GB2189237A

GB2189237A - Anaerobic digester

Info

Publication number: GB2189237A
Application number: GB8606974A
Authority: GB
Inventors: Martin Arthur Alder
Original assignee: WESSEX WATER AUTHORITY
Current assignee: WESSEX WATER AUTHORITY
Priority date: 1986-03-20
Filing date: 1986-03-20
Publication date: 1987-10-21
Anticipated expiration: 2006-03-20
Also published as: GB2189237B; GB8606974D0

Abstract

There is disclosed an anaerobic digester incorporating a venturi device (1) which has a venturi nozzle (3) through which sewage sludge from the digester (7) is circulated and returned to the digester (7). The venturi device (1) is provided with an inlet through which bio-gas is introduced into the circulating sewage sludge. A method of enhancing digestion in an anaerobic digester is also disclosed. <IMAGE>

Description

SPECIFICATION Anaerobic digester This invention relates to an anaerobic digester and is more particularly concerned with an anaerobic digester incorporating a venturi device for mixing sewage sludge in the anaerobic digester.

As a first stage on the way to disposing of raw sewage sludge, the raw sludge is digested anaerobically in a tank. During anaerobic digestion, hard deposits may develop in the volume of sludge, scum may develop at the top of the digester and grit may be deposited at the bottom of the digester.

According to a first aspect of the present invention there is provided an anaerobic digester incorporating a venturi device having a venturi nozzle through which sewage sludge from the digester is circulated and returned to the digester, the venturi device being provided with an inlet through which bio-gas is introduced into the circulating sewage sludge.

Bio-gas is the gas which is evolved during anaerobic digestion and comprises mainly methane and CO2 but also gases such as hydrogen sulphide.

In a preferred embodiment of this invention, the inlet is provided downstream of the venturi nozzle. Alternatively, the inlet may be provided upstream of the venturi nozzle or in the plane of the nozzle in which case the bio-gas must pass through the venturi-nozzle.

It may be advantageous to have two or more inlets giving two or more streams of gas through the venturi device. By providing two bio-gas inlets, one upstream and one downstream of the venturi nozzle, efficiency may be improved. The bio-gas which must pass through the venturi nozzle could be introduced at a higher pressure than the bio-gas introduced downstream of the venturi nozzle and this might give rise to an enhanced mixing effect.

It has been found that, by the provision of a venturi device in an anaerobic digester, mixing in the digester is improved and the quality of the sludge is enhanced. Moreover, bio-gas being evolved during digestion may be continually circulated through the sewage sludge, although a proportion of bio-gas will be being continually withdrawn as more bio-gas is produced.

The digesting sewage sludge is, in effect, circulated through the venturi device, at the same time being intimately mixed with bio-gas being produced from the anaerobically digesting sewage sludge. This technique ensures that digestion is even and quick.

The digesting sewage sludge is preferably fed through the venturi device by a pump. The same pump may also be used to draw raw sludge into the digester through the venturi device. This has the added advantage that raw sludge is mixed with digesting sludge before entry into the digester and this reduces the likelihood of thermal shock when the cold raw sludge enters the digester.

The venturi device preferably includes a barrel through which sewage sludge emerging from the venturi nozzle must pass before entering the body of the digester.

It is believed that mixing of the bio-gas and sludge occurs mainly at the outlet of the venturi device in the digester. The bio-gas and sludge do not significantly mix in the barrel and it is believed that a "rod" of sludge surrounded by bio-gas exists in the barrel. At the region of the outlet of the barrel, there is a significant degree of turbulence. When the sludge reaches the outlet of the barrel, sludge at the region of the outlet envelopes the biogas, the gas expands and the gas and sludge become intimately mixed in the turbulent flow.

Although the bio-gas to be intimately mixed with the sewage sludge may be pumped through the inlet in the venturi device to the region of the outlet of the venturi nozzle, it is to be appreciated that the venturi device itself produces a pressure drop at the venturi nozzle which can "suck" bio-gas through the inlet thereby avoiding the need for a separate pump for pumping bio-gas through the inlet. If the bio-gas inlet is upstream of the venturi nozzle, there may be a degree of self-entrainment also. However, it is possible that the bio-gas would need to be physically pumped through the inlet into the venturi device.

For the device to function such that bio-gas is "sucked" or self-entrained, it is important to position the device carefully in the tank.

Thus, in a typical digesting tank, 3 to 4m high and 3 to 4m in diameter, the venturi device may be positioned such that the circulating sludge is introduced into the tank about 1 to 1.5m from the top of the tank. This should ensure that the bio-gas is self-entrained. However, it is best to have the venturi device as low down the tank as possible in order to ensure thorough mixing. Although this might preclude self-entrainment of the bio-gas, it may be possible to pressurise the bio-gas being evolved from the digester such that the likelihood of self-entrainment of the bio-gas is increased.

At present, it is envisaged that the bio-gas will be permitted to self-entrain in order to reduce the items of plant necessary such as additional pumps and blowers.

At present, the preferred venturi device for the operation of this invention is the V02venturi device as manufactured and sold by Tom Maguire & Co. Ltd., Milford Haven. The venturi device may be mounted externally of the digester with the barrel of the venturi device passing through the wall of the digester into the body of the digester. This permits the pump and venturi nozzle to be serviced from outside the tank with the tank fully opera tional.

Although a single venturi device may be used, it is possible to use more than one venturi device and/or to use a venturi device having more than one barrel.

According to a second aspect of the pre sent invention, there is provided a method of enhancing digestion in an anaerobic digester comprising circulating sewage sludge from the digester through a venturi device having a venturi nozzle, and introducing bio-gas into the venturi device whereby the bio-gas and the sewage sludge become intimately mixed.

For a better understanding of the present invention and to show how the same may be carried into effect, reference will now be made, by way of example only, to the accompanying drawings in which: Figure 1 shows a venturi device for use in the present invention; Figure 2 shows an alternative embodiment of a venturi device for use in the present invention; and Figure 3 shows an anaerobic digester incorporating the venturi device as shown in Fig. 1.

With reference to Fig. 1, a venturi device 1 comprises an inlet 2 leading to a venturi nozzle 3. At the region of the outlet of the venturi nozzle 3 there is a chamber 4 into which a a bio-gas inlet tube 5 leads. Leading from the chamber 4 is a barrel 6 having a flared outlet 11 through which material passes before emerging from the venturi device 1.

A venturi device of this type may be incorporated into an anaerobic digester to accelerate and improve digestion. Thus, according to the present invention, sewage sludge is pumped from the digester through the inlet 2, through the venturi nozzle 3, subsequently emerging in the chamber 4. The venturi effect caused by the venturi nozzle 3 gives rise to a reduced pressure in the chamber which sucks or entrains bio-gas through the bio-gas inlet tube 5 and intimately mixes the bio-gas with the sewage sludge. As mentioned above, mixing of the bio-gas and sludge occurs at all regions of the venturi device downstream of the bio-gas inlet. However, the most important mixing occurs at the region of the outlet of the barrel in the digester where much turbur lence occurs. To enhance the mixing here, the outlet of the barrel is modified to include a flare 11.

In Fig. 2, an alternative venturi device 12 substantially the same as the venturi device 1 shown in Fig. 1, includes a high pressure biogas introduction tube 13 upstream of the venturi nozzle 3 and terminating in the plane of the nozzle 3. By the provision of such a tube 13, bio-gas under high pressure can be introduced into the venturi device 2. Such an arrangement may give rise to enhanced mixing.

As seen in Fig. 3, the venturi device 1 is disposed outside a digester 7 with the barrel 6 of the venturi device passing through the wall 9 of the digester 7 into the body 10 of the digester 7. A pump 8 is provided to feed digesting sludge from the body of the digester 7 into the venturi device. The action of sewage sludge pumped through the venturi nozzle 3 causes bio-gas to be sucked from above the level of sewage in the digester into the chamber 4 and there to become intimately mixed with the sewage sludge. Alternatively, or in addition, the bio-gas may be pumped into the venturi device 1 if the sucking action of the venturi 1 is inadequate.

The pump 8 may be used to feed raw sludge into the digester 7. Thus, raw sludge and digesting sludge are mixed at the pump 8 having the effect of warming the raw sludge before entry of the raw sludge into the digester 7, thereby easing thermal shock.

The pump 8 may also be used in conjunction with a heat exchanger (not shown) whereby sewage sludge being circulated is warmed to maintain the digestion temperature of about 35 C. The raw sludge being mixed at the pump 8 with the digesting sludge will also be warmed.

Claims

1. An anaerobic digester incorporating a venturi device having a venturi nozzle through which sewage sludge from the digester is circulated and returned to the digester, the venturi device being provided with an inlet through which bio-gas is introduced into the circulating sewage sludge.

2. An anaerobic digester according to Claim 1, wherein the inlet is provided downstream of the venturi nozzle.

3. An anaerobic digester according to Claim 1, wherein the inlet is disposed upstream of, or in the plane of, the venturi nozzle.

4. An anaerobic digester according to Claim 1, 2 or 3 wherein the bio-gas to be introduced into the circulating sludge is derived from the anaerobic digester of sewage sludge in the digester.

5. An anaerobic digester according to Claim 1, 2, 3 or 4, wherein the venturi device is disposed at a position in the digester such that the bio-gas is self-entrained into the venturi device.

6. An anaerobic digester according to any preceding claim, wherein the sewage sludge is circulated through the venturi device by a pump.

7. An anaerobic digester according to Claim 6, wherein the pump draws raw sludge into the circulating digesting sludge upstream of the venturi device.

8. A method of enhancing digestion in an anaerobic digester comprising circulating sewage sludge from the digester through a venturi device having a venturi nozzle, and introducing bio-gas into the venturi device whereby the bio-gas and the sewage sludge becomes inti mately mixed.

9. An anaerobic digester substantially as hereinbefore described with reference to Figs.

1 and 3; and 2 and 3.

10. A method according to Claim 8, substantially as hereinbefore described.