GB1589551A - Activated sludge process for waste water purification - Google Patents

Abstract

In biological effluent treatment by the activated sludge process, the concentration of the degradable impurities in the effluent fed is maintained so high that the formation of bulking sludge is promoted. This is effected by adjusting the space loading to a value from 1 to 2. Due to the degradation properties of the microorganisms forming the bulking sludge, more organic pollution load in the effluent can be degraded. Subsequently, the activated sludge is separated from the treated effluent and at least partially recirculated.

Description

(54) ACTIVATED SLUDGE PROCESS FOR WASTE WATER PURIFICATION (71) We, BOEHRINGER MANNHEIM GmbH., of 112-132, Sandhofer Strasse, 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: 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), 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 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 (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 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 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, overloading of the clarification plant and unfavourable reaction conditions in the plant itself, which are specific to the process. If.

swollen sludge formation occurs in a clarification plant, then the bacterial equilibrium 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 the plant, there grow or develop therebetween filamentary organisms (e.g. Sphaerotilus natans, true fungi, filamentary lactic acid bacteria, Leucotrix) 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 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; 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; h) altering the pH conditions.

This list alone clearly shows how difficult it is to deal with the problems 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 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 above-mentioned Sphaerotilus natans, have the ability better to decompose organic'components in the waste water than 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 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 purification of waste water by the activated sludge process, with separation of the activated sludge from the purified water and at least partial recycling of the separated activated sludge, wherein swollen sludge-containing waste water is withdrawn, the sludge flocks therein are comminuted and the comminuted sludge flocks and waste water are recycled together or separately. Comminution of the sludge flocks can, if desired, take place by centrifuging.

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 according to the present invention, the space loading could be increased from 0.7, which, in the run-off, ensured a BOD5/1. value of less than 25 mg., to a space loading of 4.38, without the BOD5/1. 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 BODS is the 5 day biochemical oxygen demand, i.e. the amount of oxygen consumed in 5 days at 20"C. by 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. BODs/m3 x day).

The promotion of the swollen sludge formation takes place in a simple manner by keeping high the concentration of the swollen sludge-forming components and especially of the organic components of the waste water. Swollen sludge-forming oganic 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, 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, 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 non-functional due to 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 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 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 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 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 activity is not impaired. It is assumed that the micro-organisms of the activated 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 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 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 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 centrifuge. In this case, sludge and 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 non-centrifugal device is a suitably adapted high pressure dispersion apparatus of the type used on a large scale for the digestion of micro-organisms. When recycling is carried out with a self-discharging 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 self-discharging centrifuge, a conventional settling tank can also be employed.

The process according to the present invention results in an especially rational 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, a branch pipe, means being provided in the branch pipe for comminuting sludge flocks, and a recycling pipe from said means to the activated sludge tank.

The means provided in the branch pipe for comminuting the sludge flocks can be, for example, a self-discharging centrifuge. 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 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, 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 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, be carried out by passing a part of the sludge-containing 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 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 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 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 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 drawings, in which: is a schematic illustration of a waste water clarification plant according to the present invention, with recycling of chopped up sludge.

The embodiment illustrated in the accompanying drawing comprises two activated sludge 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 one pipe and sludge via another pipe. The sludge necessary for the biological process is recycled to the tank I and excess sludge is taken off through a pipe provided for this purpose. Fresh waste water is introduced through the indicated pipe into tank I.

Furthermore, the plant additionally comprises a self-discharging centrifuge S, a part of the sludge-containing waste water being passed directly to this via pipe 6. The clarified liquid is recycled through pipe 7 to tank I and "chopped up" sludge is also returned to tank I via pipe 8. Instead of the centrifuge D, a clarification tank can also be used for this embodiment.

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 as denitrification in which nitrate ions are reduced to nitrogen when there is an oxygen deficiency. At the same time, a 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 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 0.7 was necessary in order to achieve with certainty a BODS/1. value of below 25 mg. By alteration of the process and plant according to the present invention, the SL value could 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 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 post-clarification 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 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.

The following Examples are given for the purpose of illustrating the present invention: Example I A conventional waste water plant, constructed for a loading corresponding to 17,5000 inhabitant equivalent values (IEV) (= about 1050 kg. BODS5 and a waste water volume of 400 m3/7 days, which comprises a treatment tank with a volume of 1520 m3 a post-clarification tank with a volume of 475 m3 a concentrator with a volume of 125 m3, a sludge aerating tank with a volume of 100 m3, a plant for removing water from sludge comprising a sieve band press containing a polyelectrolyte, as well as ferric chloride, and a 150 m3 volume bioreactor, forms, in the case of an increase of the space loading above 1, 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 0.7.

This plant is now changed by replacing the activator tank by two small activator tanks, connected in series, with a total volume of 524 m3. Post-clarification tank, concentrator and substantial removal of water from the sludge with bioreactor are replaced by a continuously discharging nozzle centrifuge. The sludge emerging from the centrifuge is returned directly to the first activated sludge tank and excess sludge is, without further treatment, used for agricultural purposes.

The following Table summarises the essential characteristics of the two plants and the results achieved therewith: TABLE constructional data conventional process according for 17,500 IEV process to the invention (1050 kg. BOBS) load: 17,500 IEV = ditto 1050 BOD5 vol. of waste water 4()0 m3Id 7 400 m3/d 7 space loading 0.7 2 overall (lust step: 4) sludge content: 6 g. dry 6 g. dry sub./1.

sub./1 in both stages sludge loading: 0.116 0.666 1st stage volume of the 1520 m3 524 m3 in all activator tank volume of the post- 475 m3 omitted clarification tank volume of the con- 125 m3 omitted centrator volume of the sludge 100 m3 100 m3 aeration tank centrifuges: none 2 sludge water removal: sieve band press with polyelectrolyte 1 of 150 m3 omitted precipitation + FOCI, addition bioreactor 1 omitted Example 2 In the plant according to the present invention described in Example 1, the nozzle centrifuge is replaced by a self-discharging centrifuge, a diminution of the sludge structure thereby being achieved. 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 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 8007864 (Serial No. 1589552).

WHAT WE CLAIM 15.- 1. Process for the biological purification of waste water by the activated sludge process, with separation of the activated sludge from the purified water and at least partial recycling of the separated activated sludge, wherein swollen sludge-containing waste water is withdrawn, the sludge flocks therein are comminuted and the comminuted sludge flocks and waste water are recycled together or separately.

2. Process according to claim 1, wherein comminution takes place by centrifuging.

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

4. Plant when used for carrying out the process according to any of claims 1 to 3, comprising at least one activated sludge tank, a branch pipe, means being provided in the branch pipe for comminuting sludge flocks, and a recycling pipe from said means to the

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

Claims (7)

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

   TABLE constructional data conventional process according for 17,500 IEV process to the invention (1050 kg. BOBS) load: 17,500 IEV = ditto

   1050 BOD5 vol. of waste water 4()0 m3Id 7 400 m3/d 7 space loading 0.7 2 overall (lust step: 4) sludge content: 6 g. dry 6 g. dry sub./1.

   sub./1 in both stages sludge loading: 0.116 0.666 1st stage volume of the 1520 m3 524 m3 in all activator tank volume of the post- 475 m3 omitted clarification tank volume of the con- 125 m3 omitted centrator volume of the sludge 100 m3 100 m3 aeration tank centrifuges: none 2 sludge water removal: sieve band press with polyelectrolyte 1 of 150 m3 omitted precipitation + FOCI, addition bioreactor 1 omitted Example 2 In the plant according to the present invention described in Example 1, the nozzle centrifuge is replaced by a self-discharging centrifuge, a diminution of the sludge structure thereby being achieved. 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 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 8007864 (Serial No. 1589552).

   WHAT WE CLAIM 15.- 1. Process for the biological purification of waste water by the activated sludge process, with separation of the activated sludge from the purified water and at least partial recycling of the separated activated sludge, wherein swollen sludge-containing waste water is withdrawn, the sludge flocks therein are comminuted and the comminuted sludge flocks and waste water are recycled together or separately.
2. 2. Process according to claim 1, wherein comminution takes place by centrifuging.
3. 3. Proces according to claim 1 for the biological purification of waste water, substantially as hereinbefore described and exemplified.
4. 4. Plant when used for carrying out the process according to any of claims 1 to 3, comprising at least one activated sludge tank, a branch pipe, means being provided in the branch pipe for comminuting sludge flocks, and a recycling pipe from said means to the

   activated sludge tank.
5. 5. Plant according to claim 4, wherein the comminution means is a self-discharging centrifuge.
6. 6. Plant according to claim 4 or 5, comprising at least two activated sludge tanks connected in series.
7. 7. Plant for carrying out the process according to any of claims 1 to 3, substantially as hereinbefore described and exemplified and with reference to the accompanying drawing.