CA1084639A - Two-stage activated sludge process - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A two-stage sludge process for the purification of sewage.
The process is less sensitive than those of the prior art but, never-theless, succeeds in reducing the aeration time.
The sewage is supplied to a higher loaded first activation stage, the sewage treated therein undergoes an intermediate settlement accompanied by the removal of sludge in the form of return sludge and surplus sludge, the intermediately settled sewage is fed to a lower loaded activation stage compared with the first activation stage and following final settling the material therefrom is drained off in pure form, wherein all the sewage is fed to a first aerated activa-tion stage which is operated as a maximum load stage with a space load BR of approximately 10 kg BOD5/m3/day and a sludge load BTS of at least 2 kg BOD5/kg of dry substance/day, by intermediate settling of the sewage sludge mixture removed from the first stage a strict separation of the biocenoses of the two activation stages is ensured, whereby sufficient surplus sludge is removed from the intermediate settling stage that the age of the sludge is kept low, the inter-mediately settled sewage being fed to a second aerated activation stage which is operated as a low load stage.

Description

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The invention relates to a two-stage activated sludge process for the purification of sewage, Activated sludge processes, also called sludge activation processes, constitute a qpecial form of biological purification of sewage. In the pa~t, S sewage from housing estates and industrial plants was either allowed to ` seep into the subsoil, used for irrigating agricultural land or fed into local watercourses either in a mechanically cleaned or uncleaned state, It was only at a later date that the sewage treatment plant was extended by a biological cltage, The function of sewage purification is to reduce lû to a minimum the serious dangers for the health and life of hu~nans and animals linked with inadequate purification. ~ sewage purification by means of an activated sludge process the processes of biological self-purification of a natural watercourse are compressed into the ~malle~t possible space and their time scale i~ greatly reduced. As a result of artificial aeration in special aeration tanks, and wlth the concomitant action of the activated sludge (return sludge), initially the colloidally ~:
dissolved substances are partly agglomerated around the in part organic and in part inorganic suspended matter, and optionally around any material ., .
present which has a capacity to settle, whereby flalces are formed, However, in part under the influence of the aeration there i9 a coagulation : .
and flocculation of the dissolved and semi-dissolved substances which form ,.
1~ ~ a good nutrient medium for bacteria and microorganisms. It is assumed that through the vital activity of bacteria and microorganisms a type of , suction force is exerted on the dissolved and colloidal organic contaminants ~ ' .
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of the sewage and this can be called biogenic adsorption, Thus, the biological purification action is obtained through the interaction of the surface action of the flakes and the oxidation of the adsorbed substances through said organisms with the aid of enzymes in the presence of oxygen, The water which flows out of the aeration tank, together with the activated sludge, is separated from the latter in a ~econdary settler and then flows in regular manner as biologically purified water to a draining ditch, i. e, a natural or artificial watercourse. Part of the sludge must be returned to the aeration tank as return sludge where it completes the biological decomposition of the putrefactive organic substance in the sewage, The surplus sewage is either ~tabilised in the digestion tank, mixed with the preclarification sludge or is fed to a sludge draining plant if it happens to ,: , .
be stabilised sludge, A constant rotation of the tank content is necessary so that said flakes do not ~ink to the bottom of the aeration tank and die due to lack of oxygen, One- and two-staga activated sludge plants are known for carrying -` -, ~ ~
;~ out the activated ~ludge proces~. Tha simplest form of an activated sludge ~; plant operates on the ba~is of a presettling of the sewage and a subsequent single-stage highly-loadet aeration. However, this proces~ is extremely sensitive to sewage surges and to the influences of industrial sewage ! .:
because all the sewage which has to be biologically purified is fed into the I
aeration tank so that all the activated sludge can be destroyed, A greater reliability relative to sewage ~urges and the influences of industrial ~ewage can only be obtained in one-stage activation plants through a lower sludge

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loading and long residence times.
Efforts to make the activated sludge process less sen~itive but nevertheless reduce the aeration time led to the development of processes in which there is a two-stage aeration On the basis of the latest findings, optimum results are obtained with such a two-stage biological process by proceeding in the following manner: the activated sludge of a first biological stage is used for performing all the individual processes necessary for biological decomposition such as adsorption of the dissolved organic substance , on the sludge ~lakes, their diffusion into the cell, substrate and optionally ' endogenous breathing, and mixed with the partly puriffed sewage it is supplied in the highly active state to the intermediate purification system. ~ -After separating the sludge the partly purified sewage i8 introduced into , the second biological stage operated with a lower ~pace and ~ludge load.
lS Aided by the long re~idence time and large supply of oxygen, this ~tage serve~ not only to~3ubstantially remove the remaining organic substance ' ~ .
but, dué to self-absorption, there iB alQo a significant reduction of the biomaa~ content. The conventional proc0s~ mea~ures ~uch as secondary settling and the like follow. The following are considered to be important 3 ~
proce~s parameters: a BOD5 ~ludge load between 0 5 and 1. 0 kg "
BOD5/kg TS in the first biological ~tage and a low BODS space load of below 0, 5 kgtm3/d with at the same time a low ~ludge load of approximately , ~ ~ , ,, O, 05 kg BOD5/kg TS, On implementing these parameter~ and utilising , , the biological activity of the surplus sludge of the second stage by supplying , .
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~O~ ;39 the first activation stage a high BOD5 decomposition capacity i5 intended to be achieved in both biological stages, decomposition in the first stage exceeding 85%, in order to obtain a discharge con-centration of below 40 mg BOD5/1.
The known processes of the type described hereinbefore have the disadvantage of a very considerable expenditure on energy~ The purity levels obtained continue to be unsatisfactory because through underloading the second stage and returning the surplus sludge of the second stage to the first stage the deterioration of the activated -sludge is very easily possible. It is also to be feared that the known processes will not satisfy the future water conservation regulations which are continually becoming more stringent. -There~ore, the problem of the invention is to provide a two-^ stage activated sludge process for the purification of sewage which -~ 15 are able to achieve-much improved and much more stable purity levels whilst significantly reducing energy costs, whereby the capital costs must not reach thosé of a comparable conventional sludge activation plant.
According to the invention, this problem is solved in that ~; 2Q all the sewage is fed to a first aerated activation stage which is : ..
., operated as a maximum load stage with a space load BR of approximately 10 kg BOD5/m3/day and a sludge load BTS of at least 2 kg BOD5/kg of dry substance/day, by intermediate settling of the sewage sludge mixture removed from the first stage a strict separation of the biocenoses of the two activation stages is ensured, whereby su~ficient ., surplus sludge is removéd from the intermediate settling stage that the age of the sludge is kept low, the intermediately settled sewage being fed to a second aerated activation stage which is operated as , a low load stage.

A further feature of the process according to the invention ",, ~ ,~ j f,r ~
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that compared with -the known two-stage activation processes, on the crude sewage by-pass there is no need for additional feeding of -the biocenose oE the second activation stage because in the process according to the invention the polar easily decomposable organic compounds are fed in adequate quantity to the second activation stage.
Within the scope of the present invention, the term sewage is used in its widest sense. In general, it is an aqueous system in which are dispersed organic substances, also in the presence of dissolved or suspended inorganic substanoes. The particles of the dispersed phase can be genuinely dissolved, emulsified, in colloidal and/or suspended form. The degree of dispersion of the particles to be separated is not in general of great importance. The performability of the process according to the invention is also not dependent on whether the substances will or will not settle. For example, sand and mud will settle whereas the dissolved substances and colloids which ma~e sewage turbid will not settle. The fact that sewage often contains non-putrefactive as well as putrefactive substances is also generally unimportant for the performability of the process.
Hereinafter a number of non-limitative examples of applica-~20 tion of the inventive process are given. Thus, the process can beused for the purification of domestic sewage obtained in households when washing, rinsing and bathing, as well as in toilets, for purifying municipal sewage which also contains rainwater from roads, roofs and courtyards, and for purifying purely industrial sewage. The ~ 25 process according to the present invention is particularly important ; in connection with the purification of industrial sewage. The process i according to the invention is also suitable for use when combined with other purification processés, adaptatio~ to the particular case being necessary.
The two activation stages of the inventive process are . , .:: . . . . . . . . . . . ...... . .
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_rated either by means oE compressed air or by breakiny up and splashing the water surface by means of rapidly rotating rod rollers, e.g. Kessener brushes, or by means of Simplex certrifuges, Simcar fans or other known surface fans. Furthermore, if it is advantageous -5 it is possible to combine different aerating means. -On the basis of the teaching provided, it will also be possible for the expert to select the most suitable aeration tanks for the performance of the process of the invention. The following -~
enumeration of aeration tanks must only be considered in exemplified manner and can be extended at random. Thus, for example, compressed air tanks have proved advantageous. It is also advantageously possible to use so-called Haworth channels which are aerated by means of paddle wheels which simultaneously move the water forwards, whilst it is also possible to use Kessener tanks, agitator tanks and tanks which operate according to the Simplex process.
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'~'.' : .';, ~ ' ' : ' " , ' , ' ""~, ' ' ' ' , ' , ' : ' ' , , ' ' `, .: ' ' ~ ' ' ' ' ' -~ ~O~ ;3~3 The nature of the aeration and of the aeration tanks need not be the same in the first and second activation or aeration stages according to the process of the invention, Thus, for example, it is particularly advantageous if the first activation stage (maximum load stage) involves aeration with large bubbles, whereas ~ine bubbles are used in the aeration of the second activation stage, Particularly good results can also be obtained if the second activation stage is operated with a thoroughly mixed ditch system which is independent of the construction of the aeration tank of the first activation stage. Such a settling system represents a slight modification o~ the Haworth channel, Air and/or oxygen are supplied by means of e. g, centrifuges. A high level of efficiency is regularly obtained with such plants such as e. g. low loaded oxidation ditches (roundabout plants), A further advantage is that the construction costs of such plants are about 30% lower than known plants with a comparable decomposition capacity, , In general, preliminary settling of the sewage is not necessary.
Only when there are difficult constituents such as fibres and the like may a preliminary sludge removal be necessary, and even then it is not time-consuming (t C 0. 5 h), Intermediate and final settling are performed by conventional processes, It may be necessary to finish off the process according to the invention by a filtration or flocculation filtration process in order to further reduce the BOD$-value, This reduction can exceed 50%, I~ ~uch a filtration is carried out the final ~ettling sy~tem can be m~de smallar as an exception to the ATV-Guidelines (Working Repork of ATV-Committee ~ 8-- . . . . . . ._ .,, . ... , . , ., , , , ..

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5 "Settliny Processes"), because any separatiny sludye particles are held back by the filtr~tion process. When account is taken of the smaller final settling system, filtration does not lead to significant additional construction costs. Compared with a conventional sewage treatment plant (sTs = 0.15) the present process is still cheaper and the efficiency is much higher.
The sludge from the intermediate and final settling stages must in part be returned as return sludge to the particular activation stages, whilst the remainder of the sludge is conveyed as surplus 10 -sludge to a sludge digestion tank, for example via a thickener. In connection with such sludge returns, it must always be ensured that there is a basic separation thereof. This leads not only to the advantageous separation of the biocenoses of the two activation stages but also ensures that unlike in known processes operating difficulties do not occur after only a short time.
The expert wOula also not depart from the scope of the ~ invention by modifying the process tnereof by applying conventional `~ measures of activated sludge technology which can be performed before-hand, afterwards or intermediately. Thus, for example, adsorption in the first activation stage can be improved by the addition of floccu-lating agents, whilst a general stabilisation of the system can be achieved by means of polyelectrolytes.
The addition of particular enzymes to the second active stage may prove advan-ageous, whereby the enzymes should be matched to ~, ~ -' ,' ~ .
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the sewage type, The following statements serve to provide a better understanding of the invention and relate to the basic technological interrelationships of the invention.
S It can be fundamentally assumed that readily adsorbable substances have a greater affinity to the adsorbent than solvents, in the present case water. This means that the more difficultly water-soluble, i. e, higher molecular weight, that is to say more difficultly decomposable ~ubstance~, are held back in the first activation stage or maximum load stage, whereas 10 the readily water-soluble and consequently more easily decomposable substances are supplied to the second activation stage and are biologically decomposed therein. ~ .
The essential point for a successful and reliable operation of the proces~ according to the invention is the knowledge that the sludge in the .....
. 15 maximum load stage must be constantly held in the so-called working-in phase in which the substrate breathing starts. This sludge is of a fine-~~ floccular structure with a very large specific surface area which is important for the adsorbtive action and in general has a low sludge index, ~ .
The reactions taking place in the first activation stage are mainly of a ; ~ Z0 physical~chemical nature, whilst the biological processes which are in the foreground o~ the known processes are in the background here, espite the high purifying action (above 60%) the oxygen consumption and consequently the energy requirements are relatively low, However, when performing the process according to the invention, : :.' ' ,~

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is necessary to ensure a yood intimate mixing and a high sludge load accompanied by a low sludge age in the first activation stage, whereby it has been found that a space load of BR oE approx. 10 kg ; BOD5/m /day and a sludge load BTS of at least 2 kg BOD5/kg TS/day ensure the desired technical result.
The process of the invention differs from the prior art purification processes in that without the addition of precipitation or neutralisation agents in the first high-loaded activation stage, tests have shown that in the second activation stage there is a reduction of the high pH and conductivity surges to a harmless level - for the biocenoses.
Compared with the known processes, the process of the invention also differs to a certain extent by a reversal of the puriflcation sequence. In the presently conventional biological purification processes, the complete sludge load supplied to the ' biological processes~ is converted into biologically high grade ~: , ' activated sludge with a large number of different types. The follow-,~ ing purification sequence is maintained:
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'~ ~ a) Addition (adsorption) of higher molecular weight compounds on the flakes and decomposition of the lower molecular . . ~ . .
weight (more easily decomposable) compounds, b) Absorption of the higher molecular weight compounds after , corresponding preparation through the enzyme activity in the cell, 25 c) Oxidation of the contaminants for energy production and conversion into new cell substance and separation of products of metabolism.
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10~ ;3~3 As compared with this in the maximum load/low load process according to the invention, a large proportion of the higher molecular weight compounds are not decomposed, Instead, following adsorption, in the maximum load stage they are removed after a short time with the low-age sludge and consequently do not further load the purification process for the lower molecular weight and, in particular, polar compound~
in the second activation stage, This results in a considerable energy saving, This is proved by a comparison of the oxygen consumption of the process according to the invention and the prior art processes, : -!: 10 For this purpose it is possible to use the oxygen requirement formula (see ATV-Handbook) OVR = ~ 5 ~,. BR ~ 0.1 , TSR [g 2/

in which the individual quantities have the following meaning: ~
~, OVR oxygen requirement, . ~ -: 15 7, purification capacity or proportion of decomposed :
: : substance, BR organic space load/m3/d, TSR % of solids/rn3, Taking account of sub~trate and soil breathing, as well as the i: zn oxygen requirement for nitrification and denitrification processes, the ¦ ~ following is obtained:

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~- 10~4~;3~3 Stabi- Nitri- Residual Residual lised fication BOD5 BOD5 20 mg/l 30 mg/l BTS - 05 0, 15 0, 30 0, 60 Soil and substrate breathing 0, 42 0. 56 0, 78 1, 18 OVR
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Nitrification and denitrification 0~ 05 0. 23 0, 34 0, 26 OVR 0, 47 0. 79 1, 12 1, 44 R R

Increase of soil and substrate ~, ~ breathing due to 112% 141% 144% 122'1u m 3~ nitrification and 1: denitrification : ~ ~ * Den, = denitrification, The power consumption K is functionally related to the oxygen I i , requirement OVR in accordance with the following equation:

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of aeration with medium-sized bubble~ is 1800 g of 2~
The actual total power consumption KG - KN ~ Kae . including ancillary equipn~ent, can be gathered from the following aft~r applying the ,: ~
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` ` ~()1~4639 ATV-Guidelines (see ATV-Han(lbook):
_ _ BTS~ 05 0,15 0. 300. 60 l, 0 :, TS 5 3.33.3 3.3 3-3 g/l K 21 15 11 9 8 kWh/I a I = inhabitant In this the power consumption is given in kilowatt hour~ per , inhabitant and year, '~ ~ The total power consumption which is related to the OVR- value can be gathered from the following formula:

j~ KG - KN + 0.1014 . m . r1, . a ~ 0. 0203 . m . ,~3 BTS
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The new symbols in this formula have the following meanings:
KN specific power requirements for ancillary equipment, aer, plant BTS = 0, 05 - KN = 2, S kWh/I- a, aer. plant BTS 0. 10 - KN = 3. kWhtI- a;
m additional consumption for nitrification and denitrification;
; ~, proportion of biological purification level;
purification of organic substance and , a ~pecific organic decomposable load g 13OD5/I- d~

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For the various dctivation proce~ses the factor~ can, for example, be gathered from the following:

BTS 0.05 0.15 0,30 0,60 1,00Dim, _ m 1, 12 1,41 1,44 1,22 1, 10 -_ ~,.' ' ~rl 96 94 91 86 81 %

a 57 48 45 42 42 g/I d _ _ ,~ O, 50 0. 60 0, 60 0, 70 0, 70 -_ S(dec, ) 10 15 20 30 40 mg/l .
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On the basis of these calculations the following i~ obtained for the two-~tage activated sludge proces~ (without ~ilter) according to the , ~ ~ 5 invention:
KN ~ 3, 00 kWh/I~ a KH = (1, 10 ~ 0, 09) = 1, 19 kWh/I. a KS ~ (2,27 ~ 2,16) = 4,43 kWh/I. a KN ~tand~ for the power consumption of the ancillary equipment.
K~ ~tand~ for the power con~umption of the maximum load stage reali~ed according to the invention and KS or the low load stage so that the total power con~umption i8 8, 6 kWh/I. a for the two activation ~tage~, , ~

If it i8 a~umed that both the ~ingle ~tage comparative proce~ with BTS ~ 0~1S and the proce~ according to the invention .~

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operate in the final stage with a BTs-value of 0, 15, then an energy saving of I = (15. 0 - 8, 6=) 6. 4 kWh/I~ a (without filter) is obtained, In a conventional process which finally operates with a BTs-value of 0, 15, a B~)D5-residual value of 15 mg/l is on average obtained, The ' same result is obtained with the process of the invention, If a filter `~, station is used with the process of the invention, the BOD5-value drops ~ to 8 to 10 mg/l, Thus, the following values are obtained for a connected ùp : 10 figure of 300, 000 I and IE (number of inhabitants/inhabitant equivalents):

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~: ~ Process Actual space occupied Residence time 1 ~ Ideal space occupied* t in h, -~ ~ _ ___ _ _ _ Conventional 77, 000 m , 14 ~: BTS = 0. 15 80, 000 m3 .1 , _ _ - : ' Acc, to the invention 60, 300 m 9, 1 ~;
Without filter station 68, 300 m , _ _ , Acc, to the invention 56, 000 m3 7, 6 :
With filter station 74, 500 m ~ Filter ,, . ~
~: -* The ideal space occupied was determined by relatin~ the useful volume of each treatment unit to a unit price of DM 300. --/m, Thus, the ideal volumes also constitute an estimated cost comparison . . .
'~ of the main units of the sewage treatment plant, .; : .

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10~ 39 The conventional process operates with a low load stage, followed by final settling, According to the invention, there is a maximum load stage, a low load stage, intermediate settling and final settling, whereby the process may if desired be terminated by a filtration stage, As can be gathered from the above calculations, a considerable advance in the art is provided by the process of the invention, particularly with reference to the power consumption and space occupied, whilst the purification capacity is as good o~ better than the prior art processes.
Thus, the advantages obtainable by means of the invention are that the process thereof can be used in plants which can be operated with a much lower power consumption than plants operated by conventional processes, whilst there is no disadvantageous effect on the construction costs. In fact, a plant operated in accordance with the process of the invention has a much smaller space requirement than the prior art plants.
, 15 Finally, the process according to the invention in conjunctian with the filtration stage brings about extremely high purification capacities (BOD5 c 10 mg/l), Plants operated in accordance with the process of the invention are much less sen~itive to sewage surges, Another ¦~ important advantage is the suitability of the invention for extending older or overloaded sewage treatment plants. By connecting in beforehand a maximum load plant which would have to be constructed and by converting the preliminary settling aystem into an intermediate settling system, the load on the existing activation stage can be significantly reduced, In addition, itwill also be necessary to enlarge the final settling system.
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, ~0~63~3 However, it should be noted that when using the inventive concept the old parts of the sewage treatment plant can continue to be used, Hereinafter an embodiment of the process of the invention is described relative to the drawing in order to further illustrate the invention.
: Sewage enters via an intake 1 and is fed by means of a pump 2 via a pipe ll to a preliminary sludge removal apparatus 3, After removing the difficult constituents, such as fibres and the like, the sewage passes via pipe 12 into the first aeration tank or the first activation stage 4 which is operated as a maximum load stage. The aerated medium then passes via pipe 13 into an intermediate settling apparatus 5, The clarified phase is passed via pipe 14 to the second aeration tank or the second activation stage 6 which is operated as a low load stage, A sludge is removed from apparatus 5 for intermediate settling and is supplied via a pipe Z2 and a pump 23 to pipeq 24 and 25. Pipe 24 serves to return return sludge into the first stage system, whilst pipe. 25 serves to drain off surplus sludge from the system, e, g, by means of a thickener into a . .
,~ sludge digestion tank, The same happens to the initial sludge removed . .
- via pipe Z7, At the end of biological decomposition in apparatus 6 the : 20 aqueou~ phase paqses into a inal settler 7, from which sludge is removed via a pipe 18 and a pump 19, By means of pipe Z0 this sludge is either ~: returned to the second stage sy~tem as return sludge or it is removed from the sy~tem as surplus sludge by means of pipe 21, By means of a pipe 16, . ~ ~ a pump 8 and a pipe Z6 the clarified aqueous phase i~ fed to a high speed ,~ .
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3~1 filter 9, from where the clarified water i8 supplied via a drainpipe 10 to a watercourse, By means of pipes 28 and 27 flushing water can be fed from high speed filter 9 to the second stage system.
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Claims (6)

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

1. A two stage activated sludge process for the purification of sewage, wherein the sewage is fed into a first aerated activation stage operated as a maximum load stage having a space load BR of approximately 10 kg BoD5/m3/
day and a sludge load BTS of at least 2 kg BoD5/kg of dry substance/day, the sewage treated in the first activation stage is subjected to an intermediate settling stage from which sludge is removed and is separated into a return sludge portion which is returned to the first activation stage, and a surplus sludge portion which is removed from the process in sufficient quantity to ensure that the age of the sludge in the first activation stage is kept low, the intermediately settled sewage is fed to a second aerated activation stage operated as a low load stage as compared with the first activation stage, from which the sewage proceeds to a final settling stage wherein sludge is removed and is separated into a return sludge portion which is returned only to the second activation stage, and a surplus sludge portion which is removed from the process, a strict separation of the bio-cenoses of the two activation stages being maintained by the intermediate settling stage and by maintaining strict separa-tion of the sludges returned from the intermediate and final settling stages.

2. A process according to Claim 1, wherein a floc-culating or precipitating agent is added to the maximum load stage to aid adsorption.

3. A process according to Claim 1, wherein enzymes are added to the low load stage to aid biological decomposi-tion.

4. A process according to Claim 1, wherein liquid from the final settling stage is filtered and flushing water from the filter is returned only to the second stage.

5. A process according to Claim 1, wherein the second activation stage is operated with a low load plant in the form of a roundabout oxidation ditch.

6. A process as claimed in Claim 1, wherein the second activation stage is operated at a space load BR of approximately 0.5 kg BoD5/m3/day.