CA1113193A - Method and a system for treating liquid, in particular waste water - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

This invention is directed to a method and system for treating liquids, in particular waste water, by using an aerobic purification, characterized in that the liquid under treatment, preliminary to the biological purification, is submitted to a physico-chemical pretreatment with flotation by using finely divided gas bubbles, and in that the surplus sludge of the biological purification is recycled towards the physico-chemical treatment stage, this in such a manner that the removed floating layer obtained in this physico-chemical treatment is the only separation product.

Description

~131~3 Draining wa~te water becomes an e~er increa6in~ problem.
For pur~fying such waste water biological treat~ent i~ very 6uitable, but the cost thereof iB considerable. Device~ for such a purification ha~e large dimension~q, a~d, moreover, the biological purification is very sen~itive to di6turbances. Also the purifi-¢ation degree obtained thereby is often insufficient, ~nd, moreovér, additional waste i8 produced, i.a. inacti~ated or exce~qactive sludgo. Thi~ aludge generally contains about ~9 % water, and, thus, gi~es ri~e to a new discharge proble~.
A certain relief for the biological purification can be obtained by u6ing a pre-separation by sedimentation. The sludge dopo~ited thereby has a smaller ~ater content than the ~urplus slud~e of the b~ological treat~ent, but the waste problem i8 not yet sol~ed therebyO Although such A co~pound deYice ~ill become substantiall~ lar~9r, ito e~fect is, at the current mode of ope-ration, hOt ~er~ ~ch impro~ed.
A ~urther relieS ~or the biolog~cal purification may be obtained b~ adti~ ~eparation pro~ot~ng agents, i.e. bg mea~ of phys~oo-ohe~io~l purification, a~d the produced flakes are, sub-~quentl~, s~parated b~ oedimentation. ~owe~er, the flakeq thus - obt~ined appear to ha~e only a ~ery s~all difirerence in ~pecilric ~ight in re~pect of tho liquid, ~o that the separated components contain ~ery muc~ water, and, moreo~er, the sedi~entation thereof will be di~ri¢ult~ The relief obtai~ed thereb~ ~illt again, lead to an lncrease o~ the discharge problem. Another difficulty i~
that di~turbances in the physico-ch~ical prOCoG6 will have an aiter-eff0ct in the biological purification. Moreo~er, as a conse- ~
qu0nce o~ the extension of urb~n areas and enlargement of in- -dustries, the purification of collected wa~te water will become :

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~131~3 more and more difficult, in particular ~ince, when draining ever increasing amounts of waste water~ less and less confidence can be had in the self-cleaning power of nature.
The ob~ect of the invention i~ to provide a method and a 6ystem which, without increasing the ~aste problem, will lead to a 6ubstantial improve~ent of the purification effect.
To that end use i~ made of flotation in a physico-chemically treated liquid flow, in which flakes will be or have been formed, and this by introducing into ~aid flow a liquid saturated under pre6sure with air or another gas, which liquid is depre~surized ~UBt before being introduced, which lead6 to the generation of larg~ number6 of small Bas bubble6 which will adhere to the flakes 80 as to make the latter flotatable. In this manner a ~loating layer with a high dry matter content is produced. Be-cause of this higher dry matter content, the discharge problem will already substantially be reduced in the first stage compared with physico-chemical purification by ~edi~entation~ It is often even possibl~ to a~oid an intermediary de~atering step beiore the final drying of the floating layer material. The residue can be purified ~n an accordingly smaller and, thus, cheaper aerobic biological purification st~e, which will already lead to ~ubstantial ~avings.
~he liq~id lea~ing the biological staBe iB ~ then~ ~ch cleaner than in the oa~e 0~ a much larger b~ological puri~ication device without ~hysico-ch~mical pretreatment.
Any aerobic biological purification treatment will lead to its own di8~harge problem, sin~e the ll~ing maA~ of bact~ria used th~rein incr~ases in the liquid medium vhich is favourable for its dev~l4pmont- In biological filtcrs the surplus is washed away, ~nd then separation by sedimentation can take place. In a basin containlng acti~e slud~e the whole bacterial mass is to be separa-ted by sedimentation, and a part thereof must be reintroduced for ~aintainin6 the sludge mass. The surplus has, in both cases, a very hi~h water content, and contains only little dry matter, viz.
about 1 %, which, again, causes a serious discharge problem.
In the mo~hod and system of the invention the latter problem does not exi~t, since, now, the aqueou~ surplu~ 61udge is returned towards the input separation ~tage in which, together ~131~3 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 ~tep i8 solved.
In tnis mode of operatlon still more advantages will be obtained. A remarkable fact $6 that the floa~ing layer, when adding the surplufi sludge of the biological purification, appears to have a higher dry matter content and, Ihus, a lower water content, and, at the same time, the surplus sludge separated by flotation appears to be thicke~ed. The presence of both kinds of sludge obviously leads to a mutual dewatering. For, after all, the surplus ~ludge contains much water, 80 that it should be expected that adding thi~ sludge would lead to an increased water content.
Another additional advantage is the favourable twofold effect on the purification. Not only the diRsolved organic matter content (measured as the biological or chemical oxygen demand) in the liquid leaving the p~y~ico-che~ical treatment stage will be lowered, but aiso the suspended matter content is reduced, in spite of the highter load on thi~ stage. The reduction of the di~-solved matter content is mùch larger than can be attributed to the dilution ae a consequence of the addition of the aqueoua surplussludge, and is probably a ¢onsequence of a residual activity of the bacteria prosent in the ~urpluR which oan absorb di6solved orga~ic 8ubstances. Thia can lead to addi~onal sa~ings in the ~dditi~as roqulrcd for the phys~co-chemicAl treatment. The re-duction Of the suspended ~atter conte~t i~ di~ectly related wlth the improved ~lotation e~ect s~nc~, apart ~rom a thickening, also a better flake generation i8 obtained.
It has already baen remarked that the biological purifi-cation is sensitive to disturbances. This i~ e~pecially true for surge loads attended with acidity fluctuations. Such 6urges can be noticed in the physico-che~ical treatment ~tage and can be largely oliminated, and, if neceesary, alarm can be ~iven in time.
The dlscharged float~ng layer can, furthermore, be dried ~n a aimple manner, and often comprises ~atter which is useful for preparing cattle ~odder or fertillzer, which can be sold so that tho purification cost vill be low~red accordingly. But also when the sludge iB no longer useful, the transport or deRtruction cost ~131~3 will be substantially lowered, since the water content of the floating layer i~ smaller than in the current methods.
In plants ~hich are idle during weekends, the disad~an-tag~ exists that during these periods the biologically active mass will not receive the nourish~ent required for its ~aintenance.
Wh~n using the method of the invention, in ~hich a floating layer cantaining much dry mass is produced, a part of this floating material can be stored, which can be supplied to the biological purification stage during such periods in order to maintain the living organi~ms therein. Because of the relatively low water con-tent of this material, a ~torage tank therefor needs not have verg large dimensions.
The separation of the active sludge from the liquid leaving the biological puri~ication stage i8t in particular, per-for~ed by means o~ a plate separator, in ~hich the residence time i8 suf$iciently ~hort 80 as to avoid anaerobic effects attend~d ~ith gas de~elopment opposing sedimentation.
- Suomarizing it can b~ said that by combining a physico-chemical flotation treatment and an aerobic biological purification a considorably better purification with con~iderably less waste ramo~al problems is obtained, which can be realized in devices with s~aller dimonsion3. In already existing device~ of one kind it is possible t~ obtain a considerably i~provement of the purifi--oation b~ ~dding a devi~u of the other kind without requirin2 much ~pace.
The i~entio~ will be ~lucidated below by re~urenc~ to ~a drawing showine i~
Fig. 1 a dia~r~3matic representation of a sy~t~m accor-din~ to the invention; and Fig. 2 and 3 schematic cross-sections of practical em-bodiments of t~o stage~ of the system of fig. 1.
The syate~ sho~n in fig. 1 comprise~ a supply 1 ~or the liquid to be treated, e.~. waste ~ater. After re~o~in~ coarse i~-purities, the liquid i~ supplied to a stage 2 in ~hich additional substances can be addud to the liquid~ as indicated at ~, e.g. a coagulant (FeC13), agents re~ulating the acidity, and agents ;~poly-lectrolytes) for promoting flocculation of co~ponents present . . . ~

5 ~ 3 in the liquid. This stage can, moreover, comprise an auxiliary device for brin8ing about coalescen~e of these component~, The outflow duct 4 of the stage 2 leads to a presepara-tor 5 having a discharge duct 6 through which the pretreated liquid i6 discharged. The duct 6 i8 provided with a branching duct 7 in which a co~pres~ion pump 8 i~ included which, at 9, can ~uck in air. The liquid is saturated under pre~sure with air, and is, at 10t returned into the preseparator 5. Ju~t before the intro-duction the liquid i8 returned to the normal pres6ure by means of a deco~pression valve 11, ~hich will lead to the generation of a great number of small bubbles in the liquid in the preseparator 5.
These bubbles adhure to the rlakes present in the liquid, which are or have been formed as a consequence Or the pretre~tment in stago 2~ 80 that these flakee become li~hter than the liquid and will float as a floating layer on the liquid, which layer can haYe ~ rather large dry ~atter content (about 10 ~). By mean~ of a suitable ski~mer 12 this floating layer i8 re~oved from the liquid - surface, and is led by mean6 of a duct 13 towards a dryer 14, and subsequently the tried floating material iB di6charged at 15.
The discharge duct 6 al~o leads to a biological purifi-cation device 16 prov~ded with an aerator 17, in which de~ice the liquid tre~ted in the preseparator 5 i~ additionally purified by ~erobic purificat~on. The purified liquid is led through a duct 18 towarde an aftèr-~eparatOr 19, in which t~o active sludge taken ~long by th~ liquld ~o ro~o~ed th~refrO~. The puri~ied liquid is discharg~d at 20, and ~y be subJootod to ~urther treatoent6 if ~-quired. The 6eparated slud6e l~ discharged throu6h a duct 21. I~
the de~oe 16 is a simple tank in which the acti~e slud6e is pre~ent~ the liquid discharged throu6h the duct 18 will contain ~ore sludge than iff produced b~ grovth of the bacterial ma~s, 60 ~that, in order to prevont a decrease Or the a~ount of active sludge, part of the ~ludg~ s~parated in th~ de~ica 19 iB to be returned towards tho device 16. To that end a ductl~s branched off the duct 21, the foro~r, if nece~sary, being pro~ided with a regulator or pu~p 2~, and leading again toward~ the de~ice 16. If, however, the de~ice S6 i8 A biological purification tower in which the liquid iB allowed to flow through or over aoti~e ~aterial on a substrate, .-~ ' .
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6 ~ ~ 1 3 1 ~ 3 air being si~ultaneously supplied, only the exce~s of active material will be wa~hed away which, then, is to be removed in the after-separator 19. In that case the duct 22 i6 superfluous. The duct 21 can be proYided with a pump 24, and leads back towards th~
entry side of the system, e.g. point 25 of the ~upply duct 1, 80 that, then, the separated ~ludge having a high water content (about 99 X) will be supplied to the stags 2 together with the liquid to be treated. It is also possible to lead the duct 21, as shon at 21', towards a point 25' in ths duct 4, 80 that the sepa-rated sludgo is not ~ub~ected to the treatment in the 6tage 2.Moreover it is po~sible to introduce this duct, as shown at 21", somewhere into sta~e 2, 80 that, then, the sludge will undergo a partial treat~ent in that stage.
If such a syste~ iB designed for purifying waste ~ater fro~ a factory which iB, for instance, idle during weekends, the micro-organi6ms in the biological puri~ication device 16 will not rocel~o nourish3ent during that period, 80 that, then, the active mass in this device will considerably dcteriorate. In order to pro-v-nt thio, a part of tho floating lay~r ~aterial separated during the precoding period in the presoparator 5 can be stored, and can bo ~upllied to the biological purification device 16 during that ldle perlod. This is poBBible bOOBU8e the flo~ting layer material has a hi8h dry ~attor oontent, and, thus, a relatively low water content, ~o that stOrAge thereof does not require a ~ery large oontainer-Thia is indicated in tho drawing with interruptod lines.In tho duct 13 a 3-way val~e 26 i8 included vhich is connected to - a branch duct 27 leadin6 to a storage tank 28. The latter is, on tho othor hand, connected to a discharge duct 29 which may be pro- -vlded with a pu~p 30~ and leads to the de~ice 16. By switching over t~e valve 16, tho ~loating layer ~aterial ~rom the duct S3 is not dirocted towards the drying device 14 but towards the storage tank 28, rrom which this ~aterial, a~ needed, can be led through the ~- duct 2~ towards the device 16. This return flov can be made auto-~atic in a simple manner.
In such a sy~tem the previously mentioned advantsges may be complotely attained. The execution of the various part6 depends, : ' .
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_ __ _ . _ _ . _ . . _ . _ . .. _ . _ _ . . _ _ . _ .. _ _ . ~ _, _ _ .. _ _ _ .. _ _ .. ~ _ .. _ _ ~ . . . .. ... , . . . _ . .. _ _, _ _ _. . _ _ _ . . _, __,. _. ._,_ _ . __, ",, . _ __ . ,_ . . .__ ~31~3 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 automatical, 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 the prior Canadian patent No. 1,036,718, in which, apart from means for introducing air bubbles, also a plate separator is present, which improves the separation degree still further. An embodiment thereof is schematically shown 10 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 flotatable by adhered air bubbles are collected as a floating layer 37, and 20 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 the prior Canadian patent No. 1,076,965, which is specifically suited fa3r 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 30 for gradually driving the floating layer towards an inclined overflow 39, and the other for pl.Ishing the floating matter over said overflow. These sets of scraper blades move so that - 8 - ~ ~131~3 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 17 can also comprise a plate separator, e.g. according to the prior Canadian patent No.
959,767**, 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 or 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 that such noxious side-effects will not occur.
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 said assembly 40. Sludge taken along with the liquid will be separated in counter-current 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 aGcording to prior Canadian patent 959,767 prevent intermixing of the supplied liquid and the separated sludge, and a dip baffle 45 submerged into the collected sludge prevents that liquid will flow via the sludge compartment and disturb the sediment flow, as described in the before-mentioned prior patent.

**Issued: 24 December, 1974;
Patentee: Pielkenrood-Vinitex B.V.

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: -' ` - ~ . -. ' ' ., .. , . ~ : . , , : .. ., ,. . - : - - .: .: - -1~13193 - 8a -In the pre-separator 5 about 80...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 is to be removed accordingly.
The preseparator 5 produces, moreover, a floating layer with about 10 % dry matter, so that drying théreof 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 pre-separator 5 appears to be obtained thereby. The dry material removed from the floating layer may often be usefully applied.
If tile biological purification is adapted to break down about 90...95 ~ of the supplied components, the total purification degree-. . . ~

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., 9 ~131~3 ~will be 98...99,5 X- Th~se figure~ apply to the case ~ithout re-~cycling the ~ludge.
i A n~l~erical example will elucidate the ~oregoing. At a ~upply rate o~, for instance, 100 ~3/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 ~ill provide an excess sludge amou~t of 1 m3/h with a dry matter content of 1 ~.
,The amount of purified liquid is, than, 98 ~3/h, and 2 m3Jh waste liquid i8 for~ed, which, in total, contains 100 ~ 10 = 110 kg~h solid matter. The a~erage dry matter content of the waste flow i8, then, 5,5 %. If, in stage 5, a purification 0~fect of 85 %, and in sta~e 16 a purification degree of 90 ~ i8 obtained, the liquid ~discharged at 20 ~ill be purified for 98,5 X. These figureB apply to purific~tion without recyclin~ the sludg~.
If the excess ~ludge i~ recycled, only one ~aste flow is obtained, namely 1,05 m3/h, in which the dry matter content appears ~to be neArly io,6 %~ the wasto flow a~ount being only slightly more th~n that o~ the floating layer alone, a~d the dry matter content being appreciably larger than that of the floating layer alone.
~This a~ount ia, numely, increased fro~ 110 kg~h to 111 kg/h. The ~aste flow of 1,05 ~3fh ie no~ b~ 0,95 m~h less than in thu case vithout roc~cling~ wh~ch i8 equi~alent to an accordingly larger ~ount of oleaned liquid at the output 20~ ~hich will now be more ,than 99 ~7~h. Wherea~ without recgcling, the purification deeree of thiB l~quid W~8 a~ready 98~5 ~ it ~ill, ~y the increase of the a~ount ~t a~ equ~ purlty a~ountS no~ baco~o abo~t 99 Fro~ th~ fore~oine it follo~8 that the excess sludge o~
1 m3~h with 1 % dry matter i8, a~ it were, thickened to 0,05~m3/h, Ii.e. to a dry ~atter content of 20 %.
~ By the abo~e-mentioned assembly of ~easures the dis-char~e problo~ of the products obta~ned in the purification treat-me~t has been solved, and an excellent purification i8 obtained by ~oa~s of ~ system which require~ ~uch les~ ~pace, and can, there-or~, be manu~actured in a cheaper manner. Horeo~er the cost of the biological purification will be reduced accordiu~ly, and-al~o ~aYings in the phy~ico-che~ical pretreatment can be obtained. Al~o the proceeds o~ the dried matter will so~eti~es contribute to a . . .

~i:o. .. .

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~31~3 reduction of the expense6.
In thi6 manner, therefore, an effecti~e purification can bo obtained in ~ very profitable manner, which purification is es-pecially suitable for the purification Or sewer water or wAste water of different kind~ of industries, in particular meat pro-cessing plants, 6uch as slaughter-houses or canneries, and, fur-thermore, paper and textile work6 etc. For the rest this method and system are ~uitable for purifying any liquid which may be - ~treated by means of biological purification.

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Claims (30)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method for treating liquids, by using an aerobic biological purification, characterized in that the liquid under treatment, preliminary to the biological purification, is sub-mitted to a physico-chemical pretreatment with flotation by using finely divided gas bubbles, and in that surplus sludge separated from the liquid after the biological purification is recycled towards the physico-chemical treatment stage, in such a manner that the removed floating layer obtained in this physico-chemical treatment is the only separation product.

2. The method of claim 1, characterized in that the re-moved floating layer is directly led towards a drying device.

3. The method of claim 1, characterized in that a part of the removed floating layer is stored, and is supplied to the biological treatment stage during the period that no liquid to be treated is supplied, in order to keep alive the active organisms in said stage.

4. The method of claim 1, characterized in that the separation of active sludge from the liquid leaving the bio-logical treatment stage is separated by means of a plate separator, in which the residence time is sufficiently short for avoiding aerobic effects.

5. A method for treating an incoming liquid, in parti-cular waste water, by aerobic biological purification, com-prising the steps of:
introducing a liquid saturated with a pressurized gas into the incoming liquid to preseparate at least a part of the matter suspended in the incoming liquid by dissolved air flotation to obtain a preseparated liquid and a floating layer;

- Page 1 of Claims - 11 removing and disposing of said floating layer;
subjecting the preseparated liquid to aerobic biological purification by an aerated mass of micro-organisms to obtain a biologically treated liquid;
subjecting the biologically treated liquid to a post-separation for removing sludge from the biologically treated liquid to obtain a purified liquid;
returning a sufficient fraction of the removed sludge to the aerated mass to maintain the aerated mass at a pre-determined amount;
combining the remainder of the removed sludge with the incoming liquid.

6. The method of claim 5 wherein said step of removing and disposing further comprises leading the removed floating layer to a drying device.

7. The method of claim 5 further comprising the steps of:
storing a fraction of the removed floating layer; and, adding a portion of the stored removed floating layer to the aerated mass of micro-organisms to keep the latter alive during an interval when no incoming liquid to be treated is supplied.

8. The method of claim 5 wherein the biologically treated liquid is subjected to a post-separation by passage through a separator with sufficiently short residence time that anaerobic effects are avoided.

9. A system for treating impure liquids comprising a pre-separation device, a device for performing an aerobic bio-logical purification, and an after-separator for removing from the liquid the active sludge entrained by the liquid from the - Page 2 of Claims - 12 biological purification device, provided with means for re-cycling a part of the sludge towards the biological purifica-tion device for maintaining the required amount of active sludge, characterized in that the pre-separation device is adapted to perform a physico-chemcial pretreatment in that said pre-separation device is provided with means for introducing gas bubbles in such a manner that the components separating from the liquid are made flotatable by adhesion of these gas bubbles, and in that a sludge discharge duct of the after-separator is returned towards the inlet of the pre-separation device.

10. A system for treating impure liquids comprising a pre-separation device, a device for performing an aerobic bio-logical purification, and an after-separator for removing from the liquid the active sludge entrained by the liquid from the biological purification device, provided with means for re-cycling a part of the sludge towards the biologcial purifica-tion device for maintaining the required amount of active sludge, characterized in that the pre-separation device is adapted to perform a physico-chemical pretreatment in that said pre-separation device is provided with means for introducing gas bubbles in such a manner that the components separating from the liquid are made flotatable by adhesion of these gas bubbles and in that a sludge discharge duct of the after-separator is returned towards the inlet of the pre-separation device.
- Page 3 of Claims - 13

11. The system of claim 9, characterized in that the pre-separation device comprises a pre-separation stage, adapted for adding auxiliary substances such a coagulation, pH-adjusting and flocculation agents, and in that the return duct for the surplus sludge ends before, in or after said pre-treatment stage.

12. The system of claim 9, characterized in that the return duct from the after-separator is provided with a branch duct returning towards the biological treatment device, and in that means are provided allowing the adjustment of the ratio between the sludge flows in the duct returning towards the input and this branch duct respectively.

13. The system of claims 10 or 11, characterized in that the return duct from the after-separator is provided with a branch duct returning towards the biological treatment device, and in that means are provided allowing the adjustment of the ratio between the sludge flows in the duct returning towards the input and this branch duct respectively.

14. The system of claim 9 characterized in that the after-separator is provided with a plate separator.

15. The system of claims 11 or 12, characterized in that the after-separator is provided with a plate separator.

16. The system of claim 9 characterized in that the pre-separating device is provided with a plate separator adapted to separate components remaining in the liquid after gas bubble separation.
- Page 4 of Claims -

17. The system of any one of claims 10, 11 or 12, characterized in that the pre-separating device is provided with a plate separator adapted to separate components remaining in the liquid after gas bubble separation.

18. The system of claim 14, characterized in that the pre-separating device is provided with a plate separator adapted to separate components remaining in the liquid after gas bubble separation.

19. The system of claim 9, characterized in that the floating layer discharge of the pre-separation device is directly connected to a drying device.

20. The system of any one of claims 10, 11 or 12, characterized in that the floating layer discharge of the pre-separation device is directly connected to a drying device.

21. The system of claim 14, characterized in that the floating layer discharge of the pre-separation device is directly connected to a drying device.

22. The system of claim 16, characterized in that the floating layer discharge of the pre-separation device is directly connected to a drying device.

23. The system of any one of claims 10, 11 or 12, characterized in that the pre-separating device is provided with a plate separator adapted to separate components remaining in the liquid after gas bubble separation and further characterized in that the floating layer discharge of the pre-separation device is directly connected to a drying device.

- Page 5 Of Claims -

24. The system of claim 9, characterized by a storage container for removed floating layer material,and by means for returning material stored in this container towards the biological purification device.

25. The system of any one of claims 10, 11 or 12, characterized by a storage container for removed floating layer material, and by means for returning material stored in this container towards the biological purification device.

26. The system of claims 14 or 16 characterized by a storage container for removed floating layer material, and by means for returning material stored in this container towards the biological purification device.

27. The system of any one of claims 10, 11 or 12, characterized in that the pre-separationg device is provided with a plate separator adapted to separate components remaining in the liquid after gas bubble separation and further characterized by a storage container for removed floating layer material, and by means for returning materials stored in this container towards the biological purification device.

28. The system of claim 18 characterized by a storage container for removed floating layer material, and by means for returning material stored in this container towards the biological purification device.

29. The system of claim 19 characterized by a storage container for removed floating layer material, and by means for returning material stored in this container towards the biological purification device.

- Page 6 of Claims -

30. The system of claims 21 or 22 characterized by a storage container for removed floating layer material, and by means for returning material stored in this container towards the biological purification device.