GB1589552A - Activated sludge process for waste water purification - Google Patents

Description

PATENT SPECIFICATION ( 11) 1 589 552

N" 1( 21) Application No 7864/80 ( 22) Filed 14 Nov 1977 ( 19) 1 ( 62) Divided out of No 1589551 ( 31) Convention Application No 2652229 ( 32) Filed 16 Nov 1976 in Q ( 33) Fed Rep of Germany (DE) ' Ufl ( 44) Complete Specification Published 13 May 1981 S; -A ( 51) INT CL ' CO 2 F 3/12 1/38 ( 52) Index at Acceptance CIC 311 431 432 435 43 Y J ( 54) ACTIVATED SLUDGE PROCESS FOR WASTE WATER PURIFICATION ( 71) We, Boehringer Mannheim G m b H, of 112-132 Sandhofer Sfrasse, 6800 Mannheim-Waldhof Federal Republic of Germany, a Body Corporate organised under the laws of the Federal Republic of Germany, do hereby declare the invention, for which we pray that a patent may be granted to us and the method by which it is to be performed, to be particularly described in and by the following statement: 5

The present invention is concerned with a process for the biological purification of waste water by the activated sludge process, as well as with a plant for the carrying out of this process.

Microbiological clarification plant, frequently also more generally called biological clarification plant, are based on the principle that "activated sludge" (active biomass), 10 which consists of micro-organisms carrying out the decomposition of impurities present in the waste water, is brought into intimate contact with the water to be purified in an activator tank, usually with the addition of oxygen The organic impurities are thereby partially broken down into carbon dioxide and water and partially converted into bacterial mass The separation of the solid biomass phase from the liquid phase takes place in 15 conventional plant in a post-clarification tank or on filter beds containing growing plants.

The separation of the solid phase from the liquid phase is of prime importance for the correct functioning of a biological clarification plant Thus, if activated sludge passes from the post-clarification tank into the flow-off of the clarification plant, then the purification effect is, in part, again negated and the expelled clarification sludge loads the drainage plant 20 ( 02 drift) Furthermore, biomass necessary for the purification is lost so that the purification capacity of the plant is also reduced.

One of the main causes for the driving off of activated sludge from the clarification plant is the formation of swollen sludge, this being an activated sludge, the settling properties of which have become impaired to such an extent that it can no longer be retained in the 25 post-clarification tank A measure for the settling properties of the sludge is the sludge volume index, this being the volume which one gramme of dry sludge substance takes up in a water-containing state after 30 minutes In the case of clarification plant which has no swollen sludge the sludge volume index is usually between about 50 and 100 Beyond an index of about 150 swollen sludge is present Extreme swollen sludges can assume sludge 30 volume index values of 1000 and more, which leads to an extraordinary dilution Many causes can participate in the formation of swollen sludge in clarification plant.

Thus, for example, there may be mentioned special materials present in the waste water which promote swollen sludge formation, over-loading of the clarification plant and unfavourable reaction conditions in the plant itself, which are specific to the process If 35 swollen sludge formation occurs in a clarification plant, then the bacterial equlibrium of the activated sludge is displaced, in which many types of bacteria are present in symbiosis, in favour of a type of micro-organism which, under these special conditions, has better growth conditions than the other micro-organisms present The sludge thereby degenerates, i e.

starting from the compact bacterial flocks which are necessary for the orderly functioning of 40 the plant, there grow or develop therebetween filamentary organisms (e g Sphaerotillis natans, true fungi, filamentary lactic acid bacteria, Lelicotrix) which prevent the flocks from coming close during the settling process Thus, for example, Sphaerotilus natans has the property of stringing out the individual cells in a slimy envelope to give long threads.

In order to combat the swollen sludge-forming organisms, the following measures have 45 1 ZQO CZ 1 2 1 J 07 JJ 2 already been suggested:

a) reducing the organic impurities in the waste water, when possible; b) removing certain components from the waste water, when possible; c) adding precipitation or flocculation agents for artificial aggregation of the sludge flocks; 5 d) adding turbid or muddy water; e) increasing the intensity of aeration; f) introducing anaerobic phases:

g) adding chemicals, such as chlorine or hydrogen peroxide or aqueous sodium chlorite solution, for the direct combating of the swollen sludge: 10 h) altering the p H conditions.

This list alone clearly shows how difficult it is to deal with the problem of swollen sludge formation and, in particular, how difficult it is to find a practical solution Thus, many of the proposed measures can only be effected, if at all, under certain conditions which, in practice, are frequently not present Therefore, a large number of activated sludge 15 clarification plant suffer from the occurrence of swollen sludge In general, it can be said that hitherto attempts have been made to combat swollen sludge formation in clarification plant Now, however, in the course of biological decomposition experiments, we have found that swollen sludge-forming organisms, for example the abovementioned Sphaerotills natans, have the ability better to decompose organic components in the waste water than 20 the actual desired activated sludge without filament-forming organisms The decomposition experiments which we have carried out have shown that these organisms have substantially better decomposition characteristics than the "normal" sludges.

Therefore it is an object of the present invention to overcome the problem of swollen sludge formation in microbiological clarification plant and also to utilise the outstanding 25 decomposition properties of the swollen sludge-forming micro-organisms In principle, therefore, the present invention depends upon the recognition of the fact that swollen sludge formation in such microbiological clarification plant must not be combatted but rather promoted.

Thus, according to the present invention, there is provided a process for the biological 30 purification of waste water by the activated sludge process with separation of the activated sludge from purified waste water and at least partial recycling of the separated activated sludge, wherein the concentration of the decomposable impurities in the waste water fed in is kept so high that swollen sludge formation is promoted and swollen sludge is centrifuged off from the purified waste water If desired, centrifuging can be carried out in at least two 35 stages.

According to a preferred embodiment of the process according to the present invention, the pressure, residence time and/or acceleration in the case of the centrifuging are regulated so that the sludge flocks are comminuted and filament-forming microorganisms, such as Sphaerotilus natans and true fungi, are comminuted 40 By means of the process according to the present invention, the result is achieved that, on the basis of the high space loading which occurs in the activator tank, swollen sludge formation automatically occurs after a short time In this way, per space unit, considerably more organic material can be decomposed in the waste water (space loading SL) Thus, for example, in an existing microbiological clarification plant, after introduction of the process 45 according to the present invention, the space loading could be increased from O 7, which, in the run-off, ensured a BO Ds/l value of less than 25 mg, to a space loading of 4 38, without the BOD 5/1 values in the run-off increasing This means more than a sixfold increase of the space loading and thus also of the capacity of the plant.

(The BOD 5 is the 5 day biochemical oxygen demand, i e the amount of oxygen 50 consumed in 5 davs at 20 'C bv the micro-organisms in order to break down the organic materials contained in the water.

Space loading is a measure of the supplying to clarification plant of organic impurities and is expressed as kg BO Ds/in' x day).

The promotion of the swollen sludge formation takes place in a simple manner by 55 keeping high the concentration of the swollen sludege-forming components and especially of the organic components of the waste water Swollen sludge-forming organic components of waste water include carbohydrates, solvents, such as mono- di and trihydroxy aliphatic alcohols containing up to 5 carbon atoms (e g ethanol and propanol), proteins and materials favouring the growth of fungi in particular As a rule, waste water from breweries, 60 sugar factories and jam factories have ab initio a composition which favours swollen sludge formation It is important that the space loading, i e in practice the concentration of these materials, is kept sufficiently high We have found that, as a rule, this condition is fulfilled by a space loading of over 1.

In order to achieve the most favourable possible conditions for swollen sludge formation 65 1 589 552 the space loading is preferably adjusted to above 2.

Important for the process according to the present invention is the separation of the solid phase from the liquid phase If, as according to the present invention, swollen sludge formation is promoted then separation of sludge in a post-clarification tank is omitted because the plant would, within a short period of time, become nonfunctional due to 5 expelled sludge Therefore, the activated sludge must be centrifuged off When carrying out the centrifuging, care should be taken that the separation of the solid phase and of the liquid phase is complete, i e the clarified liquid must be completely clear but the micro-organisms must not lose their biological activity in the course of separation.

It is known that a separation of the solid phase from the liquid phase is, in the case of 10 micro-organisms, possible by centrifuging Separation with continuous centrifuges, such as are primarily considered for a waste water clarification process, is, however, problematical because of the high pressures which prevail in centrifuges Bacteria from monocultures from fermentation processes can thereby be damaged to such an extent that they can no longer be used for further culturing i e for recycling to the activated sludge stage Damage 15 to the micro-organisms can indeed, be so great that the cells burst and the cell content escapes into the medium from which the micro-organisms are to be separated.

For this reason, centrifuges have hitherto been only used in clarification technology, after separation of excess sludge in settling tanks, for concentration purposes since the concentrated sludge is as a rule, removed from the process and, therefore, an impairment 20 of viability is no longer of importance.

However, we have, surprisingly, found that swollen sludge, when passed through a centrifuge, such as a self-discharging centrifuge, can admittedly undergo substantial structural change but without loss of biological activity The structural change in the case of swollen sludge-forming organisms, such as Sphaerotilus natans, results in the filaments 25 being chopped up, the swollen sludge thus losing its characteristic structure without, however, sacrifice of its biological activity The micro-organisms hereby break down into their individual cells which are strung out in bacterial sheaths The micro-organisms which do not form swollen sludge hereby also temporarily lose their sludge structure but their biological actvity is not impaired It is assumed that the micro-organisms of the activated 30 sludge, when they are present as swollen sludge, are protected by their sludge flock structure (mucus envelope) and therefore, can withstand the passage through a centrifuge at the high pressures there prevailing, without being damaged According to a preferred embodiment of the process according to the present invention, centrifuging is, therefore, so regulated that the swollen sludge flocks are broken down as far as possible One possibility 35 of achieving this consists in appropriate selection of pressure, residence time and/or accelerating in the centrifuge As a rule, it can easily be determined by preliminary experiments for particular types of centrifuges whether and how these conditions can be achieved.

The chopped up sludge emerging from the centrifuge has the appearance of a 40 voluminous, uniform paste However, when this paste is returned to the activator tank, it again rapidly loses this consistency and in the activated sludge tank again forms the typical activated sludge flocks.

The principle according to the present invention of chopping up the sludge can also be employed in conventional clarification plant which have to contend with problems of 45 swollen sludge formation In this case, it is sufficient to provide, in a shunt circulation pipe of the activator tank, a device which chops up the filaments of the swollen sludge-forming micro-organisms and then to return the chopped up sludge to the activator tank, a separation of sludge and clarified water not being necessary This can be achieved by means of devices such as the above-mentioned self-discharging centrifuges In this case, sludge and 50 water are subsequently remixed and recycled However, instead of a centrifuge, other known non-centrifugal devices can also be employed which are capable of producing such high pressures that the filamentary micro-organisms are torn apart but the other micro-organisms are not damaged An example of such a known noncentrifugal device is a suitably adapted high pressure dispersion apparatus of the type used on a large scale for the 55 digestion of micro-organisms When recycling is carried out with a selfdischarging centrifuge and with the use of a nozzle centrifuge which does not lead to chopping up, the sludge can be separated from the clarified water Instead of a selfdischarging centrifuge, a conventional settling tank can also be employed.

The process according to the present invention results in an especially rational 60 combination of activator tanks and centrifuges in such a manner that a post-clarification tank and a sludge concentrator can be omitted Therefore, the present invention also provides a plant for carrying out the process according to the present invention, which comprises at least one activated sludge tank and at least one centrifuge for the separation of sludge following the activated sludge tank 65 1 589 552 The sludge which emerges from the centrifuge has such a high concentration that only a small investment is necessary for subsequent further water removal, if this is desired.

However, as a rule, activated sludge emerging from the centrifuge as recycle sludge is returned directly to the activator tank and excess sludge can be used, for example, for agriculture, after aerobic stabilisation or in the form of liquid sludge: subsequent removal of 5 water can then be omitted.

According to a preferred embodiment of the plant according to the present invention, it has at least two centrifuges connected in series If desired, the centrifuges can be of different modes of construction By appropriate combination of various types of centrifuge, the index value of the activated sludge can then be controlled as desired Thus, for example, 10 by means of a continuously discharging nozzle centrifuge, the sludge mixture can be separated without the sludge structure of the activated sludge being substantially changed since, in the case of the short residence times which hereby occur, in spite of high pressures, practically no chopping up of the flocks takes place On the other hand, by means of a self-discharging centrifuge, the sludge structure is substantially changed by comminution of 15 the flocks By a combination of a continuously discharging nozzle centrifuge with a self-discharging centrifuge, the desired sludge volume index can, therefore, be adjusted.

This can, for example, by carried out by passing a part of the sludgecontaining waste water from the activated sludge tank to a continuously discharging nozzle centrifuge and a part to a self-discharging centrifuge, the separated sludges obtained then being returned together 20 to the activated sludge tank, insofar as it is not a question of excess sludge By variation of the amounts of waste water passed to the two types of centrifuge, the sludge volume index can also be changed Alternatively, the total waste water can first be passed through the continuously discharging nozzle centrifuge, with removal of only a part of the sludge contained therein, and subsequently the waste water with the residual sludge is passed 25 through the self-discharging centrifuge Here, too, the sludge volume index can be controlled by the amount of sludge which is or is not separated by the nozzle centrifuge introduced into the circuit.

Examples of self-discharging centrifuges which can be used within the scope of the present invention include the nozzle centrifuge DA 100, obtainable from the firm Westfalia 30 Separator AG, 4740 Oelde Federal Republic of Germany In the case of such nozzle centrifuges, sludge is continuously removed In contradistinction thereto, centrifuges with self-emptying drums throw out the separated-out sludge particles at periodic intervals of time at full rotation They have, for example, a self-emptying drum with a back and forth-moving sleeve valve which opens and closes a peripheral slot in the centrifugal 35 chamber Upon opening the drum, the sludge is suddenly thrown out.

For the better understanding of the present invention, reference is made to the accompanying drawing, in which is a schematic illustration of a waste water clarification plant with sludge separation according to the present invention.

The embodiment illustrated in the accompanying drawing comprises two activated sludge 40 tanks I and IV, which are connected together by a simple overflow, aeration pumps T supplying the necessary amount of air From the tank IV a pump P passes the active sludge-containing suspension to a continuous nozzle centrifuge D From this, clarified water is taken off via pipe I and sludge via pipe 2 Whereas the sludge necessary for the biological process is recycled via 3 to the tank 1, excess sludge is taken off through pipe 4 45 Fresh waste water is introduced through pipe 5 to tank 1.

Numerous advantages are achieved by the process and device of the present invention.

Thus, by the use of centrifuges for the separation of activated sludge from clarified waste water, there is achieved a complete independence from the sludge index of the activator tank and from other processes in which gases are formed, for example of the process known 50 as denitrification in which nitrate ions are reduced to nitrogen when there is an oxygen deficiency At the same time, substantially higher sludge concentration is achieved than is possible with the previously conventional post-clarification tanks Whereas in the case of the latter, the sludge separated off contains, as a rule, 1 to 2 % by weight of dry matter, according to the present invention sludge concentrations of 5 to 10 % by weight of dry 55 matter can easily be achieved Furthermore, in the case of a given clarification capacity, the process according to the present invention makes possible a substantial diminution of the size of the plant As already mentioned above, in the case of the present invention, the space loading in the activated sludge tank can be considerably increased For example, in the case of a given plant, the maintenance of a space loading SL of at most O 7 was 60 necessary in order to achieve with certainty a BOD'/1 value of below 25 mg By alteration of the process and plant according to the present invention, the SL value be increased more than sixfold to 4 38 On the other hand, the result of this is that, according to the present invention, it is possible to use substantially smaller activator tanks for a plant which is to work up a particular amount of impurities in waste water Furthermore, the certainty of 65 1 589 552 functioning of the plant is increased since there is no danger that, in the case of a larger amount of waste water having to be clarified, sludge passes through to the drainage plant, which can easily happen in the case of the previously conventional postclarification tanks.

Furthermore, the amount of oxygen passed into the activated sludge tank can be reduced so that aerators of lower capacity can be employed, which reduces the investment 5 expenditure Further substantial savings are achieved by the omission of the postclarification tank and of the concentrator In cases where excess sludge is to be used directly for agricultural purposes, the additional costs of plant for a substantial removal of water from the sludge, for example a sieve band press and the possibly otherwise necessary bioreactor (for composting the sludge), are saved 10 The following Examples are given for the purpose of illustrating the present invention:Example 1

A conventional waste water plant, constructed for a loading corresponding to 17,500 inhabitant equivalent values (IEV) (= about 1050 kg BODS and a waste water volume of 15 400 m 3/7 days), which comprises a treatment tank with a volume of 1520 m 3 a post-clarification tank with a volume of 475 m 3 a concentrator with a volume of 125 mi, a sludge aerating tank with a volume of 100 mi, a plant for removing water from sludge comprising a sieve band press containing a polyclectrolyte, as well as ferric chloride, and a 150 m 3 volume bioreactor, forms, in the case of an increase of the space loading above 1, 20 within a short time ( 7 to 14 days), such large amounts of swollen sludge that intolerable operational disturbances arise The sludge volume index values are above 500 In the case of an SL below 1, admittedly no more swollen sludge formation occurs but the normal activated sludge then formed only breaks down sufficiently the waste water when the SL is reduced to O 7 25 This plant is now changed by replacing the activator tank by two small activator tanks, connected in series, with a total volume of 524 M 3 Post-clarification tank, concentrator and substantial removal of water from the sludge with bioreactor are replaced by a continuously discharging nozzle centrifugue The sludge emerging from the centrifuge is returned directly to the first activated sludge tank and excess sludge is, without further treatment, 30 used for agricultural purposes.

The following Table summarises the essential characteristics of the two plants and the results achieved therewith:

1 SQO SS) 1 JO CtJ 6 TABLE constructional data conventional process according for 17,500 IEV process to the invention ( 1050 kg BOD 5) 5 load: 17,500 IEV = ditto 1050 BOD 5 volume of waste water 400 m 3 Id 7 400 m 3/d 7 10 space loading 0 7 2 overall ( 1st step: 4) sludge content 6 g dry 6 g dry sub /1 15 sub /l in both stages sludge loading 0 116 0 666 1st stage volume of the 1520 m 3 524 m 3 in all 20 activator tank volume of the post 475 m 3 omitted clarification tank 25 volume of the con 125 m 3 omitted centrator volume of the sludge 100 m 3 100 m 3 aeration tank 30 centrifuges none 2 sludge water removal:

35 sieve band press with polyelectrolyte 1 of 150 m 3 omitted precipitation + Fe C 13 addition 40 bioreactor I omitted Example 2

In the plant according to the present invention described in Example 1, the nozzle 45 centrifuge is replaced by a self-discharging centrifuge, a diminution of the sludge structure thereby being acheived The sludge volume index before the centrifuge is 250 After the centrifuge treatment, the index is, after two weeks, 100 and after three weeks is 70.

Thereafter, the latter index can be maintained without difficulty and without a drop in biological activity The chemical oxygen demand value before the centrifuge is 170 mg and 50 after the centrifuge is also 170 ( mg.

A similar process for the biological purification of waste water by the activated sludge process is described and claimed in our copending patent application number 47289/77 (Serial No 1589551).

Claims (13)

WHAT WE CLAIM IS: 55

1 Process for the biological purification of waste water by the activated sludge process, with separation of the activated sludge from purified waste water and at least partial recycling of the separated activated sludge, wherein the concentration of the decomposable impurities in the waste water fed in is kept so high that swollen sludge formation is promoted and swollen sludge is centrifuged off from the purified waste water 60

2 Process according to claim 1 wherein the pressure, residence time and/or acceleration in the case of the centrifuging are regulated so that the sludge flocks are comminuted and filament-forming micro-organisms, such as Sphaerotilus natans and true fungi, are comminuted.

3 Process according to claim 1 or 2, wherein the space loading in the activated sludge 65 A 1 589 552 stage is adjusted to a value above 1.

4 Process according to claim 3, wherein the space loading is adjusted to a value above 2.

Process according to any of the preceding claims, wherein centrifuging is carried out in at least two stages

5

6 Process according to claim 1 for the biological purification of waste water, substantially as hereinbefore described and exemplified.

7 Plant when used for carrying out the process according to any of claims 1 to 6, comprising at least one activated sludge tank and at least one centrifuge for the separation of sludge following the activated sludge tank 10

8 Plant according to claim 7, comprising at least two centrifuges of different modes of construction.

9 Plant according to claim 7 or 8, comprising at least one continuously discharging nozzle centrifuge and at least one self-discharging centrifuge.

10 Plant according to claim 9, wherein the centrifuges are connected in parallel 15

11 Plant according to claim 9, wherein the centrifuges are connected in series.

12 Plant according to any of claims 7 to 11, comprising at least two activated sludge tanks connected in series.

13 Plant for carrying out the process according to any of claims 1 to 6, substantially as hereinbefore described and exemplified and with reference to the accompanying drawing 20 VENNER, SHIPLEY & CO, Chartered Patent Agents, Rugby Chambers, 2, Rugby Street, 25 London WC 1 N 30 U.

Agents for the Applicants.

Printed for Her Majesty's Stationery Office, by Croydon Printing Company Limited, Croydon Surrey, 1981.

Published by The Patent Office, 25 Southampton Buildings, London, WC 2 A IAY, from which copies may be obtained.