GB2093727A

GB2093727A - Treatment of sludge

Info

Publication number: GB2093727A
Application number: GB8106160A
Authority: GB
Original assignee: Thames Water Authority
Current assignee: Thames Water Authority
Priority date: 1981-02-26
Filing date: 1981-02-26
Publication date: 1982-09-08
Also published as: GB2093727B

Abstract

The invention relates to a process for thickening digested sewage sludge, comprising the steps of removing gases from the sludge either by vacuum, agitation or passing a gas through the sludge, and then thickening the de-gassed sludge under gravity to provide a thickened, digested sludge in which the solids content is enhanced. The figure illustrates the rapid settlement of aerated sludge when compared to conventional settlement, and indicates that deeper settlement tanks may be used. <IMAGE>

Description

SPECIFICATION Treatment of sludge TECHNICAL FIELD OF THE INVENTION The invention relates to the treatment of sludge, in particular sewage sludge, so as to prepare it for disposal.

BACKGROUND ART Sewage sludge is usually, after mechanical removal of coarse components, subjected to a primary sedimentation to produce a primary sludge component and a supernatant liquor.

The sludge component may then be digested anaerobically to produce a product with a relatively acceptable odour.

The anaerobically digested sludge is usually allowed to settle in order to reduce its aqueous content as much as possible prior to disposal, or subjected to some other form of treatment (ie mechanical dewatering) to reduce the water content.

Most digested sludge has a solids content of about 2.5% by weight and it is desirable to increase this percentage. This is because most of the cost of disposing of sludge is taken up in transporting these sludges from a treatment plant to a point of disposal, eg at sea. Thus of the 20 Million total spent on sludge treatment and disposal in the area covered by the Applicants, a large proportion is accounted for by transport. The transport cost, it is estimated, could be halved if moisture content was reduced to 96% to 92% as opposed to the present 973%.

DISCLOSURE OF THE INVENTION It is accordingly an object of the invention to seek to increase the solids content of anaerobically digested sludge, which expression includes anaerobically digested primary sludge and sludges resulting from a primary sedimentation process to which has been added recycled sludge recovered from other sewage treatment processes.

According to one aspect of the invention there is provided a process for thickening digested sludge, comprising the steps of removing gases from the sludge by passing a gas through the sludge and then thickening the de-gassed sludge under gravity to provide a thickened, digested sludge in which the solids content is enhanced. Alternative methods of de-gassing the sludge may include vacuum degassers, gas stripping towers, surface aerators and spray aerators.

It has been found that using the invention it is possible to obtain a thickened digested sludge which has greater than 5% by weight solids in a period of as little as 2 days. Using prior processes, it has taken from many days to many weeks to obtain a digested sludge which has a 5% solids content.

The gas used in the gas removal step may be air, oxygen or other gas such as nitrogen.

The volume of air may be between 1 m3 to 5m3 per m3 of sludge per hour, passed for about two hours or less. This provides a satisfactory thickened sludge in which the solids content is enhanced.

The process may be a continous, semicontinuous or batch process.

The de-gassing, or gas stripping, step may be controlled from the pH of the sludge.

Alternatively, the de-gassing, or gas stripping step may be controlled by a dissolved oxygen control system.

The process provides a supernatant liquor fraction after thickening, and this liquor may be subsequently treated to remove ammonia.

According to a second aspect of the invention there is provided a thickened digested sludge made by a process as hereinbefore defined.

A process embodying the invention is hereinafter described, by way of example, with reference to the accompanying drawing, which is a graph of the effect of aeration prior to thickening.

Referring to the drawing, there is shown the advantageous result achieved using a process embodying the invention. The drawing shows a graph of the effect of aeration prior to gravity thickening of a digested sludge as a plot of number of days settlement to achieve 50% consolidation (Y-axis) against depth of sludge thickener in metres (X-axis). The plot marked "A" shows that for conventional digested sludge which is thickened without aeration in large relatively deep tanks a large time is required to thicken the sludge while the plot marked "B" shows that a sludge treated by a process embodying the invention can be thickened in a reltively short time in a deep thickening tank. In other words, the economic benefit of an aerating or degassing step prior to the thickening step are greatest when a deep thickening tank, or plurality thereof, is used to thicken the sludge (plot B).

This is because conventionally (plot A), the rate of consolidation, or thickening slows with increasing depth of tank used for thickening.

This is because in a conventional sludge (not de-gassed) continuing digestion in the sludge actually produces gases which, together wjth existing gas microbubbles, hold up the sludge flocs and maintain them in suspension so no, or at least a very slow, consolidation takes place.

The process of the invention operates because it removes gases in the sludge, both those trapped in pockets in the sludge and those which would otherwise be produced by continued digestion. The process terminates the methane producing stage of digestion. A corollary of this is that tall thickening tanks, for example existing secondary digestion tanks, are to be preferred in the process of the invention, in other words ones in which the height greatly exceeds the lateral diminsion, or in other words, tanks in which the volume greatly exceeds the cross-sectional area as considered transversely of the longitudinal axis of the tank. Such tall tanks therefore do not require such a large area as opposed to conventional processes where large open tanks covering several acres are required. This is entirely wasteful of land and is therefore very expensive.A process embodying the invention can be carried out in a plant which occupies a relatively small site area.

In a typical process, 1 m3 to 5m3, preferably about 5m3, of air per m3 of digested sludge per hour is passed continously, for 2 hours, through sludge to be thickened. This degasses the sludge, which can then be transferred to a thickener. Cessation of the degassing or gas-stripping may be controlled automatically by monitoring the pH of the sludge. When it reaches a certain value, the flow of stripping gas is ceased.

Alternatively, the de-gassing or gas stripping step may be controlled by monitoring the dissolved oxygen content of the sludge. When this reaches a pre-determined value, the flow of stripping gas is ceased.

Although the use of air has been referred to for the de-gassing or gas stripping step, any suitable gas such as oxygen or nitrogen could be used.

In every embodiment, a thickened, de-watered sludge is obtained which has an enhanced solids content.

Claims

1. A process for thickening digested sludge, comprising the steps of removing gases from the sludge either by vacuum, agitation or passing a gas through the sludge, and then thickening the de-gassed sludge under gravity to provide a thickened, digested sludge in which the solids content is enhanced.

2. A process according to Claim 1, comprising passing air through the sludge in the de-gassing step.

3. A process according to Claim 2, comprising passing about 1 m3 to 5m3 of air per m3 of sludge per hour for about two hours through the sludge in the de-gassing step.

4. A process according to any precedng claim, which is continuous.

5. A process according to any one of Claims 1 to 4, which is semi-continuous.

6. A process according to any one of Claims 1 to 4, which is a batch process.

7. A process according to any preceding claim, in which the de-gassing step is controlled by the pH of the sludge.

8. A process according to any of Claims 1 to 6, in which the de-gassing step is controlled by a dissolved oxygen control system.

9. A process according to any preceding claim, in which the thickening step is carried out in a thickening tank the height of which greatly exceeds its lateral extent.

10. A process according to Claim 1, for thickening digested sludge, substantially as herinbefore described.

11. A thickened sludge whenever pro- duced by a process according to any preceding claim.