Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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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
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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
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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;
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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
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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
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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
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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
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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-
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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
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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
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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
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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.
