessel via the feed to the pump.

By using one or more jets of sludge to agitate the sludge in the vessel, the oxygen can be introduced in any manner such that it becomes intimately dispersed in the sludge in the vessel. Thus, one or more dispersers can be provided. Preferably, these will be located close to or adjacent the issuing sludge jets so that oxygen may be entrained thereby and so carried throughout the sludge in the vessel.

It is, however, preferred to pass at least some of the oxygen-containing gas into the sludge mass in admixture with the sludge jets. Thus, for example, the oxygen can be supplied direct to the pump so that a mixture of oxygen and sludge is compressed by the pump, it being probable that at least some of the oxygen would then dissolve under pressure in the sludge. As the oxygenated sludge is driven from the nozzles as a jet into the surrounding sludge, the release in pressure can cause any dissolved oxygen to come out of solution in the form of a host of very small bubbles. The oxygen permeates through, and is vigorously mixed with, the sludge so that the sludge in the vessel is very efficiently oxygenated.

Whilst the oxygen-containing gas can be supplied to the pump itself, it can instead be supplied upstream or downstream of the pump, in the latter case before the sludge jet(s) impinge on the sludge mass. Thus, for example, the gas can be mixed with the sludge in the nozzles. For this purpose, a nozzle may be used which includes a chamber having a sludge inlet connected to the pump and a sludge outlet through which, in use, the sludge issues as a jet into the vessel, and wherein the gas-introducing means communicates with the chamber so that, in use, the gas is mixed with the sludge in the chamber. Alternatively, the sludge can pass over one or more oxygen diffusers located -in a conduit, before issuing as a jet into the sludge mass.

For convenience, where (as is usual) two or more nozzles are used, these nozzles may be mounted on a common manifold supplied with sludge under pressure from the pump.

The highly efficient mixing of the oxygen with the sludge, which can be achieved in the present invention, leads to a number of substantial advantages. Firstly, it is possible to arrange the oxygen feed rate and other relevant parameters, that a substantial proportion of the oxygen supplied is absorbed by the digesting sewage. It is then unnecessary to provide a closed vessel for recirculating the non-absorbed oxygen, although such re-circulation can still be effected if desired. Secondly, the excellent dispersal of oxygen in the sludge causes the rate of digestion of the mass of sludge in the vessel to be high, which means that less time is required for treating a given batch of sludge. By operating according to this invention, a batch of sludge is digested in five to seven days (as compared with about 28 days for conventional anaerobic digestion). Furthermore, the process is autothermic, i.e. the spare heat generated by the exothermic fermentation is such as to maintain the digesting sludge at a temperature of 45 to 650C. This is advantageous because the micro-organisms involved are thermophilic and react faster at these higher temperatures. In addition, pathogens in the sludge are killed by high temperature. The vessel containing the sludge may, if desired, be thermally insulated to reduce heat loss.

Another advantage of the very efficient mixing of the sludge and oxygen is that, overall, less oxygen can be required. It is preferred in the process of the invention to use pure oxygen or an oxygen-enriched gas (containing for example at least 90% by volume oxygen) rather than air, principally because the reaction can be more efficiently carried out in this way. Thus, for example, the non-oxygen components of air do not react with the sludge but merely pass through the sludge, and in so doing can effect a significant and undesirable cooling effect on the sludge.

The sludges which can be treated by the present invention include all aerobically digestible sewage and water treatment sludges, and also other similarly digestible sludges such as brewery wastes, abattoir wastes, yeast extracts, pharmaceutical wastes and vegetable wastes. The sludges are preferably pre-macerated and screened (to avoid clogging of the pump, nozzles and conduits).

The aerobic digestive process of the invention may be operated on a batchwise or continuous basis. In the latter, portions of the digested sludge may be removed periodically (e.g. daily) and replaced by untreated sludge.

In order that the invention may be more fully understood, three embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a schematic vertical sectional view of one form of apparatus of the invention (parts of the apparatus being omitted for clarity); Figure 2 is a perspective schematic view of a suitable submersible pump for use in the apparatus of Figure 1; Figure 3 is a vertical sectional schematic view of a second form of the apparatus of the invention (parts of the apparatus being omitted for clarity); Figure 4 is a vertical sectional schematic view of a third form of apparatus of the invention (parts of the apparatus being omitted for clarity); Figure 5 is a plan view of the manifold of Figures 3 and 4; Figure 6 is a section on the line AA of Figure 5; and Figure 7 is a longitudinal sectional view of a nozzle of Figure 6.

Referring to Figure 1, there is shown a vessel 1 containing sludge 2, the vessel having a cover 3. On the bottom of the vessel, submerged in the sludge, is a pump 4 (more fully detailed in Figure 2) to which oxygen is supplied via line 5.

In operation, the sludge to be treated is placed in the vessel. (Means for supplying and removing sludge are not shown). The sludge may be preheated, say to about 30"C, to allow the sludge more quickly to reach a high equilibrium temperature during the process. The cover 3 is placed over the vessel. This serves merely to avoid undue heat loss by convection. If desired, means can be provided (not shown) for recirculating oxygen passing through the sludge.

The pump is energised and oxygen supplied thereto in line 5. The pump ejects a pressurised jet of oxygenated sludge from nozzle orifice 5'. (Further description of the pump is given below.) This jet impinges on the sludge in the vessel 1 and causes it to circulate with turbulence, thus mixing the oxygen throughout the sludge. The pump and oxygen supply are continued until the digestion is complete. The temperature of the sludge increases to about 60"C or above.

In order to assist this temperature rise, thermal insulation can be provided around the vessel. When the process is operating on a continuous basis the average residence time of sludge in the vessel may be as little as 4 days.

It will be appreciated from the preceding general description, that in the Figure 1 process, the oxygen can be supplied not to the pump but instead to one or more diffusers in the sludge mass.

Referring now to Figure 2, one suitable form of pump comprises a motor in housing 1, and an impeller in housing 2. Housing 2 s mounted on legs 5 and has a sludge inlet 4. An oxygen supply line 6 connects to housing 2. A nozzle 3 is provided for the jet outlet.

In operation, sludge is drawn through inlet 4 into impeller housing 2 where it is mixed under pressure with oxygen (from line 6) and forced out as 'a ,jet from nozzle 3. In the impeller 2, at least some of the oxygen (or air) dissolves in the sludge. One suitable form of pump/oxygenator for use in the present invention, is described in U.K.

Patent Application No. 44785/76 (Serial No.

1,520,536) by ABS Pumpen.

The apparatus of Figure 3 is essentially similar to that of Figure 1, but the pump and nozzle arrangement is different. In Figure 3, there is shown a vessel 1, with insulation 2, having a removable access cover 3 with a vent 4. In the base of the vessel is located a pump 5 with an annular manifold 6 on which are mounted nozzles 7 (this arrangement being more fully described hereinafter). Electrical supply cables 8 and the oxygen-supply conduit 9 enter through ports in eccess cover 3.

The apparatus of Figure 4 is essentially similar (and like numerals indicate like parts to Figure 3), but the pump is located exteriorly of the vessel 1. Thus, in Figure 4, pump 5 is connected to manifold 6 by conduit 10 (passing through a side wall of vessel 1), and a stand-by pump 5' is provided. Isolating valves 11 and non-return valves 12 are also included as indicated. A sludge outlet 13 is provided in vessel 1, and connected to the feed port of each pump as indicated by line 14. Oxygen is fed to the nozzles 7 by line 15.

The arrangement of manifold 6 and nozzles 7 for Figure 3 is shown in Figures 5 and 6 (in which like numerals indicate like parts). The manifold 6 is an annular tubular member located around the pump 5.

The outlet 20 of the pump 5 connects to the manifold inlet port 21 to deliver sludge under pressure internally into the manifold.

Six nozzles 7 (in practice more or less than six can be used depending on circumstances) are located outwardly around the periphery of the manifold 6 by feed pipes 23 and receive sludge from the manifold.

Oxygen is also fed to each nozzle via a conduit 22.

The construction of each nozzle 7 is illustrated in Figure 7. The nozzle is conical in shape and defines an internal chamber 30 formed by conical wall 31 and a base 32.

At the apex of the cone-shape is an outlet orifice 33 of diameter "D". The base 32 comprises an annular duct 40 around feed pipe 23, formed by walls 34 and 35 and side walls 36 and 37. Oxygen supply duct 22 opens into annular duct 40. The front wall 34 has a series of orifices 50 therein to allow oxygen to pass from duct 40 into chamber 30. (Alternatively, the wall could be a sintered ceramic or metal plate or diffuser, for example.) Rear wall 35 is bolted to flange 38 on feed pipe 23. Side-walls 37 lie snugly adjacent wall 31 of chamber 30. Feed pipe 23 terminates in chamber 30 and the diameter of the pipe is the same as, or closely similar to, the diameter "D" of outlet orifice 33.

The arrangement illustrated in Figures 3 and 5 to 7 operates as follows. The pump 5 pumps sludge under pressure' into manifold 6. From manifold 6, the sludge passes via feed pipes 23 into the nozzle chambers 30.

Oxygen is supplied via conduit 22 into the ducts 40 under sufficient pressure for it to pass through orifices 50 into chamber 30, and there mix with the sludge. On entering chamber 30, the sludge experiences a reduction in pressure and after mixing with the oxygen is expelled through outlet orifice 33 as a jet to impinge on the surrounding sludge and cause turbulence and mixing of the oxygen therewith. Sludge in chamber 30 is prevented from entering gas orifices 50 by the pressure of the oxygen gas issuing therefrom.

It will be understood that the number of nozzles used, their orientation in the vessel, their size and the other operating conditions are variable and will be chosen according to the particular circumstances prevailing in any particular case. By way of example, in a 150,000 gallon vessel, we have found that orifice 33 diameter D may suitably be about 4 cm., with 16 orifices 50 each of 6 mm.

diameter.

The arrangement of Figures 4 to 7 operates in essentially the same manner as that of Figure 3, but the pump 5 is external of the vessel 1 and manifold 6 is supported by some suitable means in vessel 1. The pump 5 (or 5') draws sludge from vessel 1 via line 14 (instead of, in Figure 3, from the immediately surrounding sludge mass).

By way of example only, we have successfully tested the apparatus of Figures 3 and 5 to 7, using a 150,000 gallon vessel 1 (but not thermally insulated and without a complete cover) under the following conditions: pump having 25 horsepower motor; manifold overall diameter of 1 metre; six nozzles symmetrically arranged as shown in Figure 5; sludge velocity in the jets 15 metres/ second; oxygen input (commercial oxygen gas) 2 cubic metres per minute (expressed at NTP); a mixed sludge comprising primary and secondary sludge, activated sludge and septic tank sludge; a nozzle outlet orifice of 4 cm. diameter; 6 mm. diameter oxygen orifices into nozzle chamber; 16 oxygen orifices in each nozzle; sludge solids content (initially) of 4 to 5 % solids (treated sludge output about 2% % solids). Excellent aerobic digestion was obtained in accordance with the method of the invention.

It will be appreciated that, in many of the prior proposals for aerobic digestion of sludge, a specially constructed vessel is required, particularly where gas recirculation is essential. In the present invention, however, a special vessel is unnecessary. Furthermore, in certain prior proposals, particularly those involving recirculation, the gases in the vessel above the sludge surface, contact moving parts of the equipment such as stirrer rods. This is a potentially dangerous situation which is avoidable according to the present invention.

WHAT WE CLAIM IS:- 1. A continuous autothermic process for digesting an aerobically digestible sludge at an elevated temperature to produce a pathogen-free stable sludge, which comprises: a) forming a mass of aerobically digestible sludge in a vessel, the sludge containing thermophilic micro-organisms for aerobic digestion of the sludge, which organisms are active in the range 450 to 65QC; b) continuously supplying oxygen or an oxygen-containing gas to the sludge for the aerobic digestion thereof whilst continuously agitating the sludge to thoroughly distributethe oxygen or oxygen-containing gas throughout the sludge in the vessel, wherein the sludge is agitated by directing at least one jet of sludge under pressure into the sludge from at least one nozzle disposed in the vessel below the surface of the sludge; c) periodically removing pathogen-free digested sludge from the vessel at a rate such that the residence time of sludge within the vessel is from 5 to 7 days.

d) periodically supplying to the vessel fresh sludge to be treated to maintain the level of sludge in the vessel; e) the temperature of the sludge in the vessel being in the range from 450 to 650C, the arrangement being such that the aerobic digestion process continuously occurring in the vessel is autothermic in that it gives out sufficient heat to maintain the sludge continuously within the said temperature range without the external supply of any extra heat, fresh sludge in step (d) being supplied at ambient temperature.

2. A process according to claim 1 wherein the or each said nozzle is supplied with sludge under pressure by a pump located interiorly of the vessel and submerged in the mass of sludge, the pump drawing sludge feed from the surrounding mass of sludge in the vessel.

3. A process according to claim 1 wherein the or each said nozzle is supplied with sludge under pressure by a pump located exteriorly of the vessel, the pump being fed with sludge from the mass of sludge in the vessel.

4. A process according to claim 1, 2 or 3, wherein at least some of the said oxygen containing gas is passed into the mass of sludge in admixture with the said sludge jet(s).

5. A process according to claim 4, where in the said gas is supplied to the pump for admixture therein with the sludge.

6. A process according to claim 4 where in the said gas is supplied to the pressurised sludge down-stream of the pump but prior to impingement of the sludge jet(s) on the sludge mass.

7. A process according to claim 6 where in the pressurised sludge is mixed with the gas in one or more of the said nozzles, from whence the mixture passes as one or more jets into the mass of sludge.

8. A process according to any of claims 1 to 3 wherein at least some of the said oxygen-containing gas is supplied to the mass of sludge at a point adjacent a nozzle so that the sludge yet from the nozzle immedi ately mixes with the said gas.

9. A process according to any preceding claim wherein two or more nozzles are arranged on a pressurised sludge manifold fed with pressurised sludge, 10, A process according to any preceding claim wherein the 'oxygefi-containing gas comprises at least 90% by volume of oxygen.

11. A process according to any preceding claim wherein the vessel is thermally insulated to reduce heat loss.

12. A process according to any of claims 1 to 11, wherein the sludge comprises a water treatment sludge.

13. A process for the aerobic digestion of sludge substantially as herein described with reference to Figures 1 and 2, or Figures 3 to 7 of the accompanying drawings.

14. Pathogen free sludge which has been digested by the process of any of claims 1 to 13.

15. Apparatus for the continuous auto thermic digestion of an aerobically digestible sludge at an elevated temperature to produce a pathogen-free stable sludge, which com prises: a vessel for holding a mass of aerobically digestible sludge for containing thermophilic micro-organisms for aerobic digestion of the sludge, which organisms are active in the range 45" to 650C; means for continuously supplying oxygen to the sludge for the aerobic digestion there of; at least one nozzle disposed in the vessel below the surface of the sludge whereby the sludge is continuously agitated to thoroughly distribute the oxygen throughout the sludge in the vessel; means for periodically removing pathogen free digested sludge from the vessel at a rate

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

aerobic digestion was obtained in accordance with the method of the invention.

It will be appreciated that, in many of the prior proposals for aerobic digestion of sludge, a specially constructed vessel is required, particularly where gas recirculation is essential. In the present invention, however, a special vessel is unnecessary. Furthermore, in certain prior proposals, particularly those involving recirculation, the gases in the vessel above the sludge surface, contact moving parts of the equipment such as stirrer rods. This is a potentially dangerous situation which is avoidable according to the present invention.

WHAT WE CLAIM IS:-

1. A continuous autothermic process for digesting an aerobically digestible sludge at an elevated temperature to produce a pathogen-free stable sludge, which comprises: a) forming a mass of aerobically digestible sludge in a vessel, the sludge containing thermophilic micro-organisms for aerobic digestion of the sludge, which organisms are active in the range 450 to 65QC; b) continuously supplying oxygen or an oxygen-containing gas to the sludge for the aerobic digestion thereof whilst continuously agitating the sludge to thoroughly distributethe oxygen or oxygen-containing gas throughout the sludge in the vessel, wherein the sludge is agitated by directing at least one jet of sludge under pressure into the sludge from at least one nozzle disposed in the vessel below the surface of the sludge; c) periodically removing pathogen-free digested sludge from the vessel at a rate such that the residence time of sludge within the vessel is from 5 to 7 days.

d) periodically supplying to the vessel fresh sludge to be treated to maintain the level of sludge in the vessel; e) the temperature of the sludge in the vessel being in the range from 450 to 650C, the arrangement being such that the aerobic digestion process continuously occurring in the vessel is autothermic in that it gives out sufficient heat to maintain the sludge continuously within the said temperature range without the external supply of any extra heat, fresh sludge in step (d) being supplied at ambient temperature.

2. A process according to claim 1 wherein the or each said nozzle is supplied with sludge under pressure by a pump located interiorly of the vessel and submerged in the mass of sludge, the pump drawing sludge feed from the surrounding mass of sludge in the vessel.

3. A process according to claim 1 wherein the or each said nozzle is supplied with sludge under pressure by a pump located exteriorly of the vessel, the pump being fed with sludge from the mass of sludge in the vessel.

4. A process according to claim 1, 2 or 3, wherein at least some of the said oxygen containing gas is passed into the mass of sludge in admixture with the said sludge jet(s).

5. A process according to claim 4, where in the said gas is supplied to the pump for admixture therein with the sludge.

6. A process according to claim 4 where in the said gas is supplied to the pressurised sludge down-stream of the pump but prior to impingement of the sludge jet(s) on the sludge mass.

7. A process according to claim 6 where in the pressurised sludge is mixed with the gas in one or more of the said nozzles, from whence the mixture passes as one or more jets into the mass of sludge.

8. A process according to any of claims 1 to 3 wherein at least some of the said oxygen-containing gas is supplied to the mass of sludge at a point adjacent a nozzle so that the sludge yet from the nozzle immedi ately mixes with the said gas.

9. A process according to any preceding claim wherein two or more nozzles are arranged on a pressurised sludge manifold fed with pressurised sludge, 10, A process according to any preceding claim wherein the 'oxygefi-containing gas comprises at least 90% by volume of oxygen.

11. A process according to any preceding claim wherein the vessel is thermally insulated to reduce heat loss.

12. A process according to any of claims 1 to 11, wherein the sludge comprises a water treatment sludge.

13. A process for the aerobic digestion of sludge substantially as herein described with reference to Figures 1 and 2, or Figures 3 to 7 of the accompanying drawings.

14. Pathogen free sludge which has been digested by the process of any of claims 1 to 13.

15. Apparatus for the continuous auto thermic digestion of an aerobically digestible sludge at an elevated temperature to produce a pathogen-free stable sludge, which com prises: a vessel for holding a mass of aerobically digestible sludge for containing thermophilic micro-organisms for aerobic digestion of the sludge, which organisms are active in the range 45" to 650C; means for continuously supplying oxygen to the sludge for the aerobic digestion there of; at least one nozzle disposed in the vessel below the surface of the sludge whereby the sludge is continuously agitated to thoroughly distribute the oxygen throughout the sludge in the vessel; means for periodically removing pathogen free digested sludge from the vessel at a rate

such that the residence time of the sludge within the vessel is from 5 to 7 days; means for periodically supplying to the vessel fresh sludge to be treated to maintain the level of sludge in the vessel; wherein the apparatus is so arranged that, in use, the temperature of the sludge in the vessel is in the range from 450 to 650 C, and the aerobic digestion continuously occurring in the vessel is autothermic in that it gives out sufficient heat to maintain the sludge continuously within the said temperature range without the external supply of any extra heat, fresh sludge being supplied at ambient temperature.

16. Apparatus according to claim 15 wherein the pump means are located externally of the vessel and conduit means are provided to supply (in use) sludge from the vessel to the pump, and to convey sludge under pressure from the pump to the said nozzles in the vessel.

17. Apparatus according to claim 15 wherein the pump means are located internally of the vessel at a depth such that, in use, it is below the sludge level.

18. Apparatus according to claim 15, 16 or 17, wherein the gas-introducing means communicates with the pump, whereby in use gas is mixed with sludge in the pump and the mixture is passed to the nozzle(s).

19. Apparatus according to claim 15, 16 or 17 wherein the nozzle includes a chamber having a sludge inlet connected to the pump and a sludge outlet through which, in use, the sludge issues as a jet into the vessel, and wherein the gas-introducing means communicates with the chamber so that, in use, the gas is mixed with the sludge in the chamber.

20. Apparatus for the aerobic digestion of sludge substantially as herein described with reference to Figures 1 and 2 of the accompanying drawings.

21. Apparatus for the aerobic digestion of sludge substantially as herein described with reference to Figures 3 to 7 of the accompanying drawings.