GB2205825A

GB2205825A - Sludge thickening

Info

Publication number: GB2205825A
Application number: GB08713766A
Authority: GB
Inventors: Martin Arthur Alder
Original assignee: WESSEX WATER AUTHORITY
Current assignee: WESSEX WATER AUTHORITY
Priority date: 1987-06-12
Filing date: 1987-06-12
Publication date: 1988-12-21
Anticipated expiration: 2007-06-12
Also published as: GB8713766D0; GB2205825B

Abstract

The invention relates to a method for improving the settlement of anaerobically digested sludge by aeration of the sludge in the vessel, the aeration being conducted in at least two stages; an initial aeration stage, at a first aeration intensity, when the vessel is only partially filled with digested sludge; and a final aeration stage, at a second aeration intensity, when the vessel is fully filled with digested sludge. Optionally, further aeration stages may be employed between the initial and final aeration stages.

Description

SLUDGE THICKENING This invention relates to sludge thickening and is more particularly, but not exclusively, concerned with an improved method of thickening sludge by passing a gas, such as air, through the sludge, and with a thickened sludge.

British Patent Specification No. 1244237 provides a broad outline for a process by which anaerobically digested sludge may be dewatered (thickened) after aeration with a free-oxygen containing gas under conditions such that the sludge retains the character of digested sludge as opposed to activated sludge whilst gas formation due to anaerobic organisms is substantially inhibited. Inhibition of further gas formation accelerates consolidation of the sludge for the reason that any gas evolved necessarily upsets the consolidation having a detrimental effect on settling.

British Patent Application Nos. 8106160 and 8405024 both seek to improve on the process of British Patent Specification No. 1244237 by specifying the volume of air to volume of sludge which gives improved settlement results. British Patent Application No.

8405024 provides a modified process in which, inter alia, the aeration may be controlled by monitoring the pH of the digested sludge during aeration.

According to the present invention there is provided a method for improving the settlement of anaerobically digested sludge by aeration of the sludge in a vessel, said aeration being conducted in at least two stages, an initial aeration stage, at a first aeration intensity, where the vessel is only partially filled with digested sludge and a final aeration stage, at a second aeration intensity, where the vessel is fully filled with digested sludge.

By "fully filled" vessel, it is intended to mean that the vessel is filled to its normal full capacity whilst "partially filled vessel" means that the vessel is only partially filled to its normal full capacity.

In general, it has been found satisfactory to employ three separate aeration stages (stepped aeration), the three steps being the initial aeration of sludge in the partially filled vessel as described above, a second aeration of sludge in the vessel partially filled beyond the level for the first aeration and the final aeration stage, of sludge in the full vessel. A third aeration, at a vessel capacity between the capacity employed for the second and final aerations may also be included. Optionally, further periods of aeration may be employed before the vessel is fully filled.

Although filling of the vessel may be halted for aeration at partial capacities of the vessel, it is preferred that filling of the vessel is continuous with aeration continuing during filling, with digested sludge drawn from the digestor being continually introduced into the vessel. As the volume of sludge being drawn from the digester is relatively small, the actual volume change during a period of aeration is likely to be under 5%.

The air may be introduced into the vessel via a venturi aerator. Such an aerator may include a pump which extracts sludge from the vessel and returns the sludge to a separate region of the vessel via a venturi. The venturi effect draws air through an inlet in the region of the venturi and the air becomes entrained in the sludge. In addition, the pump may also be used to pump fresh digested sludge, from an anaerobic digester, into the vessel, again via the venturi. However, at present, it is envisaged that the fresh digested sludge should be introduced into the vessel via a separate inlet to the vessel.

Alternatively, a simple aerator may be provided in the vessel, with anaerobically digested sludge optionally being continuously fed into the vessel from a digester by a separate inlet. More than one aerator may be employed.

The aeration intensity for each of the initial, second, third (and optionally further) and final aerations may be the same or different. Thus, for instance, the aeration intensity for the initial aeration stage may be up to about 8m3air/m3 sludge per hour, preferably about 4m3air/m3 sludge per hour, whilst the aeration intensity for the final aeration may be up to about 5m3 air/m3 sludge per hour, preferably between 1 and 2m3 sludge/m3 air per hour.

The actual figure for the aeration intensity will normally depend upon the volume of sludge in the vessel. Clearly, using a standard aerator, the intensity of aeration will be greater when the tank is partially filled, than it will be when it is fully filled.

As a general rule, the initial aeration may be with the vessel filled to between 30% and 60% of full capacity, preferably about 50% of capacity, whilst for the second aeration, the vessel may be filled to between 65 and 85% of capacity, preferably about 75% of capacity. If a further aeration between the second and final aeration is required the vessel may be filled, for example, to 30 - 50%, preferably 40% for the first aeration, 50 - 70%, preferably 60% for the second aeration and 70 - 90%, preferably 80% for the third aeration. The aerations at partial capacity should be for a time sufficient to halt anaerobic digestion, without initiating aerobic digestion. For example, aeration at partial capacity should for up to 5 hours, preferably for up to 3 hours.

By aerating at partial capacity, using a relatively high aeration intensity than is possible during aeration at full capacity, it is possible to deliver an overall, time-averaged, aeration intensity higher than would normally be possible. Moreover, since aeration may continue during filling, the overall time required for aeration is reduced, relative to the known process in which a vessel is filled to capacity and only then aerated at full capacity.

It has also been found that, by stepped aeration, the likelihood of the sludge not consolidating following aeration is reduced.

Typical sizes for the vessel are up to 3000 m3, preferably in the range of from 2000 - 2500 m3. It has been found that the process of this invention is best suited to deep, rather than shallow vessels, especially tanks of lOm depth or greater. The aspect ratio (i.e.

ratio of depth to diameter) of vessels used should preferably be 1 or greater than 1.

As mentioned above, the aerator may be a venturi aerator in which the venturi effect is used to entrain air in sludge which is being circulated from one region of the vessel to another region by a pump. Using such an aerator, once aeration of the sludge in the fully filled vessel has been completed, the flow of air to the venturi may be ceased; however, it has been found to be advantageous to keep the pump turning over after the shutting off of the air supply to the venturi.

This turns over the sludge in the vessel and assists in releasing bubbles of air from the sludge. A consolidated sludge of higher solids content is achieved. The pumps may be left running until all bubbles of air entrained in the sludge are released.

This may be from 1 to 2 hours following the ceasing of air flow through the sludge.

Once the pump or pumps have been stopped, the sludge is allowed to consolidate. After a period determined empirically by assessment of the state of the consolidating sludge, aeration may be repeated.

Thus, if and when anaerobic digestion starts in the vessel, which will disturb the consolidating sludge and prevent further thickening, aeration may be recommenced for a length of time, again determined empirically, sufficient to stop that anaerobic digestion without initiating aerobic digestion. The recommenced aeration may, in effect, be conducted on a partially filled vessel since, during consolidation, the supernatant from the gradually consolidating sludge will have been decanted. It is not, at this stage, envisaged that further digested sludge would be added before recommencing aeration.

The process of the present invention works best on adequately digested sludge which has been retained in a digester for a minimum of 20 days retention time.

Although this is a longer retention time than has been previously preferred, it has been found that the percentage of solids in the settled sludge is increased over that achieved using the process of this invention on digested sludges having had a shorter residence time in the anaerobic digester. By adequately digested sludge is meant sludge which has been digested to the extent that the majority of anaerobically digestible material has been destroyed. This reduces the likelihood that further, recommenced aeration will be necessary and reduces the amount of aeration required during stepped aeration.

For a better understanding of this invention, reference will now be made, by way of Example, to the accompanying Figure which shows,- diagrammatically, a process in accordance with this invention. An anaerobic digester 2 is provided with an outlet 4. A conduit 6, which includes a valve 8, from the digester 2, communicates with a vessel 10. Anaerobically digested sludge in the vessel 10 is aerated by means of a venturi aerator 12. The venturi aerator includes a pump 14 which circulates sludge from the vessel 10 through a venturi 16 which entrains air, via an air inlet, in the sludge. Aeration is conducted at roughly 1/3, 2/3 and full capacity of the tank, as shown by reference numerals 18, 20 and 22 respectively. The flow of digested sludge from the digester 2 may or may not be ceased, by closing the valve 8, for example, during periods of aeration.

Claims

1. A method for improving the settlement of anaerobically digested sludge by aeration of the sludge in a vessel, said aeration being conducted in at least two stages: an initial aeration stage, at a first aeration intensity, where the vessel is only partially filled with digested sludge; and a final aeration stage, at a second aeration intensity, where the vessel is fully filled with digested sludge.

2. A method according to claim 1, wherein after the initial aeration stage, the sludge is aerated in a first, intermediate aeration stage in which the vessel is partially filled beyond the level for the initial aeration.

3. A method according to claim 2, wherein during the initial aeration stage the vessel is filled to between 30% to 60% of full capacity and wherein for the first, intermediate aeration stage, the vessel is filled to between 65% to 85% of full capacity.

4. A method according to claim 1 or 2, wherein a further, second, intermediate aeration stage is employed before the final aeration stage, at a vessel capacity between the capacity employed for the first intermediate stage and the final aeration stage.

5. A method according to claim 4, wherein for the initial aeration stage the vessel is filled to 30% to 50% of full capacity, for the first intermediate aeration stage the vessel is filled to between 50% to 70% of full capacity and for the second intermediate aeration stage the vessel is filled to between 70% to 90% of full capacity.

6. A method according to claim 1, 2 or 4, wherein further periods of aeration are employed before the vessel is fully filled.

7. A method according to any preceding claim, wherein the air to aerate the sludge is introduced into the vessel via a venturi aerator.

8. A method according to claim 7, wherein sludge is circulated from the vessel, through the venturi aerator and returned to the vessel, thereby agitating the sludge.

9. A method according to any preceding claim, wherein the aeration intensity for the initial aeration stage is no greater than 8m3air/m3 sludge per hour.

10. A method according to any preceding claim, wherein the aeration intensity for the final aeration stage is no greater than 5m3air/m3 sludge per hour.

11. A method according to any preceding claim, wherein the vessel employed has a depth of at least 10 meters.

12. A method according to claim 11, wherein the aspect ratio of the vessel is at least 1.

13. A method according to claim 8 or any one of claims 9 to 12 when appendant to claim 8, wherein, after aeration of the sludge in the fully filled vessel has been completed, the flow of air to the venturi is ceased whilst circulation of sludge is maintained.

14. A method for improving the settlement of anaerobically digested sludge substantially as hereinbefore described, with reference to the accompanying drawing.