CA1071543A - Overload protector for sewage treatment device - Google Patents

Abstract

Abstract A purification device comprising supply means and dis-charge means connected to, respectively, the inlet and outlet ends of an intermediate separation means for separating impurities from a liquid flowing from the inlet to the outlet end, and an emer-gency discharge means for diverting a temporary excessive liquid supply in said supply means from said separation means, character-ized by means dividing the flow path from the supply towards the discharge means into a first and a second part communicating, res-pectively, with the supply and discharge means, and by intercon-necting means between said first and second parts which are adapt-ed to limit the liquid flow rate therethrough to about the maximum allowable flow rate through the separation means, the emergency discharge means being connected to said first part. In this way, the purification device will not or not substantially be over-loaded when excessive liquid supply is provided, as happens with conventional purification devices having normal overflow weirs at the inlet end over which is diverted the excess supply towards a buffer vessel or directly towards a normal discharge duct.

Description

~7~ 3 In many branches of industry polluting waste liquids are produced which cannot be drained as such in a sewer or water-course, and generally it is to be avoided that this waste reaches the groundwater. Before being drained the waste liquid should, therefore, be stripped of noxious components as well as possible, e.g. by sedimentation or flotation, if necessary with addition of separation promoting agents, by biological purification, and the like.
For this purpose different kinds of purification devices are known which, in general, operate in such a manner that the purification effect is the better as the residence time in such a device is longer.
It can happen, however, that the liquid supply increases in such a manner that the purification device is not able to operate in the required way, e.g. when, because of a failure in a duct or container, heavy rains or the influx of extinguishing water in case of fire, an excessive supply occurs which cannot be processed in the required way by the purification device.
Such an excessive supply can be many times the normal liquid flow rate, but the probability of its occurrence is very low. It is, therefore, not justified to design the purification device for such an excessive flow rate, but nevertheless provi-sions should be made for preventing a substantial pollution by the drained liquid in the occurrence of such an exceptional con-dition. In some cases it is possible that at such an excessive supply the degree of dilution of the impurities increases so that, as such, a direct draining in the normal discharge means would not be objectionable, but, in that case, the purification device would be disturbed by the strong flow, e.g. in the case of bio-logical purification when the danger exists that the active siltis entrained by the strong liquid flow, and then the purification device will become inoperative, and its recovery will require some time after the normal conditions have been restored again.
The usual purification devices having an inlet and an outlet and intermediate means for effecting the separation of impurities from a liquid are9 therefore, often provided with an emergency outlet which is able to divert, in such circumstances, a very considerable part of the excess supply either towards a buffer vessel or directly towards a normal discharge duct, and in the former case the contents of the buffer vessel can be sub-jected to a purification treatment again after the emergency situation has ended. Such an emergency outlet can communicate, by means of an emergency overflow weir, with the inlet end of the purification device, which overflow weir is so much higher than the normal liquid level as corresponds to a level rise as a consequence of the excess supply.
Now the flow resistance of an overflow weir is mainly de-termined by the border friction and similar boundary effects, so that, as the thickness of the overflowing liquid layer increases, this resistance will increase gradually at a lower rate, which has the consequence that, at an increasing flow rate, the level rise will become smaller. As the length of the ovPr~low weir is - larger, the smaller will be the level rise as a consequence of an increasing supply, so that at an increasing length of the weir its characteristic curve will become flatter. Thus for obtain-ing a favourable emergency discharge a large length of the emer-gency overflow weir will be required in order to limit the level rise at an excessive supply, since a level rise will also lead to an increasing flow rate through the purification device which should even be avoided.
Generally an overflow weir is present at the outlet end of a purification device determining the liquid level in this de-vice. When a separated impurity will float on the liquid, it is desired to keep the liquid level as constant as possible so as 5~
to ensure, under all circumstances, a good discharge of the float-ing layer by means of an open collecting trough or the like with-out mixing with the carrier liquid, so that it is desired to use an outlet overflow weir with a relatively flat flow characteris-tic, or at least to operate in a relatively flat part of its characteristic. However this is unfavourable in the said emer-gency cases, since, as a consequence of this flat characteristic of the outlet overflow weir, a considerable increase of the flow rate will already take place at a relatively small level rise, unless the length of the emergency overflow weir would be made very large.
A draw-back of a large length of the emergency overflow weir is that such a weir requires much space and material which will lead to considerably higher cost, and, as said before, the probability that such an emergency overflow weir will have to -become operative, is relatively small.
The invention provides a device of this kind which is constructed in such a manner that, at an excessive supply rate, the separation means will not or not substantially be overloaded, to which end this device, apart from a buffer vessel or another emergency outlet, is provided with means for limiting the liquid ; supply towards the separation means to about the normal flow rate.
This can be obtained, according to the invention, in a number of ways, such as by including means in the normal flow path having a flow resistance which considerably increases as the flow rate therethrough increases, or by using means such as pumps which are designed to maintain a substantially constant liquid flow rate through the separation means. The former means com-prise, for instance, rather narrow and/or long tubes or speciallydesigned overflow weirs, and can be arranged at the inlet or out-let end or in an intermediate part of the device.

Such special weirs comprise in particular a normal overflow weir and an auxiliary weir having a lower edge which is parallel to the upper edge of the former weir and being situated at a given distance therefrom so that this lower edge corresponds to the normally maximum level in the device. Such a composite weir has the characteristic that, as soon as the liquid reaches the lower edge of the auxiliary weir, the flow resistance suddenly increases. Ordinary tubes, on the other hand, show a gradual increase of the flow resistance. In both cases, however, the increasing flow resistance will cause a level rise so that the greater part of the excess liquid supply will flow off via the emergency outlet rather than through the separation means so that the latter will not be substantially overloaded.
The invention will now be elucidated by reference to a - drawing, showing in:
Figure 1 a highly simplified section of a known purification device;
Figure 2 a graph of the relationship between the liquid discharge flow over an overflow weir and the liquid level for two different overflow weirs of this device;
Figure 3 a schematic section of a special overflow weir according to the invention;
Figure 4 a graph corresponding to Figure 2 of the operation when using the overflow weir of Figure 3;
Figure 5 a section corresponding to Figure 1 of another embodiment of the device according to the invention;
Figure 6 a graph corresponding to Figure 4 of the operation of the device of Figure 5;
Figures 7 and 8 modifications of the device of Figures 1 and 5 resp.;
Figure 9 a schematic representation of the liquid flow through the special weir of Figure 3;

'~j .`~

` ~7~5~3 Figures 10 and 11 schematic sections and a schematic view, resp., of modifications of the weir of Figure 3;
Figures 12 and 13 schematic sections of a special modification of the overflow weir of the invention;
Figures 14 and 15 schematic sections of still another embodiment of the device of the invention;
Figure 16 a top view of a practical embodiment of the device of Figure 5; and Figures 17 and 18 sections on the lines XVII - XVII and XVIII

- XVIII resp. of Figure 16.
In Figure 1 a known purification device is shown in an extremely simplified manner. This device comprises a purification vessel l with an inlet chamber 2 to which the liquid to be treated, generally polluted waste water, can be supplied by means of a supply duct 3. A separator 4 9 by means of which the impurities can be separated from the carrier liquid, communicates with the chamber 2.
Examples of such separators are sedimentation and/or flotation basins or plate separators, the latter preferably with inclined and in particular corrugated plates, or finally means for effecting a biological purification. Such separators have the property that the separation effect decreases as the flow rate therethrough increases, so that the device should be designed for a given maximum supply rate, and on exceeding this maximum the separation effect will deteriorateO The kind of separators used is of no importance for the present problem, and will therefore not be indicated in detail.
At the outlet end of the separator 4 an outlet chamber 5 is provided which connects, by means of an overflow weir 6, to a discharge channel 7 communicating with a sewer 8 or aduct leading to a water-course or a waste pit. For the sake of simplicity the chambers 2 and 5 are shown at the same level, but this is not ,P~

necessarily always the case.
In the outlet chamber 5 a collecting trough 9 for remov-ing separated components floating on the liquid is shown, and a dip baffle 10 ensures that the floating components do not flow off into the discharge channel 7. The trough 9 can also be arrang-ed in the inlet chamber 2, as shown at 9', in which case a dip haffle 10' can be provided in this chamber again. This will de-pend on the manner in which the separator 4 operates.
The inlet chamber 2 communicates, by the intermediary of an emergency overflow weir 11, with a buffer vessel 12, so that in case of an excessive supply rate and a corresponding rise of the liquid level in the chamber 2, the excess liquid can be removed. Under some circumstances it is not necessary to store the excess liquid in a buffer vessel, for instance in the case of such a dilution of the impurities in the excess liquid that the liquid can be directly drained in a sewer 8 or a similar dis-charge means.
Figure 2 shows a graph of the relationship between the liquid flow rate Q over a weir and the liquid level height h.
The curve 13 shows this relationship for the weir 6 having a height h . As a consequence of border friction and similar boundary effects the resistance will initially be such that the liquid level will rise rather sharply at an increase of the sup-ply rate, but as the overflowing liquid layer grows thicker the friction will be reduced so ~hat the level rise will be reduced accordingly. The maximum liquid flow rate which will flow over the weir 6 under normal conditions, which substantially corres-ponds to the maximum flow rate which can be processed in the separator 4 with the desired separation effect, is indicated by Q . The corresponding liquid level is indicated by h . The weir 11 has, now, a height which is equal to or slightly larger than h , so that, on exceeding the maximum supply rate, the weir 11 will become operative. The weir 11 has a considerably larger length than the weir 6 so that the slope of the curve 14 of the former is much smaller than the slope of the curve 13. As the weir 11 becomes operative, the total discharge rate is the sum of the discharge rates over both weirs as indicated by the curve 15.
From Figure 2 it clearly appears that, at an exceptional-ly high excessive supply rate Q , a considerable part Qll will flow off over the weir 11 and a correspondingly smaller part Q6 over the weir 6, but the latter part Q6 is still about two times ]arger than Q . This means that the separator will be considerably over-loaded, and the separation effect will then be smaller accordingly.
In the case of biological purification the considerably increased flow velocity will have the consequence that active silt will be entrained towards the outlet end so that the future separation ef-fect will be considerably impaired. The ratio between Qll and Q6 can be improved by making the curve 14 still flatter, i.e. by in-creasing the length of the weir 11, but this will require much space, even if this weir is assembled from partial weirs arranged in a comb or saw-tooth fashion. A certain improvement can be ob-tained by giving a steeper characteristic to the weir 6, but this will lead to considerable level fluctuations during the normal operation which is undesirable9 especially when floating compo-nents are to be removed by means of a trough 9 or 9' without en-training carrier liquid.
In order to avoid this draw-back an overflow weir accord-ing to Figure 3 may be used, comprising, apart from the normal weir 6, an inverted extension weir 16 positioned above the former one, and between both an aperture 17 is defined, the lower edge of the upper weir 16 being situated at the level h . During the normal operation of the device this assembly operates in the same manner as the single weir 6 of Figure 1. However, as soon as the level h ax has been reached, the liquid will contact 1~7~ 3 the lower edge of the upper weir 16, and the flow resistance against this edge and other boundary effects will then influence the flow.
Figure 4 shows the effect of such an additional weir on the operation of the device. Until the level h has been reach-ed, the relationship between Q and h is given by the same curve 13 as in Figure 2. On reaching the lower edge of the weir 16, however, the flow resistance increases so that the level will rise more sharply accordingly, and, moreover, the liquid discharge flow rate will decrease as shown at 18. At a further level rise the discharge flow rate will increase but now according to a much steeper curve 13'. At the same time the weir ll has become opera-tive which occurs rather abruptly as a consequence of the abrupt level rise. The latter weir will, thus, be able to absorb the growing liquid supply very quickly. The sum of the flows over both weirs is, again, represented by the curve 15'. The curve 14' of the weir 11 is, in this case, steeper than the curve 14 of Flgure ~, which indicates that the length of the weir 11 is smal-ler than in the case of Figure 1. Nevertheless, at the same ex-ceptional supply flow rate Q , the discharge flow rate Qll overthe weir ll will be substantially equal to the liquid flow rate increase, since Q6 is hardly larger than Qmax- According as the curve 13' is steeper, the difference between Q6 and Q will be smaller. In this manner an effective protection of the separator 4 against overloading can be obtained with a rather short emer-gency overflow weir 11.
When the liquid supply decreases again, the liquid flow rate over the weirs 6 and 11 will be reduced along the curves 13' and 14' resp. The flow over the weir 6 decreases along the curve 13' until the level h is reached. At a further reduction of the supply rate the working point will change more or less abrupt-ly towards the curve 13 as indicated at 18'. The curve shape at ~L~7~

a rising level is, therefore, different from that at a falling level. This hysteresis depends on the structure of the device, in particular on the slope of the curve 13', and can be much smaller than shown in Figure 4.
It will be clear that, in this manner, a normal operation with relatively small level fluctuations can be combined with a very favourable ratio between the discharge flows over both weirs.
Figure 5 shows another embodiment of the purification de-vice of Eigure 1 in which the same reference numerals are used for indicating similar parts.
This embodiment differs from the former one in that the duct 3 is a tube with a given length which forms a connection be-tween the inlet chamber 2 and a supply channel 24 which, on the other hand, communicates by means of an emergency overflow weir 11 with a buffer vessel 12 which, at an excessive supply rate, can absorb the excess liquid flowing over the weir 11.
Figure 6 shows a graph of the operation of this device, the curve 13" giving the relationship between the liquid level in the channel 24 and the liquid flow rate in the duct 3, and the curve 14" representing the relationship between the liquid flow over the weir 11 and the liquid level. The flow resistance of the tube 3 increases as the flow rate increases which leads to a cor-responding level rise in the channel 24 in respect of the level in the vessel 1 determined by the outflow weir 6. When in the point 18 the level hmaX ~ which corresponds to the height of the overflow weir 11 and is the highest level normally occurring, has been reached, the liquid will flow off over the latter weir at a further increase. The point 18" lies on a steep part of the curve 13" so that the flow rate increase in the duct 3 at a given level rise will be low. The curve 15" represents, again, the sum of the flow rates according to the curves 13" and 14". An excep-tionally large liquid flow rate Q corresponds to a level h and then a flow rate Qll will flow over the weir 1] and a flow ~7~ 3 rate Q3 through the tube 3, the latter then being only a little higher than Q so that the separator 4 will only slightly be overloaded.
This operation mainly corresponds to that according to Figure 4, but differs therefrom in that the curve 13' is steeper than the curve 13 of Figure 2 so that during the normal operation the level fluctuations in the channel 24 will be larger, but this is not objectionable since such fluctuations are only to be avoided in those parts where floating components are to be re-moved. Another difference is that the transition is less sharp.The solution of Figure 5 is, therefore, very suitable for using at the inlet side of the purification device.
If, however, a sharper transition is desired, it is also possible, as shown in Figure 7, to use between the channel 24 and the chamber 2 instead of the tube 3 a weir 6' having a height h and an auxiliary weir 16' between which an opening 17' is defined as in the case of Figure 3. The operation of such an assembly is the same as shown in Figure 4.
Figure 8 shows another embodiment corresponding to that of Figure 5, but now the outlet chamber 5 is connected by means of a tube 3' with the discharge channel 7. This will, however, lead to level fluctuations in the vessel 1, so that this solu-tion will only be used when such fluctuations are not objection-able.
Figure 9 shows a liquid flow 19 through the opening 17 between the weirs 6 and 16 of Figure 3. The liquid is driven upwards before these weirs, and flows through the opening 17 with a certain height drop. It will be clear that the location of the auxiliary weir 16 is of importance for the effect obtained. When transversely shifting the weir 16, the height oE the lower edge is to be chosen accordingly, as shown at 16a and 16b~ An adjust-ment of the operation can be obtained by moving the weir 16 verti-~7~ 3 cally and/or horizontally.
It will be clear that, instead of both weirs 6 and 16,also a plate can be used in which one or more corresponding aper-tures are provided.
The effect of these weirs depends, moreover, on their thickness. When increasing the thickness, the contact area with the liquid flow is increased, and, thus, the friction resistance.
Figures lOA, B and C show three possible examples of thicker weirs, and the thicknesses of the weirs 6 and 16 can be equal or different, depending on the desired effect on the flow. Such weirs can, of course, be mutually movable for obtaining an adjust-ment.
Figure 11 shows another embodiment in which the weirs 6 and 16 are toothed, enabling to influence the flow resistance variation. The teeth can be arranged in phase or phase opposi-tion, and it is also possible to vary the phase relationship by longitudinally shifting the weirs. Instead of the saw-tooth shape also a rectangular toothing or a corrugation can be used.
Figure 12 shows another embodiment which is an intermediary so-lution between those o,F Figure 1 and 7, on the one hand, and 5and ~ on the other ~and. Instead of the overflow weir 6 or 6' a closed baffle 20 with holes 21 is provided, in which holes curv-ed tubes 22 are fixed, having an upper rim 6" at the same height as the upper edge of the weir 6 or 6'.
In the case of Figure 12A the tubes 22 are situated, for instance, in the outlet chamber 5. The liquid flows over the rims 6" of the tubes 22, and the effect is substantially the same as in the case of the weir 6 of Figure 1. However, as soon as a given flow rate is exceeded, substantial turbulences and vortices will occur in the liquid which lead to a considerable increase of the flow resistance, and then the effect described in respect of ; Figure 4 will occur. Moreover the wall friction in the tube will . -- 11 --:' ~L~97~5~3 cause a certain Elow resistance depending on the flow velocity.
In the case of Figure 12B the tubes 22 are situated, for instance, in the discharge channel 7, and then the effect of turbulences and vortices is somewhat smaller.
Figure 13 shows additional means for influencing the flow resistance in the tubes of Figure 12. Above the extremity 6" of the tubes 22 auxiliary parts are provided in the shape of tube sections 16p with the same or a different diameter as the tubes 22, or a plane plate 16q or a closed or open conical part 16r, a passage 17" remaining free between the tube and the auxiliary part in question. It will be clear that still other shapes of these auxiliary parts are possible, and that the tubes 22 can also be arranged between the duct 24 and the inlet chamber 2.
In Figures 14 and 15 a different solution of the present problem is shown. In Figure 14 the supply duct 3 opens in the buffer vessel 12, which is connected to the inlet chamber 2 by means of a pump 23, e.g., as shown, a screw pump. The outlet of this pump is higher than the highest :Level in the vessel 12. At a given driving speed the pump has a fixed maximum yield indepen-dent of the level in the vessel 12. If the supply becomes ex-cessive, the level in the vessel 12 will rise, but the quantity of liquid transferred towards the vessel 1 remains unchanged, which quantity is adapted to the capacity of the separator 4 so that the latter will never be overloaded. If necessary the ves-sel 12 can be connected to a larger buffer vessel by means of an emergency overflow weir.
In case of Figure 15 the pump 23 is arranged at the outlet - end instead of at the inlet end, and the fixed pump yield is, again, adapted to the normal liquid supply rate. When the supply rate increases, the discharge rate remains unchanged, so that the level in the vessel 1 will rise but the flow rate through the separator 1 is determined by the flow through the pump 23 so that 5~3 such a level rise has no influence on the separation effect.
When an emergency overflow weir is present, the excess liquid will be discharged over that weir as soon as the level over that weir is exceeded.
In Figures 16 ... 18 a practical example of a device ac-cording to Figure 5 is shown. The normal separator 4 is, in this case, a plate separator, which, in the first place, serves to separate flo~ating components. The tube 3 is branched from the supply channel 24, and opens in that part of the inlet chamber 2 which is situated above the separator 4, and is, there, provided with one or more injection nozzles 25. A baffle 26 separates the chamber 2 from the outlet chamber 5 which connects via an over-flow weir 6 with the discharge duct 7. A suction duct 27 con-nects with the channel 7 and is provided with a compression pump 28 which can suck in air, and the pressure side of this pump is connected by means of a pressure reduction valve 29 to the in-jection part of the duct 3. In this manner a pressurized mixture of purified liquid and air can be injected together with the li-quid to be purified, the air forming, after decompression in the valve 29, air bubbles which will entrain above the nozzle 25 eas-, ily separable particles towards the liquid surface. Subsequent-ly the liquid flows through the separator 4 in which the remain-ing separable particles are separated. The purified liquid flows, then, towards the channel 7, and a scraper 30 removes the float-ing components towards a collecting basin 31.
A basin 12 is connected to the channel 24, and a number of plate separators 32 is arranged therein. The separators 32 are designed for purifying large quantities of liquid in which ; the purification can be less critical than in the case of the normal liquid supply, and is just sufficient for preventing ser-ious pollution. The outlet chambers 33 of these separators are connected to the channel 7 by means of an overflow weir 34. This , ~

'''' ~L~7~59~3 weir is so high that, at the normal liquid supply rate, the liquid level remains below the edge thereof, so that, then, the separa-tors 32 are inoperative. As soon as the supply rate considerably increases, the level will rise by the resistance in the duct 3 in such a manner that the liquid flows over the weir 34, and the separator 4 will be hardly overloaded. For the rest the weir 34-can also be provided at the inlet side of the separators 32.
The additional separators 32 can also be constructed as a common emergency outlet for several independent separators 4, and can also be used with the other embodiments according to the preceding Figures.
It will be clear that, when the normal separator 4 accord-ing to any one of the preceding Figllres consists of a series con-nection of two or more separators, the means for limiting the li-` quid flow through these separators can also be arranged between - two separators of such a series connection instead of at the inlet or outlet end. Although, in the preceding examples, always flo-tating components have been mentioned, it will be clear that the devices in question can also be used in the case of sedimentating components. Since, then, possibly occuring substantial level fluctuations at the outlet for the separated components are not objectionable, the embodiments in which such fluctuations may occur are particularly suitable for the purpose.
Since the probability of a very excessive supply flow is, in general, very small, a number of purification devices can be connected in common to one buffer vessel or another emergency outlet. By using the invention considerable space and cost sav-ings are possible, without detracting from the safety requirements.

Claims (17)

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

1. A purification device comprising an inlet for contaminated liquid, an outlet for purified liquid, separation means connected between the inlet and outlet for separating impurities from a liquid, flowing from the inlet to the outlet, flow-restricting means between the inlet and outlet in series with the separation means and operative to limit the liquid flow rate through the separation means to a flow rate which is substantially not greater than the maximum design flow rate for the separation means, and an emergency overflow weir associated with the liquid flow path on the upstream side of the flow-restricting means such that at temporary excessive supply rates several times in excess of the maximum design flow rate, the liquid level upstream of the flow-restricting means will rise to such a level that the excess liquid will be discharged via the emergency overflow weir, whereby such temporary excessive supply rates do not cause a correspondingly increased flow rate through the separation means.

2. A device according to claim 1, in which the flow-restricting means comprises a flow passage having such flow characteristics that, at flow rates exceeding said normally allowable maximum flow rate, the flow resistance increases considerably.

3. A device according to claim 2, in which the flow-restricting passage is defined by a second overflow weir, and an inverted weir above said second overflow weir, the lower edge of the inverted weir extending substantially parallel to the upper edge of the second overflow weir and being situated at a height which substantially corresponds to the level of the emergency overflow weir.

4. A device according to claim 3, wherein the second and inverted weirs are formed by a baffle provided with apertures, the lower and upper edges thereof corresponding to, respectively, the overflow edge of the second weir and the lower edge of the inverted weir.

5. A device according to claim 3, wherein the inverted weir is movable relative to the second overflow weir, in order to allow the size of the passage to be adjusted.

6. A device according to any one of claims 3 to 5, wherein at least the edge part of one of the second and inverted weirs has a greater thickness in the flow direction, than that of the other.

7. A device according to claim 3, wherein the edge part of at least one of the second and inverted weirs is toothed or corrugated.

8. A device according to claim 2, wherein the flow passage comprises one or more tubes.

9. A device according to claim 2, wherein the flow passage comprises one or more substantially L-shaped tubes each having a substantially vertical leg with a horizontal upper edge.

10. A device according to claim 9, wherein the substantially vertical leg is situated at the outflow side of the or each tube.

11. A device according to claim 9, wherein the substantially vertical leg is situated at the inlet side of the or each tube.

12. A device according to claim 9, wherein a tubular, conical or plate-shaped part is arranged above the upper opening of the or each tube and defines together with the upper edge thereof a passage.

13. A device according to claim 12, wherein the said part is movable with respect to the associated tube.

14. A device according to claim 1, wherein the flow-restricting means comprises a pump arranged to pump liquid only in the direction of flow from the inlet to the outlet, the pump output being independent of the liquid level upstream of the pump and corresponding to a required flow rate in the separation means.

15. A device according to claim 1, comprising a buffer vessel for storing excess liquid supplied whereby the stored excess liquid can be fed through the separation means for treatment, after resumption of normal supply conditions.

16. A device according to claim 1, comprising one or more additional separation means connected in parallel with the first-mentioned separation means to receive the excess liquid flow, the capacity of the additional separation means being sufficient to enable the additional separation means to treat the anticipated excess flow.

17. A plurality of devices according to claim 1, wherein there is a common emergency overflow weir for each of the devices.