GB1593132A

GB1593132A - Treatment of waste water or waste water-containing liquid

Info

Publication number: GB1593132A
Application number: GB3781/78A
Authority: GB
Original assignee: Pielkenrood Vinitex BV
Current assignee: Pielkenrood Vinitex BV
Priority date: 1977-02-16
Filing date: 1978-01-31
Publication date: 1981-07-15
Also published as: BR7800917A; BE864005A; CA1113193A; AU515362B2; DE2805461A1; ES467000A1; DK70778A; NL7701608A; CH626864A5; PT67653B; FR2380990B1; AU3297578A; PT67653A; NO780526L; IT7803331A0; FR2380990A1; JPS53103660A; IT1105861B; SE7801737L; AT360448B

Abstract

In the process, the waste water is cleaned by, before an aerobic biological treatment, carrying out a physico-chemical pretreatment, the impurities being floated using finely disperse gas bubbles and then skimmed off. The excess sludge from the aerobic biological treatment is fed back into the physico-chemical treatment stage. The resulting floated layer to be discharged from this physico-chemical treatment stage forms the only separation product.

Description

(54) IMPROVEMENTS IN OR RELATING TO THE TREATMENT OF WASTE WATER, OR WASTE, WATER-CONTAINING, LIQUID (71) We, PIELKENROOD-VINITEX B.V.

a Dutch Body Corporate of Industrieweg 13, Assendelft, Netherlands, do hereby declare the invention, for which we pray that a patent may be granted us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to the treatment of waste water or waste, water-containing, liquids.

To purify waste water biological treatment is very suitable, but the cost thereof is considerable. Devices for such a purification have large dimensions, and, moreover, the biological purification is very sensitive to disturbances. Also the degree of purification obtained thereby is often insufficient, and, additional waste is produced, i.e. inactivated or excessive sludge. This sludge generally contains about 99% of water, and, thus, gives rise to a new discharge problem.

A certain assistance for the biological purification can be obtained by effecting a preliminary separation by sedimentation.

The sludge deposited thereby has a smaller water content than the surplus sludge of the biological treatment, but the waste problem is not yet solved thereby. Although such a compound device will become substantially larger, its effect is, at the current mode of operation, not very much improved.

A further assistance for the biological purification may be obtained by adding separation-promoting agents, i.e. by means of physico-chemical purification, and the produced flakes are. subsequently, separated by sedimentation. However, the flakes thus obtained appear to have only a very small difference in specific weight with respect to the liquid, so that the separated components contain a large amount of water, and, the sedimentation thereof will be difficult. The benefits obtained thereby will, again, lead to an increase of the discharge problem.

Another difficulty is that disturbances in the physico-chemical process will have an aftereffect in the biological purification.

As a consequence of the extension of urban areas and the enlargement of industries, the purification of collected waste water is becoming increasingly difficult, in particular since, when draining ever increasing amounts of waste water, less reliance can be had in the self-cleaning power of nature.

According to the invention, there is provided a method for treating a liquid (as hereinafter defined) by using an aerobic biological purification, wherein the liquid under treatment, preliminary to the biological purification, is submitted to a treatment which provides a floating layer of separated components using finely divided gas bubbles, said floating layer being removed, and surplus sludge from the biological purification is recycled to be subjected to the preliminary treatment.

Further according to the invention there is provided a device for carrying out the above method comprising a preseparation device, a device for performing an aerobic biological purification, and an after-separator for removing from the liquid active sludge entrained by the liquid from the biological purification device, the preseparation device including means for introducing gas bubbles in such a manner that the components separating from the liquid are made flotable by adhesion of these gas bubbles, and means for feeding sludge discharged from the afterseparator to the preseparation device.

By the term "liquid" there is meant waste water, or waste, water-containing, liquid.

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings. in which: Figure I is a block diagram of a treatment system in accordance with the invention; and Figures 2 and 3 are schematic crosssections of practical embodiments of two stages of the system of Figure 1.

The system shown in Figure I comprises a supply I for the liquid to be treated, e.g.

waste water. After removing coarse impurities, the liquid is supplied to a pretreatment stage 2 (i.e. a preliminary treatment stage) in which additional substances can be added to the liquid, as indicated at 3, e.g. a coagulant (FeCI) agents regulating the acidity, and agents (polyelectrolytes) for promoting flocculation of components present in the liquid.

This stage can, moreover, comprise means for effecting coalescence of these components.

The outflow duct 4 of the stage 2 leads to a preseparator 5 (i.e. a preliminary separator) having a discharge duct 6 through which the pretreated liquid is discharged. The pretreatment stage 2 and the preseparator 5 collectively form a preseparation device (i.e. a preliminary separation device) in which a physico-chemical treatment is effected. The duct 6 is provided with a branch duct 7 in which a compression pump 8 is included which, at 9, can draw in air. The liquid is saturated under pressure with air, and is, at 10, returned into the preseparator 5. Just before the introduction the liquid is returned to the normal pressure by means of a decompression valve 11, which will lead to the generation of a great number of small bubbles in the liquid in the preseparator 5.These bubbles adhere to the flakes present in the liquid, which are or have been formed as a consequence of the pretreatment in stage 2, so that these flakes become lighter than the liquid and will float as a floating layer on the liquid, which layer can have a rather large dry matter content (about 10%). By means of a suitable skimmer 12 this floating layer is removed from the liquid surface, and is led by means of a duct 13 towards a dryer 14, and subsequently the dried floating material is discharged at 15. Apart from the purified water, this constitutes the only matter discharged from the system.

The discharge duct 6 also leads to a biological purification device 16 provided with an aerator 17, in which device the liquid treated in the preseparator 5 is additionally purified by aerobic purification. The purified liquid is led through a duct 18 towards an after-separator 19, in which the active sludge taken along by the liquid is removed therefrom. The purified liquid is discharged at 20, and may be subjected to further treatments if required. The separated sludge is discharged through a duct 21. If the device 16 is a simple tank in which the active sludge is present. the liquid discharged through the duct 18 will contain more sludge than is produced by growth of the bacterial mass, so that. in order to prevent a decrease of the amount of active sludge, a part of the sludge separated in the device 19 is to be returned towards the device 16. To that end a duct 22 is branched off the duct 21, the former, if necessary. being provided with a regulator or pump 23. and leading again towards the device 16. If, however. the device 16 is a biological purification tower in which the liquid is allowed to flow through or over active material on a substrate. air being simultaneously supplied, only the excess of active material will be washed away which, then, is to be removed in the after-separator 19. In that case the duct 22 is superfluous. The duct 21 can be provided with a pump 24, and leads back towards the entry side of the system, e.g.

point 25 of the supply duct 1, so that, then, the separated sludge having a high water content (about 99%) will be supplied to the stage 2 together with the liquid to be treated.

It is also possible to lead the duct 21, as shown at 21', towards a point 25' in the duct 4, so that the separated sludge is not subject to the treatment in the stage 2. Moreover it is possible to introduce this duct, as shown at 21", somewhere into stage 2, so that, then, the sludge will undergo a partial treatment in that stage.

If such a system is designed for purifying waste water from a factory which is, for instance, idle during weekends, the microorganisms in the biological purification device 16 will not receive nourishment during that period, so that, then, the active mass in this device will considerably deteriorate. In order to prevent this, a part of the floating layer material separated during the preceding period in the preseparator 5 can be stored, and can be supplied to the biological purification device 16 during that idle period.

This is possible because the floating layer material has a high dry matter content, and, thus, a relatively low water content, so that storage thereof does not require a very large container.

This is indicated in the drawing with dashed lines. In the duct 13 a 3-way valve 26 is included which is connected to a branch duct 27 leading to a storage tank 28. The latter is, on the other hand, connected to a discharge duct 29 which may be provided with a pump 30, and leads to the device 16.

By switching over the valve 16, the floating layer material from the duct 13 is not directed towards the drying device 14 but towards the storage tank 28, from which this material, as needed, can be led through the duct 29 towards the device 16. This return flow can be made automatic in a simple manner.

The precise details of the various parts of the system will depend, of course, on the character of the liquid to be treated. The supply of additional agents can, at least for a substantial part, be made automatic, and may. for instance, be controlled by means of acidity sensors or the like.

The preseparator 5 can be constructed in the manner shown in our earlier British patent Specification no. 1 490227 in which, apart from means for introducing air bubbles, also an inclined plate separator is present. which improves the separation degree still further. An embodiment thereof is schematically shown in fig. 2 showing an inclined plate assembly 31 disposed in the tank of the preseparator 5 between two chambers separated from one another by a baffle 32 so that the liquid to be treated must flow through said plate assembly 31. The liquid to be treated is pumped by means of a pump 34 from the duct 4 towards a nozzle 35 in the deepest point above said assembly 31, where also the nozzle 10 for introducing air bubbles is situated. The latter nozzle 10 is connected to the pump 8 shown in fig. 1. The suspended particles made flotable by adhered air bubbles are collected as a floating layer 37, and residual light or heavy particles are separated in the passages of the assembly 31. The liquid leaves the separator via an overflow 38.

The skimmer 12 of fig. 1 can be constructed in the manner shown in our earlier British patent Specification no. 1 554439, which is specifically suited for removing the floating layer without entraining carrier liquid, and in which, moreover, because of the special manner of removal, a further reduction of the liquid content may be obtained.

Fig. 2 shows at 12 such a skimmer, which may comprise two sets of scraper blades, one for gradually driving the floating layer towards an inclined overflow 39, and the other for pushing the floating matter over said overflow. These sets of scraper blades move so that the blades are inserted substantially in their own longitudinal direction into the floating layer, then move only over a part of the displacement path, and are retracted again substantially in their own direction, the floating layer thus being moved onwards step-wise so as to avoid too much compression, which would cause the floating matter to escape below the blades.

The after-separator 19 can also comprise an inclined plate separator, e.g. according to our earlier British patent Specification no. 1 295 123, in which a very efficient sludge separation within a relatively short time can take place. When using simple sedimentation the residence time of the active sludge in the sedimentator will be rather considerable, so that, since air supply no longer takes place, an anaerobic action can develop. This may lead to formation of gas bubbles which counteract the sedimentation, so that the efficiency of such an after-treatment may be poor. In a plate separator the residence time will be so short as to avoid anaerobic effects and the consequent problems of this. Fig. 3 shows such an after-separator 19 which comprises an inclined plate assembly 40 and a transverse baffle 41 dividing the interior of the tank of the separator 19 into two parts so that the liquid supplied at 18 must flow through the assembly 40. Sludge taken along with the liquid will be separated in countercurrent in this assembly, and is collected in a funnel 42. The clear liquid flows off via an overflow weir 43 towards duct 20. Specially designed guiding ducts 44 according to Brit ish Patent Specification 1295 123 prevent intermixing of the supplied liquid and the separated sludge, and a dip baffle 45 sub merged into the collected sludge prevents liquid from flowing via the sludge compartment and disturbing the sediment flow, as described in the before-mentioned patent specification.

In the preseparator 5 about 80 to 90% of the components suspended in the liquid may be separated, so that the biological purification device 14 can be made smaller accordingly, which leads to substantial space and energy savings, and less active sludge needs to be removed. The preseparator 5 produces, moreover, a floating layer with about 10 % dry matter, so that drying thereof can take place in a simpler manner. Recycling the excess sludge separated in the after-separator 19, which generally comprises only about 1 % dry matter, towards the input end of the system, leads to a further thickening of this sludge, and, moreover, a favourable effect on the separation effect in the preseparator 5 appears to be obtained thereby. The dry material removed from the floating layer may often from a usable by-product. If the biological purification is adapted to break down about 90 to 95 % of the supplied components, the total degree of purification will be 98 to 99,5 %. These figures apply to the case without recycling of the sludge.

A numerical example will elucidate the foregoing. At a supply rate of, for instance, 100 m3/h liquid, a floating layer of 1 m3/h with a dry matter content of 10 % will be obtained in the preseparator 5, and the biological purification will provide an excess sludge amount of 1 m3/h with a dry matter content of 1 %. The amount of purified liquid is, then, 98 m3/h, and 2 m3/h waste liquid is formed, which, in total, contains 100+ 10= 110 kg/h solid matter. The average dry matter content of the waste flow is, then 5,5 %. If, in stage 5, a purification effect of 85 %, and in stage 16 a purification degree of 90 % is obtained, the liquid discharged at 20 will be purified for 98,5 %. These figures apply to purification without recycling the sludge.

If the excess sludge is recycled, only one waste flow is obtained, namely 1,05 m3/h, in which the dry matter content appears to be nearly 10,6 %, the waste flow amount being only slightly more than that of the floating layer alone, and the dry matter content being appreciably larger than that of the floating layer alone. This amount is. namely, increased from 110 kg/h to 111 kg/h. The waste flow of 1,05 m'/h is now by 0,95 ml/h less than in the case without recycling, which is equivalent to an accordingly larger amount of cleaned liquid at the output 20, which will now be more than 99 m3/h. Whereas, with out recycling, the purification degree of this liquid was already 98,5 %, it will, by the increase of the amount at an equal impurity amount, now become about 99 %.

From the foregoing it follows that the excess sludge of 1 m3/h with 1 % dry matter is, as it were, thickened to 0,05 m3/h, i.e. to a dry matter content of 20 %.

By the above-mentioned assembly of measures the discharge problem of the products obtained in the purification treatment has been solved, and an excellent purification is obtained by means of a system which requires much less space, and can, therefore, be manufactured in a cheaper manner. Moreover the cost of the biological purification will be reduced accordingly, and also savings in the physico-chemical pretreatment can be obtained. Also the proceeds of the dried matter will sometimes contribute to a reduction of the expenses.

The system particularly described, permits substantial improvement of the purification effect without increasing the waste problem.

The system uses flotation in a physicochemically treated liquid flow, in which flakes will be or have been formed, by introducing into the flow a liquid saturated under pressure with air (or another gas), which liquid is depressurized just before being introduced, which leads to the generation of large numbers of small gas bubbles which will adhere to the flakes so as to make the latter capable of floating. In this manner a floating layer with a high dry matter content is produced. Because of this higher dry matter content, the discharge problem will already substantially be reduced in the first stage compared with physico-chemical purification by sedimentation. It is often possible to avoid an intermediary dewatering step before the final drying of the floating layer material. The residue can be purified in an accordingly smaller and, thus, less expensive aerobic biological purification stage, which will lead to substantial savings. The liquid leaving the biological stage is, then.

much cleaner than in the case of a much larger biological purification device without physico-chemical pretreatment.

Any areobic biological purification treatment will normally lead to its own discharge problem. since the living mass of bacteria used therein increases in the liquid medium which is favourable for its development. In biological filters the surplus is washed away.

and then separation by sedimentation can take place. In a basin containing active sludge the whole bacterial means is to be separated by sedimentation. and a part thereof must be reintroduced for maintaining the sludge mass. The surplus has. in both cases, a very high water content. and contains only little dry matter, viz. about l"'(', which, again. causes a serious discharge problem.

In system described the latter problem does not exist, since, now, the aqueous surplus sludge is returned towards the preliminary separation device in which, together with the supplied liquid, an effective separation to a high dry matter content takes place.

In this manner also the discharge problem of the biological purification step is solved.

In this mode of operation still more advantages will be obtained. A susprising result is that the floating layer, when adding the surplus sludge of the biological purification, appears to have a higher dry matter content and, thus, a lower water content, and, at the same time, the surplus sludge separated by flotation appears to be thickened. The presence of both kinds of sludge apparently leads to a mutual dewatering.

This result is surprising since the surplus sludge contains much water, so that it would be normally expected that adding this sludge would lead to an increased water content.

Another additional advantage is the favourable twofold effect on the purification.

Not only the dissolved organic matter content (measured as the biological or chemical oxygen demand) in the liquid leaving the physico-chemical treatment stage will be lowered, but also the suspended matter content is reduced, in spite of the higher load on this stage. The reduction of the dissolved matter content is much larger than can be attributed to the dilution as a consequence of the addition of a residual activity of the bacteria present in the surplus which can absorb dissolved organic substances. This can lead to additional savings in the additives required for the physico-chemical treatment.

The reduction of the suspended matter content is directly related with the improved flotation effect since, apart from a thickening, also a better flake generation is obtained.

It has been said earlier that the biological purification is sensitive to disturbances. This is especially true for surge loads accompanied by acidity fluctuations. Such surges can be noticed in the physico-chemical treatment stage and can bem largely eliminated, and, if necessary, alarm can be given in time.

The discharged floating layer can, furthermore, be dried in a simple manner, and often comprises matter which is useful for preparing cattle fodder or fertilizer, which can be sold so that the purification cost will be lowered accordingly. But also when the sludge is no longer useful. the transport or destruction cost will be substantially lowered, since the water content of the floating layer is relatively small.

Summarizing it can be said that by combining a physico-chemical flotation treatment and an aerobic biological purification a considerably better purification with considerably less waste removal problems is ob tained and thus can be realized in systems with smaller dimensions. In already-existing treatment devices of one kind it is possible to obtain a considerable improvement of the purification by adding a device of the other kind without requiring much space.

The system described is specially suitable for the purification of sewage water or waste water from various sources, such as meat processing plants, for example slaughterhouses or canneries, paper-making and textile plants. The system can also be used for purifying any liquid which contains water and which can be treated by means of biological purification.

WHAT WE CLAIM IS: 1. A method for treating a liquid (as hereinbefore defined) by using an aerobic biological purification, wherein the liquid under treatment, preliminary to the biological purification, is submitted to a treatment which provides a floating layer of separated components using finely divided gas bubbles, said floating layer being removed, and surplus sludge from the biological purification is recycled to be subject to the preliminary treatment.

2. A method according to claim 1, comprising the step of drying the removed floating layer.

3. A method according to claim 1 or claim 2, comprising the step of storing a part of the removed floating layer for subsequent supply to the biological treatment stage during a period when no liquid to be treated is supplied, in order to keep alive the active organisms in the biological treatment state.

4. A method according to any one of claims 1 to 3, wherein the active sludge is separated from the liquid leaving the biological treatment stage by means of an inclined plate separator, in which the residence time is sufficiently short to avoid anaerobic effects.

5. A system for carrying out the method claimed in claim 1, comprising a preseparation device, a device for performing an aerobic biological purification, and an afterseparator for removing from the liquid active sludge entrained by the liquid from the biological purification device, the preseparation device including means for introducing gas bubbles in such a manner that the components separating from the liquid are made flotable by adhesion of these gas bubbles, and means for feeding sludge discharged from the after-separator to the preseparation device.

6. A system according to claim 5, wherein the preseparation device comprises a pretreatment stage in which auxiliary substances are added to the liquid, and the feed means for the discharged sludge is operative to feed the sludge to the preseparation device, before, in or after the pretreatment stage.

7. A system according to claim 5 or 6, further comprising means for feeding sludge discharged from the after-separator to the biological treatment device, and means for adjusting the ratio between the sludge flows to the preseparation device and to the biological treatment device.

8. A system according to any one of claims 5 to 7, wherein that the after-separator comprises an inclined plate type separator.

9. A system according to any one of claims 5 to 8, wherein the preseparation device comprises an inclined plate type separator.

10. A system according to any one of claims 5 to 9, comprising means for feeding to a drying device, floating components removed from the liquid in the preseparation device.

11. A system according to any one of claims 5 to 10, comprising means for storing floating components removed from the liquid in the preseparation device, and means for feeding the stored components to the biological purification device.

12. A method of treating a liquid substantially as hereinbefore described with reference to the accompanying drawings.

13. A system for treating a liquid substantially as hereinbefore described with reference to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. tained and thus can be realized in systems with smaller dimensions. In already-existing treatment devices of one kind it is possible to obtain a considerable improvement of the purification by adding a device of the other kind without requiring much space. The system described is specially suitable for the purification of sewage water or waste water from various sources, such as meat processing plants, for example slaughterhouses or canneries, paper-making and textile plants. The system can also be used for purifying any liquid which contains water and which can be treated by means of biological purification. WHAT WE CLAIM IS:

1. A method for treating a liquid (as hereinbefore defined) by using an aerobic biological purification, wherein the liquid under treatment, preliminary to the biological purification, is submitted to a treatment which provides a floating layer of separated components using finely divided gas bubbles, said floating layer being removed, and surplus sludge from the biological purification is recycled to be subject to the preliminary treatment.