Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates to a method for per-
forming biological waste water purification according to the
activated sludge method, comprising feeding waste water into
at least one oxygen addition stage from which a liquid flow
with relatively low sludge concentration is led into a
sludge separation unit, separated sludge from the latter
unit being returned to the oxygen addition stage,
Systems for performing biological waste water
purification according to the activated sludge method may
be designed in a variety of ways. However, all these sys-
tems are provided with one or more stages for the addition
of oxygen to the inflowing water in the presence of acti-
vated sludge, which thereby grows by utilizing the nutrition
present in the waste water, aerobic conditions prevailing,
i.e., with an excess of oxygen dissolved in the water. The
systems also comprise a sludge separation unit into which
the water to which oxygen has been added is led for removing
accompanying sludge. The water from which sludge has been
removed may be further purified (e.g., chemically purified -;
20 according to any precipitating method) and discharged into ;~
a receiver. Part of the separated sludge, corresponding to
the continuous growth in the oxygen addition stage, is re-
moved from the system and the remainder is returned to the
oxygen addition stage.
Considering the rigid regulations for the quality
of the water discharged into receivers from waste water
plants, it is as necessary to achieve a high degree of re-
duction of the BOD of the waste water as to remove sludge
to a high degree in the sludge separation unit to make the
outflowing water sufficiently free from suspended matter.
This means that the sludge separation unit, which in most
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cases comprises one or more sedimentation basins, must cover
a very large area in view of the low specific yield obtain-
able due to the voluminous character of the sludge and its ;
low sedimentation rate. Also, it is not possible to achieve -
a high dry substance concentration in the sedimented sludge.
Thus, the sludge which has been returned to the oxygen addi-
tion stage ~i.e., the return sludge wherein the dry substance
content is not above 0.8%) consists of nearly 100% water
that is already purified and which dilutes the waste water
fed to the oxygen addition stage for purifying. This will
cause the concentration of activated sludge in the oxygen
addition stage to be low, with limited yield as a result.
It is obvious that both the oxygen addition stage
and the sludge separation unit require much space according
to methods hitherto used.
The present invention has for an object to avoid
the drawbacks inherent in prior methods of the above-
mentioned type for biological purification of waste water, ;
and to provide a method which enables a substantial increase
in the capacity of present systems and a substantial reduc-
tion of space demand for new systems for such purification.
According to the invention, a method of the above-
mentioned type is characterized in that the liquid flow from
the oxygen addition stage is fed to a sludge-thickening de-
vice within the sludge separation unit, where the liquidflow is divided into two parts. The first of these two
parts is a sludge phase with a relatively high dry substance
concentration (i.e., at least 2~), at least some of which
phase is returned to the oxygen addition stage. The second
of these two parts is a liquid phase that is substantially
free from sludge, which phase is fed to a clarification
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stage where the sludge residue is removed from the liquid
phase.
According to one preferred embodiment of the in-
vention, the sludge-thickening device consists of a cen-
trifugal separator, such as a centrifuge which provides forintermittent periodic or automatic sludge-sensing discharge.
In a further embodiment which has proved especial-
ly suitable for the performance of the biological waste
water purification, the sludge thickening device comprises
.
a coarse screen and a fine screen through which the liquid
flow from the oxygen addition stage passes successively be- ~
fore it is fed into the centrifugal separator. The fine ;~ -
screen may be of a design disclosed in Swedish Patent
Specification No. 357,992, the centrifugal separator being ~i-
provided with permanently open nozzles, circumferentially
arranged in the rotor, for continuous discharge of sludge. `
A fine screen of the type mentioned is marketed under the
trade mark MICROSORTER and is designed for the fractionating
of paper pulp. Surprisingly, it solves the problem of pre-
separating a fraction of the activated sludge down to a
particle size which enables a centrifugal separator with t ~ '~
permanently open circumferential nozzles in the rotor to be
utilized for concentrating the activated sludge with the
smallest particle size. There is a risk of such centrifugal
separators having their nozzles clogged by coarser particles,
which means a serious operational disturbance.
In a particularly suitable embodiment of the
method according to the invention, the mesh size of the fine ~
screen is within the range 0.1-0.8 mm, and the opening width -
of the circumferential nozzles of the centrifugal separator
is within the range 0.9-1.5 mm. -
.
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In an alternative embodiment of the method accord-
ing to the invention, the sludge-thickening device consists ~-~
of a flotation system.
The clarification stage may consist of one or more
sedimentation basins, which can be designed with a much
smaller bottom area as compared with a traditional system
with the same amount of waste water fed for biological puri- ~ -
fication.
The liquid flow emerging from the sludge-thickening
device may also be clarified by flotation.
With the method according to the invention, sludge
accompanying the li~uid flow from the oxygen addition stage
to the sludge separation unit can be separated and returned
to the oxygen addition stage with substantially higher dry
substance concentration than has hitherto been possible.
Only minor amounts of sludge will accompany the liquid flow
from the centrifugal separator to the clarification stage
for final clarification, so that the latter task requires
substantially smaller resources for a given waste water
load. As the return sludge, compared with the sludge ob-
tained from prior methods for active sludge treatment of
waste water, is accompanied by a substantially smaller
amount of purified water, the yield of the oxygen addition
stage is increased due to reduced dilution of waste water
fed into the system.
The invention will now be described more in de-
tail, reference being made to the attached drawings in which
Fig. 1 is a schematic flowsheet of the method according to
the invention, and Fig. 2 illustrate$ schematically an em-
bodiment of the sludge-thickening device shown in Fig. 1.
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In Fig. 1, reference numeral l designates an oxy-
gen addition stage, 2 a sludge separation unit, 3 a sludge-
thickening device which is part of the unit 2, and 4 a
clarification stage. An inlet line 5 is provided for feed
5 of waste water to the system, and a line 6 is provided for r
the liquid flow from oxygen addition stage l to sludge-
thickening device 3. A line 7 connects the latter to the ;
clarification stage 4, from which a line 8 discharges the
purified water from the system. A line 9 connects the
10 sludge-thickening device or zone 3 to oxygen addition stage
1 for returning of separated sludge. Excess sludge may be
discharged from the system through a bifurcated line 10, and
a line 11 from the clarification stage 4 has the same pur- ~;
pose.
In oxygen addition stage 1, a biomass is main- ;
tained (i.e., activated sludge suspended in water). Oxygen
is fed continuously to this stage, generally in the form of
air, as shown in the drawing by an arrow 12. Waste water
is generally fed continuously to oxygen addition stage 1,
and the activated sludge grows by utilizing the nutritives
available in the waste water. A liquid flow, generally con-
tinuous and containing some sludge, is led through line 6 to
sludge-thickening device 3 where the flow is divided into
two parts, of which the first is a liquid flow having a
25 small residue of sludge and which passes to clarification -
stage 4. The second part is a thickened sludge flow, of
which a portion is returned through line 9 to the oxygen
addition stage 1, while excess sludge is discharged through
bifurcation line 10. Separated sludge is discharged through
the line ll from clarification stage 4.
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Referring to Fig. 2, the sludge-thickening device
3 comprises a pre-screen 13, a fine screen 14 of the type
disclosed in Swedish Patent Specification No. 357,992, and - -
a centrifugal separator 15 having a rotor (locus of cen-
trifugal force) provided with circumferentially arranged,
permanently open nozzles for sludge discharge. Line 6 is
arranged to convey the liquid flow, emerging with a rela-
tively low sludge concentration, from the oxygen addition
stage 1 to pre-screen 13. A line 16 connects pre-screen 13
with fine screen 14, and a line 17 connects fine screen 14
to centrifugal separator 15, which in turn is connected via
line 7 to clarification stage 4. Lines 18, 19 and 20 from
pre-screen 13, fine screen 14 and centrifugal separator 15
are provided to convey sludge phase from the sludge
thickener 3 to line 9 for returning sludge to the oxygen
addition stage and discharging excess sludge through bifur-
cation line 10, -
Examples
For further elucidation of the present invention,
data will now be given, by way of example, from a system
for the performance of a conventional biological purifica- ;~
tion method for waste water from a fermentation process, and
corresponding data from two different systems constituting
two embodiments for the performance of the method according
to the invention. The first one (I~ utilizes a centrifugal
separator with intermittent sludge discharge combined with
a sedimentation basin, while the other one (II) operates
with flotation for the sludge thickening and also for the
clarification.
Waste water
BOD7 = 10,000 mg/l ~ -
Flow: 30 m3/h corresponding to 300 kg BOD/h or -
7200 BOD/24 h (100 p.e.)
System for a conventional wast~
water purification according
to the activated sludge method
Aeration basin: 2400 m3 ~ -
Concentration of activated sludge: 3000 mg/l
Load: 3 kg BOD/m3.24 h.
Sludge separation unit i
Sedimentation basin, 4m deep, 60 m2 bottom area
Sedimented sludge for returning, dry substance
conc. 0.4
Residual sludge concentration in discharged puri~
fied water: ca 30 mg/l.
System for performing the method
according to invention embodiment I
Oxygen addition stage
Aeration basin: 500 m3 ~ -~
Concentration of activated sludge: 15,000 mg/l
Load: 15 kg BOD/m3.24 h.
Sludge separation unit ;`
Sludge thickening device ~ -
Centrifugal separator with intermittent sludge
discharge.
Centrifugated sludge for returning, dry substance
concentration: ca 6% -
Clarification stage
Sedimentation basin, 4m deep, 30 m2 bottom area. ~ !
Residual sludge concentration in discharged puri-
fied water: 30 mg/l.
. .
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System for performing the method
according to invention embodiment II
Oxygen addition~st_ge
Aeration basin: 750 m3
Concentration of activated sludge: 10,000 mg/l
Load: 10 kg BOD/m3.24 h.
Sludge separation unit
Sludge thickening device
Flotation plant (thickening operation) I ;
3 m deep, 30 m2 bottom area
Flotated sludge for returning, dry substance con- ;`
centration: 2%
Clarification stage
Flotation plant (clarification operation) II
3 m deep, 6 m2 bottom area
Residual sludge in discharged, purified water:
30 mg/l.
