GB2132184A - Biological treatment of liquids - Google Patents

Abstract

In the biological treatment of an aqueous liquid, the liquid is passed in contact with one of more movable supports (65, 75, 85) on which micro-organisms are grown. The supports are moved to sequentially expose the micro-organisms to air and then to re-immerse them in the liquid. The arrangement is such that the said movement of the supports automatically causes shearing or sloughing of excess biomass from the supports. Preferably, the supports are rotatable intermeshing discs mounted on axles. <IMAGE>

Description

SPECIFICATION Biological treatment of liquids This invention relates to a method and apparatus for biologically treating aqueous liquids by contacting them with aerobic micro-organisms attached to an inert support.

The biological treatment of aqueous liquids using aerobic micro-organisms attached to an inert support is known, and is used, for example, in the pharmaceutical industry for the production of antibiotics by fermentation, and in the water industry for the treatment of aqueous effluents such as sewage. In one such process, the micro-organisms are attached to small solid inert bodies (e.g. particles of sand) in a tank, and air is bubbled into the tank as the liquid under treatment is passed through the tank. In another process, the necessity for supplying air directly into the liquid is avoided by attaching the micro-organisms to a support surface which itself is moved into and out of the liquid. When it is out of the liquid, the micro-organisms absorb oxygen from the surrounding atmosphere and are thus able to effect treatment of the liquid when they are re-immersed therein.

There have been various proposals made for suitable movable support members, probably the most satisfactory of which has been the proposal to use disc-shaped members as supports, successive immersion and aeration being achieved by mounting the disc generally vetically so that the lower half only is immersed in the liquid, and then slowly rotating the disc. A plurality of such discs can be mounted on a common driven axle across a treatment tank.

A problem arises in the use of movable supports such as the discs described, in that over a period of time, the amount of biomass generated on the supports becomes excessive.

This can result in insufficient aeration (leading to anaerobic sludges) and, usually more importantly, can result in overloading of the mechanical structures which eventually causes breakdown. These problems are conventionally dealt with by shutting down the plant and manually cleaning off the excess biomass from the supports.

It will be appreciated that it is very undesirable to have inefficient biological action and mechanical breakdowns, and also it is costly to have to close a plant at frequent intervals for biomass removal. We have now found, according to a first aspect of the present invention, that these problems can.be overcome in a simple but effective manner, namely by utilising the movement of the support to cause excess biomass to be removed automatically.

Thus, in one aspect, the invention provides a method of biologically treating an aqueous liquid in which the liquid is passed in contact with an inert support to which aerobic microorganisms are attached, and wherein the said support is moved to sequentially immerse the micro-organisms in the liquid and then withdraw them from the liquid for exposure to air for the absorbtion of oxygen, wherein said movement of the support is effective to remove therefrom excess biomass.

The invention also provides apparatus for biologically treating an aqueous liquid which comprises a vessel for receiving the liquid, at least one movable inert support surface for the growth of aerobic micro-organisms thereon, means for moving said surface whereby (in use) the micro-organisms thereon are successively immersed in the liquid in the vessel to effect treatment and withdrawn therefrom to absorb oxygen from the atmosphere, the arrangement being such that said movement of said support causes excess biomass to be removed therefrom.

The automatic removal of excess biomass by the movement of the support can be achieved in a number of ways. Among the simplest is the provision of a member, adjacent the path of movement of the support and closely spaced therefrom, so that as the support moves, excess biomass extending outwardly from its surface is sheared or sloughed off by engagement with the said member.

Thus, for example, in the case of disc supports, tines or similar fixed edges may be mounted parallel to, and spaced from, the disc surface so that as the disc rotates, the excess biomass is engaged by the fixed edge and removed. The edge will be spaced from the disc so as to leave on the disc the desired thickness of biomass, e.g. up to about 1 .5mm. With moving supports other than discs, e.g. rotating arms and the like, similar arrangements can be made so that the movement of a support relative to another member causes removal of the excess biomass.

We have further found, and this constitutes a second aspect of the present invention, that in the case of disc or similar rotatable supports, the provision of a fixed edge or some other extra member, is unnecessary and that the discs can be so arranged as to be selfregulating in regard to the amount of biomass. The new arrangement also has a number of other significant advantages over the prior art.

According to this second aspect of the invention (which will be hereafter described with reference to disc supports but is equally applicable, mutatis mutandis, to other like supports), a first series (two or more) of discs is mounted on a first common#axis, and second series of discs is mounted on a second common axis parallel to and spaced from the first axis so that a first series of disc is interposed between each pair of second series discs (and vice versa). The two axes are spaced apart by a distance greater than the radius of the discs but less than their diameter.The discs are rotated so that, in the areas of overlap, there is a relative movement between the overlapping surfaces of adjacent first and second discs, and by arranging the gap between the adjacent first and second series discs to be suitably small, each disc will automatically slough off from adjacent discs excess biomass thereon. Thus, in use, the discs are self-regulating as to the thickness of biomass thereon.

Preferably, in accordance with this second aspect of the invention, there will be several series of discs, each "intermeshing" with the adjacent series. This gives an additional advantage over the prior art in that, without using any more space, extra series of discs can be used. Thus, in the prior art, each series of discs is well spaced from the next series (the common axes are more than one disc diameter apart). In the present invention, the common axes are much closer and the discs "intermesh", so a greater number of series of discs can be accommodated within a given space. This, in turn, means that the liquid can be treated more effectively.

Apart from the discs mounted at the ends of some of the series, each disc of one series lies (partly) between two discs of an adjacent series. The discs are rotated so that there is relative movement in the overlap area. This can be achieved by rotating each series of discs with the same angular velocity since then, in the overlap area, one series disc surface will be moving generally downwardly and the other generally upwardly. However, the discs can be driven at different angular velocities if desired, provided the necessary removal of excess biomass occurs.

The gap between the surfaces of adjacent discs in the overlap region will be chosen to give the desired thickness of retained biomass. Generally, the gap will be about 3mm, thus allowing a layer of biomass up to about 1.5mum thick to form on each surface. Greater or smaller gap settings can of course be used.

Any discs at the ends of each series which are not sandwiched between two discs of an adjacent series can be provided with a fixed cleaning edge as previously described.

A preferred way of operating the method of the invention is to pass the liquid to be treated through a tank, for one end thereof to the other, the series of discs being mounted on axles extending across the tank transversely of the direction of liquid flow.

The number of discs, their thickness and diameter will depend on the quantity and composition of liquid to be treated. In the case of sewage the loading rate to produce a 30:20 (SS:BOD) effluent (units are mg/l) would not exceed about 10g BOD/m2 surface area of disc per day (at 1 0 C.). All the discs are in part submerged in the liquid. Generally, there will be at least three series of discs per tank for sewage treatment purposes. The driven intermeshed discs automatically pre vent excessive accumulation of biological film on their surfaces.

Biomass which is sheared from the discs collects in the bottom of the tank and can be automatically or manually withdrawn at regu lar intervals, i.e. often enough to avoid col lected biomass (sludge) filling the tank to a level such that the outer edge of the sub merged part of any disc is in contact with the sludge.

In the case of sewage treatment, the treated water may be discharged from the tank into a settlement vessel for removal of any sus pended solids, but this is only necessary if high quality effluent is required.

Raw sewage, after removal of grit and coarse debris (possibly by screening), or set tled sewage can be effectively treated in the method of the invention. Effluent quality of better than 30:20 (SS:BOD), including oxida tion of ammoniacal nitrogen, may be achieved at lower loading rates (about 5 g/m2 d) (at 100).

To avoid excessive loss of heat, the tank can be insulated with suitable materials such as expanded polystyrene or glass fibre, and a cover provided for the discs and any motor drive units. The motor can be located under the tank on a section not insulated so that any heat loss from the motor is transferred to the waste water.

The discs are preferably all of equal size and made of a corrosion-resistant material, e.g. plastic or glass-reinforced plastic. The diameter of discs may vary for different units e.g. from about 0.5 to about 5m. Any num ber of series of discs can be used, e.g. from 3 to 30, and the length of each series can vary widely, but will normally be from about 3 to about 30m. Axle shafts over 3m in length usually have to be supported by intermediate bearings at regular intervals along their length and may be driven by individual motors at tached to each shaft.

The discs, particularly when made of a plastics material, are not usually completely rigid and, especially at larger diameters, tend in use to flex. Also, over a period of time, discs may warp or buckle slightly so as not to be planar. This can result in uncontrolled abrasion between adjacent discs, and in varia tions in the gap size between adjacent discs, i.e. some gaps will be smaller than desired and some greater. In order to avoid this problem, disc surfaces can be provided with upstanding spacer members, designed to bear upon an adjacent disc to maintain the desired minimum gap between the surfaces. For example, in the case of plastics discs, the surfaces can be formed with one or more integral upstanding ribs or like portions appro priately sized to achieve the desired gap set ting. Thus, an annular rib can be provided at the outer edge of a disc.For the larger size discs, a second such annular rib may be desirable of, for example, about half the disc radius.

In practice, there may from time to time be periods when the, or one of the, motors driving the discs is inoperative (e.g. when there is a power supply failure to an electric motor, or when some fault develops). During such periods, the discs are stationary, and biomass will continue to accumulate on their submerged portions. At subsequent start-up of the motor, the accumulated biomass may so clog the discs that the motor is temporarily overloaded and may burn out, or some part of the linkages become damaged.To reduce this possibility we prefer to provide, between the or each motor and the respective driven axle shaft(s), a torque limiter or torque converter to prevent excessive start-up load on the motor(s) The intermeshing of the discs avoids excessive growth of biomass which in prior art systems has frequently caused breakage of shafts as a result of out-of-balance forces reaching critical values. Such intermeshing also increases the total surface area of the discs, on which biomass growth, within a given volume.

In order that the invention may be more fully understood, one preferred embodiment of apparatus in accordance with the invention will now be described, by way of example only, together with its use in the method of the invention. The embodiment is illustrated in the accompanying drawing in which: Figure 1 is vertical sectional view of a treatment tank with three series of discs; and Figure 2 is a top view of a horizontal section through the axles of Fig. 1.

Referring to the drawing, a tank 1 receives liquid to be treated at inlet 2, the liquid travelling through the tank to outlet 3. The bottom 4 of the tank 1 slopes to a sludge withdrawal port 5. Mounted across the tank 1 are three axles 6, 7 and 8 driven in rotation (in the arrowed directions) by motor unit 9 (the drive chains are not shown). Each axle has a series of discs 61,62 etc., 71,72 etc., 81,82 etc. mounted thereon. (In Fig. 2, only the first few discs on each axle are shown.

Each series of discs extends from one side of the tank to the other side.) As can be seen, the discs on axle 7 intermesh the discs on axles 6 and 8. The gap 10 between the disc surfaces is about 3mm. (In the drawings, no biomass is shown).

Claims (1)

1. A method of biologically treating an aqueous liquid in which the liquid is passed in contact with an inert support to which aerobic micro-organisms are attached, and wherein the said support is moved to sequentially immerse the micro-organisms in the liquid and then withdraw them from the liquid for exposure to air for the absorption of oxygen, wherein said movement of the support is effective to remove therefrom excess biomass.

2. A method according to claim 1, wherein said support comprises a plurality of discs mounted on an axle, and means are provided closely spaced from each disc surface to shear or slough from each said surface excess biomass thereon, upon rotation of said discs.

3. A method according to claim 1 wherein said support comprises a first series of discs mounted on a first axle, and a second series of discs mounted on a second axle, the axles being parallel and spaced so that each disc (except end discs) of each series extends between a pair of discs of the other series and is closely spaced therefrom whereby, as the or each series of discs is rotated, excess biomass on each each disc is automatically sheared or sloughed off by the adjacent discs.

4. A method according to claim 3, wherein at least a third series of discs mounted on a third axle, is also provided, said third axle being parallel to said first and second axles and spaced from said first or second axle so that each disc (possibl excepting end discs) of said third series extends between a pair of discs of the first or second series and is closely spaced therefrom whereby, during relative rotation of the third discs and the respective first or second discs, excess biomass is sheared or sloughed from the discs.

5. A method according to claim 3 or 4, wherein the gap between adjacent disc surfaces is about 3mm.

6. A method according to claim 2, 3, 4 or 5, wherein the aqueous liquid is passed through a tank in a generally horizontal direction and each axle is fixed generally transversely of the tank, the discs being partly submerged in the liquid.

7. A method according to claim 1 substantially as herein described with reference to the accompanying drawings.

8. Apparatus for biologically treating an aqueous liquid which comprises a vessel for receiving the liquid, at least one movable. inert support surface for the growth of aerobic micro-organisms thereon, means for moving said surface whereby (in use) the micro-organisms thereon are successively immersed in the liquid in the vessel to effect treatment and withdrawn therefrom to absorb oxygen from the atmosphere, the arrangement being such that said movement of said support causes excess biomass to be removed therefrom.

9. Apparatus according to claim 8 wherein there are at least two said movable supports, each comprising a series of discs mounted on a common axle, the axles being parallel, the discs on each axle intermeshing with the discs on the next adjacent axle(s) with the overlap ping disc surfaces being closely spaced whereby, in use, excess biomass thereon is automatically sheared or sloughed off during rotation of the discs.

11. Apparatus according to claim 8 substantially as herein described in the accompanying drawings.