Note: Descriptions are shown in the official language in which they were submitted.
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METHOD AND APPARATUS FOR BIOLOGICAL REMOVAL OF NITROGEN
COMPOUNDS FROM WATER
The present invention relates to a method of removing
nitrogen and its compounds from water by means of
a biological process in an activated sludge suspension, as
well to an apparatus for realizing said method.
BACKGROUND OF THE INVENTION
At biological removal of nitrogen compounds from water
a denitrification process is used for the removal of
nitrates and nitrites. The nitrates and the nitrites serve
as a source of oxygen for microorganisms and they are
reduced by its metabolism down to a gaseous nitrogen. At the
same time, the microorganisms consume a suitable
biodegradable substrate and oxidize the same using the
received oxygen. The microorganisms themselves may serve as
a substrate for the denitrification in the process of a so
called endogenic respiration, e.g. in case the supplying of
the biodegradable substrate is irregular.
To make the denitrification run, anoxide conditions are
necessary, i.e. there must be a lack of molecular oxygen in
the medium. In case there is any molecular oxygen available
in the medium, the microorganisms orientate on the
consumption of the same and they change to the
denitrification only when there is no more oxygen. The
anoxide conditions are usually reached by preventing any
access of oxygen when the concentration of microorganisms is
sufficient, and said microorganisms themselves create
anoxide conditions by consuming the present oxygen.
Generally, the biological processes may be employed
either with microorganisms attached to a solid basis or with
microorganisms being in an activated sludge suspension
("attached - growth process" and "suspended - growth
process" - see Metcalf and Eddy, Wastewater Engineering,
McGraw-Hill, New York, 1985?. On grounds of several
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practical reasons, processes with microorganisms being in an
activated sludge suspension are used more frequently.
During the denitrification process in an activated
sludge suspension the suspension is led away along with the
purified water, from which it must be separated and returned
back into the denitrification process. However, the
separation of the suspension is complicated by the bubbles
of separated nitrogen, which stick at the same. Removal of
said bubbles is realized by an intense mechanical movement
in the suspension, which is done usually by aeration, in the
known methods.
When purifying waste water the aeration serves mostly
the purpose of an aerobic activating water purification and
at the same time to create conditions for a nitrification of
ammonia and organic nitrogen, i.e. to transform any
remaining nitrogenous contamination to nitrates. The result
of connection of the denitrification and the nitrification
is the biological removal of all nitrogen compounds from
water. The activated sludge, separated from the purified
water flowing away, is usually also returned to the
denitrif ication.
However, said denitrification has a number of
disadvantages. For the biological processes in an activated
sludge it is necessary that the activated sludge is in the
state of suspension. During the aerobic activation the
suspension of the sludge is ensured automatically due to the
aeration. During the denitrification the suspension of the
sludge is secured usually by mechanical mixing. It is
disadvantageous that an agitator and a source of movement
- e.g. an electric motor with a gearing - is necessary for
the mechanical mixing. Said mechanical parts increase the
investment costs and in case of any defect the repair is
complicated due to their arrangment in the denitrification
zone. Another disadvantage of the above denitrification is
represented by a limitation for the concentration of the
activated sludge in the denitrif ication only to the values
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given by mixing of the components entering the
denitrification, which limits the rate of the
denitrification related to a volume unit of the
denitrification zone.
SIJN~IARY OF THE INVENTION
The above disadvantages are eliminated by the method
according to the invention, the essence of which is that the
nitrogen in the form of nitrates and nitrites is removed by
means of a denitrification proceeding in a fluidized layer
od sludge blanket, the layer being formed by a suspension of
biological sludge and being kept in a fluidized condition.
It is also essential that the activated biological
sludge is led into the denitrification zone by streaming
from below upwards, wherein the streaming rate in the
direction from below upwards is slowed down at least in the
lower part of the denitrification zone.
For the employment of the method according to the
invention it is significant that the superfluous activated
sludge in the forth of flocks is led away from the upper part
of the denitrification zone and then, it is subjected to an
aeration, separated form water and returned into the lower
part of the denitrification zone.
It is also essential that the streaming rate in the
upwards direction is slowed down at least in.the lower part
of the fluidized layer, wherein the water containing
nitrates, the activated sludge, and the substrate, which is
biologically oxidized during the denitrification, are led
into the lower part of the fluidized layer.
It is advantageous that after the denitrification, the
water and the superfluous activated sludge are led away from
the upper part of the fluidized layer and the activated
sludge, which has been led away, is freed from the sticking
nitrogen bubbles by means of an aeration, it is separated
from the purified water and it is returned into the lower
part.
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It is a contribution to a better removal of nitrogen
from the water to be purified that during the aeration,
nitrogen in the form of ammonia and organic nitrogen are
biologically oxidized to nitrates and the water containing
these nitrates is returned into the denitrification process
along with the activated sludge.
The essence of the apparatus for performing the method
according to the invention consist in that the apparatus
comprises an upwards widening denitrification zone, into the
lower part of which an admission for the water containing
nitrates, an admission for the activated sludge and an
admission for the substrate, which has been biologically
oxidized during the denitrification, mouth.
It is significant for the reliability of the method
that the denitrification zone is interconnected in its upper
part with the activation zone, which comprises aeration
elements and is interconnected with the separation zone, in
the upper part of which an outlet for the purified water is
arranged, the separation zone being provided with
a withdrawal for separated sludge which is provided with
a means for a forced movement and mouths into the lower part
of the denitrification zone.
It is also important that the withdrawal for separated
sludge is positioned in the lower part of,the separation
zone near to its interconnection with the activation zone,
the outlet for the water with nitrates from the activation
zone designed to return it into the denitrification zone
being realized by means of identical structural members, and
that the admission for the water containing nitrates, the
admission for the activated sludge and the admission for the
substrate, which has been biologically oxidized, are
realized as a common admission.
BRIEF DESCRIPTION OF THE DRAWINGS
The apparatus according to the invention meant for
purifying the home sewage is presented at the drawings,
~ . i . r
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wherein Fig. 1 depicts a cross section of the apparatus and
Fig. 2 the ground plan of the apparatus.
PREFERRED EMBODIMENT OF THE INVENTION
The apparatus consists of a vertical cylindrical tank
with a casing ~ and a bottom ~,, wherein an upwards funneling
denitrification zone $ is fornned in said tank by a vertical
partition wall ~, a first semi-conical wall 4_ and a part 2'
of the bottom 2_ between the lower edges of the partition
wall ~, and of the first semi-conical wall ~. Further, there
is a separation zone 7 formed by the vertical partition wall
$ and a second semi-conical wall C in the other part of the
cylindrical tank. An activation zone $ is formed between the
casing _1 and both semi-conical walls 4_ and $. An inlet .~ for
purified water with an outlet 1~ is arranged in the upper
part of the separation zone 7. The water level ~ is
determined by the position of the inlet $ for purified
water. There is an interconnection ~ between the
denitrification zone $ and the activation zone $ realized in
the upper part of the first semi-conical wall 4. Said
interconnection ~,~. is foamed by two circular apertures, the
upper edges of which are positioned above the water level
,~ and the lower edges below the water level ,~. However,
the interconnection 12 may be formed by other means, e.g. by
recessions in the upper edge of the first semi-conical wall
4_ or by another embodiment of the overflow formed by
lowering the upper edge of the first semi-conical wall ~. An
opening ,~,'~ is prepared in the lower part of the second
semi-conical wall $, said opening ~ providing the
interconnection between the separation zone 7 and the
activation zone $. There is a vertical semi-circular piping
~4 arranged in the denitrification zone $ near to the
vertical partition wall $, said piping ~ forming the common
admission for the biodegradable substrate, activated sludge
and nitrates into the lower part of the denitrification zone
A basket ~ is positioned in the upper part of the
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semi-circular piping ~4_, wherein a sewage water admission
1~ is situated above said basket ~. There is a drain mouth
17 for the separated activated sludge provided in the
partition wall ~, opposite the opening ~, in the lower part
of the separation zone 7, said mouth ~7 leading into an
air-lift pump ~$. The air-lift pump "1$ is substantiall~.-
inserted into the semi-circular piping ~ and leads below
the basket ~ iFig.l). A pressurized air supply ~ is
provided in the lower part of the air-lift pump 1~. Aeration
elements 2Q are positioned in the lower part of the
activation zone $ near to the bottom 2_. A large bubble
auxiliary aeration element ~ is positioned in the lower
part of the denitrification zone $ near to the part ~ of
the bottom 2_.
The method according to the invention and the function
of the above apparatus are described in the following part.
The sewage water containing nitrogen compounds, e.g.
urea, ammonia, nitrogen in organic materials, and further
various biodegradable materials, which will be further
designated as the substrate, flows through the sewage water
admission 1~ into the basket ~. Larger pieces, e.g. paper,
are retained in the basket ~,~.. These pieces are constantly
held up by the air coming from the air-lift pump 1~ below
the basket ~, which prevents the basket ,~ from clogging.
If said pieces are biodegradable, their biodegradation and
thereby their destruction occurs effected by the air and
activated sludge coming from the air-lift pump ~$. Pieces,
which are not biodegradable, e.g. polyethylene bags and so
on, remain in the basket ~ and they are removed therefrom
during a periodical inspection of the apparatus.
The sewage water, i.e. the water with biodegradable
substrate, after flowing through the basket ~, is mixed
with water, which contains nitrates and returned activated
sludge and which is led by air-lift pump ~$ below the basket
~. The mixture of Water, biodegradable substrate, nitrates,
and activated sludge is led by the semi-circular piping ~,~.
...... .....__ ~,~.~..~.._ . . r i
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into the lower part of the denitrification zone ~, where the
direction of its movement is changed so that said mixture in
the denitrification zone ~ flows upwards. As the
denitrification zone $ becomes wider in the upward
direction, the flow rate becomes lower in the upward
direction. This causes the flocks of the activated sludge to
be held in the denitrification zone $ by the gravitational
force and to form therein a fluidized layer of a sludge
blanket, through which water with nitrates and biodegradable
substrate flows. The oxygen which is present in the water is
quickly consumed by the microorganisms already in the lower
part of the denitrification zone ~, which circumstances
create anoxide conditions in the whole denitrification zone
5, or at least in its prevailing part. As a result of the
anoxide conditions the denitrification occurs in the
denitrification zone $, during which the nitrates present in
the water are reduced to gaseous nitrogen. At the same time,
a part of the biodegradable substrate is consumed. Another
part of the substrate is retained by the activated sludge
and consumed later in the period, when no substrate is
supplied from outside. The water from which nitrates and
most of the biodegradable substrate has been removed,
overflows from the denitrification zone $ through the
interconnection ~ into the activation zone $. Particles of
the activated sludge, at which the bubbles of separated
nitrogen produced by denitrification stick, are raised
upwards from the fluidized layer of the sludge blanket in
the denitrification zone $ as they are held up by said
bubbles, and they are entrained by the flowing streaming
water through the interconnection ~ into the activation
zone 8. Due to the operation of the air-lift pump ~,
further and further activated sludge is supplied through the
semi-circular piping ~, into the lower part of the
denitrification zone $, and therefore the surplus activated
sludge also overflows through the interconnection ~ into
the activation zone after the whole denitrification zone $
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has been filled by the fluidized layer of the sludge
blanket. The pressurized air is led into the aeration
elements ~Q, bubbles through the activation zone ~ upwards,
and creates a turbulence, which removes the sticking bubbles
of nitrogen from the particles of the activated sludge
suspension. At the same time, the water in the activation
zone is saturated with oxygen and a streaming is formed,
which keeps the activated sludge in suspension and mixes the
same evenly in the whole activation zone $. The water, which
has got from the denitrification zone ~ through the
interconnection 12 into the activation zone $, is mixed
gradually in the latter, as well. At the presence of oxygen,
the mixed activated sludge oxidizes all available nitrogen
compounds to nitrates and removes the rest of the dissolved
biodegradable materials from the water. At the same time, an
aerobic stabilization of the activated sludge occurs.
The pressurized air is blown into the air-lift pump
by the supply ~.2, gets through the air-lift pump 1$ upwards
and creates there the known air-lift pumping effect. As
a result, the activating mixture consisting of water with
nitrates and mixed activated sludge is sucked from the
adjacent part of the activation zone $ by means of the mouth
through the opening ~ and is led below the basket 1,~ and
through the semi-circular piping ,~4_ into the lower part of
the denitrification zone ~, as described above. So the
air-lift pump ~$ serves as the source of positive movement,
by means of which a closed circulation circuit is provided,
said circuit serving the purpose of returning the nitrates
formed in the activation zone ~ back into the
denitrification zone ~.
The sewage water flows through the admission ~. in
shocks and irregularly. At the period, when no sewage water
is flowing in, the nitrates are led through the described
circulation circuit from the activation zone ~ into the
denitrification zone ~, where they are removed by the
denitrification. As a biodegradable substrate for the
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denitrification serves first of all the substrate retained
before in the activated sludge in the fluidized layer of the
sludge blanket in the denitrification zone ,~. In case, the
retained substrate has been exhausted, further
denitrification proceeds on the endogenic respiration of the
activated sludge. The concentration of the nitrates in the
activation zone $ is lowered constantly down to a very low
value.
When a certain volume shock of sewage water flows in
through the admission ~, said water forces out the same
volume of water from the denitrification zone ~, through the
interconnection 12 into the adjacent part of the activation
zone. Than, the water drives out the same volume of the
activating mixture through the opening ~, from the adjacent
part of the activation zone $ into the lower part of the
separation zone 2. The activating mixture transferred in
said way forces out clear water from the upper part of the
separation zone 7 into the inlet $ for purified water,
wherefrom the water flows away through the outlet ~Q. The
activated sludge in the expelled activating mixture forms
during the flow a fluid layer of sludge blanket in the
separation zone 7 and it is separated from the water.
If the flowing of the sewage water through the
admission ~ takes a longer time, the activating mixture
flows through the formed fluid layer of sludge blanket and
the activated sludge is retained and attaches to the flocks
in the sludge blanket. The formed large flocks fall in the
separation zone 2 down and they are drained off by means of
the mouth ~ through the air-lift pump ,~$ into the
semi-circular piping Wig, through which they get into the
lower part of the denitrification zone $. So the separated
sludge returns into the denitrification zone $. After the
described separation of the activated sludge the purified
water flows away from the separation zone Z through the
inlet $ for purified water into the outlet ~,,Q.
After. the flowing of sewage water through the admission
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7~ is stopped, the leading of the purified water away
through the outlet .~ is also stopped. Then, the fluidized
layer in the separation zone 7 falls and its flocks are
drained off by means of the mouth ~7, along with the
activating mixture drained off through the opening ~ from
the activation zone $. So, all the activated sludge, which
has been separated from the water of the activating mixture
forced out into the separation zone 7, is drained off along
with the drained activating mixture. Only the purified water
remains in the separation zone 7 above the opening ~, and
said water is forced out through the inlet $ for purified
water, next time, when the sewage water fitfully flows in
through the admission ~, as described above. The purified
water remaining in the separation zone 7 is then the water
of the activating mixture in that part of the activation
zone $, which is adjacent to the opening ~, and it has got
through the denitrification zone $ and activation zone $
before and from which biodegradable materials and nitrogen
compounds have been removed in that way.
Through the described operation of the exemplifying
embodiment of the apparatus the sewage water flowing into
the apparatus through the admission ,~ is striped of
biodegradable materials and nitrogen compounds and the water
flows out from the apparatus through the, outlet ~ as
a purified water.
Due to the fact that the activated sludge, which is
constantly led in due to the operation of the air-lift pump
accumulates in the fluid layer of the sludge blanket in
the denitrification zone .~, and only the sludge in
suspension goes over into the activation zone $, and the
superfluous sludge comes into the activation zone $ only
after the sludge blanket has filled the whole
denitrification zone $, it could happen in case of a smaller
amount of the activated sludge in the whole apparatus, e.g.
in case of initiating the operation that practically all
sludge would accumulate in the denitrification zone $ and
~.r
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a permanent lack of activated sludge would arrive in the
activation zone $. This would result in a disruption of the
nitrification in the activation zone ,~ and thereby in
a disruption of the operation of the apparatus. Also in case
of a fall-out loss of electrical energy, which results in
stopping the operation of the air-lift pump ,~$ and in
stopping the flow in the denitrification zone $,
a disruption of the operation of the apparatus could occur,
as the fluidized layer in the denitrification zone $ would
settle and would form an sediment, which would form a gel in
the course of time, and said gel would not be brought in
suspension after the flow restoration. An auxiliary aeration
element 21 is provided to prevent such cases and it is
installed in the denitrification zone ~ near to the part
of the bottom ~. At starting the auxiliary aeration element
2~ the air going out of it breaks all incidental
accumulations, gets them from the part 2~ of the bottom 2_ in
suspension and mixes the fluidized layer in the
denitrification zone $ so that the activated sludge goes
over from the denitrification zone ~, into the activation
zone $ along with the flowing water. So a short-time,
sporadic switching-on of the auxiliary aeration element 2_1
may serve as a prevention against the above mentioned
failures.
To document the function of the exemplifying apparatus
the results of its test operation are provided in the Table
below, wherein the results have been won in a series of
withdrawals from the operation of ten test units used in
family houses, where the number of inhabitant ranged from
2 to 7 persons. The functional spaces of said test unit had
the following volumes: the denitrification zone ,~ - 0,25
m3, the separation zone 7 - 0,4 m3, the activation zone
$ - 1,1 m3.
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TABLE
Date COD BODS SS N-NHQ~ N-N03.
28/08/96 15,4 2,7 1,4 0,00 5,6
04/09/96 23,2 3,9 2,8 0,50 5,2
11/09/96 13,8 4,0 4,3 0,55 4,9
18/09/96 12,1 1,8 3,7 0,30 6,1
02/10/96 30,0 5,0 6,0 0,78 5,2
16/10/96 40,1 8,5 7,2 0,81 8,9
23/10/96 24,2 4,8 8,7 0,60 5,6
06/11/96 10,0 7,0 7,0 0,92 7,6
13/11/96 15,0 8,0 11,0 0,85 3,6
21/11/96 30,0 15,0 14,0 1,22 8,7
27/11/96 20,0 11,0 7,2 0,72 6,2
11/12/96 18,3 4,3 5,3 0,50 5,9
17/12/96 15,1 5,4 3,9 0,45 5,1
10/01/97 21,2 7,5 6,3 0,62 6,7
14/01/97 23,0 8,2 7,9 0,73 8,3
22/01/97 16,7 5,1 4,9 0,49 5,7
29/01/97 13,9 3,2 2,6 0,30 4,8
10/02/97 19,2 5,7 3,3 0,52 5,3
12/02/97 15,0 4,2 2,8 0,41 4,9
16/02/97 22,3 7,6 7,9 0,49 7,9
23/02/97 18,0 6,3 6,3 0,31 6,1
04/03/97 15,3 5,4 5,4 0,39 5,7
., ......._... r .. ~,r
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14/03/97 21,2 8,1 7,3 0,85 8,4
21/03/97 18,4 6,5 6,2 0,60 6,7
28/03/97 15,7 4,1 5,1 0,35 5,1
Average 19,5 6,1 5,9 0,57 6,2
In the Table means COD chemical oxygen demand
BODE biological oxygen demand
SS suspended solids
N-NHQ' nitrogen in the forth of ammonia
N-N03 nitrogen in the form of nitrates
All stated values are in mg/1.
If we take the known tabular data for the daily water
consumption and production of pollution for an equivalent
inhabitant as the input values (150 1, 60 g BODS, 12 g N),
the achieved average values correspond to the removal of
91,58 of nitrogen and 98,5% of HOD
5.
The method and apparatus according to the invention
have numerous advantages. The concentration of the activated
sludge in the fluidized layer of the sludge blanket in the
denitrification zone $ is higher than the concentration of
the activated sludge in the activation zone $, Which results
in the fact that the denitrification rate in a volume unit
is higher than in a mechanically mixed denitrification zone.
The substrate, which is brought into the denitrification
zone ,~ and serves the denitrification, is retained there in
the fluidized layer of the sludge blanket so that its
utilization for denitrification is higher than in
a mechanically mixed denitrification zone. There is no
mechanical apparatus necessary to keep the activated sludge
in suspension inside the denitrification zone ,~. All these
facts lower the investment costs of the apparatus and raise
the effectiveness of the purifying processes.
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INDUSTRIAL APPLICABILITY
The method and apparatus according to the invention is
not limited to the waste water purification and to the
described exemplifying apparatus only. It is possible to use
it for other purposes, e.g. to remove nitrates from
a potable water, where a hygienically suitable organic
compound, e.g. alcohol, sugar, and so on, will be added as
the substrate. Also the apparatus for realizing the method
according to the invention may be of various internal
arrangment, wherein the basic principle of the invention is
essential, i.e. that the apparatus comprises an upwards
extended denitrification zone, in the lower part of which an
admission for water containing nitrates, activated sludge
and a substrate for denitrification is provided. It is not
essential whether the admission for the water containing
nitrates, the activated sludge and the substrate for
denitrification is common, as it is in the described
exemplifying apparatus, or there are independent admissions
for the individual components into the lower part of the
denitrification zone ~. All kinds of utilization of the
basic principle of the above described inventive idea do not
digress from the essence of the invention.
i .
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List of parts
1 casing
2 bottom
2~ part of bottom
3 partition wall
4 first semi-conical wall
5 denitrification zone
6 second semi-conical wall
7 separation zone
8 activation zone
9 inlet for purified water
10 outlet
11 water level
12 interconnection
13 opening
14 piping
15 basket
16 sewage water admission
17 drain mouth
18 air-lift pump
19 pressurized air supply
20 aeration elements
21 auxiliary aeration elements
