Note: Descriptions are shown in the official language in which they were submitted.
1063263
In the operation of a biological effluent treatment plant,
the ~lctivated-~ludge bnsin has to be ~upplied with the quantlty
of oxygen required for the metabolism of the mlcroorganisms and
a certain concentration o~` oxygen mu~t be ~aintalned to permlt
nero~ic bacterial culture. The oxygen requir~d for this purpose
is delivered to the litluid from the gas phase. The rate at which
o~ygen is supplied to the liquid from the ga8 i8 ~ on the one
h~nd, g~overned by the size of the interface between the gaR and
liquid phase and by the turbulence prevailing there, while on
the other hand it i~ directly proportional to the concentration
difference ~ c between the oxygen saturation concentration
and the concentration of 2 dissolved in the liquid.
The phase interface is increased inter alia by using sur-
face aerators or by introducing a gas, generally air, into
the liquid and dispersing it in the liquid, optionally by
means of stirrers or liquid ~ets (ejectors; different kind of
nozzles). In the first case, the pressure in the gas-treatment
zone is normal, while in the second case basins from 3 to 7
meters deep are generally used, so that a slight hydrostatic
2C pressure prevails in the gas dispersion zone.
In general, the described method of gas treatment is
carried out in open basins, with the result that the waste
air escaping contains, in some cases in conslderable quantities,
readily volatile foul-smelling constituents which pollute the
surroundings. Since atmospheric oxygen is only very incompletely
utilized in the described aeration processes, the quantity
of air required to satisfy the oxygen demand and, hence, the
quantity of waQte air as well i8 very considerable so that
deodorization by heat treatment is very expensive.
The supply of oxygen to the liquid can be considerably
accelerated by increasing the concentration difference ~ c .
Le A 16 290 - 1 -
1063Z63
In practice, this increase in the concentration difference ~ c
can only be obtained by increasing the 02-partial pre~sure in
the gas. This can be done either by raising the system pressure
or by changing from air to oxygen or an oxygen-enriched gas.
However, the use of pure oxygen or of an oxygen-enriched gas
requires, ~or economic reasons, that the gas stream be largely
deprived of oxygen as it passes through the installation. When
working under normal pressure, this re~ult may be obtained by
repeatedly introducing the gas stream into the li~uid and/or
by passing the gas through successive absorption stages (a so-
called "multistage activated-sludge basin"). The production
of a narrow residence-time distribution of the gas throughput
by cascading the treatment zones is the sub~ect of the process
according to German Auslegeschrift No. 2,032,535.
The system pressure may be raised to increase the concen-
tration difference ~ c by designing the activated sludge
stage in the form of a shaft. However, this requires intensive
circulation of the liquid in order to ensure that, on the one
hand, the low-oxygen layers enter the intensive oxygen transport
zone and, on the other hand, that the liquid is never saturated
with oxygen in that zone in order to prevent outgassing and,
hence, flotation of the activated sludge in areas where a lower
hydrostatic pressure prevails. Circulation of the liquid may be
obtained by dividing the shaft with partitions into an ascending
path and a descending path (German Offenlegungsschrift No.
2,423,085).
The ob~ect of the present invention is to develop a single-
stage process for the treatment of effluent in activated-sludge
basins, in which the advantage of a high hydrostatic pressure
is combined with the advantage of the absence of backmixing
Le A 16 290 - 2 -
1063263
in regard to the gas phase so that the oxygen present in the gas is utilized
to a high degree during only a single passage through the effluent containing
activated sludge. In the single-stage activated-sludge basin, the oxygen-
containing gas is passed into only a single absorption stage where it is
largely deprived of oxygen. It has been found that, in the case of large
treatment plants, it is not economical to carry out gas treatment outside the
activated sludge basins, for example in a deep ~haft, because in that case
the supply of dissolved oxygen requires a liquid circuit that is almost
imposæible to establish. Thus, the input of oxygen must of necessity take
place within the activated sludge stage. However, cost factors preclude
designing the entire activated sludge stage in the form of a deep shaft.
Accordingly, the present invention rel ates to an apparatus for
the biological treatment of effluent or for fermentation processes in a basin
having a height of about 10 to 40 m, a height to diameter ratio between about
5:1 and 0.5:1, comprising a plurality of gas-inlets with an individual cross-
sectional areas of from about 0.1 to 0.5 m2 arranged substantially equidis-
tantly from another at a height up to about 1 m above the base of the basin
and separated from one another by a distance of about 2 to 10 m, as measured
from the middle point of each gas-inlet, wherein each gas-inlet is loaded
with 100 to 300 effective m3 of gas/m2 of cross-sectional area per hour of
gas, containing at least 50% by volume of oxygen, thereby obtaining the
formation of chimneys with intensive ascending and descending circulation of
liquid.
The apparatus of the invention is useful in a process for the
continuous introduction of air or oxygen-containing gases into an effluent
containing activated sludge, the oxygen-containing gas largely being consumed
by the effluent containing activated sludge in a single absorption stage,
distinguished by the fact that the oxygen-containing gas is introduced into
the effluent containing activated sludge, which is under its own hydrostatic
3Q presaure, in zones where a hydrostatic pressure of at least about 0.9 bar
prevails, the gas pressure of the gas introduced being about 0.01 to 0.5 bar
above the hydrostatic pressure prevailing at the gas inlet, each gas inlet
1063Z63
having a gas-swept cross-sectional area of at least about 0.01 m2 which is
loaded by at least about 100 effective cubic meters of gas per square meter
of cross-sectional area per hour.
The gas inlets are conveniently arranged substantially equidistantly
from one another in the base or just above the base, preferably up to about
0.5 to l meter above the base.
It has been found that, to absorb oæygen to a high degree (80 to
90 %) in a single stage, it is essential to ensure, through intenslve
circulation of the liquid and through rapid ascent of the swarm of gas
bubbles, that no appreciable oxygen concentration gradients occur in the
activated-sludge basin, and that the liquid entrained by the swarm of gas
bubbles on the air-lift pump principle is not saturated with oxygen to such
an eætent that degassing can occur in zones where a relatively low hydro-
static pressure prevails. The measures taken in accordance with the invention
provide for the treatment of effluent containing activated sludge in a single
stage during which the oxygen is consumed.
It has surprisingly been found that, in cases where air is used, a
waste gas containing about 8 % by volume of oxygen can be obtained. By
virtue of the effective utilization of oxygen in the activated sludge basin,
the input of air and, hence, the quantity of waste gas as well can be reduced
to one half to one third of the level normally encountered in conventional
processes using air. This factor also enables the fuel demand (for example
fuel oil) required for thermally deodorizing the waste air at around 1000C
to be reduced to between one half and one third of the quantity which would
normally be necessary for combustion.
In accordance with one embodiment of the invention, oxygen-contain-
ing gases containing more than S0 % by volume of oxygen, including oxygen
per se, are only introduced through a certain number of gas inlets, preferably
through more than about 5 gas inlets. These gas inlets may be arranged
eith~r in the bottom of the activated sludge basin or just above it, prefer-
ably up to about 1 meter above the bottom of the activated-sludge basin. The
gas enters the effluent containing activated sludge through these gas inlets,
1~63Z63
preferably vertically upwards, in the form of a swarm of gas bubbles. This
results in the formation of "chimneys" with intensive ascending and descend-
ing circulation of liquid, the effect of which does not ha ve to be supported
by any fittings.
sased on the cross-sectional area of the activated sludge basin,
one gas inlet is provided for every 0.2 to 15 m2 of cross-sectional area.
These gas inlets for the oxygen-containing gas are preferably circular or
annular in shape. However, they may also assume other forms, for example
a quadratic or equilateral triangular form. The oxygen-containing gas is
introduced through these gas inlets into the activated sludge basin in
throughputs of about 100 to 300 effective cubic meters per unit area (m2)
and per unit of time (hours) (cross-sectional load of a gas inlet).
The gas inlets may be designed in known manner in the form of
nozzles, ejectors, perforated plates, etc., which enable the oxygen-contain-
ing gas introduced to be effectively dispersed into fine gas bubbles.
Eiectors are preferred. All the gas inlets are preferably situated in zo~es
of equal hydrostatic pressure and are spaced at a distance of about 2 to 10
meters (as measured from the middle point of a gas inlet). In addition, the
gas inlets should be arranged as uniformly as possible over the entire cross-
sectional area of the activated sludge basin.
In accordance with ~ertain preferred combination of conditions, in
`~ one set of conditions of which~Rxample ~ hereinbelow is illustrative, each
gas inlet has a gas-swept cross-sectional area of about 0.1 to 0.5 m2, pre-
ferably about 0.1 to 0.3 m , which is loaded by about 100 to 300, preferably
about 150 to 250, effective cubic meters of gas per square meter of cross-
sectional area per hour, the gas inlets being spaced from one another by about
2 to 10 meters, preferably about 3 to 6 meters. One gas inlet is provided
for about every 8 to 12 m2 of cross-sectional area. The gas introduced con-
tains more than 50 % by volume of oxygen.
The finene~s of the gas bubbles initially produced in the gas-
treatment zone is governed by the energy applied for dispersing the gas
throughput into gas bubbles. In the case of eiectors and single-orifice
1063Z63
plates (Chemie-Ingenieur-Technik) 43 (1971) 6, 329-335), this energy is
generally applied by a certain liquid througllput through the gas dispersion
unit. In one special embodiment, therefore, a liquid acting as propulsion
liquid is introduced through the gas inlet together with the oxygen-contain-
ing gas. This liquid throughput amounts to between about 10 and 30 % by
volume of the gas throughput under normal conditions. It is particularly
advantageous to use the effluent containing activated sludge as the propelling
li~uid.
While the present invention will be set forth with special regard
to biological treatment of effluent it easily can be performed similarly in
other biological processes, where oxygen supply is necessary, e.g. fermentation
processes.
The invention is further described in the accompanying schematic
drawing of an apparatus for carrying out the treatment of effluent.
In the drawing, 10 is a cylindrical activated sludge basin provided
with numerous gas e~ectors 12 to which oxygen containing gas is supplied
via 14. Waste gas escapes at 16, passing to a burner (not shown). Effluent
overflows at 18 and through a pump 20 part of this effluent is delivered to
the ejectors 12 as the propulsion liquid.
1063Z63
The invention is further described in the following illustrative
example relating to a process carried out with an apparatus as described.
Example 1 (using technical pure 2 as aerating gas):
An activated sludge basin with a liquid volume of 6200 m3 is charged
with 800 m3/h of effluent. me oxygen demand of this
1063263
installation amounts to 45,6 tons/day. The oxygen concentration
of the liquid amou~ts to 6 mg/l and the water temperature to
30C. The liquid level in the activated sludge basin amounts
to 20 meters, the cylindrical ac~ivated sludge basin having a
diameter of also 20 meters (cross-sectional area of the activ-
ated sludge basin 314 m2). According to the invention, oxygen
is introduced into the liquid containing activated sludge at
only 30 inlets (one inlet to about every 10 m2), these gas in-
lets being arranged equidistantly (imagined as being at the
corners of equilateral triangles). Ejectors (cross-sectional
area: 0.1 m2) arranged about 0.5 meter above the bottom of the
basin are used as the gas inlets. The throughput per ejector
amounts to 19.4 effective cubic meters /h of gas and 6 m3/h
of effluent containing activated sludge. This effluent containing
activated sludge is taken from the activated sludge basin
and delivered to the e~ectors as the propulsion liquid through
delivery pump(s). The excess pressure of the gas introduced
at the gas inlet amounts to about 0.01 bar above the hydrostatic
pressureO The degree of utilization amounts to 80 % of the quan-
tity of oxygen introduced.
It will be appreciated that the instant specification andexamples are set forth by way of illustration and not limitation,
and that various modifications and changes may be made without
departing from the spirit and scope of the present invention.
Le A 16 290
