GB2207668A

GB2207668A - Thermophilic aerobic digestion

Info

Publication number: GB2207668A
Application number: GB8817141A
Authority: GB
Inventors: Barry Oliver; David William Musco; Alan Michael Bruce
Original assignee: Water Research Centre
Current assignee: Water Research Centre
Priority date: 1987-07-20
Filing date: 1988-07-19
Publication date: 1989-02-08
Also published as: GB8817141D0; GB8717098D0

Abstract

In the thermophilic aerobic digestion of an aqueous sludge in an insulated vessel (1), sludge is withdrawn from the vessel and pumped (5) in a circuit to a level above the surface (7) of the sludge in the vessel, aerated, and then returned into the vessel through a downcomer (10). The aeration is effected in a horizontal or downflow portion of the circuit using an aerator (9). <IMAGE>

Description

THERMOPHILIC AEROBIC DIGESTION This invention relates to a process and apparatus for thermophilic aerobic digestion of aqueous sludges (by which term we include sewage sludges and aqueous industrial effluents containing biodegradable materials such as fermentation liquors).

The thermophilic aerobic digestion (TAD) of sludges is known and one such process is described, for example, in U.K. specification 1587718. In these processes, micro-organisms present in the sludge digest the biodegradable materials in the presence of oxygen, giving off heat. By insulating the reaction vessel, the temperature of the contents can rise to above 500 C, for example, and thereby have a sterilising effect on pathogenic organisms therein.

One of the difficulties in the practical operation of TAD processes is the provision of oxygen in the digesting sludge. U.K. specification 1587718 describes the use of an aerated sludge jet injected from a nozzle below the sludge surface. This and other similar proposals have some advantages and also some disadvantages, among which are that the system is relatively complex and also not easy to repair and maintain without shutting down the whole process.

In our British patent no. 2072027B, we have described a way of dissolving oxygen in an aqueous liquid, in which the liquid is flowed downwardly at a velocity greater than 0.1 m/s to entrain therein small bubbles of oxygen-containing gas from an aerator.

We have now found that this aeration technique has a number of special advantages when used in a particular way in connection with a TAD process. In one aspect, the present invention provides a method of TAD of an aqueous sludge which comprises passing oxygen (which term as used herein includes an oxygen-containing gas such as air) into a thermally insulated vessel containing the sludge, whereby TAD occurs, and wherein sludge is withdrawn from the vessel and pumped upwardly to an aerator disposed externally of the vessel and above the surface level of sludge (in the vessel), and thereafter caused to return to the vessel through a downcomer which opens in the vessel below the said surface level, said flowing sludge entraining small bubbles of oxygen from said aerator.

The invention also includes an apparatus for the thermophilic aerobic digestion of sludge, which comprises a thermally insulated digester vessel, pump means for withdrawing sludge from the vessel, conduit means for passing the withdrawn sludge exteriorly of the vessel to a level above the surface level of sludge in the vessel, and a downcomer for returning the sludge into the vessel, said downcomer opening in the vessel below the said surface level (in use), an aerator disposed in said conduit means above the surface level of sludge in the vessel, to introduce oxygen or an oxygen-containing gas into the sludge in the conduit, the aerator being in a horizontal or downflow portion of the conduit.

In an arrangement of this type, the pump, the aerator, the connecting pipework and the top part of the downcomer can all be readily accessible for maintenance and repair, without having to drain the contents of the vessel.

Furthermore, by placing the aerator at a level above that of the sludge surface, very efficient aeration is achieved, i.e. air entrainment is maximised. Preferably, the aerator is on the downflow or on a horizontal flow, rather than on the upflow of the sludge.

The downcomer which carries the aerated sludge back into the body of sludge in the vessel, opens below the sludge surface. Preferably, the downcomer increases in diameter at or towards its open end, eg. it is flared or bell-shaped, to reduce any turbulence to a minimum. Also, the downcomer preferably extends to close to the bottom of the vessel in order to maximise the period of retention of gas bubbles in the sludge.

The vessel is thermally insulated and is preferably closed to retain as much of the heat as possible.

Thus, there will preferably be a closure on the top of the vessel, the closure also preferably being thermally insulated.

The closure (or the upper side wall(s) of the vessel above the sludge) is provided with a gas vent to permit the off gases to pass out of the vessel. Means for deodorising the off gases such as for example, a peat filter or similar system, may be provided if desired. In accordance with a preferred feature of the invention, means are provided to analyse the vented gases and, from the analysis, to provide data for overall control of the process (which may be automated). For example, if the oxygen concentration in the vented gases rises to a predetermined value, this may indicate low biological oxygen demand (BOD) in the sludge and hence that the treatment process is substantially complete.

The process of the invention can be operated batch-wise or continuously and can be controlled automatically.

In some circumstances, there may be a tendency for foam to form on the sludge surface, in excessive quantities.

An excess is plainly undesirable and mechanical or other means can be provided to destroy the foam. For example, some of the recirculating sludge can be injected (before aeration) into the vessel at a level above the sludge surface, in order to impinge on and physically break down the foam. Alternatively, a simple mechanical cutter can be installed in order to collapse excess foam.

The process of the invention can be carried out to completion, viz. to produce a pathogen-free, stable digested sludge, or the process can be used to achieve only partial digestion. The partially digested hot sludge can then, for example, be fed to a conventional anaerobic digester for further treatment. In fact, the partial TAD process of the invention is an excellent way of heating sludge prior to anaerobic digestion.

In the process of the invention, the aerator can be a venturi or a nozzle aerator, for example, or any other type of aerator. Our patent no. 2072027B describes some suitable arrangements. As will be understood by those skilled in the art, in large scale installations it may be desirable to supply air under pressure (from an air blower) to the aerator instead of relying solely on aspiration under atmospheric pressure.

In order that the invention may be more fully understood, it will now be described, by way of illustration only, with reference to the accompanying drawings, wherein: Figure 1 is a schematic illustration of one embodiment of apparatus of the invention; Figure 2 shows the temperature in a digester operated in accordance with the invention over a nine month period; and Figure 3 shows the temperature in a digester operated in accordance with the invention on a single batch of sludge over eighteen days.

Referring to Figure 1, there is shown a thermally insulated vessel 1 containing a body of sludge 2 (to be treated). The sludge is fed into the vessel through motorised valve inlet 3. Near the bottom of vessel 1 is a valved outlet 4 communicating with recirculating pump 5.

Recirculating pipework 6 extends from pump 5 upwardly to above the level of the surface 7 of the sludge 2 and foam 8 in the vessel 1, and then descends to a venturi aerator 9.

Aerator 9 is external of vessel 1. From the lower part of aerator 9, downcomer 10 extends into the vessel to below the surface 7 of the sludge and foam.

In the top 11 of vessel 1 is mounted a vent 12 to allow gases to escape from vessel 1. Also, a foam level sensor 13 is provided in top 11.

Pipework 6 includes a valved spur 14 opening into vessel 1 in the region occupied by the foam 8.

In use, sludge is admitted to vessel 1 through valved inlet 3, and then pump 5 is operated to withdraw sludge from vessel 1 via outlet 4 and pump it up pipework 6 and down through aerator 9 back into vessel 1 via downcomer 10. Tiny bubbles of air are thus continuously being entrained in the recirculating flow of sludge and the sludge in the vessel is kept adequately aerated. As digestion proceeds, heat is generated and the temperature of the contents of the vessel rises to an equilibrium value eg. 50 to 60 C.

Gases which are not absorbed by the sludge rise through the sludge and may cause formation of foam 8. If the amount of foam increases so that sensor 13 is actuated, motorised valve spur 14 is opened (via control 20) to allow some of the circulating sludge to pass therethrough into the foam 8 in vessel 1 thus destroying foam and reducing the total amount thereof.

Gases exiting from vent 12 may be analysed (oxygen meter 15) to provide data for control of the process. The process may be operated automatically.

Treated sludge is removed from the vessel at appropriate intervals by opening motorised valve 15 and allowing the required quantity to be discharged either for further treatment or for storage before disposal.

Following removal of treated sludge an equivalent quantity is admitted to the vessel via valve 3. Both removal and replenishment of sludge can be effected automatically using an appropriate control system.

Figure 2 shows the temperature in a 21m3 digester vessel over a nine month period during which the digester was operated continuously on a semi-batch procedure, i.e.

the sludge in the digester was aerated continuously with periodical removals of part of the digested sludge and replacement with raw sludge. The digester was substantially the same as in Figure 1.

Figure 3 shows temperature data for digestion of a single batch of sludge aerated continuously over 18 days.

The digester and process conditions were the same as for Figure 2, although the sludge was from a different source.

Claims

CLAIMS:

1. A method of thermophilic aerobic digestion of an aqueous sludge, which comprises passing oxygen gas or an oxygen-containing gas into a thermally insulated vessel containing the sludge for said digestion to occur, and wherein during digestion sludge is withdrawn from the vessel and pumped upwardly to an aerator disposed externally of the vessel and above the surface level of sludge in the vessel, and thereafter caused to return to the vessel through a downcomer which opens in the vessel below the said surface level, said flowing sludge entraining small bubbles of said gas from said aerator.

2. A method according to claim 1, wherein the aerator is on a horizontal or downflow path of the withdrawn sludge.

3. A method according to claim 1 or 2, wherein the downcomer increases in diameter at or towards its immersed open end to reduce turbulence in the vessel.

4. A method according to claim 1,2 or 3, wherein the downcomer extends to close to the bottom of the vessel.

5. A method according to any of claims 1 to 4, wherein the vessel has a top closure.

6. A method according to any preceding claim, wherein off gases from the digesting sludge are analyzed.

7. A method according to any preceding claim, wherein some of the withdrawn sludge is returned into the vessel at a level above the sludge surface to impinge on, and physically break down, foam on the surface.

8. A method according to any preceding claim, wherein the sludge is digested for a period sufficient to produce a pathogen-free stable sludge.

9. A method according to any of claims 1 to 7, wherein the sludge is digested for a time sufficient only for partial digestion, and the partially digested hot sludge is removed from the vessel.

10. A method according to claim 9, wherein the removed partially digested hot sludge is then subjected to anaerobic digestion.

11. #Apparatus for the thermophilic aerobic digestion of sludge, which comprises a thermally insulated digester vessel, pump means for withdrawing sludge from the vessel, conduit means for passing the withdrawn sludge exteriorly of the vessel to a level above the surface level of sludge in the vessel, and a downcomer for returning the sludge into the vessel, said downcomer opening in the vessel below the said surface level (in use), an aerator disposed in said conduit means above the surface level of sludge in the vessel, to introduce oxygen or an oxygen-containing gas into the sludge in the conduit, the aerator being in a horizontal or downflow portion of the conduit.