CA1114964A - Plant for the treatment of waste water by the activated-sludge process - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

All of the sewage and waste water to be treated is introduced into a maximum-loading ambient-air aeration basin (after coarse screening) and is subjected to activated-sludge degradation (cracking or transformation) of decomposition-resistant hydrocarbons to more readily decomposable substances. The liquid decanted from the sludge of this basin in an intermediate clarifier, serving to strictly separate the biozones of the two stages from one another, is treated in a low-loading basin with a gas having at least 50 volume percent oxygen. The effluent is again separated is again separated in a clarifier and is discharged.

Description

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PLANT FOR THE TREATMENT OF WASTE WATER
BY THE ACTIVATED-SLUDGE PROCESS

SPECIFICATION

The present invention relates to an installation, plant or system for the treatment of waste water by the activated-slutge process and, re particularly, to improvements in multistage activated-sludge treatment of waste water and sewage.
It is known in the activated-sludge process for the treatment of waste water to provlde an actlvation basin for a first activatlon stage, an lntermedlate clarifler, an activation basin for a second clarlfier stage, and a flnal clarifier.
In such processes, all of the waste water to be treated is introduced into the flrst activation vessel for aeration in a first maximum-load aeratlon (maximum volumetrlc loadlng - see pages 485ff of WASTE WATER ~TG~NNERING, Metcalf and Eddy, second edition, McGraw-Hlll Book Co., New York, 1979~. The recovered sludge from the high-load aceivation stage i8 recycled only to the flrst actlvatlon stage from thls intermedlate clarifler andlor is sub~ected to a sludge treatment.
me clarlfled phase of the lntermediate clarlfier i9 lntroduced into the vessel for the second actlvatlon stage and the latter basln is operated as a low-load activation vessel.
me term "sludgetreatment" is used in the sense set forth in the aforementloned publlcatlon to provlde a sludge capable of lmmediate disposal, e.g. use as a fertllizer.
The term "activation basin" as used in the framework of the present dis losure ls intended to include multivessel systems as well as slngle vessels. In other words, the first activatlon basln may be X

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a plurallty of actlvatlon vessels whlch are functlonally united. Thls applles as well to the second actlvatlon vessel. The term "vessel" as lt may be used hereln ls lntended to refer to any body of the llquld to be treated and usually refers to a basin, pool, lagoon, tank or like unlt commonly applied in waste-water treatment.
Thus, while we will use the terms "activation vessel" and "activation basin" in the singular hereinafter, it should be understood that this expression is lntended to a plurality of actlvatlon basins as well.
The^term "waste water" as used herein is intended in the broadest sense. In general, the waste water wlll be an aqueous system in which organic substances, with or without soluble or suspended inorganic substances, are dlspersed.
The partlcles of the dlspersed phase can be ln par~ solubilized, emulslfled, ln colloldal or/and other suspended forms, or can be present in any combination of these forms ln the aqueous phase. The organic substances can be settlable or sedlmentable or can be so suspended that they cannot be sedlmented. They can lnclude putrlfiable or non-putrifiable wastes, ~uch as sewage or lndustrlal wastes.
me waste water to be treated can be sub~ected, prlor to introduction lnto the maximum-load aeration basin, to a coarse mechanical cleaning, e.g. a screening as described in the aforementioned publication or as otherwlse known in the art.
In conventional apparatus of the aforedescribed type, not only is the activation basin of the first aeration stage a vessel provided wlth classical aeration with atmospheric alr, but the basin of the second actlvation stage can be sub~ected to atmospheric air aeration as well.

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.' ~' . '. ' The maximum-loadlng aeration basln of the earller systems are vessels wlth a volumetric loadlng ~. cit., page 472) of about 10 kg BOD5/m3/day and wlth a sludge loadlng Ld (dry sludge) of at least 2 kg BOD5 (mean Ld8 ~ 5-0 kg) per kg dry substance and per day.
In such systems, ~o much excess sludge ls wlthdrawn from the intermediate clarifier that the sludge in the maxlmum-loading aeration basln achieves only a minimum sludge age (mean vessel residence time).
The low-loading actlvation basin is, on the contrary, operated with a comparatively low volumetrlc loading and with a comparatlvely low sludge loadlng. In thls second stage a high-sludge age (mean vessel or cell resldence time) ls achieved.
Since lts nutrlent (food) content is scant, substances which are difficult to decompose and which are not held back in the first stage are decomposed to a signlflcant degree in the second stage ln the presence of easlly decomposable hydrophilic and usually polar organic compounds whlch are not removed in the first stage. Moreover, the higher the sludge age which can be maintained, the higher will be the degree of decompositlon of the difficult-to-decompose substances.
Such systems have generally given good results. When, however, the waste water to be treated contains a high concentration of dlfflcult-to-decompose hydrocarbon compounds, their decomposltlon is not always satlsfactory. It i~ also a problem that thesludgeof the second decomr position stage is of relatively low density. The second stage sludge ls slmilar to the sludge of a classical intermediate-loading to low-loading biographlcal decomposltion unlt.
It has also been proposed to treat waste water ln an actlvated-sludge process by the lntroduction of oxygen (oxygen system). Here a slngle-stage or two-stage apparatus is used ln w~lch the blologlcal ~ ' .

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actlon i8 carrled out in closed vessels or chambers, not wlth atmospherlc air but with a gas containing at least 50 volume percent oxygen. To distlngulsh the orlglnally descrlbed aeratlon basins from these vessels, the latter will be characterlzed as oxygen-activation vessels or tanks.
In the flrst stage of an oxygen process, the ma~or part of the easily decomposable hydrocarbon compounds are decomposed whlle in the second stage the maJor part of the nltrogen compounds are decomposed.
Separation of the biozones ls not malntalned. Because of the hlgh energy consumption of such oxygen systems they have not found wldespread acceptance, nor is it possible to control readlly the operations of the two biological stages.
It is the prlncipal obiect of the present invention to provide an improved plant of the type originally described, i.e. comprising a first-stage aeration basin, an intermedlate clarifier, a second-stage aeration basin and a final clarifier, whereby the disadvantages of the earlier systems are avoided~
It ls a more speciflc ob~ect of the invention to provlde an lnstallatlon of the latter type which is especially effectlve in de-composlng dlfficult-to-decompose hydrocarbon compounds when the latter are present to a slgniflcant degree in the waste water to be treated without exces6ive energy consumptlon.
Here dlsclosed ls a system of the latter type which can be operated wlth especially low energy requlrements, even when the waste water contalns dlfflcult-to-decompose hydrocarbon compounds to a slgnlficant degree, and whlch generates in the second stage a unlform and relatively dense sludge wlth good settling and dewatering character-lstlcs exceeding those of conventional systens.
Further dlsclosed i8 a plant of the class last referred to ~; -.
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whlch glves rise ln the second stage to a relatlvely heavy sludge wlth a slgnlflcantly reduced sludge lndex (op. cit., page 507).
Brlefly, therefore, here dl~closed ln a plant of the type flrst referred to, 19 a system for the actlvated sludge treatment of water whlch comprlses a flrst actlvatlon basln for a flrst actlve-sludge treatment stage, an lntermedlate clarlfler, a second actlvatlon basin for a second actlvatlon stsge, and a flnal clarlfier, ln which all of the waste water to be treated, after separatlon of large lmpurltles by mechanlcal means, is lntroduced lnto the flrst actlvatlon basin whlch 19 operated as a hlgh-loadlng aeration unlt; all of the sludge of the flrst stage recovered from the lntermedlate clarlfler 19 recycled to this first stage and/or is sub~ected to sludge treatment while the clarifled phase of the lntermedlate clarlfier ls introduced into the second actlvatlon stage, the sludge from the latter belng recovered ln the final clarlfler and belng recycled to the second activatlon stage and/or sub~ected to sludge treatment.
This apparatus is improved by a comblnatlon of the followlng features:
(a) the hlgh-loading aeratlon basin of the first activation stage ls constructed and arranged and operated to effect partlal elimina-tion of the difficult-to-decompose hydrocarbons, and (b) the activation basin of the second actlvatlon stage ls constructed, arranged and operated as an oxygen-activation vessel for the oxygen gasification of the contents thereof, a biological decomposi-tion of the remaining hydrocarbon compounds being carried out concurrent-ly with the biological decomposition of the nitrogen compounds.
Moreover, the high-loading aeration basin (flrst stage) is ~' operated 80 as to partially ellmlnate the dlfflcult-to-decompose hydro-carbon compounds whlle the oxygen-actlvatlon basln ls operatet ln the manner descrlbed to Benerate ammonla ln such relatlonshlp that the ammonla of the oxygen-actlvation basln substantlally completely neutra-llzes the excess carbon dloxlde generated thereln.
The dlffuslon of oxygen lnto the oxygen-actlvatlon basln can be carrled out ln any conventlonal manner, preferably in covered oxygen-activatlon chambers (tanks) connected in cascade with lncreaslng oxygen partlal pressure from the first cascade stage to the last.
The parameters of the two stages whlch are ad~usted so that the nltrogen generated in the 3econd stage neutrallzes practlcally all of the carbon dloxlde resultlng from the oxygen actlvatlon of the second stage include the aeration and oxygen-dlffuslon rates, the ad~ustment of the sludge age or mean residence time ln the respectlve basins, i.e.
the mean resldence tlmes of the waste water to be treated or the res-pective phases resulting from the treatment, and, wlth strict separation of the two biozones of the two activation stages, the feed into the second activation stage.
The elements required for controlllng these parameters can be any conventional system-control elements, such as valves, welrs, diffusers and the like and the settlngs which will give the result indicated, namely, substantially complete neutralization of the generated ammonia by the surplus carbon dioxlde from the oxygen-activation basin can be readily determined empirically and will, of course, depend upon the source and composition of the waste water.
When the water to be treated has a very high concentration of difficult-to-decompose (refractory) hydrocarbon compounds, according to a feature of the invention, the maximum-loading aeration basin is . ,. . -' ~

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operatet with facultative aerobic microorganisms and a reduced oxygen content toxygen deflclency) and the dlfflcult-to-decompose hydrocarbon compounds are split and/or transformed into easlly decomposable organic compounds .
~hig spllttlng phenomenon, which can also be referred to as cracklng, is a surprising effect whlch gives rlse to surprising effects ln the oxygen-activatlon basin as will be described below.
The oxygen deficiency causes the facultative aerobic micro-organi~m~ to operate anaerobically. A similar aerobic impact 18 obtalned with a corresponding oxygen content.
When the waste water to be treated has a normal content of difficult-to-decompose hydrocarbons, e.g. is the usual municipal waste water, a preferred embodiment of the invention provides that the maximum-loadlng aeration basin is operated with aerobic microorganisms and with a sufficlent oxygen content (up to oxygen surplus). In the latter case, the maximum-loading aeration basin is operated to decompose the difficult-to-decompose hydrocarbon compounds and to remove them from the supernatant ~ liquid preferably by adsorption, coagulation and flocculation in a self-; filtering action. The result of this operation also can be seen in a surprising effect in the oxygen-activation basin.
Naturally, both of the effects can be combined and thus the maxlmum~loading aeration basin or a number of stages into which the maximumrloading aeration is subdivided, can be operated alternately or in alternat~ng chambers with aerobic or~acultativeaerobic processes.
It is also a feature of the lnvention that the maximum-loading aeration basin is operated in the transition region between aerobic and faculta-tive aerobic processes.
Best results are obtained when the maximum-loading aerat~on basin , .

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is operated to effect ellminatlon of about 30~ to 70% of the hydrocarbon compounds, especially the dlfflcult-to-decompose hydrocarbon compounds and practlcally all coarsely dlspersed substances and collolds and practically all hlgh molecular-weight compounds. Surprlsingly these operatlons also effect in the maxlmum-loadlng basin an ellmination of nitrogen compounds which can pose difficultles in the oxygen-activation basin or whose decomposition products can create difficulties during the oxygen activation stage.
The oxygen-activation basin is, according to the invention, preferably operated with a sludge loading of LdSc 2 kg BOD5, preferably < 0.5 kg BOD5/kg of dry substance and per day. Within these operating parameters, most biologically decomposable and household waste waters can be treated in the maximum-loading aeration basin with a residence time of 20 to 30 minutes. With higher concentratlons of difficult-to-decompose hydrocarbons in the raw waste water, however, longer residence times are employed. The oxygen-activation basin of the second-activation stage with an ordinary concentration of raw waste water of 300 mg BOD5/m3 is operated with a residence time of 1 to 3 hours. The oxygen-activition basin can, of course, have a plurality of compartments or can be formed by a plurality of tanks. The abbreviation "BOD5" designates biological oxygen requirement ln five dayq.
The treatment in the msximum-loading aeration basin is so ad~ust-ed that the activated sludge in the oxygen-activation basin has uniform loading properties, stable decomposition characteristics and good runoff characteristics. The energy consumption is remarkably low in both activation stages. The operation of the plant will be more readily apparent from the functional description below:
(1) By eliminatlon of obstructing substances in the first-activatlon stages, the degree of decompositlon and process stablllty ln the oxygen-actlvation stage is markedly increased. Practically all coarse-dispersed substances, practlcally all collolds, the hlgh- lecular-welght substances and compounds which are hydrophobic and nonpolar and which would tend to obstruct the oxygen activatlon are ellmlnated or held back.
There ls also a selectlve ellmlnatlon of reslstant organlc compounds whlch generally do not possess nitrogen groups.

(2) In the first-activation stage so-called poi30ns are elimin-ated from the biomass so that bacteria, such as nitrosomonas and nitro-bacter, which effect a nitrification of the ammonia, are afforded suitable living conditlons in the second-activatlon stage even wlth high sludge loading.

(3) By eliminatlon of about 30% to 70% of the hydrocarbon compounds and especially the dlfficult-to-decompose hydrocarbon compounds ln the fir~t-actlvation stage, the raw substrate, i.e. the waste water, is so modified that the remainlng organic load, after intermediate clarlficatlon, i9 readily decomposed in the oxygen-actlvatlon stage with reduced carbon dioxide development. As a result, there is a reduction in the carbonic acld concentration in the second-activation stage which is important because otherwise the substrate fed to the second-activation stage must be brought to a basic pH to maintain the desired condltions for the nitrifying bacteria and to maintain biological activity. An excess acldificatlon, which can occur in a conventional system, can no longer occur here.

(4) The clarlfied phase recovered from the intermediate clarifier and dlscharged from the first-activation stage has about twice the nitrogen/carbon (N/C) ratlo of the raw substrate ~o that only a reduced hydrolyzation of the organic nitrogen compounds is effected in the first _ 9 _ ,;, ,.

and hardly any ammonla strlpplng occurY thereln. The ~lgnlflcant hydro-lysls of the organlc nltrogen compounds ls effected ln the second-actlva-tlon stage wlth oxygen ln~ectlon. However, because of the necessarlly closed constructlon of the oxygen-actlvatlon system used ln accordance wlth the present lnventlon, there can be relatlvely llttle strlpping of ammonla. The hlgher than usual amounts of ammonia generated ln the second-actlvatlon stage serve to neutralize the carbon dloxlde resulting from the use of oxygen ln this activation phase and additionally reduces the acidification of the substrate in this closed phase of the process.

(5) The ellmination of selected substances, as described in paragraph (1) above changes the characteristics of the clarified phase introduced into the second-activation stage such that a mass development ofheterotrophicorganisms i9 precluded and the growth rate of these organisms in combination with the specific sludge yield is reduced with oxygen introduction. These factors ensure, for a given sludge loading, a higher sludge age slnce the sludge yield is indlrectly proportional to the sludge aging (mean resldence time).

(6) Because of the elimination of the hydrocarbon compounds in the first-activatlon stage and the concomitant reductlon ln the carbon dloxide content in the second-activatlon stage, the partlal pressure relationshipA of the thermodynamics of the system ensure a shift in the dlffusion equilibrlum ln an advantageous manner for the oxygen supplled to the second-actlvatlon stage. This results in improved incorporation of oxygen and hence a higher oxygen content in the diffus-ate. In addition, the oxygen yield (efficlency) in the second-activation stage ls significantly increased.

(7) Also because of the elimination of the hydrocarbon compounds in the first-activation stage and because of the neutrallzation of ,~' carbon acid wlth the ammonia genersted in the oxygen-actlvation ~tage, the treated substrate 18 subJected to such changes ln characteristlcs that the heterotrophlc organlsms have a smaller growth rate than the nitrification rate and nitrification commences earlier even with greater sludge loadings than in conventlonal processes.
~ 8) The substrate-sludge mlxture fed from the oxygen-actlvation stage to the final clarifler contains a relatively heavy sludge which can be readily separated from the supernatant liquid with small residence time and high surface charge.
The sludge which is removed from the first-activatlon stage has a relatively low sludge age and i9 constltuted practically exclusively of primary digesting microorganisms. These, in facultative operation with oxygen deficiency, presumably cause the breakdown of the difficult-to-decompose hydrocarbon compounds. Probably, however, in facultative (i.e. anaerobic) as well as in aerobic operation, also enzymes and metabolism products are freed and diffused through the cell walls out-wardly to effect a biogenic flocculation and adsorption on the floccu-late. Apparently for storage of the nutrient, semisolubilized high-molecular-weight compounds and the suspended substances, even those not sedimentable heretofore, are to large measure flocculated out by depositlon on the cell and are removed by the filtration through the intercellular cell structure. As a result, with very short residence times, which is a rule for ordinary waste water concentrations in the maximum-loading activation basin, reductions in the organic loading of 30% to 80% can be achieved. What is moYt important, however, is that the first stage should establi3h the optimum composition of the clarified phase which will pass from the intermediate clarifier into the oxidation-activation stage.

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Specific embodiments of the invention will now be described having reference to the sole FIGURE of the accompanylng drawing which ls a flow diagram illustrating, schematically, a plant for carrying out the invention.
The plant of the drawing comprises a first-actlvatlon basln 1 for the flrst-actlvatlon stage I whlch ls separated from the second-actlvatlon stage II wlth strlct separatlon of the biozones represented by the regions I and II ln the manner previously described. Between the basin 1, which can be conventional open aeration basin, pool, lagoon or tank, and the closed basin (tan~) 3 for oxygen activation of the biomass, there is provided an intermediate clarifier 2. A final clarifler 4 is provided downstream of the basin 3. The units 1 - 4 can be activa-tlon basins for aeratlon or the lntroductlon of oxygen lnto the biomass and clariflers, as required, which can have any of the constructlons descrlbed in the aforementioned publicatlon using the aeratlon and oxygen-lntroduction diffusers there described. In additlon, the sludge processlng or wastlng systems of thls publlcation may also be used for the excess sludge from each zone.
All of the waste water to be processed is lntroduced lnto the actlvatlon basln 1 which is of the atmospheric-air aeration type and which is operated as a maxi~um,loading aeration basin.
The intermediate clarifier 2 serves to separate the sludge which sediments from the substrate from the clarified phase whlch is decanted into the oxygen-activation basln 3. Thus the intermediate clarifler 2 serves as a separator between the biozones of the first-activatlon stage I and the second-activation stage II.
In order to ensure isolatlon of the two zones, all of the sludge recovered from the lntermedlate clarlfier 2 ls recycled to the first-~,~

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activatlon stage, e.g. by a pump 14, and/or is wasted, l.e. sub~ected to sludge treatment at 5.
The clarlfler sludge from unlt 4, however, ls only fed to the second actlvation stage 3 by the pump 19 and/or ls sub~ected to sludge wastlng or treatment at 5.
In the manner prevlously descrlbed, the maxlmum-loadlng aeratlon basln 1 ls operated aeroblcally or facultatlvely aeroblcally, l.e. wlth oxygen excess or oxygen deficiency. It can also be operated in the transltlon region between aerobic and facultatlve aerobic.
The actlvation basln 3 of the second stage ls designed for oxygen activation as represented by the arrow 3a representlng the connectlon to an oxygen source so that a gas consisting of at least 50%
by volume oxygen may be introduced into the activation basin 3 through any conventional diffuser arrangement, preferably one of those described for the pure oxygen system in the last-mentioned publication.
From the lntermediate clarifier 2, the clarifier phase enters the oxygen-activatlon basin 3 and the treated product of the latter passes lnto the clarlfler 4 from whence the clarified phase may be dis-charged whlle the sludge i8 recycled in the manner descrlbed.
More speclflcally, the waste water to be treated, l.e. any municlpal or lndustrlal waste which contains relataelvely large a unts of difflcult-to-decompose hydrocarbon compounds, is fed via an inlet 6 by a pump 7 through a line 8 to a coarse filter 9 for desanding and coarse-sollds removal, e.g. by mechanical filtering. When the products removed by thls coarse fllterlng at 9 lnclude organic components, they may be transferred at 17 to the sludge treatment process represented at 5.
After removal of substances which may be detrimental further , I .

~ 't on in the process, especlally sand, fibers and other coarse ~ollds, the waste water iB dellvered by llne 10 to the maxlmum-loadlng activstlon basln 1 of the flrst-activation stage I.
Basln 1 can be aerated wlth atmospherlc alr as descrlbed ln the aforementioned publicatlon.
The aerated medlum is then transferred via llne 11 lnto the intermediate clarlfier 2. The clarified phase is supplied via line 12 to the activation basin 3 of the second-activation stage II which is operated as a low-loading stage but with introduction of gaseous oxygen into the medium. This basin is covered and can be subdivided into a plurality of chambers ~tanks) operated with progressively increasing oxygen partial pressure in a so-called oxygen aeration cascade.
The sludge recovered from the, intermediate clarifier 2 is led by line 13 and the pump 14 to the lines 15 and 16. Line 15 serves to recycle this first-stage sludge to the first-stage activation basin 1 while line 16 delivers surplus sludge to a sludge wasting system which has been represented at 5 in the ~orm of a sludge treatment system.
Any conventional sludge treatment process may be used and, in general, this can lnclude first the thickening of the sludge and then the drying or inclneration thereof or the transformation of the sludge into a useful product such as fertilizer.
After termination of the biological decompositlon in the second-activation stage II wlth lntroductlon of oxygen, the aqueous phase is fed to the clarifier 4 from which the sludge is withdrawn uia a llne 18 and the pump 19.
Thlssludge ls delivered via llne 20 in the form of a recycled sludge to the oxygen-activation basin 3 and/or vla llne 21 as excess sludge to the sludge treatment system 5.

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' The clarified llquld i8 fed vla llne 22 and a pump 23 to a fast fllter 25 from whlch the clarlfled llquld i8 delivered vla an overflow pipe 26 to a body of water for u'~eimate dlspo~al. From the fast fllter 25, rinsing water is returned to the activation basin 3 of the second stage II.
A controller 30 may be provlded to respond to a sensor 31 which measures the ammonla concentratlon ln the gas wlthin the oxygen-actlvation basin 3, the controller operating the pumps 7, 14, 19, 23 and flow controllers 32 and 33, lf desired, to maintain the balance between the decomposition in the first zone and the oxygen actlvation of the second zone so that sufficient ammonla is generated to practically neutralize all of the carbon dioxlde generated ln excess ln the oxygen-actlvatlon zone, thereby neutralizing the carbonlc acids resulting from the carbon dioxlde. This will practically always be the case if a slight excess ; of ammonia is detected within the oxygen-actlvatlon chamber. Naturally, the controller can respond to the pH of the substrate ln the oxygen-activation zone since thls too is a function of dlssolved carbon dioxide.
Such controllers are, of course, symbolic of any means for performing the indicated functions which may, in part, be controlled manually.

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Claims (3)

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

1. A method of operating an activated-sludge waste-water-treatment plant having a first activation basin for a first digestion stage, an intermediate clarifier connected to the first activation basin, a second activation basin for a second digestion stage connected to said intermediate clarifier, and a further clarifier connected to said second basin, said method comprising the steps of:
(a) introducing all of the waste water to be treated into said first activation basin and operating same selectively in an aerobic and facultative aerobic mode while aerating the contents of said first basin exclusively with ambient air in a maximum loading condition to form a first sludge and a first liquid phase whereby hydrocarbons are transformed into readily decomposable organic compounds;
(b) maintaining strict separation of biozones represented by said first and second basins by separating said first sludge from said first liquid phase in said intermediate clarifier and passing said first liquid phase into said second basin for digestion with a biomass therein while preventing transfer of said first sludge to said second basin;
(c) aerating the contents of said second basin exclusively with oxygen or oxygen-enriched air to form a second sludge and a second liquid phase;
(d) controlling the digestion in said first basin with elimination of about 30% to 70% of difficult-to-decompose carbon compounds so that ammonia generated in said second basin substantially completely neutralizes excess carbonic acid formed by the digestion therein; and (e) separating said second sludge from said second liquid phase in said further clarifier and discharging said second liquid phase as a clarified effluent while subjecting said first and second sludges to sludge treatment.

2. The method defined in claim 1 wherein said second basin is operated with a sludge loading ? 0.5 kg BOD5/kg times day of dry substance, said first basin is operated with a residence time of 20 to 30 minutes, and said second basin is operated with a residence time of 1 to 3 hours.

3. The method defined in claim 1 or 2 wherein said first activation basin is operated with facultative aerobic microorganisms.