GB2034296A

GB2034296A - Biological Purification of Liquids

Info

Publication number: GB2034296A
Application number: GB7926887A
Authority: GB
Original assignee: Pielkenrood Vinitex BV
Current assignee: Pielkenrood Vinitex BV
Priority date: 1978-08-02
Filing date: 1979-08-02
Publication date: 1980-06-04
Also published as: GB2034296B; CA1130020A; AU536288B2; MY8500020A; NL7904891A; ES483011A1; FR2432486A1; DE2930489A1; AU4938479A; BR7904942A; SG53583G

Abstract

In the biological purification of a liquid, a batch of the liquid is contacted in a treatment space with micro-organisms present in that space from a preceding treatment. During or after supply of the liquid, oxygen is supplied to the treatment space until the desired degree of purification has been reached, after which the liquid becomes quiescent and the substances suspended in the liquid can sedimentate. The clarified liquid present above the sediment is drawn off, and the slit remaining in the treatment space and containing micro-organisms is stored therein under such conditions that their activity remains conserved to a sufficient degree until the supply of the next liquid batch. The amount of the stored silt is varied according to what is needed for treating the next batch, the surplus silt being removed.

Description

SPECIFICATION The Biological Purification of Liquids The invention relates to a method and a device for biologically purifying aqueous liquids.

Purifying waste water by means of microorganisms at a simultaneous supply of oxygen, so that these micro-organisms will feed themselves on the digestible components present in the water, and convert these substances into cell matter, mineral substances and metabolic products such as CO2 and water, is a generally known and much used method for mainly aerobic purification of water.

Of course care should be taken then that the number and kind of the micro-organisms in the treating device are sufficient for digesting the nutricial components offered to them within the available time period. Generally this will require retaining the active micro-organisms in the treatment device, or separating the microorganisms present in the treated and discharged liquid and returning them towards the treatment space. Since micro-organisms multiply themselves, and their amount in the treatment space will gradually increase towards inadmissible values, removal of excess cell matter and mineral substances is always required.

It is generally desired that the micro-organisms which are able to digest the offered nutrients will accumulate themselves in the system in a sufficient degree. This is, however, not always sufficiently the case in practice. In particular micro-organisms may develop which are able to digest the offered nutrients indeed, but cannot be sufficiently separated from the supplied water, so that, on the one hand, an unallowable loss of micro-organisms may occur, and, on the other hand, the discharged purified water will be polluted in an unallowable degree by such nonseparable microorganisms. In order to obtain, nevertheless, a satisfactory operation in such cases, the separation part is often overdimensioned when designing a purification system.The badly separable matter will, then, be sufficiently separated from the water, but the concentration of the micro-organisms in the liquid to be returned will, then, often be very small, so that the amount of liquid to be returned will be large in relation to the amount of water to be purified. Therefore not only the treatment device, but also the separation portion will be more heavily loaded accordingly, and the consequence thereof may be an insufficient purification effect.

Since it is difficult or hardly possible to promote the growth of the desired kinds of microorganisms and to suppress the growth of other kinds without reconstructing or unlarging the device, the purification effect of existing devices is often far from optimal.

The biological purification of aqueous liquids comprising a mixture of various digestible components will often take place for the various components under different optimal conditions and, for each component, by means of different micro-organisms. Also with a single digestible substance the biological purification will sometimes take place in partial steps, and for each step there exist different optimal conditions.

In order to obtain the desired degree of purity, either a compromise should be found so as to obtain averagely acceptable conditions for the various micro-organisms, or a number of biological stages should be arranged in series, in each of which the conditions can be made as favourable as possible for optimally realising a given biological effect. Sometimes it is tried to create different conditions in different points of the same treatment device, so that in one device the various biological activities take place simultaneously but more less spatially separated.

If the required purification cannot be realised completely in the biological manner, the biological treatment can be followed by a physico-chemical treatment, e.g. a dephosphatising treatment, generally called the third stage.

In order to reduce the load of the biological stage in a simple manner, a presedimentation will be applied in many instances, in order to remove non-soluble substances, which will only be digested by the micro-organisms after a long period.

Moreover an aerobic biological purification can be preceded by a physico-chemical purification which relieves the biological purification to a large extent. Finally an aerobic purification, irrespective of the character of a possibly preceding operation, can be followed by an anaerobic purification.

The devices for performing such a biological purification can be realised in many ways. The oxygen supply nearly always takes place by introducing an oxygen containing gas such as air.

When, in the following description, oxygen or oxygen supply are mentioned, this will mean the oxygen present in the oxygen containing gas as required for the biological process. This oxygen supply, for instance, can take place by means of surface aerators, by means of nozzles positioned in the liquid, to which oxygen containing gas is supplied under pressure, or by means of liquid circulated by a pump in which an oxygen containing gas is introduced by means of a mixing venturi or the like. Furthermore a distinction can be made in the size of the introduced gas bubbles, and the oxygen supply can take place in a locally restricted or in a spatially distributed manner. The biological by active material can, for instance, be suspended in the treatment space in the liquid to be treated (so-called active silt), and can also be adhered to a carrier suspended in the liquid.It is also possible to bind the active material to a solid carrier, thus forming a so-called biological filter, which carrier can also be movable. In the devices of the first kind, the surplus silt, i.e. the increase in the silt amount by multiplication of the microorganisms and by production of mineral substances, will, after separation of the active silt from the purified water, be divided from the amount of silt to be returned, and, when using bound micro-organisms, the surplus silt will be repelled and taken along with the purified liquid, and should be separated from the latter.

Such systems have, from the beginning of their development (1914), been manufactured as continuously operating systems which will take in the liquid varying in amount and composition as it is supplied, and the purified liquid will be removed from the device by displacement. The liquid content in the complete system is, then, substantially constant. Even if the liquid to be treated is produced batch-wise, the operatior of the system as generally is described. In that case often buffer vessels are used so as to maintain an uninterrupted average flow rate towards the biological device. It is true that, in the laboratory, experiments for investigating the possibilities for biological purification are performed by operating batch-wise, but, as soon as the design parameters of a biological system are to be determined, a continuously operating model will be used also on laboratory scale.

The invention is based on the insight that also on a practical scale a batch-wise operation is possible, and the invention provides a method in which a biological purification device is operated in an interrupted manner. The effect thereof is that in a simple device a biological purification will be obtained, which is equivalent to or even better than what is possible in the current continuously operating devices. Without extension or modification of the batch-wise operating device of the invention it appears that the most complex biological purification process can be performed therein, for which in the current continuously operating devices, additions are required which make the device complicated and/or require modifications which are difficultly to operate or to control, and at any rate, give rise to substantial increases in the cost.The batchwise operation according to the invention is suitable for any type of biological purification, i.e.

either with suspended or bound micro-organisms, and for any type of oxygen supply.

The method according to the invention for biologically purifying an aqueous liquid, in which this liquid, if necessary after being freed in some manner from separable impurities, is being contacted in a treatment space, under supply of an oxygen containing gas, with micro-organisms present in that space, after which the micro organisms present in the liquid are separated therefrom by sedimentation, is characterised in that a batch of the liquid to be treated is supplied to the treatment space in which space the micro organisms of the previous treatment have been left, in that during or after supplying the liquid batch, the supply of oxygen containing gas towards the treatment space is switched on, and is only switched off, eventually, as soon as the desired purification degree has been reached, after which the liquid is allowed to come to rest so that the substances suspended in the liquid can sedimentate in the same treatment space, in that the purified liquid present above the sediment is drained off, and in that, finally, the micro-organisms remaining in the treatment space are kept there under such conditions that their activity is sufficiently maintained until the next liquid batch is supplied.

Thus all the steps required for the biological treatment take place in the same space but, now, succeeding one another in time. When the biological purification takes place in different partial steps, these will succeed one another, possibly with some overlapping, and the conditions can, then, be adapted optimally to the organisms which are most effective on that moment, or to desired effects.This can often take place automatically, since, for instance, at first certain micro-organisms will digest substances suitable as nutrients at a high speed from the supplied liquid, so that the oxygen consumption will be high and the concentration of dissolved oxygen will be low, after which, as soon as these substances will have disappeared for the greater part, the oxygen consumption will decrease, and the oxygen concentration will increase accordingly, so that, then other organisms present in the liquid will automatically meet with more favourable conditions, for instance for oxydizing nitrogen present as ammonia, which ammonia may, for instance, have originated from organic nitrogen.Such subsequent operations may, however, also be promoted, if necessary, by an artificial change in the conditions, and the moments at which this has to take place can be determined, if necessary, by means of measurements. This artificial change in conditions can sometimes be brought about by adding additional substances, and often it can be sufficient to store a small part of the untreated liquid separately, and to return it towards the treatment space after depletion of the nutrients for the micro-organisms, in order to start or support therein an other specific effect. This may, for instance, be the case with the anaerobic treatment which will be discussed below.

If after the biological treatment an additional physico-chemical treatment is required, this can take place again in the same space, and either before or after sedimentation of the microorganisms. The products produced by this physico-chemical process are taken up by the mass of micro-organisms, and often have, there, an additional useful function as a weighting mass, so that the sedimentation will be improved and will take place in a shorter time. It is also possible to add special substances just before the sedimentation which enhance the flocculation of the micro-organisms and/or the sedimentation of mineral substances.

If, for obtaining a complete purification, the aerobic biological treatment is to be followed by an anaerobic treatment, this too can be done in the same space, and the micro-organisms in suspension can be kept in suspension then by means of very short gas supply periods or by replacing the oxygen containing gas by an inert gas, or also by using a suitable agitator. If the oxygen supply takes place by means of a circulation pump with oxygen being introduced into the circulated liquid, the desired stirring action can also be obtained by shutting off the oxygen supply towards the liquid but keeping the pump in operation.

The surplus of micro-organisms obtained by multiplication and the mineral components formed can be removed from the treatment space in different ways, e.g. by removing a part of the sediment periodically after sedimentation, or by drawing off a part of the batch and the matter suspended therein during at least a part of the oxygen supply period or during a part of the sedimentation period. It is, then, possible to keep, at choice, heavy material for instance in the treatment space by allowing it to sedimentate, and to remove only then the liquid with the lighter components. This may, for instance, be favourable when the micro-organisms are adhered to bigger and/or heavier particles, or when the microorganisms coalesce to larger aggregates which sedimentate more easily, and which, preferably, should be stored for the next operation.If a preclarifier is used, at least a part of the removed surplus can be returned towards this preclarifier.

The device according to the invention comprises a container which is adapted to perform therein all the required steps of the method of the invention successively.

The invention will now be elucidated below by reference to a drawing, showing in: Fig. 1 a schematical representation of a first embodiment of the device according to the invention; Fig. 2 a simplified schematical representation of a compound device comprising a plurality of devices of Fig. 1; and Fig. 3 a schematical representation of another embodiment of the device according to the invention.

The device according to the invention schematically shown in Fig. 1 is intended for biological purification without a preceding preclarification, and the liquid to be purified is supplied batch-wise. This liquid may, for instance, originate from a factory which produces waste water only during a part or specific parts of the day. This is, for instance, the case with slaughterhouses and the like.

The liquid to be treated is supplied by a duct 1 which can be provided with a valve 2, and which leads to a basin 3, which constitutes the most important part of the device according to the invention.

Near the bottom of the basin 3 outflow openings 4 provided in a tube 5 are shown, which tube is connected to an air pump 6 so as to allow air to be introduced into the liquid present in the basin 3. This tube is shown only by way of example, and can be replaced by any other aerator which is able to introduce air into the liquid at a more or less variable liquid level.

In the basin 3 micro-organisms are present which are left from the preceding batch-wise treatment, and which are required for the purification of the liquid to be treated.

In or near the lateral wall of the basin a tapping point 7 is provided which is, by the intermediary of a valve 8, connected to a discharge duct 9.

Above the opening 7 a second tapping point 7' is indicated with interrupted lines, which, by means of a valve 8', is connected to the duct 9 too. The number of such superposed tapping points is not restricted to two. If more than one opening 7 is present, one can tap at different levels as desired.

Other tapping means can be used instead, e.g.

overflow ducts which can follow the changing liquid level in the basin 3, or other suitable tapping means.

In the basin 3 an opening 10 for surplus silt can be provided, which communicates with a duct 1 1 provided with a valve 12, and leads towards a batch-wise operating silt treatment stage 13 in which a separation is brought about into concentrated surplus silt, on the one hand, which is discharged at 14, and a quantity of clarified water, on the other hand, which is discharged at 15, and the discharge duct can communicate with the duct 9.

The silt treatment stage 13 is shown, in Fig. 1, at a lower level than the basin 3, but this is not necessarily the case, and this treatment stage can also be positioned at the same level as the basin 3, and can, for instance, be separated from the latter by means of partitions. In particular this may be the case if the silt discharge takes place at times that the basin 3 is completely or nearly completely filled, so that the silt discharge towards the stage 13 can be effected by the pressure of the overlying water. The clarified water separated in the stage 13 can flow off by its own level difference towards the discharge duct 9.

The operation of this device takes place in substantially four steps: (1) At the beginning of the filling, the active silt left back reaches, for instance, the level 16 in the basin 3. The liquid to be treated will then more or less gradually fill the basin 3. Already during filling the aeration can be started, but it is also possible to wait until the basin 3 is completely filled.

(2) After switching on the aeration, the biological purification begins, and is continued until the liquid is sufficiently purified.

(3) As soon as the desired purity is reached, the aeration is interrupted, and the silt mass which was kept in suspension by the aeration, is then allowed to settle. During a part of the second step or also during a part of the third one, surplus silt can be drawn off through the opening 10 or the higher opening 10' indicated in Fig. 1 with interrupted lines, so as to ensure that not too much silt will be present during the treatment of the next batch.

(4) After completing the sedimentation, the clarified liquid can be discharged through the opening 7. If higher discharge openings are present, discharge can begin already during sedimentation at that level where no longer micro-organisms are present. This can also take place if use is made of a discharge which is always situated near the liquid surface. After removing the clarified liquid, a quiescent time can follow if necessary, during which the activity of the remaining silt can be maintained by periodic aeration.

The basin 3 operates therefore, on the one hand, as a biological purification vessel, and, on the other hand, as a sedimentation vessel. Since, during a substantial part of the biological treatment (step 2), the concentration of nutrients is large in respect of that in comparable continuously operating devices, the velocity of the biological conversion will be particularly high, so that this device can be made particularly small as compared to the known devices. Moreover the device of the invention is particularly simple.

The present device can be controlled in a very simple manner, and the control lends itself also to automatisation. This can, for instance, be done by means of time switches, but it is preferred to design the control means in such a manner that one proceeds to the next step only after completion of the preceding one. The end point of step (2), for instance, can be determined by establishing the increase of the dissolved oxygen concentration at the end of the clarification. In some cases, when the liquid contains ammonia or ammonia is produced therein, the concentration thereof can be determined.For as soon as the nutrients for the micro-organisms present in the liquid have been digested, oxygen will become available for oxidising ammonia by means of other micro-organisms, so that a decrease of the ammonia content is an indication for the fact that the digestible substances have been digested.

Also the discharge of clarified liquid can be controlled by means of measurements, in particular in the case of discharge openings situated at different heights or of an adjustable discharge with a controllable immersion depth.

The control can, for instance, be performed in conformity with a turbidity measurement near the discharge, so that liquid will be drawn off at a lower level only as the turbidity in that point will have decreased below a certain threshold value.

the silt treatment stage 13 of this device is very small in comparison with the settlers used in the current devices, as sedimentation takes already place in the basin 3, and in the stage 13 only surplus silt will be thickened.

The higher discharge opening 10' schematically indicated in Fig. 1 with the associated duct 11' and valve 12' may be used, for instance, if one wishes to obtain, at first, the sedimentation of a silt layer and to remove the surplus silt in a higher point. The heavier sediment will, then, not be removed. This can, for instance, be favourable when the micro-organisms are adhered to heavier carrier particles, and the adhered micro-organisms should be kept in the basin 3, whereas repelled organisms together with mineral substances should be removed as surplus silt. Moreover it has surprisingly appeared that also without using heavier carrier substances and with systematically removing lighter silt particles as surplus silt, automatically a quickly settling active mass will be formed which has more favourable settling properties than the smaller silt particles.The amount of surplus silt to be removed depends, of course, on the amount of active silt required for treating a subsequent liquid batch, further on the growth of the microorganism population, and finally on the amount of produced mineral substances.

It is possible to add, at a suitable moment, additional substances from a container 17 to the liquid to be treated, so as to bring about an additional effect. An example thereof is the addition of agents promoting the sedimentation or flocculation of the micro-organisms and the produced or present mineral substances so as to accelerate settling. Moreover the particles of mineral substances settled in this manner may serve as adhesion nuclei for micro-organisms so as to enhance sedimentation thereof, or, when reused, to improve the effect of the microorganisms. It is, of course, also possible, as mentioned above, to adhere the micro-organisms to heavier particles already from the outset, or to obtain larger particles by a certain coalescence of the micro-organisms themselves.Moreover the operating conditions can be modified also in other ways, for-instance by varying the temperature or the acidity.

Besides micro-organisms suspended in the liquid, adhered or not to heavier particles, use can also be made of organisms adhered to fixed carrier surfaces in a known manner not further described. These surfaces may have the shape of plates or discs immersed into the liquid, which can be removed from the liquid at the end of the operation. This can, for instance, also be done by constructing these plates as rotatable discs which only partly extend into the liquid. By rotation thereof these micro-organisms will be periodically introduced into the liquid, and are subsequently exposed to the air. As the micro-organisms on such plates multiply, a part thereof will be repelled from the carrier surface together with the produced mineral product, and will land in the liquid. After the interruption of the biological purification process, these substances landed in the liquid can settle, and can be removed at least partly from the liquid. Of course sedimentation will, then, be faster than in the case that all the micro-organisms are suspended in the liquid.

It is sometimes favourable to add, at a later moment, a suitable nutrient for the microorganisms, e.g. a part of the liquid to be treated, to the liquid in the vessel 3, which is indicated in Fig. 1 by the auxiliary container 18, in which a part of the supplied liquid can be stored. This may have a favourable effect on the purification, for instance by starting and promoting an additional activity by other micro-organisms, or by maintaining the activity of micro-organisms during the time that other micro-organisms are bringing about another effect.

This is particularly relevant in the case that, after the aerobic biological treatment, an anaerobic biological treatment should take place in the basin 3, e.g. for denitrifying aerobically produced nitrate. Of course the micro-organisms suspended in the liquid which are suitable for this anaerobic treatment should be kept in suspension to that end. This can be obtained by stirring by means of an additional agitator. It is also possible to bring about the required liquid movement by means of short air blasts for the nozzles 4 which are insufficient for substantially increasing the oxygen content of the liquid. Also an inert gas can be introduced into the liquid instead of air through the nozzles 4.If the oxygen supply takes place by introducing water mixed with air by means of a liquid pump, it is sufficient to shut off the air supply, so that only water is introduced which will keep the liquid mass in the basin 3 in motion.

In the silt treatment stage 13 a level sensor may be provided, by means of which the valve 12 or 12' will be closed as soon as a given level has been reached. In this manner the amount of silt can be automatically controlled. If this level sensor is made adjustable, an adaptation to any prevailing condition can be obtained.

If a plurality of tapping openings 7, or a tapping means to be kept near the liquid surface is used, discharging the clarified water can already begin before settling has been fully completed. The complete settling of the silt as far as the level 16 may sometimes last rather long in proportion to the time available until the supply of the next batch, so that, accordingly, little time is left for removing the clarified liquid. This can be unfavourable if it is not possible to discharge the clarified liquid fastly, for instance in view of the capacity of the drain, in particular in the case of draining the clarified water into a sewer or waterway.It is advisable, then, to begin with removing clarified liquid as soon as possible after the beginning of the sedimentation, and to that end the above-mentioned discharge means can be used which allow to begin the discharge near the liquid surface where settling is completely terminated soon. In particular use can be made, as mentioned above, of a turbidity meter which allows to determine when the sedimentation has proceeded sufficiently far.

In the device described above for a batch-wise treatment of batch-wise supplied amounts of water to be purified (e.g. once a day), the volume of the basin 3 can easily be calculated from the size of the batch and the required amount of remaining active silt, the volume of this silt mass being relatively small, and being, generally, less than 20% of the liquid mass. With a basin 3 having a volume which is only 20% more than the batches to be treated, a purification effect can be obtained which can only be obtained in a current continuously operating device if its liquid volume is several times larger.

The advantages of this batch-wise operation are so great that, even when the liquid to be cleaned is supplied in a substantially uninterrupted manner, a division into separate batch-wise operating parts will offer great advantages.

Fig. 2 shows a schematical representation of a device suitable for that purpose, which can, for the rest, also be used when the device of Fig. 1 requires extension, and the plural device shown in Fig. 2 can also be used for taking in rarely occurring impact loads.

The plural device shown comprises a plurality of basins 3 according to Fig. 1, which are shown in a highly simplified fashion. All reference numerals correspond to those of Fig. 1. Although only two basins 3 have been shown, their number can be chosen arbitrarily large.

The supply valves 2 of the various basins 3 are connected in parallel to the single supply duct 1.

Also the discharge valves 8 of these basins are connected in common to one single discharge duct 9. The aeration tubes 5 of the various basins are, now, provided with a valve 19, which valves are connected in parallel to the pump 6. Also the silt discharge valves 12 are connected in common with one single silt treatment vessel 13, and the water discharge 15 is returned towards the discharge duct 9.

By alternately opening the supply valves 2, the basins 3 can be filled with liquid charges. After filling a basin 3, the associated valve 19 is opened so as to bring about the aeration of the contents of this basin. At the end of the purification period, the valve 8 in question is opened, and, if silt is to be drawn off, the associated valve 12 is opened.

The various valves may be actuated by means of a programme switch in the correct succession, and, if necessary, level sensors in the various basins may be used for closing the associated supply valve 2 as soon as the desired level in the basin 3 has been reached. As a matter of fact each basin 3 can be provided with an associated air pump 6, and, if desired, several silt treatment stages 1 3 may be used. The number of stages 1 3 is not necessarily equal to the number of basins 3, if only suitably operated switches are used.

Fig. 3 shows another embodiment of the device of Fig. 1, in which, again, corresponding parts have been indicated by the same reference numerals, which parts are represented in a high simplified manner.

The supply duct 1 is, again, provided with a valve 2, which, however, does not lead directly towards the basin 3, but towards a preclarifier 20.

In this preclarifier a part of the impurities is removed from the supplied liquid in a nonaerobic-biological manner. Examples thereof are sedimentation, if necessary by means of auxiliary substances and/or air bubbles, physico-chemical treatment, anaerobic treatment and the like. For the purposes of the description of the invention the character of the preclarifying stage is not important, so that its structure need not be discussed in detail.

Matter separated in the preclarifier 20 is discharged through a duct 21. The preclarified liquid is led through a duct 22 towards a basin 3 according to Fig. 1. The branching 22' indicated schematically in Fig. 3 indicates in which manner, in a compound device according to Fig. 2, the preclarifier 20 can be coupled to a plurality of basins 3.

The basin 3 is, for the rest, constructed in the manner of Fig. 1, with as the only difference that the silt discharge duct 1 1 with a valve 12 does not lead towards a silt treatment stage 13, but communicates with a return duct 23. At 23' a branching is shown again, indicating how in the case of a plural device according to Fig. 2 a number of basins 3 can be connected to a single return duct 23. This duct leads back towards the entry side of the preclarifier 20, but can also open into the duct 1 beyond the valve 2. The basin 3 is, moreover, connected to an aerating pump 6, if necessary together with other basins 3, and the discharge for clarified water 7, if necessary by the intermediary of a valve 8, connects again with a discharge 9 which, as indicated 9', can also connect to the discharge sides of the other basins 3.

The surplus silt is, in this device, not thickened in a separate stage, but is returned towards the preclarifier 20, which often hardly means an additional load for the preclarifier. The biological surplus silt is, then, removed together with the substances separated by the preclarification process.

If necessary a pump can be included in the duct 23, and this duct may also open in any other suitable point in the preclarifier, for instance when a certain succession of treatment steps takes place therein, and the introduction of silt can better be done in a more remote step.

Also in this modified embodiment the basin 3 is used for a plurality of successive process steps.

It will be clear that within the scope of the invention many modifications are possible. The essence of the invention, however, is that all the process steps to which the liquid to be purified is to be submitted, with the exception of the not always required preseparation, take place in the same space, viz. in the basin 3, so that the device according to the invention requires much less space than the known devices, and is much simpler. In particular, in contrast to the known devices, the number of vessels need not to be equal to the number of process steps, since these steps can now be performed consecutively in the same basin.Also when using several basins 3 in parallel for treating batches which succeed one another faster, and, in particular, for treating a substantially continuous supply, the total volume of the device will be smaller than in a device of the known type designed for the same capacity, since, in the known devices, in contrast to the device according to the invention, only small level fluctuations will occur.

It is, furthermore, possible to convert a known device with several basins or vessels, into, for instance, a plural device according to Fig. 2, in which, then, the relatively large aftersedimentators can be provided with aerating means and, then, can be used as basins 3 for biological purification. In this manner an existing device can be adapted to treating substantially larger amounts, which, if an existing device has become insufficient because of the increasing liquid supply, will lead to substantial cost and space savings.

In the preceding description it has already been said that the active silt to be used for a next batch can be kept alive in the basin 3 by aeration. At longer interruptions of the supply it may be advisable to stabilise the silt. This is done by a strong aeration which, since nutrients are absent, has the effect that a part of the micro-organisms is digested by others, non-digestable mineral substances then being produced, and no organic substances subject to putrefaction will remain.

This leads to an aerobic stabilisation and extensive mineralization of the silt to be stored.

The existing continuously operating biological treatment devices are often capable of satisfactorily treating waste water or the like to be treated, if the supply rate and/or impurity concentration are substantially constant, but in practice the supply rate and/or concentration will show peaks, so that, at intervals, the purification will be less than optimal, although the average polution degree remains below the maximally allowable level. Such a device is, then, to be extended, if such a less satisfactory purification during load peaks should be avoided, which means that the device will be underloaded during a very considerable time.

According to the invention a current continuously operating device can be made suitable for absorbing such load peaks, in that a batch-wise operating device of the kind described above is connected in parallel thereto. This additional device needs only to absorb a peak load a few times a day, and, as described already above, the structure and control of such a device are very simple. The continuously operating original device is then relieved of these peak loads, and can operate in the most optimal manner. Since the additional batch-wise operating device of the invention needs only relatively little space, such an extension is generally realisable without any difficulty, and, moreover, the cost will be substantially lower than those of a second continuously operating device which, moreover, will operate in the average uneconomically.

It will be clear that such an extension is not restricted to one single additional device. After some extensions the situation of a compound device of Fig. 2 will be reached to which a continuously operating device is connected in parallel which absorbs the constant and, in relation to the total load, possibly small basic load.

Inversely, if a certain constant basic flow is developing, a device of the invention can be extended by means of a relatively small continuously operating device. In a new device to be constructed one may also choose this structure from the outset.

In such a compound device which operates partly continuously and partly batch-wise, one will switch over from the first towards the second kind of operation when the supply rate exceeds a given value or if, by means of the current measuring and control apparatus, it is established that the oxygen demand in the aeration stages of the continuously operating part of the device increases beyong a certain limit. In this manner the operation of the continuously operating part can be stabilised in a very considerable degree, whereas the impact or peak loads will be absorbed in a very effective manner by the batchwise operating part.

Claims

1. A method for biologically purifying a liquid, in which the liquid, if necessary after being stripped of separable components, is contacted in a treatment space with micro-organisms present in that space under supply of an oxygen containing gas, after which the micro-organisms and reaction products present in the liquid are separated from the liquid by sedimentation, characterized in that a batch of the liquid to be treated is supplied to the treatment space, in which space the micro-organisms of a preceding treatment have been left, in that during or after supplying the liquid batch, the oxygen supply towards the treatment space is switched on and is, eventually, switched off again as soon as the desired purification degree of the liquid has been reached, after which the liquid comes to rest and the substances suspended in the liquid can sedimentate, in that the clarified liquid present above the sediment is drawn off, and in that, finally, the micro-organisms remaining in the treatment space are stored therein under such conditions that their activity remains conserved in a sufficient degree until the supply of the next liquid batch, the amount of the stored silt being adapted to what is needed for treating the next batch, and the surplus silt is removed.

2. The method of claim 1, characterised in that the working conditions in successive steps or partial steps, as the case may be, are adapted in the most favourable manner to the operation taking place in each step or partial step, and in particular in accordance with measurements of the state of the liquid being treated, in particular of the oxygen concentration and/or demand, the adjustment of the working conditions being obtained, for instance, by adding additional substances.

3. The method of claim 2, characterised in that a part of the liquid to be treated is stored, and is added to the liquid in the treatment space at a suitable moment.

4. The method of any one of claims 1-3, characterised in that before or after the aerobic treatment a treatment without substantial oxygen supply is performed, the liquid during said anaerobic treatment being kept in motion by stirring by introducing into the liquid an oxygen depleted or oxygen-free gas or liquid.

5. The method of claim 4, in which the oxygen supply during the aerobic treatment takes place by injecting an oxygen containing liquid, characterised in that during the anaerobic treatment, the oxygen supply towards the injected liquid is interrupted.

6. The method of any one of claims 1-5, characterised in that during or after the biological treatment a physico-chemical treatment is performed in the same treatment space, comprising, in particular, adding during or after the oxygen supply additional substances enhancing the sedimentation of the microorganisms and of mineral components present in the liquid or produced therein.

7. The method of any one of claims 1-6, characterised in that the surplus silt produced by multiplication of micro-organisms and/or production of mineral components is removed from the treatment space during an arbitrary period of the treatment as needed by drawing off liquid with salt suspended therein from the treatment space.

8. The method of claim 7, characterised in that the discharge of surplus silt takes place above an already settled silt layer, in order to remove lighter silt with preference over heavier silt.

9. The method of any one of claims 1-8, in which the liquid to be treated is treated first in a preseparator, characterised in that at least a part of the removed surplus silt is returned to the preseparator.

10. The method of any one of claims 1-9, characterised in that the treated liquid is discharged from the liquid portion present above the sedimentating material already before termination of the sedimentation.

1 The method of any one of claims 1-10, characterised in that a substantially constant part of the supplied liquid is submitted to a continuous biological purification treatment, and the remaining variable part is treated batch-wise.

12. A device, obviously intended for executing the method of any one of claims 1-11, comprising a container adapted to be used as a treatment space for treating a liquid supplied thereto with micro-organisms, and provided with a liquid supply, means for introducing an oxygen containing gas, in particular air, into the liquid, and means for discharging the treated liquid, characterised in that the container is adapted for admitting liquid batches supplied at intervals, in that this container is, furthermore, provided with means for collecting and discharging sediment, and in that the container is, finally, provided with means for controlling the supply of liquid and oxygen containing gas and for discharging liquid in such a manner that the gas supply can be interrupted for allowing the liquid to come to rest and the sedimentatable material to sedimentate therefrom, before the next liquid batch is supplied.

13. The device of claim 12, characterised by an auxiliary container connected to the liquid container, in which a part of the supplied liquid can be temporarily stored.

14. The device of claim 12 or 13, characterised by means for keeping the liquid present in the container in motion without a substantial oxygen supply, in order to allow an anaerobic process to take place.

15. The device of claim 14, characterised in that the means for keeping the liquid in motion are adapted to introduce an oxygen depleted or oxygen-less gas or liquid into the liquid.

16. The device of any one of claims 12-1 5, characterised by one or more silt discharge ducts connected in the container and provided with a valve.

17. The device of claim 16, provided with a preclarifier included in the supply duct, characterised in that one or more silt discharge ducts are connected to the preclarifier.

18. The device of any one of claims 12-17, characterised in that the liquid discharge comprises a plurality of discharge ducts opening at different heights in the container, each duct being provided with a valve.

19. The device of any one of claims 12-17, characterised in that the liquid discharge is adapted to discharge the treated liquid from near the liquid surface.

20. The device of any one of claims 18 or 1 9, characterised by a sensor for determining the turbidity degree of the liquid near the liquid discharge, which sensor is adapted for controlling the discharge in correspondence to the turbidity degree thus determined.

21. The device of any one of claims 12-20, characterised by a plurality of treatment containers connected in parallel to a supply and a discharge duct, the supply and discharge connections thereof being provided with valves which are adapted to be alternately actuated, so as to supply to these containers alternately a liquid batch and to discharge therefrom treated liquid respectively.

22. The device of any one of claims 12-21, for executing the method of claim 11, characterised in that the device is connected in parallel to a device for executing a continuous biological purification, and is provided with valves which are adapted for branching off, as needed a part of the supplied liquid flow.

23. The device of claim 22, characterised by means for measuring the flow range and/or the impurity concentration or the oxygen demand in the continuous flow, and for controlling the branching valves in conformity therewith.

24. A method substantially as hereinbefore described with reference to the accompanying diagrammatic drawing.

25. A device substantially as hereinbefore described with reference to the accompanying diagrammatic drawing.