U.S. patent application number 10/073467 was filed with the patent office on 2002-08-08 for method and apparatus for wastewater treatment.
This patent application is currently assigned to NKK CORPORATION. Invention is credited to Baba, Kei, Endo, Shinichi, Miyata, Jun, Takechi, Tatsuo, Tanabe, Masahisa, Tsubone, Toshiaki, Udagawa, Satoru.
Application Number | 20020104798 10/073467 |
Document ID | / |
Family ID | 27478156 |
Filed Date | 2002-08-08 |
United States Patent
Application |
20020104798 |
Kind Code |
A1 |
Takechi, Tatsuo ; et
al. |
August 8, 2002 |
Method and apparatus for wastewater treatment
Abstract
Phosphorus in wastewater is removed by using an apparatus
including a settling tank, an anaerobic tank, an aerobic tank, and
a final settling tank. Nitrogen in wastewater is removed by using
an apparatus including a settling tank, an anaerobic tank, a
nitrification tank, and a final settling tank.
Inventors: |
Takechi, Tatsuo; (Yokohama,
JP) ; Tanabe, Masahisa; (Sagamihara, JP) ;
Tsubone, Toshiaki; (Tokyo, JP) ; Miyata, Jun;
(Yokohama, JP) ; Baba, Kei; (Yokohama, JP)
; Udagawa, Satoru; (Yokohama, JP) ; Endo,
Shinichi; (Hachioji, JP) |
Correspondence
Address: |
FRISHAUF, HOLTZ, GOODMAN &
LANGER & CHICK, PC
767 THIRD AVENUE
25TH FLOOR
NEW YORK
NY
10017-2023
US
|
Assignee: |
NKK CORPORATION
TOKYO
JP
|
Family ID: |
27478156 |
Appl. No.: |
10/073467 |
Filed: |
February 11, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10073467 |
Feb 11, 2002 |
|
|
|
09383314 |
Aug 25, 1999 |
|
|
|
Current U.S.
Class: |
210/605 |
Current CPC
Class: |
Y02W 10/15 20150501;
C02F 3/1215 20130101; Y02W 10/10 20150501; C02F 3/308 20130101 |
Class at
Publication: |
210/605 |
International
Class: |
C02F 003/30 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 3, 1998 |
JP |
10-249319 |
Sep 3, 1998 |
JP |
10-249320 |
Oct 15, 1998 |
JP |
10-293786 |
Oct 23, 1998 |
JP |
10-302235 |
Claims
What is claimed is:
1. A method for treating wastewater comprising the steps of:
removing phosphorus from wastewater, using a wastewater treatment
apparatus comprising a settling tank, an anaerobic tank, and an
aerobic tank; crushing at least a portion of sediment in the
settling tank; and supplying the crushed sediment to the anaerobic
tank.
2. A method for treating wastewater according to claim 1, wherein
the step of crushing comprises grinding a portion of the sediment
in the settling tank.
3. A wastewater treatment apparatus comprising: a settling tank for
removing solids contained in wastewater; an anaerobic tank, wherein
effluent from the settling tank is introduced and activated sludge
releases phosphoric acid ion from cells into wastewater by a
biological phosphorus-releasing reaction; an aerobic tank, wherein
effluent from the anaerobic tank is introduced and activated sludge
takes up phosphoric acid ion into cells by a biological
phosphorus-uptake reaction; a line for withdrawing sediment from
the settling tank; a crushing device for crushing the withdrawn
sediment; and a line for supplying the crushed sediment to the
anaerobic tank.
4. A wastewater treatment apparatus according to claim 3, wherein
the crushing device comprises a device for grinding the
sediment.
5. A method for treating wastewater comprising the steps of:
removing phosphorus from wastewater, using a wastewater treatment
apparatus comprising a primary settling tank, an anaerobic tank, an
aerobic tank, and a final settling tank; returning effluent from
the aerobic tank to the primary settling tank; returning sludge
withdrawn from the final settling tank to the primary settling
tank; and sending sludge withdrawn from the primary settling tank
to the anaerobic tank or the aerobic tank.
6. A method for treating wastewater according to claim 5, wherein
the step of removing phosphorus comprises removing phosphorus from
the wastewater, using a wastewater treatment apparatus comprising a
primary settling tank, an anaerobic tank, an anoxic tank, an
aerobic tank, and a final settling tank.
7. A method for treating wastewater according to claim 6, further
comprising the step of sending sludge in the primary settling tank
to the anoxic tank.
8. A method for treating wastewater comprising the steps of:
removing phosphorus from wastewater, using a wastewater treatment
apparatus comprising a primary settling tank, an anaerobic tank, an
aerobic tank, and a final settling tank; returning effluent from
the aerobic tank to the primary settling tank; and sending sludge
withdrawn from the primary settling tank to the anaerobic tank or
the aerobic tank.
9. A method for treating wastewater comprising the steps of:
removing phosphorus from wastewater, using a wastewater treatment
apparatus comprising a primary settling tank, an anaerobic tank, an
aerobic tank, and a final settling tank; returning sludge withdrawn
from the final settling tank to the primary settling tank; and
sending sludge withdrawn from the primary settling tank to the
anaerobic tank or the aerobic tank.
10. A wastewater treatment apparatus comprising: a primary settling
tank for removing solids contained in wastewater; an anaerobic
tank, wherein effluent from the primary settling tank is introduced
and activated sludge releases phosphoric acid ion from cells into
the wastewater by a biological phosphorus-releasing reaction; an
aerobic tank, wherein effluent from the anaerobic tank is
introduced and activated sludge takes up phosphate anions into
cells by a biological phosphorus-uptake reaction; a final settling
tank for settling the wastewater treated in the aerobic tank; a
line for returning effluent from the aerobic tank to the primary
settling tank; a line for returning sludge withdrawn from the final
settling tank to the primary settling tank; and a line for sending
sludge withdrawn from the primary settling tank to the anaerobic
tank or the aerobic tank.
11. A wastewater treatment apparatus according to claim 10, further
comprising an anoxic tank for denitrifying effluent from the
anaerobic tank, the anoxic tank being disposed between the
anaerobic tank and the aerobic tank.
12. A wastewater treatment apparatus according to claim 11, further
comprising a line for sending sludge in the primary settling tank
to the anoxic tank.
13. A wastewater treatment apparatus comprising: a primary settling
tank for removing solids contained in wastewater; an anaerobic
tank, wherein effluent from the primary settling tank is introduced
and activated sludge releases phosphate anions from cells into the
wastewater by a biological phosphorus-releasing reaction; an
aerobic tank, wherein effluent from the anaerobic tank is
introduced and activated sludge takes up phosphate anions into
cells by a biological phosphorus-uptake reaction; a final settling
tank for settling the wastewater treated in the aerobic tank; a
line for returning effluent from the aerobic tank to the primary
settling tank; and a line for sending sludge withdrawn from the
primary settling tank to the anaerobic tank or the aerobic
tank.
14. A wastewater treatment apparatus comprising: a primary settling
tank for removing solids contained in wastewater; an anaerobic
tank, wherein effluent from the primary settling tank is introduced
and activated sludge releases phosphate anions from cells into the
wastewater by a biological phosphorus-releasing reaction; an
aerobic tank, wherein effluent from the anaerobic tank is
introduced and activated sludge takes up phosphate anions into
cells by a biological phosphorus-uptake reaction; a final settling
tank for settling the wastewater treated in the aerobic tank; a
line for returning sludge withdrawn from the final settling tank to
the primary settling tank; and a line for sending sludge withdrawn
from the primary settling tank to the anaerobic tank or the aerobic
tank.
15. A method for treating wastewater comprising the steps of:
removing nitrogen from wastewater, using a wastewater treatment
apparatus comprising a settling tank and a denitrification tank;
crushing at least a portion of sediment in the settling tank; and
supplying the crushed sediment to the denitrification tank.
16. A method for treating wastewater according to claim 15, wherein
the step of crushing comprises grinding a portion of the sediment
in the settling tank.
17. A method for treating wastewater comprising the steps of:
removing nitrogen from wastewater, using a wastewater treatment
apparatus comprising a settling tank, an anaerobic tank, and a
nitrification tank; crushing at least a portion of sediment in the
settling tank; and supplying the crushed sediment to the anaerobic
tank.
18. A method for treating wastewater according to claim 17, wherein
the step of crushing comprises grinding a portion of the sediment
in the settling tank.
19. A wastewater treatment apparatus comprising: a settling tank
for removing solids contained in wastewater; a denitrification
tank, wherein effluent from the settling tank is introduced and
nitrate nitrogen or nitrite nitrogen in the wastewater is reduced
to nitrogen gas; a line for withdrawing sediment from the settling
tank; a crushing device for crushing the withdrawn sediment; and a
line for supplying the crushed sediment to the denitrification
tank.
20. A wastewater treatment apparatus according to claim 19, wherein
the crushing device comprises a device for grinding the
sediment.
21. A wastewater treatment apparatus comprising: a settling tank
for removing solids contained in wastewater; an anaerobic tank,
wherein effluent from the settling tank is introduced, nitrate
nitrogen or nitrite nitrogen in the wastewater is reduced to
nitrogen gas, and activated sludge releases phosphate anions from
cells into the wastewater by a biological phosphorus-releasing
reaction; a line for withdrawing sediment from the settling tank; a
crushing device for crushing the withdrawn sediment; and a line for
supplying the crushed sediment to a denitrification tank.
22. A wastewater treatment apparatus according to claim 21, wherein
the crushing device comprises a device for grinding the
sediment.
23. A wastewater treatment apparatus according to claim 21, further
comprising a nitrification tank, wherein effluent from the
anaerobic tank is introduced and nitrogen compounds in the
wastewater are oxidized to nitrate nitrogen or nitrite
nitrogen.
24. A wastewater treatment apparatus comprising: a primary settling
tank for removing solids contained in wastewater; a denitrification
tank, wherein effluent from the primary settling tank is
introduced, and nitrate nitrogen or nitrite nitrogen in the
wastewater is reduced to nitrogen gas; a nitrification tank,
wherein effluent from the denitrification tank is introduced and
nitrogen compounds in the wastewater are oxidized to nitrate
nitrogen or nitrite nitrogen; a final settling tank for settling
effluent from the nitrification tank; and a line for returning a
portion of effluent from the nitrification tank or a portion of
effluent from the final settling tank to the primary settling
tank.
25. A wastewater treatment apparatus according to claim 24, further
comprising an anaerobic tank disposed between the primary settling
tank and the denitrification tank, wherein effluent from the
primary settling tank is introduced, nitrate nitrogen or nitrite
nitrogen in the wastewater is reduced to nitrogen gas, and
activated sludge releases phosphate anions from cells into
wastewater by a biological phosphorus-releasing reaction.
26. A wastewater treatment apparatus according to claim 25, further
comprising a line for sending sediment in the primary settling tank
to the anaerobic tank.
27. A wastewater treatment apparatus according to claim 24, further
comprising a line for sending sediment in the primary settling tank
to the denitrification tank.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to methods for treating
wastewater and apparatuses therefor, and more particularly, to
methods for removing phosphorus and nitrogen from wastewater and
apparatuses therefor.
[0003] 2. Description of the Related Art
[0004] The activated sludge process is a typical treatment process
for removing organic substances from wastewater, and an
anaerobic-aerobic activated sludge process is used as a
conventional method for simultaneously removing phosphorus anions
and organic substances from wastewater.
[0005] An example of a wastewater treatment apparatus by the
anaerobic-aerobic activated sludge process is shown in FIG. 6. This
wastewater treatment apparatus includes a primary settling tank 2,
an anaerobic tank 3 in which activated sludge releases phosphoric
acid ion from cells into wastewater by a biological
phosphorus-releasing reaction, an aerobic tank 4 in which activated
sludge takes up phosphoric acid ion into cells by a biological
phosphorus-uptake reaction, and a final settling tank 6. The amount
of phosphorus taken up at the aerobic stage is greater than the
amount of phosphorus released at the anaerobic stage, and a
difference between these amounts corresponds to the amount of
phosphorus removed.
[0006] In the primary settling tank 2, relatively large and heavy
solids contained in wastewater 1 are removed. After being subjected
to the biological phosphorus-releasing reaction at the anaerobic
stage and the biological phosphorus-uptake reaction at the aerobic
stage, phosphorus in the wastewater is transformed into a
constituent of the sludge, and is finally discharged as excess
sludge from the wastewater treatment apparatus. Meanwhile, organic
substances in the wastewater are removed both at the anaerobic
stage and at the aerobic stage.
[0007] The phosphorus removal from wastewater using a wastewater
treatment apparatus by means of the anaerobic-aerobic activated
sludge process has the following drawbacks. That is, when the
organic substance concentration in the wastewater is decreased, for
example, by the influx of rainwater, since the wastewater is
diluted while being supplied with oxygen, the organic substances in
the wastewater, which have been oxidized and diluted, are fed to
the anaerobic stage. As a result, since the organic substance
concentration is decreased in comparison with the phosphoric acid
ion concentration, the rate of the phosphorus-releasing reaction at
the anaerobic stage and the rate of the phosphorus-uptake reaction
at the aerobic stage are decreased, resulting in deterioration in
the quality of treated water.
[0008] Since solid waste components of the wastewater are subjected
to sedimentation in the primary settling tank, the wastewater
supplied to the anaerobic tank from the primary settling tank
containing mainly soluble pollutants. Therefore, when the ratio of
the organic substance concentration to the phosphoric acid ion
concentration is low with respect to the soluble pollutants in the
wastewater to be fed to the anaerobic stage, the rate of the
phosphorus-releasing reaction at the anaerobic stage and the rate
of the phosphorus-uptake reaction at the aerobic stage are also
decreased, resulting in deterioration in the quality of treated
water.
[0009] In order to cope with such problems, a method may be
employed in which an organic chemical agent such as methanol,
together with the wastewater, is supplied to the anaerobic stage,
and thus, by compensating for the shortage of the organic substance
concentration in the wastewater, decreases in the rate of the
phosphorus-releasing reaction at the anaerobic stage and the rate
of the phosphorus-uptake reaction at the aerobic stage are
prevented.
[0010] In the conventional method of adding an organic chemical
agent to the anaerobic stage, although in general, relatively
inexpensive methanol is used, the cost for the chemical agent still
increases operating expenses. Moreover, since methanol is a Class
IV hazardous substance, safety must be taken into consideration in
handling and storage facilities, and facilities for receipt and
supply must be prepared, resulting in an increased cost of
equipment and difficulty in handling.
[0011] With respect to both the phosphorus-releasing reaction at
the anaerobic stage and the phosphorus-uptake reaction at the
aerobic stage, substantially large amounts of organic substances
are required. In the phosphorus removal treatment by the
anaerobic-aerobic activated sludge process, if there is a shortage
of organic substances, satisfactory phosphorus removal cannot be
achieved. Therefore, it is effective for improving the results of
phosphorus removal treatment to introduce the organic substances
contained in the wastewater fed into the primary settling tank and
removed in the primary settling tank, that is, organic substances
mainly composed of solids, to the anaerobic stage or both the
anaerobic stage and the aerobic stage.
[0012] As a conventional method of operating facilities having such
an effect, there is a method in which by decreasing the number of
primary settling tanks used in wastewater treatment facilities
having a plurality of treatment lines, operation is performed at
increased water surface loading with respect to the primary
settling tanks so that suspended solid (SS) concentration in the
primary settling tank effluent is increased, and thus organic
substances flowing into the anaerobic stage are increased.
[0013] However, it is difficult to accurately control the amount of
solid organic substances that flow into the anaerobic stage by
changing the number of primary settling tanks in operation.
[0014] In order to solve the problems described above, the inventor
has already applied for patents which disclose techniques for
satisfactory phosphorus removal treatment by increasing an amount
of organic substances to be supplied to anaerobic tanks. One such
method for removing phosphorus from wastewater, in which at least a
portion of sediment in a primary settling tank is fed to an
anaerobic tank, and a method for removing phosphorus from
wastewater, in which the sediment is subjected to ultrasonic
treatment before being fed to the anaerobic tank, are disclosed in
Japanese Patent Application No. 9-133989. Another such method for
removing phosphorus from wastewater, in which the sediment is
subjected to ozone treatment before being fed to the anaerobic
tank, is disclosed in Japanese Patent Application No. 9-133988.
However, when the sediment in the primary settling tank is
introduced to the anaerobic tank as it is, since particles of
solids contained in the sedimented sludge are relatively large and
have a small surface area per unit weight, the rate of
decomposition/treatment in a biological reactor such as an
anaerobic tank is relatively low. When the sediment in the primary
settling tank is fed to the anaerobic tank after ultrasonic
treatment, solids, as the sediment, contain microorganisms enclosed
by cell walls and/or cell membranes, and cellulose, which are
relatively rigid and are not easily broken by vibration treatment.
Therefore, the particle size of the solids introduced to the
anaerobic tank is not sufficiently reduced. Furthermore, when the
sediment in the primary settling tank is fed to the anaerobic tank
after ozone treatment, a portion of organic substances contained in
the sediment is oxidized and mineralized, and thus loses
effectiveness as organic substances. Since a relatively large
amount of electric power, approximately 10 to 15 kWh/kg O.sub.3, is
also consumed for the generation of ozone, operating expenses
increase.
[0015] As a conventional method for simultaneously removing
nitrogen compounds and organic substances, a biological
nitrification-denitrificat- ion process is used.
[0016] An example of a wastewater treatment apparatus by the
biological nitrification-denitrification process is shown in FIG.
14. This wastewater treatment apparatus includes a primary settling
tank 2, a nitrification tank 4 for oxidizing (nitrifying) nitrogen
compounds contained in wastewater to nitrate nitrogen or to nitrite
nitrogen, a denitrification tank 3 for reducing (denitrifying)
nitrate nitrogen or nitrite nitrogen to nitrogen gas, and a final
settling tank 7. After relatively large and heavy solids are
removed from wastewater 1 in the primary settling tank 2,
nitrification and denitrification are performed, and nitrogen is
released into the atmosphere as nitrogen gas, and thus nitrogen is
removed from wastewater. Meanwhile, organic substances in the
wastewater are removed both at the denitrification stage and at the
nitrification stage.
[0017] The nitrogen removal from wastewater using a wastewater
treatment apparatus by the biological nitrification-denitrification
process has the following drawbacks. That is, when the organic
substance concentration in the wastewater is decreased, for
example, by the influx of rainwater, since the wastewater is
diluted while being supplied with oxygen, the organic substances in
the wastewater, which have been oxidized and diluted, are fed to
the denitrification stage. As a result, since the organic substance
concentration is further decreased in comparison with the nitrogen
concentration, the rate of denitrification reaction at the
denitrification stage is decreased, resulting in deterioration in
the quality of treated water.
[0018] Since solid particles are subjected to sedimentation in the
primary settling tank, the wastewater fed to the denitrification
stage through the primary settling tank is mainly composed of
soluble pollutants. Therefore, when the ratio of the organic
substance concentration to the nitrogen concentration is low with
respect to the soluble wastes in the wastewater to be fed to the
denitrification stage, the rate of nitrogen removal reaction at the
denitrification stage is also decreased, resulting in deterioration
in the quality of treated water.
[0019] In order to cope with such problems, a method is used, in
which an organic chemical agent such as methanol, together with the
wastewater, is fed to the denitrification stage, and thus, by
compensating for the shortage of the organic substance
concentration in the wastewater, a decrease in the rate of
denitrification reaction is prevented.
[0020] In the conventional method of adding an organic chemical
agent to the denitrification stage, although in general methanol,
which is relatively inexpensive, is used, the cost for the chemical
agent still increases operating expenses. Moreover, since methanol
is a Class IV hazardous substance, safety must be taken into
consideration in handling, and storage facilities and facilities
for receipt and supply must be prepared, resulting in increased
cost of equipment and difficulty in handling.
[0021] When the ratio of the organic substance concentration to the
nitrogen concentration is low with respect to the wastewater
supplied to the denitrification stage, it is difficult to cause
satisfactory nitrogen removal reactions, in particular,
denitrification reactions, because of a shortage of organic
substances, and thus satisfactory nitrogen removal cannot be
achieved. Therefore, it is effective in improving the results of
nitrogen removal treatment to introduce the organic substances
contained in the wastewater fed into the primary settling tank and
removed in the primary settling tank, that is, organic substances
mainly composed of solids, to the denitrification stage.
[0022] As a conventional method for operating facilities for
producing such effects, there is a method in which, by decreasing
the number of primary settling tanks used in wastewater treatment
facilities having a plurality of treatment lines, operation is
performed at increased water surface loading with respect to the
primary settling tanks so that suspended solids (SS) concentration
in the primary settling tank effluent is increased, and thus the
amount of organic substances flowing into the denitrification stage
is increased.
[0023] However, it is difficult to accurately control the amount of
solid organic substances that flow into the denitrification stage
by changing the number of primary settling tanks in operation.
[0024] In order to solve the problems described above, the inventor
has already applied for patents which disclose techniques for
satisfactory nitrogen removal treatment by increasing the amounts
of organic substances to be supplied to denitrification tanks or
anaerobic tanks. One such method for removing nitrogen from
wastewater, in which at least a portion of sediment in a primary
settling tank is fed to a denitrification tank or an anaerobic
tank, and a method for removing nitrogen from wastewater, in which
the sediment is subjected to ultrasonic treatment before being fed
to the denitrification tank or the anaerobic tank, are disclosed in
Japanese Patent Application No. 9-133989. Another such method for
removing nitrogen from wastewater, in which the sediment is
subjected to ozone treatment before being fed to the
denitrification tank or the anaerobic tank, is disclosed in
Japanese Patent Application No. 9-133988. However, when the
sediment in the primary settling tank is introduced to the
denitrification tank or the anaerobic tank as it is, since
particles of solids contained in the sedimented sludge are
relatively large and have a small surface area per unit weight, the
rate of decomposition/treatment in a biological reactor, such as a
denitrification tank or an anaerobic tank, is relatively low. When
the sediment in the primary settling tank is fed to the
denitrification tank or the anaerobic tank after ultrasonic
treatment, solids, as the sediment, contain microorganisms enclosed
by cell walls and/or cell membranes, and cellulose, which are
relatively rigid and are not easily broken by vibration treatment.
Therefore, the particle size of the solids introduced to the
denitrification tank or the anaerobic tank is not sufficiently
reduced. Furthermore, when the sediment in the primary settling
tank is fed to the denitrification tank or the anaerobic tank after
ozone treatment, a portion of organic substances contained in the
sediment is oxidized and mineralized, and thus loses effectiveness
as organic substances. Since a relatively large amount of electric
power, approximately 10 to 15 kWh/kg O.sub.3, is also consumed for
the generation of ozone, operating expenses increase.
SUMMARY OF THE INVENTION
[0025] It is an object of the present invention to provide methods
for treating wastewater and apparatuses therefor, in which
effective wastewater treatment can be performed in an economical
manner.
[0026] According to a first aspect of the present invention, a
method for treating wastewater includes the steps of: removing
phosphorus from wastewater, using a wastewater treatment apparatus
including a settling tank, an anaerobic tank, and an aerobic tank;
crushing at least a portion of sediment in the settling tank; and
supplying the crushed sediment to the anaerobic tank.
[0027] According to a second aspect of the present invention, a
wastewater treatment apparatus includes: a settling tank for
removing solids contained in wastewater; an anaerobic tank, in
which effluent from the settling tank is introduced and activated
sludge releases phosphoric acid ion from cells into the wastewater
by a biological phosphorus-releasing reaction; an aerobic tank in
which effluent from the anaerobic tank is introduced and activated
sludge takes up phosphoric acid ion into cells by a biological
phosphorus-uptake reaction; a line for withdrawing sediment from
the settling tank; a crushing device for crushing the withdrawn
sediment; and a line for supplying the crushed sediment to the
anaerobic tank.
[0028] According to a third aspect of the present invention, a
method for treating wastewater includes the steps of: removing
phosphorus from wastewater, using a wastewater treatment apparatus
including a primary settling tank, an anaerobic tank, an aerobic
tank, and a final settling tank; returning effluent from the
aerobic tank to the primary settling tank; returning sludge
withdrawn from the final settling tank to the primary settling
tank; and sending sludge withdrawn from the primary settling tank
to the anaerobic tank or the aerobic tank.
[0029] According to a fourth aspect of the present invention, a
method for treating wastewater includes the steps of: removing
phosphorus from wastewater, using a wastewater treatment apparatus
including a primary settling tank, an anaerobic tank, an aerobic
tank, and a final settling tank; returning effluent from the
aerobic tank to the primary settling tank; and sending sludge
withdrawn from the primary settling tank to the anaerobic tank or
the aerobic tank.
[0030] According to a fifth aspect of the present invention, a
method for treating wastewater includes the steps of: removing
phosphorus from wastewater, using a wastewater treatment apparatus
including a primary settling tank, an anaerobic tank, an aerobic
tank, and a final settling tank; returning sludge withdrawn from
the final settling tank to the primary settling tank; and sending
sludge withdrawn from the primary settling tank to the anaerobic
tank or the aerobic tank.
[0031] According to a sixth aspect of the present invention, a
wastewater treatment apparatus includes: a primary settling tank
for removing solids contained in wastewater; an anaerobic tank, in
which effluent from the settling tank is introduced and activated
sludge releases phosphoric acid ion from cells into the wastewater
by a biological phosphorus-releasing reaction; an aerobic tank in
which effluent from the anaerobic tank is introduced and activated
sludge takes up phosphoric acid ion into cells by a biological
phosphorus-uptake reaction; a final settling tank for settling
wastewater treated in the aerobic tank; a line for returning
effluent from the aerobic tank to the primary settling tank; a line
for returning sludge withdrawn from the final settling tank to the
primary settling tank; and a line for sending sludge withdrawn from
the primary settling tank to the anaerobic tank or the aerobic
tank.
[0032] According to a seventh aspect of the present invention, a
wastewater treatment apparatus includes: a primary settling tank
for removing solids contained in wastewater; an anaerobic tank, in
which effluent from the settling tank is introduced and activated
sludge releases phosphoric acid ion from cells into the wastewater
by the biological phosphorus-releasing reaction; an aerobic tank in
which effluent from the anaerobic tank is introduced and activated
sludge takes phosphoric acid ion into cells by the biological
phosphorus-uptake reaction; a final settling tank for settling
wastewater treated in the aerobic tank; a line for returning
effluent from the aerobic tank to the primary settling tank; and a
line for sending sludge withdrawn from the primary settling tank to
the anaerobic tank or the aerobic tank.
[0033] According to an eighth aspect of the present invention, a
wastewater treatment apparatus includes: a primary settling tank
for removing solids contained in wastewater; an anaerobic tank, in
which effluent from the settling tank is introduced and activated
sludge releases phosphoric acid ion from cells into the wastewater
by a biological phosphorus-releasing reaction; an aerobic tank in
which effluent from the anaerobic tank is introduced and activated
sludge takes up phosphoric acid ion into cells by a biological
phosphorus-uptake reaction; a final settling tank for settling
wastewater treated in the aerobic tank; a line for returning sludge
withdrawn from the final settling tank to the primary settling
tank; and a line for sending sludge withdrawn from the primary
settling tank to the anaerobic tank or the aerobic tank.
[0034] According to a ninth aspect of the present invention, a
method for treating wastewater includes the steps of: removing
nitrogen from wastewater, using a wastewater treatment apparatus
including a settling tank and a denitrification tank; crushing at
least a portion of sediment in the settling tank; and supplying the
crushed sediment to the denitrification tank.
[0035] According to a tenth aspect of the present invention, a
method for treating wastewater includes the steps of: removing
nitrogen from wastewater, using a wastewater treatment apparatus
including a settling tank and an anaerobic tank, and a
nitrification tank; crushing at least a portion of sediment in the
settling tank; and supplying the crushed sediment to the anaerobic
tank.
[0036] According to an eleventh aspect of the present invention, a
wastewater treatment apparatus includes: a settling tank for
removing solids contained in wastewater; a denitrification tank, in
which effluent from the settling tank is introduced and nitrate
nitrogen or nitrite nitrogen in wastewater is reduced to nitrogen
gas; a line for withdrawing sediment in the settling tank; a
crushing device for crushing the withdrawn sediment; and a line for
supplying the crushed sediment to the denitrification tank.
[0037] According to a twelfth aspect of the present invention, a
wastewater treatment apparatus includes: a settling tank for
removing solids contained in wastewater; an anaerobic tank, in
which effluent from the settling tank is introduced, nitrate
nitrogen or nitrite nitrogen in the wastewater is reduced to
nitrogen gas, and activated sludge releases phosphoric acid ion
from cells into the wastewater by a biological phosphorus-releasing
reaction; a line for withdrawing sediment from the settling tank; a
crushing device for crushing the withdrawn sediment; and a line for
supplying the crushed sediment to a denitrification tank.
[0038] According to a thirteenth aspect of the present invention, a
wastewater treatment apparatus includes: a primary settling tank
for removing solids contained in wastewater; a denitrification
tank, in which effluent from the primary settling tank is
introduced, and nitrate nitrogen or nitrite nitrogen in the
wastewater is reduced to nitrogen gas; a nitrification tank, in
which effluent from the denitrification tank is introduced, and
nitrogen compounds in the wastewater are oxidized to nitrate
nitrogen or nitrite nitrogen; a final settling tank for settling
effluent from the nitrification tank; and a line for returning a
portion of effluent from the nitrification tank or a portion of
effluent from the final settling tank to the primary settling
tank.
BRIEF DESCRIPTION OF THE DRAWINGS
[0039] FIG. 1 is a block diagram of a biological phosphorus removal
apparatus in accordance with embodiment 1;
[0040] FIG. 2 is a block diagram of another biological phosphorus
removal apparatus in accordance with embodiment 1;
[0041] FIG. 3 is a block diagram of another biological phosphorus
removal apparatus in accordance with embodiment 1;
[0042] FIG. 4 is a side view of an experimental device for the
phosphorus-releasing reaction in accordance with embodiment 1;
[0043] FIG. 5 a graph which shows a change in the phosphoric acid
ion concentration over time in cases when ground sediment from the
primary settling tank is added and when it is not added, using the
device shown in FIG. 4;
[0044] FIG. 6 is a block diagram of a conventional biological
phosphorus removal apparatus;
[0045] FIG. 7 is a block diagram of a biological phosphorus removal
apparatus in accordance with embodiment 2;
[0046] FIG. 8 is a block diagram of another biological phosphorus
removal apparatus in accordance with embodiment 2;
[0047] FIG. 9 is a block diagram of a biological nitrogen removal
apparatus in accordance with embodiment 3;
[0048] FIG. 10 is a block diagram of another biological nitrogen
removal apparatus in accordance with embodiment 3;
[0049] FIG. 11 is a block diagram of another biological nitrogen
removal apparatus in accordance with embodiment 3;
[0050] FIG. 12 is a side view of an experimental device for the
denitrification reaction in accordance with embodiment 3;
[0051] FIG. 13 is a graph showing a change in the nitrate nitrogen
concentration over time in cases when ground sediment from the
primary settling tank is added and when it is not added, using the
device shown in FIG. 12.
[0052] FIG. 14 is a block diagram of a conventional biological
nitrogen removal apparatus;
[0053] FIG. 15 is a block diagram of a biological nitrogen removal
apparatus in accordance with embodiment 4;
[0054] FIG. 16 is a block diagram of another biological nitrogen
removal apparatus in accordance with embodiment 4; and
[0055] FIG. 17 is a block diagram of another biological nitrogen
removal apparatus in accordance with embodiment 4.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0056] Embodiment 1
[0057] In embodiment 1, the deterioration of phosphorus removal
performance in an anaerobic-aerobic activated sludge process due to
low organic substance concentration in wastewater, in particular, a
shortage of the soluble organic substance concentration, is
overcome. In accordance with embodiment 1, methods and apparatuses
for effectively and economically removing phosphorus are provided,
in which sludge containing solid organic substances generated in a
primary settling tank are supplied to an anaerobic tank, and thus
by compensating for the shortage of the organic substance
concentration of the wastewater flowing into the anaerobic tank,
organic substances generated in the waste treatment facilities are
effectively used and the deterioration of phosphorus removal
performance is avoided.
[0058] Embodiment 1 relates to methods for removing phosphorus from
wastewater and apparatuses therefor, in which phosphorus in
wastewater is removed using an apparatus including at least a
settling tank for wastewater to be treated, an anaerobic tank, and
an aerobic tank, and at least a portion of the sediment from the
wastewater in the settling tank is crushed or ground before being
supplied to the anaerobic tank.
[0059] In accordance with the method for biologically removing
phosphorus from wastewater and the apparatus therefor in embodiment
1, at least a portion of the sediment in the primary settling tank,
which contains abundant solid organic substances, that is, at least
a portion of the solid organic substances, is fed into the
anaerobic tank in order to secure an organic substance
concentration required for the phosphorus-releasing reaction at the
anaerobic stage. By crushing or grinding at least a portion of the
sediment in the primary settling tank, solid organic substances
contained in the sediment in the primary settling tank are
particulated or solubilized so that microorganisms can effectively
and promptly use the solid organic substances for the
phosphorus-releasing reaction. The present inventors have found
through experimentation that when the organic substance
concentration in wastewater is low and the organic substance
concentration required for the phosphorus-releasing reaction at the
anaerobic stage is insufficient, the oxidation-reduction potential
(ORP) at the anaerobic stage is -100 mV or more. Accordingly, an
ORP at the anaerobic stage is measured, and only when the ORP is
-100 mV or more, that is, only when the organic substance
concentration required for the phosphorus-releasing reaction is
insufficient at the anaerobic stage, sediment from the primary
settling tank which has been crushed or ground is fed into the
anaerobic tank. Thus, by supplying organic substances originating
from the solids, the organic substance concentration required for
the phosphorus-releasing reaction will be sufficient.
[0060] An example of a biological phosphorus removal apparatus
based on embodiment 1 is shown in FIG. 1.
[0061] The biological phosphorus removal apparatus based on
embodiment 1 mainly includes a primary settling tank 2, an
anaerobic tank 3, an aerobic tank 4, and a final settling tank 6.
In the anaerobic tank 3, only agitation is performed, and in the
aerobic tank 4, oxygen is supplied by an. air diffuser 5 and
agitation is performed by flow caused by the air diffusion.
[0062] In the biological phosphorus removal apparatus shown in FIG.
1, wastewater 1 is subjected to the solid-liquid separation in the
primary settling tank 2, and is then passed to the anaerobic tank 3
and to the aerobic tank 4 in that order. Effluent from the aerobic
tank 4 flowing into the final settling tank 6 is separated into
treated water 7 and activated sludge in the final settling tank 6.
At least a portion of the activated sludge separated and thickened
in the final settling tank 6 is sent to the anaerobic tank 3 as
return sludge 8.
[0063] Sediment 9 settled in the primary settling tank is subjected
to crushing or grinding treatment by a crushing device or a
grinding device 10, and is then directly sent to the anaerobic tank
3 as crushed or ground sediment 11 from the primary settling tank,
or is sent to the anaerobic tank 3 through the effluent channel of
the primary settling tank 2. Alternatively, the crushed or ground
sediment 11 is fed to the primary settling tank 2 through the
influx channel of the primary settling tank 2, and a portion,
excluding those which are settled again in the primary settling
tank 2, is sent to the anaerobic tank 3 through the effluent
channel of the primary settling tank 2.
[0064] In the anaerobic tank 3, by using organic substances mainly
constituting soluble components in the wastewater 1 and organic
substances originating from the sediment 9 in the primary settling
tank, activated sludge releases phosphoric acid ion accumulated in
cells in the activated sludge into the wastewater (the
phosphorus-releasing reaction). In the aerobic tank 4, activated
sludge takes up phosphoric acid ion in the wastewater into cells in
the activated sludge (the phosphorus-uptake reaction), and
simultaneously, organic substances are removed by oxidizing
decomposition.
[0065] After solid organic substances contained in the sediment 9
are treated by the crushing device or the grinding device 10, at
least a portion thereof is finely particulated and at least a
portion thereof becomes soluble components. That is, when the grain
sizes of the solids are reduced by crushing or grinding treatment,
the surface area increases with respect to solids having the same
weight. Microorganisms, animals, plants, and the like are contained
in the sediment 9, and these organisms are composed of cells
enclosed by cell membranes and/or cell walls. When these membranous
structures are broken by crushing or grinding treatment, since cell
fluids are released from the cells, the soluble organic substance
concentration of the wastewater into which the crushed or ground
sediment 11 is fed is increased.
[0066] In general, when microorganisms propagate by decomposing and
using organic substances, organic substances having higher
molecular weights take longer time to be decomposed and become
useful. Organic substances having lower molecular weights are
easier to be used, and the rate of the decomposing reaction is
higher. In comparison with soluble organic substances, solid
organic substances generally have larger molecular weights, and
thus the solid organic substances are more difficult to be used by
microorganisms. This is because of the fact that when
microorganisms take up organic substances through cell membranes
and/or cell walls, the organic substances must have enough low
molecular weights. In order for microorganisms to use organic
substances having high molecular weights, the organic substances
having high molecular weights must be decomposed into organic
substances having lower molecular weights, thus requiring time for
decomposition to usable states. In organisms, reactions for
decomposing organic substances having high molecular weights into
organic substances having lower molecular weights are mainly
performed by the enzymatic reactions. In such reactions, as the
surface on which an enzyme acts is increased, that is, as the grain
sizes of solid organic substances having comparable weights are
decreased, the reaction rate increases. Consequently, in embodiment
1, by subjecting the sediment 9 to crushing or grinding treatment
so that the grain sizes of solids are reduced, enzymatic reactions
which allow microorganisms to decompose and use solid organic
substances are enhanced, the rate of decomposition and use of solid
organic substances is increased, and the microbial reactions may be
efficiently completed in a reactor of limited capacity. When the
soluble organic substances generated by crushing or grinding the
sediment 9 are introduced to the anaerobic tank 3, they are also
effectively and promptly used for the phosphorus-releasing reaction
and the denitrification reaction in the anaerobic tank 3 and for
the subsequent phosphorus-uptake reaction in the aerobic tank 4.
This happens at least in the treatment of municipal wastewater.
Sediments 9 for a municipal wastewater treatment include microbial
bodies or plant cells or animal cells, which are enveloped by cell
walls, or cell membrane and cell fluids including organic
components leak out by breaking the cell structure. In this respect
also, the crushing or grinding treatment to the sediment 9
contributes to increase in the rate of decomposition and use of the
organic substances contained in solids in the influent wastewater 1
by microorganisms in the reaction tank, and this is effective to
complete the microbial reaction in a reactor of limited
capacity.
[0067] The crushed or ground sediment 11 may be directly fed into
the anaerobic tank 3, or may be fed into the effluent channel of
the primary settling tank 2 at any point from the weir to the
anaerobic tank 3. When such a method is used, all the crushed or
ground sediment 11 can be fed into the anaerobic tank 3. Since the
capacity of the channel from the supply point of the crushed or
ground sediment 11 to the anaerobic tank 3 is relatively small and
the residence time is relatively short, by changing the feed amount
of the crushed or ground sediment 11, the reaction in the anaerobic
tank 3 may be controlled over relatively short periods of time.
Whether the crushed or ground sediment 11 is directly fed into the
anaerobic tank 3 or is fed into the effluent channel of the primary
settling tank 2 at any point from the weir to the anaerobic tank 3
can be determined depending on the distance between the
installation position of the crushing device or grinding device 10
and the supplying position of the crushed or ground sediment 11,
waterhead difference, or the like.
[0068] The crushed or ground sediment 11 may be fed into the influx
channel of the primary settling tank 2 at any point, and after
removing the solids settled again in the primary settling tank 2,
components of the crushed or ground sediment 11 which are not
settled may be fed into the anaerobic tank 3 together with the
overflow from the primary settling tank 2. In such a method,
although all of the crushed or ground sediment 11 is not fed into
the anaerobic tank 3, since the capacity of the channel from the
supply point of the crushed or ground sediment 11 to the anaerobic
tank 3 is large and the residence time is increased, the
components, excluding solids which readily settle out, originating
from the crushed or ground sediment 11 can be fed into the
anaerobic tank 3 at relatively stable concentrations.
[0069] Depending on conditions, such as the phosphorus
concentration in the effluent from the primary settling tank 2
after sedimentation and the residence time in the anaerobic tank 3
and the aerobic tank 4, the phosphorus-uptake reaction must be
accelerated in comparison with the phosphorus-releasing reaction.
In such a case, the crushed or ground sediment 11 may be introduced
to the aerobic tank 4 in addition to the anaerobic tank 3.
[0070] The withdrawal of the sediment 9 from the primary settling
tank 2 is performed by opening an outlet provided on the lower
section or the bottom of the primary settling tank 2. The sediment
9 may be naturally discharged in accordance with the position of
the primary settling tank 2 or a pump may be used.
[0071] The sediment treated by the crushing device or grinding
device is in the form of a suspension or a slurry, and has a
concentration of approximately 3,000 to 12,000 mg/l, and normally
approximately 5,000 to 10,000 mg/l.
[0072] The timing of adding the crushed or ground sediment is
determined, for example, by measuring the oxidation-reduction
potential (ORP) of the anaerobic tank, and when the ORP is -100 mV
or more, the addition is performed. When the ratio of the organic
substance concentration to the phosphoric acid ion concentration in
the soluble components of wastewater 1 is always rather low, it is
therefore practical to continuously add the crushed or ground
sediment during operation.
[0073] The pump used for withdrawing the sediment 9 from the
primary settling tank is generally a slurry pump which does not
easily clog, and thus the pump is not very effective for crushing
or grinding the sediment. Therefore, in order to crush or grind the
sediment, a crushing device or a grinding device must be provided.
The crushing device or the grinding device 10 may be of various
types, such as a gear type and a multiple spindle disk type. In
order to control the degree of crushing or grinding, it is
effective to appropriately select the mesh of a screen in which the
crushed or ground sediment is to be passed. If the screen mesh is
too small, clogging easily occurs, and if it is too large, solids
having a large grain size easily pass therethrough, and thus the
screen mesh size is preferably chosen to be approximately 1 to 5
mm. Although the crushing device or the grinding device 10 may be
installed in-line or in an open channel, since the sediment 9 may
be a source of unpleasant odors, the in-line installation is
preferred. Since a general-purpose device can be used as such a
crushing device or grinding device 10, the cost of equipment as
well as the operating expenses, which mainly cover the cost of
powering motors, will be relatively low, and the grain size
reduction effect with respect to solids will be relatively
large.
[0074] The amount of the crushed or ground sediment added is,
preferably, approximately 0.1 to 1.5% on the volume of the
wastewater 1, and normally approximately 0.5 to 1.0%. The addition
may be continuously or intermittently performed.
[0075] The addition is preferably terminated when the ORP reaches
approximately -150 to -250 mV, and depending on the result,
continuous addition may be effective.
[0076] FIG. 2 shows another example of a biological phosphorus
removal apparatus in accordance with embodiment 1. With respect to
the biological phosphorus removal apparatus based on the
conventional technique shown in FIG. 6, in accordance with the
results of the inventors' experimentation, when the soluble organic
substance concentration in the wastewater 1 is high and the organic
substance concentration required for the phosphorus-releasing
reaction in the anaerobic tank 3 is obtained, the ORP in the
anaerobic tank 3 is -100 mV or less, and when the organic substance
concentration in the wastewater 1 is low and the organic substance
concentration required for the phosphorus-releasing reaction in the
anaerobic tank 3 is not satisfactory, the ORP in the anaerobic tank
3 is -100 mV or more.
[0077] Based on the above, in the biological phosphorus removal
apparatus in accordance with embodiment 1, shown in FIG. 2, the
operation of the apparatus is controlled so that when the measured
value by an ORP meter 12 mounted on an anaerobic tank 3 is
approximately -150 to -250 mV or less, the influent of crushed or
ground sediment 11 from a primary settling tank is halted or the
operation of a crushing device or a grinding device 10 is halted,
and when the measured value is -100 mV or more, the influx of the
crushed or ground sediment 11 is started. These controls are
performed by a control device 13.
[0078] The phosphorus removal method in the present invention is
also applicable to a wastewater treatment apparatus including an
anaerobic stage, a denitrification stage (an anoxic stage), and a
nitrification stage (an aerobic stage), in addition to the
wastewater treatment apparatus including the anaerobic stage and
the aerobic stage.
[0079] FIG. 3 shows another example of a biological phosphorus
removal apparatus in accordance with embodiment 1. In the
biological phosphorus removal apparatus in accordance with
embodiment 1 shown in FIG. 3, a denitrification tank 14 is added to
the biological phosphorus removal apparatus in accordance with
embodiment 1 shown in FIG. 1. The apparatus shown in FIG. 3 mainly
includes a primary settling tank 2, an anaerobic tank 3, the
denitrification tank 14, an aerobic tank 4, and a final settling
tank 6.
[0080] In the biological phosphorus removal apparatus shown in FIG.
3, in the denitrification tank 14, only agitation is performed, and
effluent from the anaerobic tank 3 and a nitrification circulating
liquid 15 are fed into the denitrification tank 14. In the
denitrification tank 14, nitrate nitrogen or nitrite nitrogen
contained in the nitrification circulating liquid 15 is reduced to
nitrogen gas (a denitrification reaction), using organic substances
in the wastewater, and thus denitrification treatment is
performed.
[0081] Additionally, in the biological phosphorus removal apparatus
shown in FIG. 3, a portion of crushed or ground sediment 11 from
the primary settling tank may be fed into the denitrification tank
14.
EXAMPLE OF THE INVENTION
[0082] An example of the biological phosphorus removal method in
accordance with embodiment 1 will be described below. FIG. 4 shows
an experimental device. In the example, a mixture of return sludge,
wastewater, and primary settling tank sediment taken from a
wastewater treatment apparatus having a flow pattern as shown in
FIG. 6 as a sample was fed into the device shown in FIG. 4, and
characteristics of the phosphorus-releasing reaction of activated
sludge under anaerobic conditions were measured. Table 1 shows the
composition of samples. The wastewater taken from the wastewater
treatment apparatus having a flow shown in FIG. 6 had a BOD
concentration of 42 mg/L, and the return sludge had an MLSS
concentration of 4,200 mg/L. In order to determine if the
wastewater treatment apparatus having a flow shown in FIG. 1
functioned effectively, the primary settling tank sediment having
an MLSS concentration of 3,200 mg/L taken from the primary settling
tank in the wastewater treatment apparatus having a flow shown in
FIG. 6 was ground for 3 minutes by a Potter-type glass homogenizer
having a capacity of 10 ml, and its effectiveness as an organic
substance source was investigated.
1 TABLE 1 Sample and Volume Sample A Sample B Return sludge (ml)
1,000 1,000 Wastewater (ml) 2,000 2,000 Ground sediment 20 0 from
primary settling tank (ml)
[0083] FIG. 5 shows a change in the phosphoric acid ion
concentration in the sample wastewater over time. A sample A in
which the phosphorus-releasing reaction was performed with the
ground primary settling tank sediment being added was compared with
a sample B in which the phosphorus-releasing reaction was performed
without the ground primary settling tank sediment. The rate of
increase of phosphoric acid ion in the wastewater was higher when
the phosphorus-releasing reaction was performed with the ground
sediment being added. That is, it has been confirmed that by adding
the primary settling tank sediment, which had been subjected to
grinding treatment in order to reduce the grain size of solids and
to solubilize a portion of organic substances, to the sample
wastewater, the rate of the phosphorus-releasing reaction was
increased.
[0084] Since crushing treatment is similar to grinding treatment,
the crushing treatment is also considered to be effective in
reducing the grain size of solids in the primary settling tank
sediment and in solubilizing a portion of the organic
substances.
[0085] In embodiment 1, the primary settling tank sediment which
has been crushed or ground is fed into an anaerobic stage, and in
some cases, to an anaerobic stage of a biological phosphorus
removal apparatus having an anoxic stage and an aerobic stage, so
that the organic substance concentration required for the
phosphorus-releasing reaction at the anaerobic stage is
secured.
[0086] Accordingly, either when the quality of treated water is
deteriorated by the decreased rate of phosphorus-releasing reaction
at the anaerobic stage because the concentration of organic
substances supplied to the anaerobic stage is decreased more than
the phosphoric acid ion concentration is decreased, such as in a
case of the influx of rainwater, or when the quality of treated
water is deteriorated by the decreased rate of the
phosphorus-releasing reaction at the anaerobic stage because
soluble pollutants in wastewater entering the anaerobic stage after
being subjected to solid-liquid separation treatment in the primary
settling tank have a low ratio of organic substance concentration
to phosphoric acid ion concentration, the supply of the organic
substances required for the phosphorus-releasing reaction is
secured. Thus, a decrease in the rate of the phosphorus-releasing
reaction at the anaerobic stage can be prevented, and the rate of
the phosphorus-uptake reaction at the subsequent aerobic stage is
effectively increased.
[0087] In embodiment 1, the ORP at the anaerobic stage is measured
and only when the ORP is -100 mV or more, the primary settling tank
sediment which has been subjected to crushing or grinding treatment
is fed into the anaerobic stage. Thus, organic substances can be
supplied only when the organic substance concentration in the
wastewater is decreased and the organic substance concentration
required for the phosphorus-releasing reaction is not secured. By
such control, the cost of power required for crushing or grinding
the primary settling tank sediment or the cost of power required to
transport the primary settling tank sediment after crushing or
grinding to the anaerobic stage can be minimized, and the organic
substance load at the anaerobic stage and the aerobic stage can be
minimized.
[0088] The primary settling tank sediment is subjected to crushing
or grinding treatment so that the grain size of organic solids
contained in the primary settling tank sediment is reduced and the
organic solids are partially solubilized, and the crushed or ground
sediment is then introduced to the anaerobic stage, and in some
cases, further to the aerobic stage or to the denitrification
stage. Thus, the crushed or ground sediment is promptly and
effectively used in microbial reactions which mainly include
phosphorus-releasing reactions and phosphorus-uptake reactions.
[0089] Since a general-purpose device can be used for crushing or
grinding treatment, the cost of equipment can be minimized, the
operating expenses, which mainly cover the cost of powering motors
can be relatively low, and large solids can be effectively broken
up.
[0090] Embodiment 2
[0091] In embodiment 2, two problems are overcome. The first
problem is that due to low organic substance concentration in
wastewater, and in particular, a shortage of the soluble organic
substance concentration, the phosphorus removal performance at a
wastewater treatment plant using the anaerobic-aerobic activated
sludge process is deteriorated. The second problem is that the
residence time at reaction tanks in the plant using an activated
sludge process represented by the conventional activated sludge
process is not sufficient for the phosphorus removal performance in
a plant using the anaerobic-aerobic activated sludge process. When
the operating expenses such as for the addition of methanol are low
and the existing activated sludge facilities for removing BOD are
converted to wastewater treatment facilities using the
anaerobic-aerobic activated sludge process, the BOD removal and the
phosphorus removal can be performed by maximizing the utilization
of volume and capacity of the existing facilities. Moreover, when a
new plant is opened, it is possible to make the residence time at
the entire reactor as compact as that in the plant using the
conventional activated sludge process.
[0092] Embodiment 2 relates to an apparatus for removing phosphorus
from wastewater including at least a settling tank in which
wastewater to be treated is settled (a primary settling tank), an
anaerobic tank, an aerobic tank, and a settling tank in which
wastewater treated in these tanks is settled (a final settling
tank). The apparatus is provided with a line for returning at least
a portion of effluent from the aerobic tank or sludge withdrawn
from the final settling tank to the primary settling tank, and a
line for feeding at least a portion of the sludge from the primary
settling tank to the anaerobic tank or the aerobic tank.
[0093] In the biological phosphorus removal apparatus in accordance
with embodiment 2, by returning at least a portion of the mixed
liquor flowing from the aerobic tank, at least a portion of the
sludge withdrawn from the final settling tank, or both to the
primary settling tank, organic substances contained in sediment and
suspended solids in the primary settling tank, and microorganisms
in the activated sludge returned to the primary settling tank are
brought into contact with each other under the anaerobic conditions
in the primary settling tank. By effectively utilizing the solid
substances in the primary settling tank and the sludge retention
time, phosphorus-releasing reactions proceed. By feeding at least a
portion of the sludge withdrawn from the primary settling tank to
the anaerobic stage, the anoxic stage, or the aerobic stage, the
sludge settled in the primary settling tank, containing the
activated sludge in which the phosphorus-releasing reactions have
proceeded and solid organic substances, is fed to the anaerobic
stage, the anoxic stage, or the aerobic stage. Thus, further
phosphorus-releasing reactions at the anaerobic stage,
phosphorus-uptake reactions, and in some cases, denitrification
reactions, at the anoxic stage, or phosphorus-uptake reactions at
the aerobic stage proceed.
[0094] That is, in the apparatuses of embodiment 2, since the mixed
liquor containing activated sludge flowing from the aerobic tank or
the sludge withdrawn from the final settling tank is returned to
the primary settling tank, activated sludge is present in the
primary settling tank. There are also solid organic substances as
sediment and suspended solids and soluble organic substances
originating from wastewater in the primary settling tank, and the
activated sludge in the primary settling tank is brought into
contact with these organic substances. Since the primary settling
tank is not aerated and the wastewater normally has a large oxygen
demand, the primary settling tank is anaerobic. That is, in the
primary settling tank, activated sludge releases phosphoric acid
ion accumulated in activated sludge cells into wastewater (the
phosphorus-releasing reaction), using the organic substances.
Subsequently, by feeding at least a portion of the sediment in the
primary settling tank to the anaerobic tank or the aerobic tank,
further phosphorus-releasing reactions in the anaerobic tank or
phosphorus-uptake reactions in the aerobic tank are caused.
[0095] When the sludge withdrawn from the final settling tank is
returned to the primary settling tank, since the sludge withdrawn
from the final settling tank has a higher activated sludge
concentration than that of the mixed liquor flowing from the
aerobic tank, the required return volume is smaller with respect to
the same amount of activated sludge, which is advantageous. When
the mixed liquor flowing from the aerobic tank is returned to the
primary settling tank, differing from the case when the sludge
withdrawn from the final settling tank is returned to the primary
settling tank, a water load to the final settling tank is not
increased, which is also advantageous. When both the method for
returning the sludge withdrawn from the final settling tank to the
primary settling tank and the method for returning the mixed liquor
flowing from the aerobic tank to the primary settling tank are
combined, an intermediate merit between the two advantages
described above can be obtained.
[0096] In the anaerobic tank, activated sludge releases phosphorus
using organic substances mainly composed of soluble components in
the wastewater, and in some cases, organic substances originating
from the sediment in the primary settling tank. When at least a
portion of the sediment in the primary settling tank is introduced
to the anaerobic tank, differing from the conventional method, the
activated sludge which has released phosphorus in the primary
settling tank may be introduced to the anaerobic tank, together
with organic solids originating from sediment in the primary
settling tank, to perform a further phosphorus-releasing reaction,
and thus the phosphorus-releasing reaction can be strengthened in
comparison with the conventional method.
[0097] In the aerobic tank, activated sludge takes up phosphoric
acid ion in the wastewater into activated sludge cells (the
phosphorus-uptake reaction), using organic substances mainly
composed of soluble components in the wastewater, and in some
cases, organic substances originating from the sediment in the
primary settling tank, and simultaneously, organic substances are
oxidized, decomposed and removed. When at least a portion of the
sediment in the primary settling tank is introduced to the aerobic
tank, differing from the conventional method, since a
phosphorus-uptake reaction is performed using even organic solids
originating from the sediment in the primary settling tank, the
phosphorus-uptake reaction can be strengthened in comparison with
the conventional method. In such a case, in order to maintain the
sufficient activated sludge concentration in the anaerobic tank,
preferably, a portion of the sludge withdrawn from the final
settling tank is returned to the anaerobic tank.
[0098] The method for removing phosphorus in accordance with the
present invention is also applicable to a wastewater treatment
apparatus including an anaerobic stage, a denitrification stage (an
anoxic stage), and a nitrification stage (an aerobic stage), in
addition to the wastewater treatment apparatus including the
anaerobic stage and the aerobic stage.
[0099] An example of a biological phosphorus removal apparatus
based on embodiment 2 is shown in FIG. 7.
[0100] The biological phosphorus removal apparatus based on
embodiment 2 mainly includes a primary settling tank 2, an
anaerobic tank 3, an aerobic tank 4, and a final settling tank 6.
In the anaerobic tank 3, only agitation is performed, and in the
aerobic tank 4, oxygen is supplied by an air diffuser 5 and
agitation is performed by a flow caused by the air diffusion.
[0101] In the biological phosphorus removal apparatus based on
embodiment 2 shown in FIG. 7, wastewater 1 is subjected to the
solid-liquid separation in the primary settling tank 2, and is then
passed to the anaerobic tank 3 and to the aerobic tank 4 in that
order. Effluent 10 from the aerobic tank 4 flowing into the final
settling tank 6 is separated into treated water 7 and activated
sludge in the final settling tank 6, and preferably, at least a
portion of the activated sludge separated and thickened in the
final settling tank 6, that is, sludge 8 withdrawn from the final
settling tank, is sent to the anaerobic tank 3 as return
sludge.
[0102] At least a portion of the effluent 10 from the aerobic tank
4, at least a portion of the sludge 8 withdrawn from the final
settling tank 6, or both is returned to the primary settling tank
2, and at least a portion of the sludge withdrawn from the primary
settling tank 2, that is, sediment 9 in the primary settling tank,
is fed to the anaerobic tank 3 or the aerobic tank 4.
[0103] Depending on the quality of wastewater and other conditions,
at least a portion of the sediment 9 in the primary settling tank
may be arranged to be sent to the anaerobic tank 3 instead of
returning the sludge 8 withdrawn from the final settling tank 6 to
the anaerobic tank 3.
[0104] FIG. 8 shows an example of a biological phosphorus removal
apparatus in accordance with embodiment 2. In the biological
phosphorus removal apparatus shown in FIG. 8, a denitrification
tank 14 is added to the biological phosphorus removal apparatus
shown in FIG. 7. The apparatus shown in FIG. 8 mainly includes a
primary settling tank 2, anaerobic tank 3, the denitrification tank
14, an aerobic tank 4, and a final settling tank 6.
[0105] In the biological phosphorus removal apparatus based on
embodiment 2 shown in FIG. 8, in the denitrification tank 14, only
agitation is performed, and effluent from the anaerobic tank 3 and
mixed liquor 10 from the aerobic tank 4 are fed into the
denitrification tank 14. In the denitrification tank 14, nitrate
nitrogen or nitrite nitrogen contained in the mixed liquor 10 from
the aerobic tank 4 is reduced to nitrogen gas (the denitrification
reaction), using organic substances in the wastewater, and thus
denitrification treatment is performed.
[0106] Furthermore, in the biological phosphorus removal apparatus
based on embodiment 2 shown in FIG. 8, the mixed liquor 10 from the
aerobic tank 4 is returned to the primary settling tank 2 and at
least a portion of the sediment 9 in the primary settling tank is
fed to the anaerobic tank 3. By introducing activated sludge to the
primary settling tank 2, a phosphorus-releasing reaction can be
caused in the primary settling tank 2, and by feeding the sediment
9 containing the sludge in which the phosphorus-releasing reaction
has proceeded to the anaerobic tank 3, a further
phosphorus-releasing reaction proceeds in the anaerobic tank 3, and
subsequently a phosphorus-uptake reaction proceeds in the
denitrification tank 14 and the aerobic tank 4.
[0107] In FIG. 8, although the mixed liquor 10 flowing from the
aerobic tank 4 is returned to the primary settling tank 2 in order
to introduce activated sludge to the primary settling tank 2, a
method of returning sludge 8 withdrawn from the final settling tank
6 to the primary settling tank 2 may be employed. Additionally, in
FIG. 8, although the sediment 9 in the primary settling tank is fed
to the anaerobic tank 3, the sediment 9 may be fed into the
denitrification tank 14 or the aerobic tank 4. In order to
accelerate the nitrification reaction, carriers for immobilizing
microorganisms may be added to the aerobic tank 4.
[0108] Effluent from the aerobic tank has a sludge concentration of
approximately 1,000 to 4,000 mg/L, and normally approximately 2,000
to 3,000 mg/L. The concentration of the sludge withdrawn from the
final settling tank is approximately 5,000 to 10,000 mg/L, and
normally approximately 6,500 to 8,500 mg/L. In the present
invention, the sludge is preferably returned to the reactor at
approximately 10 to 40% of the influent flow, and to the primary
settling tank at approximately 5 to 30% of the influent flow.
Meanwhile, in the steady state in which the sludge is being
returned, the concentration of the sludge withdrawn from the
primary settling tank is approximately 6,000 to 13,000 mg/L, and
normally approximately 8,000 to 10,000 mg/L. In the present
invention, the sludge is arranged to be sent to the anaerobic tank,
the anoxic tank, or the aerobic tank at approximately 5 to 40% of
the influent flow, and preferably, at approximately 10 to 30% of
the influent flow. When the sludge is sent to two or more tanks,
the amount described above corresponds to the total amount.
EXAMPLE OF THE INVENTION
[0109] An example of the biological phosphorus removal method in
accordance with embodiment 2 will be described below. Comparative
experiments were conducted with respect to an apparatus using the
conventional anaerobic-aerobic activated sludge process as shown in
FIG. 6 and an apparatus using the anaerobic-aerobic activated
sludge process in accordance with the present invention as shown in
FIG. 7 in which a portion of the sludge withdrawn from the final
settling tank is returned to the primary settling tank and the
sediment 9 in the primary settling tank is fed into the anaerobic
tank 3. In either case, the volume of wastewater treated was 12
m.sup.3/day, and the residence time at the primary settling tank 2,
the anaerobic tank 3, the aerobic tank 4, and the final settling
tank 6 were set at 3 hours, 1 hour, 8 hours, and 4 hours,
respectively. In the conventional method, the return sludge flow
was set at 30% of the influent wastewater flow, and in the method
of the present invention, sludge withdrawn from the final settling
tank 6 was returned to the anaerobic tank 3 and the primary
settling tank 2, at flows corresponding to 20% and 10% of the
influent wastewater flow, respectively. The flow of the sediment 9
withdrawn from the primary settling tank and fed into the anaerobic
tank 3 was set at 10% of the influent wastewater flow. As
wastewater 1, domestic wastewater was used, and the experiments
were conducted at water temperatures of 17 to 20.degree. C. In each
case, a three-month preliminary operation was carried out, and
after treatment was stabilized, the quality of treated water was
investigated. Table 2 shows the results of analysis with respect to
the treated water in each case along with the domestic wastewater
used. When the data was collected, the mixed liquor suspended solid
(MLSS) concentration in the denitrification tank and the
nitrification tank was approximately 2,600 mg/L in the conventional
case, and approximately 2,700 mg/L in the method of embodiment
2.
2 TABLE 2 Samples Treated water in Domestic conventional Treated
water in Analysis item wastewater method embodiment 2 Total
phosphorus 3.8 1.5 0.4 (mg/L) BOD (mg/L) 85.2 11.6 10.7 SS (mg/L)
89.1 5.5 5.0 PH (--) 7.4 7.2 7.2 Total nitrogen 18.6 12.2 12.0
(mg/L)
[0110] As is clear from Table 2, in accordance with the method of
embodiment 2, treated water having a lower phosphorus concentration
was obtained in comparison with the conventional method when the
same amount of wastewater was treated using the facilities having
the same capacity.
[0111] In accordance with the wastewater treatment of embodiment 2,
the biological phosphorus removal apparatus includes the primary
settling tank, the anaerobic stage, and in some cases, the anoxic
stage, the aerobic stage, and the final settling tank. At least a
portion of the mixed liquor flowing from the aerobic stage, at
least a portion of the sludge withdrawn from the final settling
tank, or both is returned to the primary settling tank, and at
least a portion of sludge withdrawn from the primary settling tank
is fed to the anaerobic stage, the anoxic stage, or the aerobic
stage.
[0112] Accordingly, either when the quality of treated water is
deteriorated by the decreased rate of the phosphorus-releasing
reaction at the anaerobic stage because the concentration of
organic substances supplied to the anaerobic stage is decreased
more than the phosphorus concentration is decreased, such as a case
of the influx of rainwater, or when the quality of treated water is
deteriorated by the decreased rate of the phosphorus-releasing
reaction at the anaerobic stage because soluble wastes in
wastewater entering the anaerobic stage after being subjected to
solid-liquid separation treatment in the primary settling tank have
a low ratio of the organic substance concentration to the
phosphorus concentration, the primary settling tank functions as a
reaction tank for the phosphorus-releasing reaction, and even solid
organic substances in the primary settling tank originating from
wastewater can be effectively used for the phosphorus removal
reaction. The sludge retention time in the primary settling tank
also effectively works in order to advance the phosphorus-releasing
reaction by activated sludge. Since solid organic substances in the
primary settling tank originating from wastewater can be
effectively used for the phosphorus-releasing reaction, or further
for the phosphorus-uptake reaction, the cost for the chemical agent
such as methanol to be added can be reduced. It is also not
required to use chemicals such as ozone which incur electric power
costs for production. The major operating expenses required for
implementing the method of embodiment 2 are electric power costs
for transporting liquid and air, which are substantially the same
as those in the conventional facilities using the anaerobic-aerobic
activated sludge process, resulting in low operating expenses.
[0113] In accordance with embodiment 2, since the primary settling
tank functions as a reactor for the phosphorus-releasing reaction,
and the sludge retention time in the primary settling tank
effectively affects the phosphorus-releasing reaction, when the
existing activated sludge facilities for removing BOD are converted
to wastewater treatment facilities using the anaerobic-aerobic
activated sludge process, the same effect can be obtained as that
when reactors are enlarged without drastically changing the basic
civil engineering structure of the existing facilities, and
moreover, the designed water flow can be treated in the primary
settling tank and the final settling tank. Thus, BOD removal
treatment and phosphorus removal treatment, and in some cases,
nitrogen removal treatment, can be performed by maximizing the
utilization of volume and capacity of the existing facilities, and
satisfactory treatment can be achieved, resulting in low
construction costs (conversion costs). Moreover, when new
wastewater treatment facilities using the anaerobic-aerobic
activated sludge process is opened, since reactors for the
biological treatment are as compact as those in the conventional
facilities using the conventional activated sludge process, the
construction costs are low.
[0114] Embodiment 3
[0115] In embodiment 3, the deterioration of nitrogen removal
performance of a wastewater treatment apparatus using the
biological nitrification-denitrification process due to low organic
substance concentration in wastewater, in particular, a shortage of
the soluble organic substance concentration, is overcome. In
accordance with embodiment 3, methods and apparatuses for
effectively and economically removing nitrogen are provided, in
which sludge containing solid organic substances generated in a
primary settling tank is supplied to a denitrification tank or an
anaerobic tank, and thus by compensating for the shortage of the
organic substance concentration of the wastewater flowing into the
denitrification tank or the anaerobic tank, organic substances
generated in the waste treatment facilities are effectively used
and the deterioration of nitrogen removal performance is
avoided.
[0116] Embodiment 3 relates to methods for removing nitrogen from
wastewater and apparatuses therefor, in which nitrogen in
wastewater is removed using an apparatus including at least a
settling tank for wastewater to be treated, a denitrification tank
or an anaerobic tank, and a nitrification tank, and at least a
portion of the sediment from the wastewater in the settling tank is
crushed or ground before being supplied to the denitrification tank
or the anaerobic tank.
[0117] In accordance with the method for biologically removing
nitrogen from wastewater and the apparatus therefor in embodiment
3, at least a portion of sediment in the primary settling tank,
which contains abundant solid organic substances, that is, at least
a portion of the solid organic substances, is fed into the
denitrification stage or the anaerobic stage in order to secure an
organic substance concentration required for the denitrification
reaction at the denitrification stage. By crushing or grinding at
least a portion of the sediment in the primary settling tank, solid
organic substances contained in the sediment in the primary
settling tank are particulated or solubilized so that
microorganisms can effectively and promptly use the solid organic
substances for the denitrification reaction. The present inventors
found through experimentation that when the organic substance
concentration in wastewater is low and the organic substance
concentration required for the denitrification reaction at the
denitrification stage is insufficient, the oxidation-reduction
potential (ORP) at the denitrification stage is 0 mV or more.
Accordingly, the ORP at the denitrification stage is measured, and
only when the ORP is 0 mV or more, that is, only when the organic
substance concentration required for the denitrification reaction
is insufficient at the denitrification stage, the sediment from the
primary settling tank which has been crushed or ground is fed into
the denitrification tank or the anaerobic tank. Thus, by supplying
organic substances originating from the solids, the organic
substance concentration required for the denitrification reaction
will be sufficient.
[0118] An example of a biological nitrogen removal apparatus based
on embodiment 3 is shown in FIG. 9.
[0119] The biological nitrogen removal apparatus based on
embodiment 3 mainly includes a primary settling tank 102, a
denitrification tank 103, a nitrification tank 104, and a final
settling tank 107. In the denitrification tank 103, only agitation
is performed, and in the nitrification tank 104, oxygen is supplied
by an air diffuser 105 and agitation is performed by a flow caused
by the air diffusion.
[0120] In the biological nitrogen removal apparatus shown in FIG.
9, wastewater 101 is subjected to the solid-liquid separation in
the primary settling tank 102, and is then passed to the
denitrification tank 103 and to the nitrification tank 104 in that
order. Effluent from the nitrification tank 104 flowing into the
final settling tank 107 is separated into treated water 109 and
activated sludge in the final settling tank 107. At least a portion
of the activated sludge separated and thickened in the final
settling tank 107 is sent to the denitrification tank 103 as return
sludge 108. A portion of effluent from the nitrification tank 104
is sent to the denitrification tank as a nitrification circulating
liquid 106.
[0121] Sediment 110 settled in the primary settling tank is
subjected to crushing or grinding treatment by a crushing device or
a grinding device 111, and is then directly sent to the
denitrification tank 103 as crushed or ground sediment 112 from the
primary settling tank, or is sent to the denitrification tank 103
through the effluent channel of the primary settling tank 102.
Alternatively, the crushed or ground sediment 112 is fed to the
primary settling tank 102 through the influx channel of the primary
settling tank 102, and a portion, excluding those which are settled
again in the primary settling tank 102, is sent to the
denitrification tank 103 through the effluent channel of the
primary settling tank 102.
[0122] In the nitrification tank 104, nitrogen compounds in
wastewater is oxidized to nitrate nitrogen or to nitrite nitrogen
by activated sludge (the nitrification reaction), and
simultaneously, organic substances are removed by oxidizing
decomposition. In the denitrification tank 103, activated sludge
reduces nitrate nitrogen or nitrite nitrogen contained in the
wastewater 101, the return sludge 108, and the nitrification
circulating liquid 106 to nitrogen gas (the denitrification
reaction), using organic substances mainly composed of soluble
components in the wastewater 101 and organic substances originating
from the sediment 110.
[0123] After solid organic substances contained in the sediment 110
are treated by the crushing device or the grinding device 111, at
least a portion thereof is finely particulated and at least a
portion thereof becomes soluble components. That is, when the grain
size of the solids are reduced by crushing or grinding treatment,
the surface area increases with respect to solids having the same
weight. Microorganisms, animals, plants, and the like are contained
in the sediment 110, and these organisms are composed of cells
enclosed by cell membranes and/or cell walls. When these membranous
structures are broken by crushing or grinding treatment, since cell
fluids are released from the cells, the soluble organic substance
concentration of the wastewater into which the crushed or ground
sediment 112 is fed is increased.
[0124] In general, when microorganisms propagate by decomposing and
using organic substances, the organic substances having higher
molecular weights take longer time to be decomposed and become
useful. Organic substances having lower molecular weights are
easier to be used, and the rate of the decomposing reaction is
higher. In comparison with soluble organic substances, solid
organic substances generally have higher molecular weights, and
thus the solid organic substances are more difficult to be used by
microorganisms. This is because of the fact that when
microorganisms take up organic substances through cell membranes
and/or cell walls, the organic substances must have enough low
molecular weights. In order for microorganisms to use organic
substances having high molecular weights, the organic substances
having high molecular weights must be decomposed into organic
substances having lower molecular weights, thus requiring time for
decomposition to usable states. In organisms, reactions for
decomposing organic substances having high molecular weights into
organic substances having lower molecular weights are mainly
performed by the enzymatic reactions. In such reactions, as the
surface on which an enzyme acts is increased, that is, as the grain
sizes of solid organic substances having comparable weights are
decreased, the reaction rate increases. Consequently, in the method
in accordance with embodiment 3, by subjecting the sediment 110 to
crushing or grinding treatment so that the grain sizes of solids
are reduced, the enzymatic reactions which allow microorganisms to
decompose and use solid organic substances are enhanced, the rate
of decomposition and use of solid organic substances is increased,
and the microbial reactions may be effectively completed in a
reactor of limited capacity. If the soluble organic substances
generated by crushing or grinding the sediment 110 are introduced
to the denitrification tank 103, they are also effectively and
promptly used for the denitrification reaction by denitrifying
bacteria. In this respect also, the crushing or grinding treatment
to the sediment 110 contributes to an increase in the rate of
decomposition and use of the organic substances contained in solids
in the influent wastewater 101 by microorganisms in the reaction
tank, and this is effective to complete the microbial reactions in
a reactor of limited capacity.
[0125] The crushed or ground sediment 112 may be directly fed into
the denitrification tank 103, or may be fed into the effluent
channel of the primary settling tank 102 at any point from the weir
to the denitrification tank 103. When such a method is used, all
the crushed or ground sediment 112 can be fed into the
denitrification tank 103. Since the capacity of the channel from
the supply point of the crushed or ground sediment 112 to the
denitrification tank 103 is relatively small and the residence time
is relatively short, by changing the feed amount of the crushed or
ground sediment 112, the reaction in the denitrification tank 103
may be controlled over relatively short periods of time. Whether
the crushed or ground sediment 112 is directly fed into the
denitrification tank 103 or is fed into the effluent channel of the
primary settling tank 102 at any point from the weir to the
denitrification tank 103 can be determined depending on the
distance between the installation position of the crushing device
or grinding device 111 and the supplying position of the crushed or
ground sediment 112, waterhead difference, or the like.
[0126] The crushed or ground sediment 112 may be fed into the
influx channel of the primary settling tank 102 at any point, and
after removing the solids settled again in the primary settling
tank 102, components of the crushed or ground sediment 112 which
are not settled may be fed into the denitrification tank 103
together with the overflow from the primary settling tank 102. In
such a method, although all of the crushed or ground sediment 112
is not fed into the denitrification tank 103, since the capacity of
the channel from the supply point of the crushed or ground sediment
112 to the denitrification tank 103 is large and the residence time
is increased, the components, excluding solids which readily settle
out, originating from the crushed or ground sediment 112 can be fed
into the denitrification tank 103 at relatively stable
concentrations.
[0127] The withdrawal of the sediment 110 from the primary settling
tank 102 is performed by opening an outlet provided on the lower
section or the bottom of the primary settling tank 102. The
sediment 110 may be naturally discharged in accordance with the
position of the primary settling tank 102 or a pump may be
used.
[0128] The sediment treated by the crushing device or grinding
device is in the form of a suspension or a slurry, and has a
concentration of approximately 3,000 to 12,000 mg/l, and normally
approximately 5,000 to 10,000 mg/l.
[0129] The timing of adding the crushed or ground sediment is
determined, for example, by measuring the oxidation-reduction
potential (ORP) of the denitrification tank, and when the ORP is 0
mV or more, the addition is performed. When the ratio of the
organic substance concentration to the nitrogen concentration in
the soluble waste in wastewater 101 is always rather low, it is
therefore practical to continuously add the crushed or ground
sediment during operation.
[0130] The pump used for withdrawing the sediment 109 from the
primary settling tank is generally a slurry pump which does not
easily clog, and thus the pump is not very effective for crushing
or grinding the sediment. Therefore, in order to crush or grind the
sediment, a crushing device or a grinding device must be provided.
The crushing device or the grinding device 111 may be of various
types, such as a gear type and a multiple spindle disk type. In
order to control the degree of crushing or grinding, it is
effective to appropriately select the mesh of a screen in which the
crushed or ground sediment is to be passed. If the screen mesh is
too small, clogging easily occurs, and if it is too large, solids
having a large grain size easily pass therethrough, and thus the
screen mesh size is preferably chosen to be approximately 1 to 5
mm. Although the crushing device or the grinding device 111 may be
installed in-line or in an open channel, since the sediment 110 may
be a source of unpleasant odors, the in-line installation is
preferred. Since a general-purpose device can be used as such a
crushing device or grinding device 111, the cost of equipment as
well as the operating expenses, which mainly cover the cost of
powering motors, will become relatively low, and the grain size
reduction effect with respect to solids will be relatively
large.
[0131] The load of the crushed or ground sediment is, preferably,
approximately 0.1 to 1.5% on the volume of the wastewater 101, and
normally approximately 0.5 to 1.0%. The addition may be
continuously or intermittently performed.
[0132] The addition is preferably terminated when the ORP reaches
approximately -50 to -150 mV, and depending on the result,
continuous addition may be effective.
[0133] FIG. 10 shows another example of a biological nitrogen
removal apparatus in accordance with embodiment 3. With respect to
the biological nitrogen removal apparatus based on the conventional
technique shown in FIG. 14, in accordance with the results of the
inventors' knowledge from experimentation, when the soluble organic
substance concentration in the wastewater 101 is high and the
organic substance concentration required for the denitrification
reaction in the denitrification tank 103 is obtained, the ORP in
the denitrification tank 103 is 0 mV or less, and when the organic
substance concentration in the wastewater 101 is low and the
organic substance concentration required for the denitrification
reaction in the denitrification tank 103 is not satisfactory, the
ORP in the denitrification tank 103 is 0 mV or more. Based on the
above, in the biological nitrogen removal apparatus in accordance
with embodiment 3 shown in FIG. 10, the operation of the apparatus
is controlled so that when the measured value by an ORP meter 113
mounted on a denitrification tank 103 is 0 mV or less, the influx
of crushed or ground sediment 112 from a primary settling tank is
halted or the operation of a crushing device or a grinding device
111 is halted, and when the measured value is 0 mV or more, the
introduction of the crushed or ground sediment 112 is started.
These controls are performed by a control device 114.
[0134] The nitrogen removal method in accordance with embodiment 3
is also applicable to a wastewater treatment apparatus including an
anaerobic stage, a denitrification stage (an anoxic stage), and a
nitrification stage (an aerobic stage), in addition to the
wastewater treatment apparatus including the denitrification stage
and the nitrification stage.
[0135] FIG. 11 shows another example of a biological nitrogen
removal apparatus in accordance with embodiment 3. In the
biological nitrogen removal apparatus in accordance with embodiment
3 shown in FIG. 11, an anaerobic tank 115 is added to the
biological nitrogen removal apparatus in accordance with embodiment
3 shown in FIG. 9. The apparatus shown in FIG. 11 mainly includes a
primary settling tank 102, the anaerobic tank 115, a
denitrification tank 103, a nitrification tank 104, and a final
settling tank 107.
[0136] In the biological nitrogen removal apparatus shown in FIG.
11, in the anaerobic tank 115, only agitation is performed, and
wastewater 101 and return sludge 108 are fed into the anaerobic
tank 115. In the anaerobic tank 115, activated sludge reduces
nitrate nitrogen or nitrite nitrogen contained in wastewater 101
and return sludge 108 to nitrogen gas (the denitrification
reaction), using organic substances mainly composed of soluble
components in the wastewater 101, and activated sludge releases
phosphate anions accumulated in cells (the biological
phosphorus-releasing reaction). In the denitrification tank 103,
agitation is also performed and nitrate nitrogen or nitrite
nitrogen contained in nitrification circulating liquid and mixed
liquor flowing from the anaerobic tank 115 is reduced to nitrogen
gas (the denitrification reaction). Furthermore, in the
denitrification tank 103 and the nitrification tank 104, activated
sludge takes up phosphate anions in wastewater into cells (the
biological phosphorus-uptake reaction). In the nitrification tank
104, organic nitrogen and ammonia nitrogen are oxidized to nitrate
nitrogen or nitrite nitrogen (the nitrification reaction). Organic
substances are consumed and treated in each of the anaerobic tank
115, the denitrification tank 103, and the nitrification tank
104.
[0137] Additionally, in the biological nitrogen removal apparatus
shown in FIG. 11 in accordance with embodiment 3, crushed or ground
sediment 112 from the primary settling tank is arranged to be fed
into the anaerobic tank 115. A portion of the crushed or ground
sediment 112 or the entire crushed or ground sediment 112 may be
fed into the denitrification tank 103.
EXAMPLE OF THE INVENTION
[0138] An example of the biological nitrogen removal method in
accordance with embodiment 3 will be described below. FIG. 12 shows
an experimental device. In the example, a mixture of return sludge,
nitrification circulating liquid, wastewater, and primary settling
tank sediment taken from a wastewater treatment apparatus having a
flow pattern as shown in FIG. 14 as a sample was fed into the
device shown in FIG. 12, and characteristics of the denitrification
reaction of activated sludge under denitrification tank conditions
were measured. Table 3 shows the composition of samples. The
wastewater taken from the wastewater treatment apparatus having a
flow shown in FIG. 14 had a BOD concentration of 44 mg/L, and the
return sludge and the nitrification circulating liquid had MLSS
concentrations of 4,300 mg/L and 1,800 mg/L, respectively. In order
to determine if the wastewater treatment apparatus having a flow
shown in FIG. 9 functioned effectively, the primary settling tank
sediment having an MLSS concentration of 3,400 mg/L taken from the
primary settling tank in the wastewater treatment apparatus having
a flow shown in FIG. 14 was ground for 3 minutes by a Potter-type
glass homogenizer having a capacity of 10 ml, and its effectiveness
as an organic substance source was investigated.
3 TABLE 3 Sample and Volume Sample A Sample B Nitrification 1,500
1,500 circulating liquid (ml) Return sludge 500 500 (ml) Wastewater
1,000 1,000 (ml) Ground sediment from 20 0 primary settling tank
(ml)
[0139] FIG. 13 shows a change in the nitrate nitrogen concentration
in the sample wastewater over time. A sample A in which the
denitrification reaction was performed with the ground primary
settling tank sediment being added was compared with a sample B in
which the denitrification reaction was performed without the ground
primary settling tank sediment. The rate of decrease of nitrate
nitrogen in the wastewater was higher when the denitrification
reaction was performed with the ground sediment being added. That
is, it has been confirmed that by adding the primary settling tank
sediment, which had been subjected to grinding treatment in order
to reduce the grain size of solids and to solubilize a portion of
organic substances, to the sample wastewater, the rate of the
denitrification reaction was increased.
[0140] Since crushing treatment is similar to grinding treatment,
the crushing treatment is also considered to be effective in
reducing the grain size of solids in the primary settling tank
sediment and in solubilizing a portion of the organic
substances.
[0141] In embodiment 3, the primary settling tank sediment which
has been crushed or ground is fed into a denitrification stage, and
in some cases, to a denitrification stage or an anaerobic stage of
a biological nitrogen removal apparatus having the anaerobic stage
and the nitrification stage, so that the organic substance
concentration required for the denitrification reaction at the
denitrification stage is secured.
[0142] Accordingly, either when the quality of treated water is
deteriorated by the decreased rate of denitrification reaction at
the denitrification stage because the concentration of organic
substances supplied to the denitrification stage is decreased more
than the nitrogen concentration is decreased, such as in a case of
the influx of rainwater, or when the quality of treated water is
deteriorated by the decreased rate of the denitrification reaction
at the denitrification stage because soluble wastes in wastewater
entering the denitrification stage after being subjected to
solid-liquid separation treatment in the primary settling tank have
a low ratio of organic substance concentration to nitrogen
concentration, the supply of the organic substances required for
the denitrification reaction is secured. Thus, a decrease in the
rate of the denitrification reaction at the denitrification stage
can be avoided.
[0143] In embodiment 3, the ORP at the denitrification stage is
measured and only when the ORP is 0 mV or more, the primary
settling tank sediment which has been subjected to crushing or
grinding treatment is fed into the denitrification stage or the
anaerobic stage. Thus, organic substances can be supplied only when
the organic substance concentration in the wastewater is decreased
and the organic substance concentration required for the
denitrification reaction is in sufficient. By such control, the
cost of power required for crushing or grinding the primary
settling tank sediment or the cost of power required to transport
the primary settling tank sediment after crushing or grinding to
the denitrification stage or the anaerobic stage can be minimized,
and the organic substance load at the denitrification stage, the
anaerobic stage, and the nitrification stage can be minimized.
[0144] The primary settling tank sediment is subjected to crushing
or grinding treatment so that the grain size of organic solids
contained in the primary settling tank sediment is reduced and the
organic solids are partially solubilized, and the crushed or ground
sediment is then introduced to the denitrification tank or the
denitrification stage such as the anaerobic tank. Thus, the crushed
or ground sediment is promptly and effectively used in microbial
reactions which mainly include nitrogen reactions.
[0145] Since a general-purpose device can be used for crushing or
grinding treatment, the cost of equipment can be minimized, the
operating expenses, which mainly cover the cost of powering motors
can be relatively low, and large solids can be effectively broken
up.
[0146] Embodiment 4
[0147] Embodiment 4 overcomes the following problems. The first
problem is that due to a shortage of the organic substance
concentration in wastewater, and in particular, a shortage of the
soluble organic substance concentration, the nitrogen removal
performance at a wastewater treatment facilities using the
biological nitrification-denitrification process is deteriorated.
The second problem is that due to a shortage of the residence time
at reaction tanks when facilities using an activated sludge process
represented by the conventional activated sludge process is
converted to facilities using the biological
nitrification-denitrifica- tion process, the nitrogen removal
performance at the wastewater treatment facilities using the
biological nitrification-denitrification process is deteriorated.
When the operating expenses such as for the addition of methanol
are low and the existing activated sludge facilities for removing
BOD are converted to wastewater treatment facilities using the
biological nitrification-denitrification process, the BOD removal
and the nitrogen removal can be performed by maximizing the
utilization of volume and capacity of the existing facilities. When
a new plant is opened, it is possible to make the residence time at
the entire reactors as compact as that in the plant using the
conventional activated sludge process.
[0148] Embodiment 4 relates to an apparatus for removing nitrogen
from wastewater including at least a settling tank in which
wastewater to be treated is settled (a primary settling tank), a
denitrification tank, a nitrification tank, and a settling tank in
which wastewater treated in these tanks is settled (a final
settling tank). The apparatus is provided with a line for returning
at least a portion of the effluent from the nitrification tank or
the effluent from the final settling tank to the primary settling
tank.
[0149] In the biological nitrogen removal apparatus in accordance
with embodiment 4, at least a portion of the mixed liquor flowing
from the nitrification tank, a portion of the treated water from
the final settling tank, or both is returned to the primary
settling tank. Thus, denitrifying bacteria contained in solids
existing as sediment and suspended matter, and solid organic
substances in the primary settling tank are effectively used for
the denitrification reaction, and the primary settling tank is also
used as a reaction tank for the denitrification reaction. When at
least a portion of the mixed liquor from the nitrification stage is
returned to the primary settling tank, if the returned volume is
large and all the sludge withdrawn from the primary settling tank
is introduced to the sludge treatment stage, the mixed liquor
suspended solid concentration may be decreased and the rate of the
denitrification reaction or the nitrification reaction, or both may
be decreased, resulting in a hindrance to treatment. Therefore, by
feeding at least a portion of sludge in the primary settling tank
to the denitrification tank or the anaerobic tank, the sufficient
concentration of mixed liquor suspended solids in the reaction tank
is obtained.
[0150] That is, in general, the mixed liquor from the nitrification
tank contains nitrate nitrogen or nitrite nitrogen due to the
treatment at the nitrification tank, and since the treated water
flowing from the final settling tank corresponds to the mixed
liquor from the nitrification tank from which solids are removed,
the treated water contains nitrate nitrogen or nitrite nitrogen.
Moreover, even if the nitrification reaction takes place
sufficiently, when the denitrification reaction is insufficient
because of a shortage of the organic substances used in the
denitrification reaction, a shortage of the residence time in the
denitrification reaction tank, or the like, the total of the
nitrate nitrogen concentration and the nitrite nitrogen
concentration will be increased in the mixed liquor from the
nitrification tank and the treated water.
[0151] One of the reasons for the insufficient denitrification
reaction is that since the ratio of the organic substance
concentration to the nitrogen concentration in soluble pollutants
in wastewater is low, most of the solid organic substances
contained in the wastewater are removed in the primary settling
tank, and thus with respect to the composition of the wastewater
entering the denitrification tank, the amount of organic substances
is low in comparison with that of nitrate nitrogen or nitrite
nitrogen. Therefore, in such a case, by returning at least a
portion of the mixed liquor from the nitrification tank, at least a
portion of the treated water flowing from the final settling tank,
or both, solid organic substances originating from wastewater which
exist as sediment and suspended matter in the primary settling tank
can be effectively used as a reducing agent for the denitrification
reaction. Since the wastewater in the primary settling tank has a
relatively high organic substance concentration and a large oxygen
demand, the primary settling tank is anaerobic, which satisfies the
requirement for the denitrification reaction. Moreover, the
denitrification reaction depends on the nitrate respiration
activity and the nitrite respiration activity by microorganisms.
Characteristics such as nitrate respiration or nitrite respiration
are widely observed in soil microorganisms and activated sludge
microorganisms. Since some of microorganisms constituting solids in
the primary settling tank have the denitrification activity, even
when the treated water as mixed liquor from the nitrification tank
from which solids are removed is introduced to the primary settling
tank, microorganisms which cause the denitrification are supplied
in the primary settling tank.
[0152] Another reason for the insufficient denitrification reaction
is that the residence time at the denitrification is insufficient.
The factors affecting the rate of the denitrification reaction
includes water temperature, sludge concentration, the quality and
amount of substrates including organic substances. Microorganisms
which cause the denitrification reaction themselves may be used as
substrates (an endogenous denitrification reaction). When the
denitrification reaction is insufficient, it is effective in
expediting the denitrification reaction to increase time (residence
time) for the coexistence of nitrate nitrogen or nitrite nitrogen
and microorganisms under the anaerobic conditions. In accordance
with the present invention, since the primary settling tank can be
used as a denitrification reaction tank, the residence time at the
primary settling tank, which is generally designed for 3 to 6
hours, effectively contributes to the satisfactory
denitrification.
[0153] In embodiment 4, at least a portion of the mixed liquor from
the nitrification tank, at least a portion of the treated water
flowing from the final settling tank, or both is returned to the
primary settling tank. When at least a portion of the mixed liquor
from the nitrification tank is returned to the primary settling
tank, since denitrifying bacteria in the mixed liquor suspended
solids, together with nitrate nitrogen or nitrite nitrogen, are
supplied to the primary settling tank, an increase in rate of the
denitrification reaction in the primary settling tank may be
achieved. In order to increase the percentage of the portion to be
supplied to the denitrification treatment in nitrate nitrogen or
nitrite nitrogen contained in the mixed liquor from the
nitrification tank, the flow amount of the mixed liquor from the
nitrification tank or the treated water to be returned to the
primary settling tank must be relatively large in comparison with
the flow amount of the wastewater. However, when the amount of the
mixed liquor from the nitrification tank returned to the primary
settling tank is large and all of the sludge withdrawn from the
primary settling tank is introduced to the sludge treatment stage,
the thinning rate of activated sludge is higher than the
propagation rate of activated sludge. The activated sludge
concentration in the denitrification tank and the nitrification
tank is decreased and the rate of the denitrification in the
denitrification tank and the rate of the nitrification in the
nitrification tank are decreased, thus adversely affecting the
treatment. When at least a portion of the treated water is returned
to the primary settling tank, since the concentration of
denitrifying bacteria in the primary settling tank is relatively
low, the rate of the reaction is relatively low. In such a case,
even if all the sludge withdrawn from the primary settling tank is
introduced to the sludge treatment stage, the thinning effect of
activated sludge, which may occur when the mixed liquor from the
nitrification tank is returned to the primary settling tank, is not
caused. When both a portion of the mixed liquor from the
nitrification tank and a portion of the treated water from the
final settling tank are returned to the primary settling tank, the
rate of the denitrification in the primary settling tank and the
activated sludge concentration in the denitrification tank and the
nitrification tank are affected at an intermediate level between
the two cases described above.
[0154] An example of a biological nitrogen removal apparatus based
on embodiment 4 is shown in FIG. 15. Embodiment 4 will be described
in detail with reference to the drawing.
[0155] The biological nitrogen removal apparatus based on
embodiment 4 mainly includes a primary settling tank 102, a
denitrification tank 103, a nitrification tank 104, and a final
settling tank 107. In the denitrification tank 103, only agitation
is performed, and in the nitrification tank 104, oxygen is supplied
by an air diffuser 105 and agitation is performed by a flow caused
by the air diffusion.
[0156] In the biological nitrogen removal apparatus based on
embodiment 4 shown in FIG. 15, wastewater 101 is subjected to the
solid-liquid separation in the primary settling tank 102, and is
then passed to the denitrification tank 103 and to the
nitrification tank 104 in that order. Effluent from the
nitrification tank 104 flowing into the final settling tank 107 is
separated into treated water 109 and activated sludge in the final
settling tank 107, and preferably, at least a portion of the
activated sludge separated and thickened in the final settling tank
107 is sent to the denitrification tank 103 as return sludge 108.
At least a portion of the effluent mixed with sludge from the
nitrification tank 104, at least a portion of the treated water 109
flowing from the final settling tank 107, or both is returned to
the primary settling tank 102.
[0157] In the nitrification tank 104, by the action of activated
sludge, and in some cases, by the action of immobilized activated
sludge on carriers for immobilizing microorganisms fed into the
nitrification tank 104, nitrogen compounds in the wastewater are
oxidized to nitrate nitrogen or nitrite nitrogen (the nitrification
reaction), and organic substances are removed by oxidizing
decomposition. In the denitrification tank 103, activated sludge
reduces nitrate nitrogen or nitrite nitrogen, contained in the
wastewater 101, the return sludge 108, and the mixed liquor from
the nitrification tank 106, to nitrogen gas (the denitrification
reaction), using organic substances mainly composed of soluble
components in the wastewater 101.
[0158] FIG. 16 shows another example of a biological nitrogen
removal apparatus in accordance with embodiment 4. In this
apparatus, a portion of the mixed liquor from the nitrification
tank is returned to a primary settling tank 102, and simultaneously
at least a portion of primary settling tank sediment 110 is
introduced to a denitrification tank 103. As described above, when
the amount of the mixed liquor 106 from the nitrification tank
returned to the primary settling tank 102 is large and all of the
sediment 110 withdrawn from the primary settling tank is introduced
to the sludge treatment stage, the activated sludge concentration
in the denitrification tank 103 and the nitrification tank 104 is
decreased, and the rate of the denitrification in the
denitrification tank 103 and the rate of nitrification in the
nitrification tank 104 are decreased, and thus hindrance to
treatment may occur. However, in this apparatus, by introducing at
least a portion of the primary settling tank sediment 110 to the
denitrification tank 103, the sufficient MLSS concentration in the
reaction tank can be obtained.
[0159] In addition to the wastewater treatment apparatus including
the denitrification stage and the nitrification stage, the nitrogen
removal method in accordance with the present invention will be
applicable to a wastewater treatment apparatus including the
anaerobic stage, the denitrification stage (anoxic stage), and the
nitrification stage (aerobic stage), and a portion of the primary
settling tank sediment 110 may be introduced to the anaerobic tank
in addition to the denitrification tank.
[0160] FIG. 17 shows another example of a biological nitrogen
removal apparatus in accordance with embodiment 4. In the
biological nitrogen removal apparatus in accordance with embodiment
4 shown in FIG. 17, an anaerobic tank 115 is added to the
biological nitrogen removal apparatus in accordance with embodiment
4 shown in FIG. 15. The apparatus shown in FIG. 17 mainly includes
a primary settling tank 102, the anaerobic tank 115, a
denitrification tank 103, a nitrification tank 104, and a final
settling tank 107.
[0161] In the biological nitrogen removal apparatus shown in FIG.
17, in the anaerobic tank 115, only agitation is performed, and
wastewater 101, return sludge 108, and at least a portion of
primary settling tank sediment 110 are fed into the anaerobic tank
115. In the anaerobic tank 115, activated sludge reduces nitrate
nitrogen or nitrite nitrogen contained in the wastewater 101 and
the return sludge 108 to nitrogen gas (the denitrification
reaction), using inflow organic substances, and activated sludge
releases phosphate anions accumulated in cells into the wastewater
(the biological phosphorus-releasing reaction). In the
denitrification tank 103, agitation is also performed and nitrate
nitrogen or nitrite nitrogen contained in nitrification circulating
liquid and mixed liquor flowing from the anaerobic tank 115 is
reduced to nitrogen gas (the denitrification reaction).
Furthermore, in the denitrification tank 103 and the nitrification
tank 104, activated sludge takes up phosphate anions in the
wastewater into cells (the biological phosphorus-uptake reaction).
In the nitrification tank 104, organic nitrogen and ammonia
nitrogen are oxidized to nitrate nitrogen or nitrite nitrogen (the
nitrification reaction). Organic substances are consumed and
treated in each of the anaerobic tank 115, the denitrification tank
103, and the nitrification tank 104. In this embodiment, solid
organic substances originating from the wastewater 101 in the
primary settling tank 102 are also effectively used as a reducing
agent for the denitrification reaction, and denitrifying bacteria
contained in the solid organic substances principally cause the
denitrification reaction. The primary settling tank 2 also
functions as a denitrification tank.
[0162] Additionally, in the biological nitrogen removal apparatus
shown in FIG. 17 in accordance with embodiment 4, although a
portion of or all of the primary settling tank sediment 110 is
designed to be fed into the anaerobic tank 115, a portion of or all
of the primary settling tank sediment 110 may be fed into the
denitrification tank 103.
[0163] In the case of sewage treatment, with respect to effluent
from the nitrification tank, the total concentration of nitrate
nitrogen and nitrite nitrogen is approximately 10 to 250 mg/L, and
normally approximately 15 to 30 mg/L. With respect to effluent from
the final settling tank, the total concentration of nitrate
nitrogen and nitrite nitrogen is approximately 1 to 15 mg/L, and
normally approximately 5 to 10 mg/L. The effluent from the
nitrification tank to be returned to the primary settling tank is
preferably set at approximately 10 to 300%, and normally
approximately 20 to 100%. The effluent from the final settling tank
to be returned to the primary settling tank is preferably set at
approximately 20 to 100%. Although the return amount may be changed
depending on the nitrate nitrogen concentration or the nitrite
nitrogen concentration of the effluent, a given amount facilitates
the operational administration. However, when the nitrate nitrogen
concentration or the nitrite nitrogen concentration in the effluent
from the final settling tank exceeds the controlled amount, all the
volume is returned to the primary settling tank or the
denitrification tank.
EXAMPLE OF THE INVENTION
[0164] An example of the biological nitrogen removal method in
accordance with embodiment 4 will be described below. Comparative
experiments were conducted with respect to a conventional
biological nitrogen removal apparatus as shown in FIG. 14 and a
biological nitrogen removal apparatus in accordance with the
present invention as shown in FIG. 15 in which a portion of treated
water 109 is returned to the primary settling tank. In either case,
the volume of wastewater treated was 12 m.sup.3/day, and the
residence time at the primary settling tank, the denitrification
tank, the nitrification tank, and the final settling tank were set
at 3 hours, 4 hours, 2 hours, and 4 hours, respectively. In either
case, in order to accelerate nitrification, hollow cylindrical
carriers composed of expandable polypropylene having an inside
diameter of 3 mm, an outside diameter of 4 mm, and a length of 5 mm
were fed into the nitrification tank at an apparent volume ratio of
16%, that is, at a true volume ratio of 4%. As wastewater 101,
domestic wastewater was used, and the experiments were conducted at
water temperatures of 17 to 20.degree. C. In the conventional
method, the total of the returned sludge flow amount and the
nitrification circulating liquid flow amount is set at 200% of the
original wastewater flow, and in the method of the present
invention, the total of the returned sludge flow amount, the
nitrification circulating liquid flow amount, and the flow amount
of the treated water to the primary settling tank was set at 200%
of the original wastewater flow. In each case, a three-month
preliminary operation was carried out, and after treatment was
stabilized, the quality of treated water was investigated. Table 4
shows the results of analysis with respect to the treated water in
each case along with the domestic wastewater used. When the data
was collected, the mixed liquor suspended solid (MLSS)
concentration at the denitrification tank and the nitrification
tank was approximately 1,600 mg/L in either case.
4 TABLE 4 Samples Treated water in Domestic conventional Treated
water in Analysis item wastewater method embodiment 2 Total
nitrogen 20.6 6.5 2.4 (mg/L) Nitrate + Nitrite 0.2 5.8 1.7 nitrogen
(mg/L) Ammonia nitrogen 19.1 0 0 (mg/L) BOD (m/L) 74.5 12.2 12.0 SS
(m/L) 55.0 8.2 8.1
[0165] As is clear from Table 4, in accordance with the method of
embodiment 4, treated water having a lower nitrogen concentration
was obtained in comparison with the conventional method when the
same amount of wastewater was treated using the facilities having
the same capacity.
[0166] In accordance with embodiment 4, the biological nitrogen
removal apparatus includes the denitrification stage, in some
cases, the anaerobic stage, and the nitrification stage, as well as
the primary settling tank and the final settling tank. At least a
portion of the mixed liquor flowing from the nitrification stage,
at least a portion of the treated water flowing from the final
settling tank, or both is returned to the primary settling tank.
When at least a portion of the mixed liquor flowing from the
nitrification stage is returned to the primary settling tank, at
least a portion of the sludge settled in the primary settling tank
is fed into the denitrification tank or the anaerobic tank.
[0167] Accordingly, either when the quality of treated water is
deteriorated by the decreased rate of the denitrification reaction
at the denitrification stage because the concentration of organic
substances supplied to the denitrification stage is decreased more
than the nitrogen concentration is decreased, such as a case of the
influx of rainwater, or when the quality of treated water is
deteriorated by the decreased rate of the denitrification reaction
at the denitrification stage because soluble wastes in wastewater
entering the denitrification stage after being subjected to
solid-liquid separation treatment in the primary settling tank have
a low ratio of organic substance concentration to nitrogen
concentration, organic substances contained in solids and
denitrifying bacteria originating from wastewater in the primary
settling tank effectively contribute to the denitrification
reaction. The retention time in the primary settling tank also
effectively works in order to advance the denitrification reaction.
Since solid organic substances in the primary settling tank
originating from the wastewater can be used as a reducing agent for
the denitrification reaction, the cost for the organic substance
such as methanol to be added can be reduced. It is also not
required to use chemicals such as ozone which incur electric power
costs for production. The major operating expenses required for
implementing the method of this invention are electric power costs
for transporting liquid and air, which are substantially the same
as those in the conventional wastewater treatment facilities using
the biological nitrification-denitrification process, resulting in
reduced operating expenses.
[0168] In accordance with embodiment 4, since the primary settling
tank also functions as a denitrification reaction tank, and the
retention time in the primary settling tank effectively affects the
denitrification reaction, when the existing activated sludge
facilities for removing BOD are converted to wastewater treatment
facilities using the biological nitrification-denitrification
process, the same effect can be obtained as that when existing
reaction tanks are enlarged, and moreover, the originally designed
water flow can be treated in the primary settling tank and the
final settling tank. Thus, BOD removal treatment and nitrogen
removal treatment can be performed by maximizing the utilization of
volume and capacity of the existing activated sludge process
facilities, and satisfactory treatment can be achieved. In
accordance with the present invention, when the existing activated
sludge facilities for removing BOD are converted to wastewater
treatment facilities using the biological
nitrification-denitrification process, the BOD removal and the
nitrogen removal can be performed without drastically changing the
basic engineering structure of the existing facilities, and without
drastically reducing the volume of the treated water, resulting in
low construction costs (conversion costs). Moreover, when new
wastewater treatment facilities using the biological
nitrification-denitrification process are opened, since biological
treatment tanks are as compact as those in the conventional
facilities using the standard activated sludge process, the
construction costs are low.
[0169] When at least a portion of the mixed liquor from the
nitrification tank is returned to the primary settling tank, since
denitrifying bacteria in suspended solids in addition to nitrate
nitrogen or nitrite nitrogen are fed to the primary settling tank,
the rate of the denitrification reaction in the primary settling
tank may be increased. On the other hand, when the volume of the
mixed liquor flowing from the nitrification tank returned to the
primary settling tank is large, and all the sludge withdrawn from
the primary settling tank is introduced to the sludge treatment
stage, the mixed liquor suspended solid concentration in the
denitrification tank 103 and the nitrification tank 104 may be
decreased, and thus the rate of the denitrification reaction in the
denitrification tank 103 and the rate of the nitrification reaction
may be decreased, resulting in a hindrance to the treatment. In
such a case, in accordance with the present invention, at least a
portion of the primary settling tank sludge is fed into the
denitrification tank or the anaerobic tank, thus preventing a
decrease in the mixed liquor suspended solid concentration in the
denitrification tank or the anaerobic tank. Consequently, while the
amount of nitrate nitrogen or nitrite nitrogen supplied to the
denitrification reaction is increased by increasing the flow amount
of the mixed liquor flowing from the nitrification tank in order to
improve the nitrogen removal rate, the stable operation of the
apparatus is enabled.
* * * * *