Description
Title of Invention: System and Method for Treating Wastewater Containing Suspended
Organic Substances
Technical Field
[0001]
The present invention relates to a system and a method for treating a wastewater
containing suspended organic substances, and especially, to a system and a method suitable
for a wastewater containing high concentrations of suspended organic substances.
Background Art
[0002]
Conventionally, when treating a wastewater containing high concentrations of
suspended organic substances, a method used comprises separating and removing
suspended organic substances from the wastewater first, and then biologically treating the
separator effluent by nitrification denitrification method, etc. The suspended organic
substances separated and removed from the wastewater are then directly dehydrated, or
disposed after being anaerobically digested for the purpose of volume reduction or methane
gas collection. Then, the filtrate or the digester effluent generated as a result of these
treatments is returned to the biologically treating step, and treated together with the
separator effluent.
[0003]
The nitrification denitrification method to decompose the ammonia nitrogen in the
wastewater to nitrogen molecules comprises steps of: nitrifying to oxidize the ammonia
nitrogen in the wastewater to nitrite nitrogen by ammonia-oxidizing bacteria, and then
oxidize the nitrite nitrogen to nitrate nitrogen by nitrite oxidizing bacteria; and denitrifying
to decompose the nitrite nitrogen and nitrate nitrogen to nitrogen molecules by
heterotrophic denitrifying bacteria.
[0004]
However, such a conventional, biological nitrification denitrification method has been
problematic in that its nitrifying step requires a great deal of oxygen, causing the cost of
electricity for a blower fan, etc. for aeration to rise, and that a great deal of methanol, etc.
needs to be added as the organic carbon source in the denitrifying step, causing the cost for
chemicals to rise, and that usage of heterotrophic denitrifying microorganisms produces a
good deal of sludge, increasing the cost for treating the excess sludge; which leads to an
increase in the running costs thereof.
[0005]
Therefore, a system for biologically removing nitrogen disclosed in the Patent Document
1 comprises: a solid-liquid separator means for separating solids and liquid in the
wastewater containing SS organic substances and ammonia nitrogen; an anaerobic digester
tank for anaerobically digesting the sludge separated by the solid-liquid separator means; a
nitrification tank for nitrifying the liquid effluent from the anaerobic digester tank to
nitrite; and a denitrification tank for mixing and denitrifying the nitrified effluent from the
nitrification tank and the separator effluent from the solid-liquid separator means.
[0006]
This system for biologically removing nitrogen anaerobically digests the sludge
separated by the solid-liquid separation from the wastewater containing SS organic
2
substances and ammonia nitrogen; and then denitrifies, by anaerobic ammonia oxidation,
the liquid obtained by the anaerobic digestion and the nitrous-acid-forming treatment after
that, and the separator effluent obtained by solid-liquid separation of the wastewater.
Background Art
Patent Document
[0007]
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2003-
245689
Summary of Invention
Technical Problem
[0008]
The anaerobic ammonia oxidation is, as expressed by the following formula, the
treatment that converts ammonia nitrogen of 1 equivalent weight and nitrite nitrogen of
1.32 equivalent weight to nitrogen molecules by denitrification reaction under anaerobic
conditions, with ammonia nitrogen produced by autotrophic denitrifying microorganisms
(autotrophic denitrifying bacteria) as an electron donor, and nitrite nitrogen as an electron
acceptor. As a result, nitrate of 0.26 equivalent weight is produced.
NH4+ ＋ 1.32NO2- ＋ 0.066HCO3- ＋ 0.13H+→
1.02N2＋0.26NO3-＋0.066CH2O0.5N0.15＋2.03H2O
[0009]
If a wastewater containing high concentrations of suspended organic substances is
treated by the system for biologically removing nitrogen of the Patent Document 1, a large
amount of sludge separated in the settling tank is anaerobically digested, and as a result, a
high concentration of ammonia is produced. The produced ammonia is converted to nitrite
in the nitrification tank, and denitrified in the denitrification tank as a result of being
anaerobically oxidized, together with the liquid separated by the settling tank.
[0010]
However, in this system for biologically removing nitrogen, if the concentration of nitrite
(the concentration of free nitrite) in the nitrification tank becomes too high, activation of
ammonia-oxidizing bacteria is inhibited, and efficiency of the nitrous-acid-forming reaction
is reduced. And, the large amount of ammonia nitrogen which was not converted to nitrite
in the nitrification tank flows into the denitrification tank, and as a result, the ammonia
nitrogen in the denitrification tank becomes too much relative to the nitrite nitrogen,
causing untreatable ammonia to leak.
[0011]
In consideration of the aforementioned problems, an object of the present invention is to
provide a system and a method for treating a wastewater containing suspended organic
substances, in a more efficient way, even when the wastewater contains high concentrations
of suspended organic substances.
Solution to Problem
[0012]
To achieve the above object, as disclosed in claim 1, a first aspect of a system for treating
a wastewater containing suspended organic substances of the present invention is that the
system comprises; a suspended-organic-substance separator for separating the suspended
organic substances contained in the wastewater; an anaerobic sludge digester for
3
anaerobically digesting the suspended organic substances separated by the suspendedorganic-
substance separator; a nitritation reactor for oxidizing the ammonia contained in
the digester effluent from the anaerobic sludge digester into nitrite under aerobic
conditions; and an autotrophic denitrification device for oxidizing the ammonia contained in
the effluent from the nitritation reactor into nitrogen gas by autotrophic denitrifying
microorganisms under anaerobic conditions; and that a first transfer route for supplying
the separator effluent from which suspended organic substances have been removed by the
suspended-organic-substance separator, or said separator effluent that has been
biologically treated, to the nitritation reactor is provided.
[0013]
The suspended organic substances separated from the wastewater by the suspendedorganic-
substance separator are anaerobically digested by the anaerobic sludge digester.
The digester effluent containing ammonia produced by the anaerobic digestion flows into
the nitritation reactor, and is oxidized to nitrite under aerobic conditions, and the resulting
liquid containing ammonia and nitrite flows into the autotrophic denitrification device.
Then, in the autotrophic denitrification device, the resulting liquid is denitrified by
anaerobic ammonia oxidation by the autotrophic denitrifying microorganisms under
anaerobic conditions, with the ammonia nitrogen as an electron donor and the nitrite
nitrogen as an electron acceptor in the liquid. When the ammonia is oxidized to nitrite in
the nitritation reactor, if the concentration of nitrite (the concentration of free nitrite)
becomes higher than a concentration, activation of ammonia-oxidizing bacteria is inhibited,
and as a result, efficiency of the converting-to-nitrous-acid reaction and the denitrification
in the subsequent autotrophic denitrification device is reduced.
[0014]
However, even in such a case, the separator effluent from which suspended organic
substances have been removed by the suspended-organic-substance separator, or said
separator effluent that has been biologically treated, is supplied to the nitritation reactor
through the first transfer route, and therefore, the concentration of nitrite (the
concentration of free nitrite) in the nitrified liquid in the nitritation reactor is adjusted to be
suitable for conversion of ammonia to nitrite, allowing for prompt of the converting-tonitrous-
acid reaction. Note that this configuration does not require preparation of any
separate equipment for supplying dilution water using tap water, etc., and the convertingto-
nitrous-acid reaction is prompted without controlling the temperature or pH. It becomes
possible to save costs of+ energy including tap water, electricity, etc. or chemicals.
[0015]
Its second aspect is, as disclosed in claim 2 and in addition to the first aspect, that a
second transfer route for supplying the separator effluent from the suspended-organicsubstance
separator, or said separator effluent that has been biologically treated, to the
autotrophic denitrification device is provided.
[0016]
In the autotrophic denitrification device, the ammonia contained in the effluent from the
nitritation reactor is oxidized to nitrogen gas by autotrophic denitrifying microorganisms
under anaerobic conditions. However, if the concentration of nitrite (the concentration of
4
free nitrite) in the effluent flows into the autotrophic denitrification device becomes high,
efficiency of the anaerobic ammonia oxidation is reduced.
[0017]
However, even in such a case, the separator effluent from which suspended organic
substances have been removed by the suspended-organic-substance separator, or said
separator effluent that has been biologically treated, is supplied to the autotrophic
denitrification device through the second transfer route, and therefore, the concentration of
nitrite (the concentration of free nitrite) in the liquid in the autotrophic denitrification
device is diluted to be suitable for anaerobic ammonia oxidation, allowing for prompt of
efficient anaerobic ammonia oxidation. As described above, this configuration does not
require preparation of separate equipment for supplying dilution water using tap water, etc.
[0018]
Its third aspect is, as disclosed in claim 3 and in addition to the first or second aspect,
that a third transfer route for returning the denitrified effluent from the autotrophic
denitrification device to the autotrophic denitrification device is provided.
[0019]
Even when the separator effluent or said separator effluent that has been biologically
treated, cannot dilute the concentration of nitrite (the concentration of free nitrite) in the
liquid in the autotrophic denitrification device to an appropriate value, the denitrified
effluent from the autotrophic denitrification device having a reduced concentration of
nitrite is returned to the autotrophic denitrification device through the third transfer route,
and therefore, it becomes possible to further dilute the concentration of nitrite (the
concentration of free nitrite) in the liquid in the autotrophic denitrification device to take
an appropriate value. As described above, this configuration does not require preparation of
separate equipment for supplying dilution water using tap water, etc.
[0020]
Its fourth aspect is, as disclosed in claim 4 and in addition to the first or second aspect,
that a heterotrophic denitrification device for reducing the nitrate, contained in the
denitrified effluent from the autotrophic denitrification device, into nitrogen gas by
heterotrophic microorganism under anaerobic conditions is provided; and that a fourth
transfer route for returning the denitrified effluent from the heterotrophic denitrification
device to the autotrophic denitrification device.
[0021]
The denitrified effluent that has gone through the anaerobic ammonia oxidation in the
autotrophic denitrification device contains nitrate. By letting such denitrified effluent flow
to the heterotrophic denitrification device, nitrate contained in the denitrified effluent will
be reduced to nitrite by heterotrophic denitrifying microorganisms under anaerobic
conditions. Since the denitrified effluent from the heterotrophic denitrification device is
returned to the autotrophic denitrification device through the fourth transfer route, the
concentration of nitrite (the concentration of free nitrite) in the liquid in the autotrophic
denitrification device can be further diluted to an appropriate value.
[0022]
A first aspect of a method for treating a wastewater containing suspended organic
substances of the present invention is, as disclosed in claim 5, is that the method comprises
5
steps of: separating suspended organic substances contained in the wastewater;
anaerobically sludge-digesting the suspended organic substances separated by the
separating step; oxidizing the ammonia contained in the digester effluent, from the
anaerobically sludge-digesting step, to nitrite under aerobic conditions; and autotrophically
denitrifying and oxidizing the ammonia contained in the effluent, from the oxidizing-tonitrous-
acid step, to nitrogen gas by autotrophic denitrifying microorganisms under
anaerobic conditions; and that the separator effluent from which suspended organic
substances have been removed by the separating step, or said separator effluent that has
been biologically treated, is transferred to the oxidizing-to-nitrous-acid step and the
autotrophically denitrifying step, to adjust the concentrations of free nitrite in the
respective steps.
[0023]
Its second aspect is, as disclosed in claim 6, that the method comprises the steps of:
separating suspended organic substances contained in the wastewater; anaerobically
sludge-digesting the suspended organic substances separated by the separating step;
oxidizing and nitrifying the ammonia contained in the digester effluent, from the
anaerobically sludge-digesting step, to nitrite under aerobic conditions; and autotrophically
denitrifying and oxidizing the ammonia contained in the effluent, from the oxidizing-tonitrous-
acid step, to nitrogen gas by autotrophic denitrifying microorganisms under
anaerobic conditions; and that the separator effluent from which suspended organic
substances have been removed by the separating step, or said separator effluent that has
been biologically treated, is transferred to the oxidizing-to-nitrous-acid step, and the
biologically-treated separator effluent, or both the separator effluent and the biologicallytreated
separator effluent, are transferred to the autotrophically denitrifying step, to adjust
the concentrations of free nitrite in the respective steps.
[0024]
Its third aspect is, as disclosed in claim 7 and in addition to the first or second aspect,
that the concentration of free nitrite in the oxidizing-to-nitrous-acid step is adjusted to be
lower than 0.2 ppm, and lower than 0.08 ppm in the autotrophically denitrifying step.
[0025]
In the oxidizing-to-nitrous-acid step, the concentration of free nitrite is adjusted to be
lower than 0.2 ppm to efficiently convert the ammonia to nitrite. In the autotrophically
denitrifying step, the concentration of free nitrite is adjusted to be lower than 0.08 ppm for
an efficient anaerobic ammonia oxidation. That is, the nitrogen can be efficiently removed
even from a wastewater containing high concentrations of suspended organic substances.
Advantageous Effects of Invention
[0026]
As explained above, the present invention provides a system and a method for treating a
wastewater containing suspended organic substances, in a more efficient way, even when
the wastewater contains high concentrations of suspended organic substances.
Brief Description of Drawings
[0027]
[Fig. 1] Fig. 1(a) illustrates an explanatory diagram of a first embodiment of a system of the
present invention, and Fig. 1(b) illustrates an explanatory diagram of the concentrations of
free nitrite obtained when the ammonia nitrogen in the anaerobic digester effluent is
6
converted to nitrite such that the concentrations of ammonia nitrogen and nitrite nitrogen
become 1:1.32.
[Fig. 2] Fig. 2 illustrates an explanatory diagram of a second embodiment of the system of
the present invention.
[Fig. 3] Fig. 3 illustrates an explanatory diagram of a third embodiment of the system of the
present invention.
[Fig. 4] Fig. 4 illustrates an explanatory diagram of a fourth embodiment of the system of
the present invention.
[Fig. 5] Fig. 5 illustrates an explanatory diagram of a fifth embodiment of the system of the
present invention.
[Fig. 6] Fig. 6 illustrates an explanatory diagram of a sixth embodiment of the system of the
present invention.
[Fig. 7] Fig. 7 illustrates an explanatory diagram of a numerical simulation performed on
the system of the present invention.
[Fig. 8] Fig. 8 illustrates an explanatory diagram of a numerical simulation performed on
the system of the present invention.
Description of Embodiments
[0028]
Now, embodiments of a system and a method for treating a wastewater containing
suspended organic substances of the present invention will be explained.
[0029]
Fig. 1(a) illustrates a first embodiment of the system for treating a wastewater
containing suspended organic substances. The system purifies a wastewater containing
suspended organic substances and comprises: a suspended-organic-substance separator 10;
an anaerobic sludge digester 20; a nitritation reactor 30; an autotrophic denitrification
device 40; and a first transfer route R1.
[0030]
The suspended-organic-substance separator 10 separates suspended organic substances
from the wastewater, and the anaerobic sludge digester 20 anaerobically digests the
suspended organic substances separated by the suspended-organic-substance separator 10.
[0031]
The nitritation reactor 30 oxidizes the ammonia contained in the digester effluent, from
the anaerobic sludge digester 20, into nitrite under aerobic conditions, and the autotrophic
denitrification device 40 converts the ammonia and nitrite contained in the nitrified
effluent, from the nitritation reactor 30, into nitrogen gas by autotrophic denitrifying
microorganisms under anaerobic conditions. Then, the separator effluent from which
suspended organic substances have been removed by the suspended-organic-substance
separator 10 is supplied to the nitritation reactor 30 through the first transfer route R1.
[0032]
The suspended-organic-substance separator 10 may include a mechanical separator such
as a settler, coagulation settler, flotation separator, screen device, membrane separator,
cyclone device, screw press, and decanter. It may also include a combination of those devices.
7
[0033]
The suspended organic substances separated from the wastewater by the suspendedorganic-
substance separator 10 are anaerobically digested by the anaerobic sludge digester
20, and turn into the digester effluent containing ammonia nitrogen.
[0034]
The digester effluent containing ammonia nitrogen flows into the nitritation reactor 30,
and is nitrified to form nitrite by nitrite bacteria under aerobic conditions, and a part of the
ammonia nitrogen is oxidized to nitrite nitrogen.
[0035]
The effluent nitrified by the nitritation reactor 30, that is the effluent containing
ammonia nitrogen and nitrite nitrogen flows into the autotrophic denitrification device 40,
and is denitrified by anaerobic ammonia oxidation by autotrophic denitrifying
microorganisms under anaerobic conditions, with ammonia nitrogen as an electron donor
and nitrite nitrogen as an electron acceptor. More specifically, the autotrophic
denitrification device 40 is an anaerobic ammonia oxidation device.
[0036]
The autotrophic denitrifying microorganisms are stored in the autotrophic
denitrification device 40, in the form of floating fungus, and they may be stored there as a
biofilm formed of a filler to which they are attached, or as immobilization media formed of
immobilization materials to which they are immobilized.
[0037]
When using a biofilm formed of a filler to which the autotrophic denitrifying
microorganisms are attached, the content of the biofilm filled in the autotrophic
denitrification device 40 is preferably 20-80 vol%, if it is a fixed-bed type. The filler may be
formed of a non-woven fabric, plastics material, sponge material or porous ceramics, etc.,
and its shape may be plate-, granule- or tube-like.
[0038]
When using immobilization media formed of immobilization materials to which
autotrophic denitrifying microorganisms are immobilized, the method of immobilizing
autotrophic denitrifying microorganisms to the immobilization materials may be
attachment immobilization or entrapping immobilization.
[0039]
In the attachment immobilization, the media may be formed of a sphere or tube, or a
string, gel, non-woven fabric, etc., and preferably, a material having a very uneven shape
allowing microbes to easily attach thereto.
[0040]
In the entrapping immobilization, microbes are generally entrapped and immobilized by
mixing and then polymerizing the microbes to be immobilized and monomers or
prepolymers as media.
[0041]
Preferably, the monomer may be formed of acrylamide, methylenebisacrylamide,
triacrylformal, etc., and the prepolymer may be formed of polyethylene glycol diacrylate or
polyethylene glycol methacrylate. Like in the attachment immobilization described above,
usage of immobilization materials having a very uneven shape, which allows them to make
8
contact with the wastewater more efficiently, makes it possible to improve the performance
of denitrification.
[0042]
As described above, the anaerobic ammonia oxidation performed by the autotrophic
denitrification device 40 converts ammonia nitrogen of 1 equivalent weight and nitrite
nitrogen of 1.32 equivalent weight to nitrogen molecules by denitrification reaction.
Therefore, for an efficient anaerobic ammonia oxidation, the nitrous-acid-forming treatment
in the nitritation reactor 30 is performed such that the mole ratio of ammonia and nitrite
ideally and preferably becomes 1:1.32.
[0043]
More specifically, the digester effluent from which high concentrations of suspended
organic substances have been digested contains high concentrations of ammonia nitrogen,
and therefore, the concentration of nitrite (the concentration of free nitrite) in the
nitritation reactor 30 tends to be high.
[0044]
It is said that in nitrification, if the concentration of Free Nitrous Acid (FNA) becomes
0.2 ppm or higher, all nitrifying bacteria are inhibited. According to the mathematical
formula by Anthonisen, et al., the concentration of Free Nitrous Acid (FNA) is expressed by
the following formula. That is, the concentration of Free Nitrous Acid (FNA) is determined
by the concentration of nitrite, temperature and pH.
[Formula 1]
[0045]
Fig. 1(b) shows the values of the concentrations of free nitrite calculated by the above
formula, obtained when the nitrous-acid-forming treatment is performed with the anaerobic
digester effluent, such that its concentrations of ammonia nitrogen and nitrite nitrogen
becomes 1:1.32.
[0046]
When the concentration of ammonia nitrogen in the anaerobic digester effluent is 1000
ppm, the nitrous-acid-forming treatment is performed such that the concentration of nitrite
nitrogen becomes 570 ppm. At this time, if the pH is 7.4 and the water temperature is 30°C,
the concentration of free nitrite becomes 0.151 ppm, which does not disturb the nitrousacid-
forming treatment. However, if the pH is decreased to 7.2, the concentration of free
nitrite becomes 0.239 ppm, which inhibits formation of nitrite.
[0047]
Conventionally, an alkaline agent is added to increase the pH, however, the system of
the first embodiment supplies the separator effluent to the nitritation reactor 30 through
the first transfer route R1, to dilute the concentration of nitrite nitrogen to some 470 ppm,
which brings about the concentration of free nitrite of a level enough for formation of nitrite.
[0048]
When the water temperature becomes 15°C, the concentration of free nitrite becomes
0.224 ppm, however, by diluting the concentration of nitrite nitrogen to some 500 ppm
without increasing the water temperature, the conditions for formation of nitrite stay good.
9
[0049]
When the concentration of ammonia nitrogen in the anaerobic digester effluent becomes
2000 ppm, the concentration of free nitrite becomes 0.302 ppm, with the pH of 7.4 and the
water temperature of 30°C. At that time, if the pH or the water temperature is adjusted to
7.6 or 48°C, the concentration of free nitrite can be controlled to be lower than 0.2 ppm,
however, a great deal of alkaline agents or energy for heating will be required for the
adjustment. However, the system of the first embodiment dilutes the concentration of
nitrite nitrogen to some 740 ppm, to obtain the concentration of free nitrite good enough for
formation of nitrite.
[0050]
As described above, by diluting the digester effluent in the nitritation reactor 30 by
supplying the separator effluent to the nitritation reactor 30 through the first transfer
route R1, it becomes possible to adjust the concentration of nitrite (the concentration of free
nitrite) in the digester effluent in the nitritation reactor 30 to be suitable for oxidation of
ammonia to nitrite.
[0051]
Fig. 1(a) illustrates an example in which the diluting fluid supplied to the nitritation
reactor 30 through the first transfer route R1 is the separator effluent from which
suspended organic substances have been removed by the suspended-organic-substance
separator 10, however, if a biotreatment device for biologically treating the separator
effluent by means of aerobic microorganisms is provided, the separator effluent that has
been biologically treated may be supplied to the nitritation reactor 30 as the diluting fluid,
instead of the separator effluent. This embodiment will be explained in detail below.
[0052]
In the nitritation reactor 30, ammonia-oxidizing bacteria can efficiently oxidizes
ammonia to nitrite without being inhibited, and therefore, ammonia can be fully oxidized to
nitrite by the nitritation reactor 30.
[0053]
The effluent from the nitritation reactor 30 is efficiently denitrified by anaerobic
ammonia oxidation by the autotrophic denitrification device 40. For adjustment of the
concentration of nitrite (the concentration of free nitrite) in the digester effluent, this
configuration uses the separator effluent or the separator effluent that has been biologically
treated, and therefore, there is no need for a separate device for supplying dilute water
including tap water, etc. Costs of electricity or chemicals to control the temperature or pH
for an efficient nitrous-acid-forming reaction can be saved.
[0054]
The above configuration carries out a method for treating a wastewater containing
suspended organic substances, the method comprising steps of: separating suspended
organic substances contained in the wastewater; anaerobically sludge-digesting the
suspended organic substances separated by the separating step; oxidizing the ammonia
contained in the digester effluent, from the anaerobically sludge-digesting step, to nitrite
under aerobic conditions; and autotrophically denitrifying and oxidizing the ammonia
contained in the effluent from the oxidizing-to-nitrous-acid step, to nitrogen gas by
autotrophic denitrifying microorganisms under anaerobic conditions; wherein the separator
effluent from which suspended organic substances have been removed by the separating
10
step, or said separator effluent that has been biologically treated, is transferred to the
oxidizing-to-nitrous-acid step, to adjust the concentrations of free nitrite in the oxidizing-tonitrous-
acid step.
[0055]
Fig. 2 illustrates a second embodiment of the system. In addition to the system of the
first embodiment described above, the system further comprises a second transfer route R2
for supplying the separator effluent to the autotrophic denitrification device 40.
[0056]
As described above, in the autotrophic denitrification device 40, the effluent from the
nitritation reactor 30 is denitrified by anaerobic ammonia oxidation by autotrophic
denitrifying microorganisms under anaerobic conditions, with ammonia nitrogen as an
electron donor and nitrite nitrogen as an electron acceptor. At that time, if the
concentration of nitrite (the concentration of free nitrite) in the autotrophic denitrification
device 40 is high, efficiency of the anaerobic ammonia oxidation is reduced.
[0057]
By supplying the separator effluent to the autotrophic denitrification device 40 through
the second transfer route R2, the denitrified effluent in the autotrophic denitrification
device 40 is diluted, and the concentration of nitrite (the concentration of free nitrite) in the
effluent is adjusted to the concentration suitable for anaerobic ammonia oxidation.
Therefore, anaerobic ammonia oxidation in the autotrophic denitrification device 40 is
efficiently performed. In this configuration, adjustment of the concentration of nitrite (the
concentration of free nitrite) is performed by using the separator effluent, and therefore,
there is no need for separate equipment for supplying dilute water including tap water, etc.
[0058]
That is, the performed is a method for treating a wastewater containing suspended
organic substances, the method comprising the steps of: separating suspended organic
substances contained in the wastewater; anaerobically sludge-digesting the suspended
organic substances separated by the separating step; oxidizing the ammonia contained in
the digester effluent, from the anaerobically sludge-digesting step, to nitrite under aerobic
conditions; and autotrophically denitrifying and converting the ammonia contained in the
effluent, from the oxidizing-to-nitrous-acid step, to nitrogen gas by autotrophic denitrifying
microorganisms under anaerobic conditions; wherein the separator effluent from which
suspended organic substances have been removed by the separating step, or said separator
effluent that has been biologically treated, is transferred to the oxidizing-to-nitrous-acid
step and the autotrophically denitrifying step, to adjust the concentrations of free nitrite in
the respective steps.
[0059]
Note that in the step of forming nitrite, the concentration of free nitrite is adjusted to be
lower than 0.2 ppm, and in the step of autotrophic denitrification, the concentration of free
nitrite is preferably adjusted to be lower than 0.08 ppm, and more preferably lower than
0.06 ppm. In the step of forming nitrite, by adjusting the concentration of free nitrite to be
lower than 0.2 ppm, the ammonia is efficiently transformed to nitrite. In the step of
11
autotrophic denitrification, by adjusting the concentration of free nitrite to be lower than
0.08 ppm, the anaerobic ammonia oxidation is efficiently performed. That is, it becomes
possible to efficiently remove nitrogen even from a wastewater containing high
concentrations of suspended organic substances.
[0060]
Fig. 3 illustrates a third embodiment of the system. In addition to the system of the
second embodiment described above, the system further comprises a third transfer route R3
for returning the denitrified effluent from the autotrophic denitrification device 40 to the
autotrophic denitrification device 40. Note that the second transfer route R2 does not
necessarily have to be provided.
[0061]
Supplement of the separator effluent only through the first transfer route R1 and the
second transfer route R2 may not be enough to dilute the liquid in the autotrophic
denitrification device 40 well. Even in such a case, by returning the denitrified effluent from
the autotrophic denitrification device 40 to the autotrophic denitrification device 40 through
the third transfer route R3, the liquid in the autotrophic denitrification device 40 can be
further diluted. Therefore, the concentration of nitrite (the concentration of free nitrite) in
the liquid in the autotrophic denitrification device 40 is certainly adjusted, without
supplying dilution water to the autotrophic denitrification device 40 from outside the
system.
[0062]
Note that untreated nitrite or ammonia, as well as nitrate, remains in the denitrified
effluent from the autotrophic denitrification device 40. It is preferable to provide the
autotrophic denitrification device 40 with a sensor for measuring nitrite ion or ammonia ion,
to adjust the returning rate according to the remaining nitrite or ammonia to meet the
discharge standard of the denitrified effluent discharged outside the system from the
autotrophic denitrification device 40.
[0063]
That is, the performed is a method for treating a wastewater containing suspended
organic substances, the method comprising the steps of: separating suspended organic
substances contained in the wastewater; anaerobically sludge-digesting the suspended
organic substances separated by the separating step; oxidizing the ammonia contained in
the digester effluent, from the anaerobically sludge-digesting step, to nitrite under aerobic
conditions; and autotrophically denitrifying and converting the ammonia contained in the
effluent, from the oxidizing-to-nitrous-acid step, to nitrogen gas by autotrophic denitrifying
microorganisms under anaerobic conditions; wherein the separator effluent from which
suspended organic substances have been removed by the separating step is transferred to
the oxidizing-to-nitrous-acid step, and the biologically-treated separator effluent, or both
the separator effluent and the biologically-treated separator effluent, are transferred to the
autotrophically denitrifying step, to adjust the concentrations of free nitrite in the
respective steps.
[0064]
Fig. 4 illustrates a fourth embodiment of the system. In addition to the system of the
second embodiment described above, the system further comprises a heterotrophic
12
denitrification device 50 for reducing nitrate contained in the denitrified effluent from the
autotrophic denitrification device 40 to nitrogen gas by heterotrophic denitrifying bacteria
under anaerobic conditions, and a fourth transfer route R4 for returning the denitrified
effluent from the heterotrophic denitrification device 50 to the autotrophic denitrification
device 40. Note that a fifth transfer route R5 for supplying the separator effluent from the
suspended-organic-substance separator 10 to the heterotrophic denitrification device 50
may be provided. The second transfer route R2 does not necessarily have to be provided.
[0065]
In the heterotrophic denitrification device 50, the denitrification reaction by
heterotrophic denitrifying bacteria progresses with nitrate nitrogen generated in the
anaerobic ammonia oxidation as an electron acceptor, and organic substances, etc. dissolved
in the separator effluent as an electron donor, and therefore, the amount of nitrate or
organic substances discharged outside the system from the autotrophic denitrification
device 40 can be reduced.
[0066]
Here, in the nitritation reactor 30, the mole ratio of ammonia and nitrite is ideally
adjusted to be 1:1.32, however, the nitrite remaining after the anaerobic ammonia oxidation
reaction can be denitrified by heterotrophic denitrifying bacteria in the subsequent
heterotrophic denitrification device 50, and therefore, the mole ratio of nitrite, relative to
ammonia, in the nitritation reactor 30 only needs to be adjusted to be larger than 1.32. In
some cases, nitrification may proceed to nitrate. However, for the sake of the efficiency of
denitrification or reduction in the amount of oxygen required for nitrification, the ammonia:
nitrite is preferably adjusted to be some 1:1.32-1.5 in the nitritation reactor 30.
[0067]
Since the denitrified effluent from the heterotrophic denitrification device 50 is returned
to the autotrophic denitrification device 40 through the fourth transfer route R4, the liquid
in the autotrophic denitrification device 40 can be further diluted. Therefore, the
concentration of nitrite (the concentration of free nitrite) in the liquid in the autotrophic
denitrification device 40 is certainly adjusted.
[0068]
Note that heterotrophic denitrifying bacteria are put into the heterotrophic
denitrification device 50, and keep the atmosphere to be anaerobic. The heterotrophic
denitrifying bacteria in the heterotrophic denitrification device 50 may be stored in the
same form as that of the autotrophic denitrifying microorganisms in the autotrophic
denitrification device 40, which was explained with the system of the first embodiment. If
organic substances contained in the denitrified effluent from the autotrophic denitrification
device 40 are not enough, organic substances can be replenished to the heterotrophic
denitrification device 50, by supplying the separator effluent from the suspended-organicsubstance
separator 10 through the fifth transfer route R5.
[0069]
In the systems of the first to fourth embodiments described above, the separator effluent
from which suspended organic substances have been removed by the suspended-organicsubstance
separator 10 is supplied to the nitritation reactor 30 through the first transfer
13
route R1, or to the anaerobic ammonia oxidation device 40 through the second transfer
route R2, however, diluting fluid supplied through both the transfer routes R1, R2 does not
have to be the separator effluent, and such diluting fluid may be the separator effluent the
concentration of organic substances in which has been reduced by biotreatment.
[0070]
Fig. 5 illustrates a fifth embodiment of the system. In addition to the system of the
second embodiment described above, the system further comprises a biotreatment device 60
for biologically treating the separator effluent from the suspended-organic-substance
separator 10, to supply the biologically treated liquid to the nitritation reactor 30. Note that
the system of any one of the first, third and fourth embodiments can employ the same
configuration.
[0071]
If the content of BOD components in the separator effluent from the suspended-organicsubstance
separator 10 is high, heterotrophic bacteria become dominant in the nitritation
reactor 30 or the autotrophic denitrification device 40, and efficiency of the nitrous-acid
formation or anaerobic ammonia oxidation may be reduced. This problem can be solved by
appropriately removing the BOD components contained in the separator effluent from the
biotreatment device. Note that an example of the biotreatment device 60 is an aeration tank.
[0072]
Fig. 6 illustrates a sixth embodiment of the system. In the system, organic liquid waste
flows into the above-described anaerobic sludge digester 20 of the system of the first
embodiment through a route that is different from that for supplying suspended organic
substances from the suspended-organic-substance separator 10. As the organic liquid waste,
food waste, sludge from effluent treatment, livestock excretion, etc. are preferable. The
systems of the second to fifth embodiments may also have the same configuration.
[0073]
With the system and method of the present invention for treating a wastewater
containing suspended organic substances, like those of the above-described first to fifth
embodiments, even if more ammonia is generated as a result of organic liquid waste flowing
into the anaerobic sludge digester from outside the system, the concentration of nitrite (the
concentration of free nitrite) in the nitritation reactor can be adjusted by the separator
effluent from which suspended organic substances have been removed by the suspendedorganic-
substance separator, and therefore, it becomes possible to efficiently remove
nitrogen by the subsequent anaerobic ammonia oxidation.
[0074]
Now, the results of the numerical simulation performed on the system of the present
invention will be explained.
[0075]
Fig. 7 shows the results of the numerical simulation performed on the system of the
second embodiment, which was explained with Fig. 2. A wastewater of 200 m3/d flows into
the system. The suspended-organic-substance separator 10 separates suspended organic
substances of 10 m3/d contained in the wastewater. For convenience, the concentrations of
14
ammonia nitrogen and nitrite nitrogen in the 190-m3/d separator effluent from the
suspended-organic-substance separator 10 are both considered to be 0 ppm.
[0076]
The suspended organic substances separated by the suspended-organic-substance
separator 10 are anaerobically digested by the anaerobic sludge digester 20. The digester
effluent contains ammonia nitrogen of 5000 ppm.
[0077]
The nitritation reactor 30 is kept to be aerobic, with the temperature of 30°C and the pH
of 7.4, and oxidizes the ammonia contained in the anaerobic digester effluent from the
anaerobic sludge digester 20 to nitrite, to obtain the mole ratio of ammonia and nitrite of
about 1:1.3. Therefore, if ammonia nitrogen of 5000 ppm is directly oxidized to nitrite,
ammonia nitrogen of 2160 ppm and nitrite nitrogen of 2840 ppm are produced. At that time,
the concentration of free ammonia is 53.8 ppm, and the concentration of free nitrite is 0.75
ppm.
[0078]
Note that as shown in the following mathematical formula 2, the concentration of free
ammonia may be calculated by the formula of Anthonisen et al. using the relationships
between the temperature and pH, by measuring the concentration of ammonium ion in the
wastewater by the diaphragm ion electrode method, etc.
[0079]
[Formula 2]
[0080]
This concentration of free nitrite, 0.75 ppm, is higher than 0.2 ppm which is said to
inhibit all nitrifying bacteria as described above, and therefore it disturbs transformation of
ammonia to nitrite.
[0081]
Therefore, 45 m3/d of the 190-m3/d separator effluent from the suspended-organicsubstance
separator 10 is supplied to the nitritation reactor 30 through the first transfer
route R1. That is, the digester effluent from the anaerobic sludge digester 20 of 10 m3/d is
diluted 5.5 times with the 45 m3/d separator effluent. As a result, the ammonia nitrogen in
the digester effluent from the anaerobic sludge digester 20 is diluted from 5000 ppm to 900
ppm.
[0082]
The 900-ppm ammonia nitrogen contained in the digester effluent after dilution is
converted, by the nitritation reactor 30, into ammonia nitrogen of 390 ppm and nitrite
nitrogen of 510 ppm. At that time, the concentration of free ammonia is 9.7 ppm, and the
concentration of free nitrite is 0.13 ppm. As described above, by supplying the separator
effluent to the nitritation reactor 30 through the first transfer route R1, it becomes possible
to reduce the concentration of free nitrite to be 0.2 ppm or lower, which does not disturb
formation of nitrite.
15
[0083]
Note that when the 10-m3/d digester effluent from the anaerobic sludge digester 20 is
diluted 4 times with the separator effluent of 30 m3/d, the ammonia nitrogen contained in
the digester effluent after dilution becomes 1250 ppm. And, nitrification of this diluted
digester effluent by the nitritation reactor 30 produces ammonia nitrogen of 543 ppm and
nitrite nitrogen of 707 ppm. At that time, the concentration of free nitrite is 0.19 ppm. That
is, by diluting the digester effluent 4 times, the concentration of free nitrite becomes lower
than 0.2 ppm.
[0084]
However, the concentration of free ammonia is 13.5 ppm, and to achieve the
concentration of free ammonia of 0.1-10 ppm which is said to selectively inhibit nitrate
bacteria, the 10-m3/d digester effluent from the anaerobic sludge digester 20 was diluted 5.5
times with the 45-m3/d separator effluent. As described above, dilution of the digester
effluent is performed such that the concentration of free nitrite becomes lower than 0.2 ppm,
and, it is more preferable to dilute to the range of the free ammonia concentrations of 0.1-10
ppm.
[0085]
The autotrophic denitrification device 40 is kept to be anaerobic, with the temperature of
30°C and the pH of 7.4, and the effluent from the nitritation reactor 30 is denitrified by
anaerobic ammonia oxidation. As described above, the concentration of free nitrite in the
effluent from the nitritation reactor 30 is 0.13 ppm. This concentration of free nitrite, 0.13
ppm, is higher than 0.08 ppm required for an efficient anaerobic ammonia oxidation, and
with this concentration, efficiency of the anaerobic ammonia oxidation is reduced.
[0086]
Therefore, 85 m3/d out of the 135-m3/d separator effluent from the suspended-organicsubstance
separator 10 is supplied to the autotrophic denitrification device 40 through the
second transfer route R2. That is, the 55- m3/d digester effluent from the nitritation reactor
30 is diluted about 2.55 times with the 85-m3/d separator effluent. As a result, the nitrite
nitrogen contained in the digester effluent from the nitritation reactor 30 is diluted from
510 ppm to 200 ppm. At that time, the concentration of free nitrite is 0.053 ppm, which is
lower than 0.08 ppm required for an efficient anaerobic ammonia oxidation. Note that the
nitrite nitrogen is diluted 1.7 times to 300 ppm, and at that time, the concentration of free
nitrite is 0.079 ppm, which is lower than 0.08 ppm, however, it was diluted to 0.06 ppm that
allows for more efficient treatment.
[0087]
Fig. 8 shows the results of the numerical simulation performed on the system of the
third embodiment, which was explained with reference to Fig. 3. The system is of the third
embodiment explained with Fig. 3. A wastewater of 200 m3/d flows into this system. The
suspended-organic-substance separator 10 separates suspended organic substances of 20
m3/d contained in the wastewater. Note that for convenience, the concentrations of
ammonia nitrogen and nitrite nitrogen in the 180-m3/d separator effluent from by the
suspended-organic-substance separator 10 are both considered to be 0 ppm.
16
[0088]
The suspended organic substances separated by the suspended-organic-substance
separator 10 are anaerobically digested by the anaerobic sludge digester 20. The digester
effluent contains ammonia nitrogen of 5000 ppm.
[0089]
The nitritation reactor 30 is kept to be aerobic, with the temperature of 30°C and the pH
of 7.4, and the ammonia contained in the anaerobic digester effluent from the anaerobic
sludge digester 20 is oxidized to ammonia and nitrite, such that their mole ratio becomes
about 1:1.3. Therefore, if ammonia nitrogen of 5000 ppm is directly oxidized to nitrite,
ammonia nitrogen of 2160 ppm and nitrite nitrogen of 2840 ppm are produced. At that time,
the concentration of free ammonia is 53.8 ppm, and the concentration of free nitrite is 0.75
ppm.
[0090]
This concentration of free nitrite of 0.75 ppm is higher than 0.2 ppm, which is said to
inhibit all nitrifying bacteria as described above, and therefore, it disturbs transformation
of ammonia to nitrite.
[0091]
Therefore, 90 m3/d out of the 180-m3/d separator effluent from the suspended-organicsubstance
separator 10 is supplied to the nitritation reactor 30 through the first transfer
route R1. That is, 20 m3/d of the digester effluent of the anaerobic sludge digester 20 is
diluted 5.5 times with the 90-m3/d separator effluent. As a result, the ammonia nitrogen in
the digester effluent from the anaerobic sludge digester 20 is diluted from 5000 ppm to 900
ppm.
[0092]
The 900-ppm ammonia nitrogen contained in the digester effluent after dilution is
converted by the nitritation reactor 30, into ammonia nitrogen of 390 ppm and nitrite
nitrogen of 510 ppm. At that time, the concentration of free ammonia is 9.7 ppm, and the
concentration of free nitrite is 0.13 ppm. As described above, by supplying the separator
effluent to the nitritation reactor 30 through the first transfer route R1, it becomes possible
to reduce the concentration of free nitrite to be lower than 0.2 ppm, which does not disturb
formation of nitrite.
[0093]
The autotrophic denitrification device 40 is kept to be anaerobic, with the temperature of
30°C and the pH of 7.4, and denitrifies the effluent from the nitritation reactor 30 by
anaerobic ammonia oxidation. As described above, the concentration of free nitrite in the
effluent from the nitritation reactor 30 is 0.13 ppm. This concentration of free nitrite of 0.13
ppm is higher than 0.06 ppm required for an efficient anaerobic ammonia oxidation, and
therefore, efficiency of the anaerobic ammonia oxidation is reduced if nothing is done.
[0094]
If the nitrite nitrogen in the effluent from the nitritation reactor 30 is diluted from 510
ppm to 200 ppm, the concentration of free nitrite is 0.053 ppm, which is lower than 0.06
ppm required for an efficient anaerobic ammonia oxidation. For that sake, the effluent from
17
the nitritation reactor 30 of 110 m3/d needs to be diluted about 2.55 times. Therefore, for
dilution of the effluent, the separator effluent of 170 m3/d is required. However, the
separator effluent the suspended-organic-substance separator 10 is only 90 m3/d in the
amount.
[0095]
Therefore, the remaining 90 m3/d of the separator effluent, from the suspended-organicsubstance
separator 10, is supplied to the autotrophic denitrification device 40 through the
second transfer route R2, and 80 m3/d of the denitrified effluent from the autotrophic
denitrification device 40 is returned to the autotrophic denitrification device 40 through the
third transfer route R3 .
[0096]
In this manner, the 110-m3/d effluent from the nitritation reactor 30 is diluted about
2.55 times with the 90-m3/d separator effluent supplied through the first transfer route R1
and the 80-m3/d denitrified affluent supplied through the third transfer route R3. As a
result, the nitrite nitrogen in the effluent from the nitritation reactor 30 is diluted from 510
ppm to 200 ppm. At that time, the concentration of free nitrite is 0.053 ppm, which is lower
than 0.06 ppm required for an efficient anaerobic ammonia oxidation.
[0097]
Note that the case explained here is to dilute, about 2.55 times, the 110- m3/d digester
effluent from the nitritation reactor 30 with the 90-m3/d separator effluent supplied
through the first transfer route R1 and the 90-m3/d denitrified effluent supplied through
the third transfer route R3, however, the separator effluent and the denitrified effluent
used for dilution of the effluent from the nitritation reactor 30 may be desirably distributed,
and for example, the 110-m3/d effluent from the nitritation reactor 30 may be diluted only
with the 170-m3/d denitrified effluent supplied through the third transfer route R3, without
using the separator effluent . Or, like the system of the fourth embodiment, if the system
comprises the heterotrophic denitrification device 50 after the autotrophic denitrification
device 40, the effluent from the nitritation reactor 30 may be diluted with the denitrified
effluent from the heterotrophic denitrification device 50.
[0098]
The embodiments described above are all mere examples of the present invention, and it
should be understood that the description does not limit the present invention, and each
component may be configured in the range the function effects of the present invention are
provided.
Explanation of Reference Numerals
[0099]
10: suspended-organic-substance separator
20: anaerobic sludge digester
30: nitritation reactor
40: autotrophic denitrification device
50: heterotrophic denitrification device
60: biotreatment device
R1: first transfer route
R2: second transfer route
R3: third transfer route
18
R4: fourth transfer route
R5: fifth transfer route

WE CLAIMS:-
A system for treating a wastewater containing suspended organic substances, the
system comprising; a suspended-organic-substance separator for separating the suspended
organic substances contained in the wastewater; an anaerobic sludge digester for
anaerobically digesting the suspended organic substances separated by the suspendedorganic-
substance separator; a nitritation reactor for oxidizing the ammonia contained in
the digester effluent from the anaerobic sludge digester into nitrite under aerobic
conditions; and an autotrophic denitrification device for oxidizing the ammonia contained in
the effluent from the nitritation reactor into nitrogen gas by autotrophic denitrifying
microorganisms under anaerobic conditions; wherein
a first transfer route for supplying the separator effluent from which suspended organic
substances have been removed by the suspended-organic-substance separator, or said
separator effluent that has been biologically treated, to the nitritation reactor is provided.
[Claim 2]
The system for treating a wastewater containing suspended organic substances of claim
1, wherein a second transfer route for supplying the separator effluent from the suspendedorganic-
substance separator, or said separator effluent that has been biologically treated, to
the autotrophic denitrification device is provided.
[Claim 3]
The system for treating a wastewater containing suspended organic substances of claim
1 or 2, wherein a third transfer route for returning the denitrified effluent from the
autotrophic denitrification device to the autotrophic denitrification device is provided.
[Claim 4]
The system for treating a wastewater containing suspended organic substances of claim
1 or 2, wherein a heterotrophic denitrification device for reducing the nitrate, contained in
the denitrified effluent from the autotrophic denitrification device, into nitrogen gas by
heterotrophic microorganism under anaerobic conditions is provided; and a fourth transfer
route for returning the denitrified effluent from the heterotrophic denitrification device to
the autotrophic denitrification device.
[Claim 5]
A method for treating a wastewater containing suspended organic substances, the
method comprising steps of: separating suspended organic substances contained in the
wastewater; anaerobically sludge-digesting the suspended organic substances separated by
the separating step; oxidizing the ammonia contained in the digester effluent, from the
anaerobically sludge-digesting step, to nitrite under aerobic conditions; and autotrophically
denitrifying and oxidizing the ammonia contained in the effluent from the oxidizing-tonitrous-
acid step, to nitrogen gas by autotrophic denitrifying microorganisms under
anaerobic conditions; wherein
the separator effluent from which suspended organic substances have been removed by
the separating step, or said separator effluent that has been biologically treated, is
20
transferred to the oxidizing-to-nitrous-acid step and the autotrophically denitrifying step, to
adjust the concentrations of free nitrite in the respective steps.
[Claim 6]
A method for treating a wastewater containing suspended organic substances, the
method comprising the steps of: separating suspended organic substances contained in the
wastewater; anaerobically sludge-digesting the suspended organic substances separated by
the separating step; oxidizing the ammonia contained in the digester effluent, from the
anaerobically sludge-digesting step, to nitrite under aerobic conditions; and autotrophically
denitrifying and oxidizing the ammonia contained in the effluent, from the oxidizing-tonitrous-
acid step, to nitrogen gas by autotrophic denitrifying microorganisms under
anaerobic conditions; wherein
the separator effluent from which suspended organic substances have been removed by
the separating step, or said separator effluent that has been biologically treated, is
transferred to the oxidizing-to-nitrous-acid step, and the biologically-treated separator
effluent, or both the separator effluent and the biologically-treated separator effluent, are
transferred to the autotrophically denitrifying step, to adjust the concentrations of free
nitrite in the respective steps.
[Claim 7]
The method for treating a wastewater containing suspended organic substances of claim
5 or 6, wherein the concentration of free nitrite in the oxidizing-to-nitrous-acid step is
adjusted to be lower than 0.2 ppm, and lower than 0.08 ppm in the autotrophically
denitrifying step.