Note: Descriptions are shown in the official language in which they were submitted.
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FILE, ~I THIS Ar~E~dD~
WO 97/08104 TE~T~RANSLATI~N PCT/AT96/00144
Process for purifying sewage
The invention relates to a process for purifying sewage by means
of activated sludge, in which the sewage to be purified is introduced
firstly into an aeratable activation vessel and then into a
sedimentation vessel in which separation of sludge and clear water
occurs, whereupon sludge is returned into the activation vessel and
clear'water is drawn off.
In processes of that kind (see AT-B-395 413) organic carbon
compounds are broken down by bacteria in a biological reactor - in the
activation vessel - with a supply of oxygen, and converted into
activated sludge. Nitrification, denitrification and the removal of
phosphorus additionally take place. In the subsequent post-settlement
vessel the activated sludge undergoes sedimentation and the clear water
can be drawn off. To maintain the procedure the sedimented activated
sludge must be pumped back into the activation vessel again. Between 5
and 10% of the sludge is removed from the system per day as excess
sludge.
A disadvantage with the known process is that the post-settlement
vessel only serves for the separation of sludge and clear water whereas
the biochemical procedures which at best take place therein are without
practical significance. On the other hand a process is certainly known
in which further and in general all biochemical and physical events
which are necessary for sewage purification also take place in a single
vessel, in addition to sedimentation. For that purpose the sludge is
firstly stirred up in the vessel. Operation can be implemented with or
without the introduction of oxygen in that stirring phase. Depending on
the presence of free or bound oxygen nitrification (aerobic medium) or
denitrification (anoxic medium) is effected. The sludge is then allowed
to undergo sedimentation, and it is only after a certain time has
expired that the operation of drawing off clear water is begun. In that
period which is referred to as the pre-settlement phase on the one hand
the sludge undergoes settlement, while on the other hand
denitrification occurs. The sludge which settles forms a filter body
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which also filters small activated sludge flakes or flocks out of the
clear water. The sludge also sinks further to the bottom during the
subsequent draw-off phase which serves to draw off clear water, and it
is capable of involving denitrification. In the total absence of oxygen
(anaerobic medium), phosphorus can also be biologically eliminated.
Substantial phosphorus elimination however requires a suitable
precipitation agent which moreover also has positive effects on the
settlement properties of the sludge.
The invention is based on the consideration that the post-
settlèment vessel which in the activation process only serves for theseparation of sludge and clear water can take over a part or all of the
functions which in the single-vessel process take place in the single
treatment vessel.
It is therefore provided in accordance with the invention that an
operating cycle takes place in the sedimentation vessel several times
during the day, in which cycle the sludge is mixed again with the water
(stirring phase), and that the draw-off phase of the operating cycle is
separated from the stirring phase by an interval (pre-settlement
phase).
If the content of the sedimentation vessel is aerated during the
stirring phase, the single-vessel process which takes place in the
sedimentation vessel thus promotes the primary functions of the
activation vessel, namely breaking down organic carbon compounds and
denitrification. In that situation the devices which serve for
introduction of nitrogen advantageously simultaneously produce stirring
in accordance with the invention of the sludge in the sedimentation
vessel.
Further details of the invention are described hereinafter with
reference to the drawing in which Figure 1 is a diagrammatic plan view
of an apparatus according to the invention, indicating three different
functional phases, Figure 2 is a cross-sectional view on an enlarged
scale through an apparatus as shown in Figure 1, Figure 3 is a cross-
sectional view through a detail on an enlarged scale from Figure 2,
Figure 5 is a cross-sectional view through a second embodiment, and
Figure 4 is a plan view of a further embodiment.
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The spatially adjacent arrangement of an activation vessel 1 and
a sedimentation vessel 2 is common to all illustrated embodiments. The
separating wall 17 between those two vessels is respectively provided
with a lower opening 6 which is continuous over the entire width of
the vessels so that the vessels 1, 2 behave as communicating vessels.
Disposed in the activation vessel 1 is a respective fan or aerator 3
for the introduction of oxygen; the operation of swirling up the
sludge in the sedimentation container 2 can be effected by a propeller
4 which at the same time generates a flow which homogenises the
contènt of the vessels 1 and 2 so that the two vessels have
approximately identical dry substance contents and approximately equal
sludge ages.
In the apparatus shown in Figure 1 the propeller 4 generates a
circulatory flow around a guide wall 5 which stands up from the bottom
in the center of the vessel, that flow being maintained only in a
stirring phase which involves homogenisation of the container contents
and recycling of sludge back into the activation vessel 1. The
duration of the stirring phase can be greatly varied depending on
weather conditions, in which respect rainy weather indicates a
reduction (for example to ten minutes) and dry weather indicates an
extension (for example to seventy minutes).
As can be seen from Figure 1 the stirring phase, the R-phase, is
followed by an interval (pre-settlement phase or V-phase) in which the
sludge sinks in the sedimentation container 2 without clear water
already being drawn off. In order in particular in this phase to
prevent sludge settlement from being disturbed by the aerator 3 in the
activation vessel 1, a baffle wall 7 is desirably disposed in front of
the opening 6. The baffle wall 7 provides that air bubbles escape
upwardly in the intermediate space between the baffle wall 7 and the
separating wall 17 between the two vessels so that - apart at most
from the stirring phase - only bubble-free sludge can pass into the
sedimentation vessel 2. In the V-phase the level of sludge falls to
such an extent ( > 50cm) that clear water can be drawn off in the
subsequent draw-off phase (A-phase). A typical value for the duration
of the V-phase, with a total cycle duration of 200 minutes, is about
40 minutes.
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The discharge flow from an installation consisting of the vessels
l and 2 occurs only in the A-phase. The feed flow can be implemented
continuously in all three phases or also only in one or two phases.
The flow through an installation occurs in a free fall mode if the
feed and discharge flows occur only in the A-phase and are equal (Qin
= Qout). A plurality of installations can be operated in phase-
displaced relationship in such a way that there is always at least one
A-phase available. The settling installation discharge then
corresponds to the settling installation feed.
'Figure 3 shows a possible clear water draw-off situation. In the
A-phase, when the level 8 of sludge is already more than 50 cm below
the water level 9, clear water is drawn off by way of round discharge
flow openings lO of a diameter of about 8 cm, which are arranged at a
spacing of l.0 m and which are disposed about 30 cm below the water
level and which are closed off with flaps. The clear water then passes
into a pressure passage ll which can be closed off with an electrical
slider 12. The discharge flow from the sedimentation vessel 2 is
regulated by the slider 12 (open and closed position), downstream of
which is arranged a fixed overflow or spillway 13 for maintaining a
desired water level in the vessels. By introducing small amounts 14 of
clear water into the pressure passage, it is possible to avoid sludge
from penetrating into the draw-off system when the slider is closed.
To calculate the admissible draw-off amount from the
sedimentation vessel 2 (Qout) it is to be assumed that, throughout
the entire A-phase, the level of sludge is always to be at least 50 cm
below the water level. As an approximation and to be on the safe side,
it can be assumed that the amount of clear water formed in the A-phase
can also be drawn off. The V-phase then serves to produce the clear
water body of at least 50 cm in thickness, that is required for the
draw-off operation. The creation of clear water is then governed by
the speed of downward movement of the level of sludge VS which can be
assumed approximately constant. That gives as an admissible draw-off
amount QOut ~m3/hj = F[m2] x vs[m/h] with vs[m/h] x ISV~ml/9] x
TS[k9/m3] = 725 (see also Kayser, gwf, No 12, 1995).
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With a continuous feed flow the water level in the vessels rises
during the R-phase and the V-phase. In such a case it is possible to
operate with a pre-settlement phase which is divided into two, in
which case firstly the level of liquid in the vessels rises and then
it is kept constant by means of a fixed overflow. That procedure
provides that the draw-off amount is reduced in the subsequent A-phase
and the draw-off time can be reduced.
With a mixed water feed flow to an installation according to the
invention, it is possible to operate with a dry weather mode (DW) and
a ràiny weather mode (RW), in which case the A-phase is prolonged in
the case of rainy weather, at the expense of the R-phase, as a
percentage. The sedimentation vessel is then used more for the
biochemical procedures in the case of dry weather and more for the
hydraulic procedures in the case of rainy weather.
Several tasks have to be managed in the R-phase. The sludge which
is transported from the activation vessel 1 into the sedimentation
vessel in the V-phase and the A-phase has to be returned to the vessel
1 again. Sludge which has settled and thickened at the bottom of the
vessel 2 has to be swirled up again so that there is approximately
equal dry substance in the vessel. In addition any floating sludge or
scum which has possibly been formed must be incorporated into the
sludge body again. Finally it is advantageous if oxygen can also be
introduced into the sedimentation vessel. The four tasks referred to
above are to be solved in different ways depending on the respective
size of the settlement installations. In the case of large
installations for example it is possible to operate with the
circulatory principle, that is to say all vessels are operated as
circulatory vessels and the stirring mechanisms are so situated that
the flows shown in Figure 4 are produced. In that case guide walls 5
provide for a uniform flow at a speed of about 0.5 m/sec.
In the case of small settlement vessels aeration in the
activation vessel 1 and the mixing effect in the sedimentation vessel
can be effected with a single piece of equipment, as shown in Figure
5. A jet aerator 3 which is located in the activation vessel 1 and
which is rotatable about a vertical axis permits aeration and
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circulation in both vessels. In the R-phase the water jet which is
enriched with small air bubbles is directed on to the opening 16,
which is provided with a non-return flap, between the two vessels, so
that turbulence and circulation acts primarily in the sedimentation
vessel 2. Any floating sludge or scum which may be formed is passed
from the sedimentation vessel 2 into the activation vessel 1 through a
return flow 15 at the height of the water level, and in the activation
vessel 1 is incorporated into the sludge body again. In the other two
phases the water jet is directed away from the opening 16. The jet
aeration effect then only acts in the activation vessel 1. Instead of
turning the jet aeration effect away, the opening 16 can also be
closed with a slider.
A particular advantage of the invention is that homogenisation of
the content of the vessels 1 and 2 makes it possible to measure the
dry substance content in that vessel - the sedimentation vessel - from
which the excess sludge is drawn off. At the end of the V-phase,
before the beginning of the A-phase, the level of sludge must already
have moved down to such a depth (for example about 75 cm) that solids-
free clear water can be drawn off in the A-phase. In that time, a
floating excess sludge pump is briefly brought into operation (for
example automatic excess sludge draw-off for about between 1 and 2
minutes). If the level of sludge is still too high, excess sludge is
concentratedly drawn off, while it is already too low, the draw-off of
clear water predominates. The drawn-off sludge-clear water mixture
passes into a sludge reservoir where the sludge settles. The solids-
free cloudy water is passed into the activation vessel again by way of
a return. The sludge which thickens in the sludge reservoir is to be
drawn off from time to time and removed.
Another possible way of automatically drawing off excess sludge
involves measuring the level of sludge at the end of the V-phase with
a sludge level measuring apparatus and effecting the operation of
drawing off excess sludge, in dependence on the position of the level
of sludge. That method can be effected for example by means of a
floating density measuring apparatus and a sludge conduit which is
arranged at the bottom of the vessel and which is provided with an
electrical slider. If for example at the end of the V-phase the level
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of sludge is below 75 cm below the water level, no sludge is drawn
off, while if it is thereabove, then sludge is drawn off. That method
- primarily suitable for larger sewage treatment installations - has
the advantage that only sludge which has already thickened is drawn
off and there is no need for a sludge reservoir for the separation of
sludge and clear water.
It is possible to connect upstream of an installation according
to the invention a sewage or process water reservoir or storage means,
a screening or sieving installation, a sand and grease or fat
sepa'rator, a preliminary sedimentation means, an anaerobic vessel or
only individual ones of those parts of an installation. A filter for
more extensive purification can be connected downstream of the
installation.
In order to increase biological phosphorus elimination, it is
possible to provide a sufficiently long anaerobic medium in the
activation vessel during the A-phase. The content of the activation
vessel is only stirred, and no oxygen is introduced, in that period.
A particular situation provides that the activation vessel is
neither stirred nor aerated during the A-phase or during the V-phase
and the A-phase. The activation vessel then acts as a sewage reservoir
or storage unit. It is only when the level of sludge has reached the
communicating opening, arranged at the vessel bottom, between the two
vessels, that raw sewage can pass into the sedimentation vessel. That
prevents raw sewage from penetrating through into the discharge.
The advantage of the process according to the invention lies in
the high level of purification efficiency, the very small volume
required for vessels and the machine equipment, which is reduced to a
minimum, thus affording a high level of economy. The aeration devices
are only required in the activation vessel and are practically always
in operation, under oxygen control. The operation of swirling up the
sedimented sludge and circulating the sludge which has been
transferred into the sedimentation vessel is effected by simple
stirring mechanisms. It is also possible to operate with very high dry
substance (between 5.0 and 8.0 g/l) in the system. Finally the
sedimentation vessel is used in the optimum fashion as the biochemical
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"
procedures can be preferred with a low level of hydraulic loading (dry
weather DW) and the hydraulic processes can be preferred with a high
level of hydraulic loading (rainy weather RW).
