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Description  |
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The present invention relates to biological treatment of waste water, and
more particularly relates to biological treatment of waste water in which
no substantial amount of excess waste sludge is produced.
Recently, biological treatment techniques of waste water have been widely
used for preventing water pollution resulting from the accumulation of
organic substances which is indicated as B.O.D. (Biochemical Oxygen
Demand). Known biological treatment methods of waste water typically
include: (i) an activated sludge type process wherein waste water and
flocks of microorganisms suspended therein are contacted with each other
to purify the waste water, and; (ii) a biological fixed film type process
wherein waste water is contacted with biological fixed films attached onto
a support to purify the waste water.
Among these processes an activated sludge type process in which an aqueous
suspension of microorganisms is utilized is most prevalent for purifying
waste water such as sewage, various kinds of industrial waste water and
the like. However, the known activated sludge type processes have the
disadvantages that a large amount of excess waste sludge is produced and
the cost of the treatment of the excess waste sludge thus produced is very
high. The amount of the excess waste sludge produced in the activated
sludge type processes is generally within the range of from 40 to 80% by
weight, based on the amount of the B.O.D. components removed from the
waste water. The typical method of treating excess waste sludge generally
includes the steps of concentration, dewatering, drying and incineration.
Particularly, since recent energy costs have been rapidly increasing, it
has been very much desired to reduce the amount of excess waste sludge
produced in the water treatment system. Although it has been practical to
use either an aerobic or anaerobic method to digest excess waste sludge in
order to reduce the solid contents of aqueous excess waste sludge, such
digestion methods for treating excess waste sludge are unsatisfactory from
the point of view of digestion efficiency and digestion rate, and the
steps of treating the excess waste sludge become complicated and the
operational control of the digestion treatment becomes difficult due to
the post-treatment of the digested aqueous sludge and liquid. For
instance, with respect to anaerobic digestion, even if the aqueous excess
sludge is digested for a long period of time, such as from 20 to 40 days,
the digestion efficiency of the process is at most 60%, and further,
complicated steps are required, such as solid-liquid separation, washing
of the digested sludge, flocculation of the washed sludge, dewatering,
incineration and treatment of the separated liquid from the digested
aqueous sludges. On the other hand, with respect to aerobic digestion,
even if aqueous excess sludge is digested for a long period of time, such
as from 5 to 20 days, the digestion efficiency of the process is at most
40-50%. In addition, the digested sludge is inferior in terms of
concentration and dewatering properties, as compared to the original
sludge, and the liquid removed from the digested sludge becomes turbid and
white. Although a so-called total oxidation process which produces no
substantial amount of excess sludge has already been proposed in the art,
little of this process can be applied to practical use due to the fact
that an extremely long period of aeration and a large space for installing
the required apparatus are required.
The amount of excess waste sludge produced by the biological fixed film
type process is generally within the range of from 30 to 50% by weight
based on the amount of the B.O.D. components removed from the waste water,
which amount is somewhat smaller than that of the activated sludge type
process. However, this process also has problems in that a large amount of
excess waste sludge is produced and the treatment of the excess waste
sludge is troublesome and costly.
Accordingly, the objects of the present invention are to obviate the
above-mentioned disadvantages and problems of the conventional biological
treatment of waste water and to provide an improved biological treatment
of waste water which produces no substantial amount of excess waste
sludge.
Other objects and advantages of the present invention will be apparent from
the following description.
In accordance with the present invention, there is provided a method for
biologically purifying waste water comprising the steps of:
(a) contacting the waste water containing biodegradable substances with
microorganisms in a biological fixed film type reactor under conditions
sufficient to degrade said biodegradable substances;
(b) separating and concentrating an aqueous waste sludge contained in the
biologically treated waste water discharged from the biological fixed film
reactor by using a mechanical type separator or floatation equipment type
separator;
(c) aerobically digesting the concentrated aqueous sludge, and;
(d) returning the digested aqueous sludge to the biological fixed film
reactor or the upstream line thereof.
In accordance with the present invention, there is further provided a
method for biologically purifying waste water containing biodegradable
substances comprising the steps of:
(a) treating a portion of said waste water by using suspended
microorganisms in water in an activated sludge type reactor under
conditions sufficient to degrade said biodegradable substances (stage A);
(b) simultaneously and separately treating the remainder of said waste
water by using a biological fixed film type reactor under conditions
sufficient to degrade said biodegradable substances (stage B);
(c) aerobically digesting a mixture of (i) an aqueous excess waste sludge
discharged from the activated sludge type reactor of step (a) and
separated from the biologically treated waste water and (ii) an aqueous
excess waste sludge discharged from the biological fixed film type reactor
of step (b) and separated from the biologically treated waste water by
using a mechanical type separator or floatation equipment type separator,
(stage C) and;
(d) returning the digested aqueous sludge to the activated sludge type
reactor of step (a), the biological fixed film type reactor of step (b) or
both the reactors of steps (a) and (b) (stage D).
The present invention will be better understood from the following
description with reference to the accompanying drawings, but is by no
means limited to these drawings, in which:
FIGS. 1 through 5 are schematic drawings illustrating the various
embodiments of the method of the present invention in which waste water is
biologically treated by using biological fixed film type reactors;
FIG. 6 is a schematic drawing illustrating a conventional method for
purifying waste water by using an activated sludge type reactor, and;
FIG. 7 is a schematic drawing illustrating a preferred embodiment of the
method of the present invention in which waste water is biologically
treated by using, in parallel, an activated sludge type reactor and a
biological film type reactor.
The biological fixed film type reactors used in the present invention for
purifying the waste water are known and are typically classified into the
three groups, that is, (i) a trickling filter, (ii) a rotating disc
contactor and (iii) an immersion type contact oxidation reactor. Any
conventional biological fixed film reactor or contactor can be used in the
present invention. Among the biological fixed film reactors, the inventors
of the present invention recommend using the immersion type contact
oxidation reactor in the present invention. The contact oxidation reactor
is disclosed in detail in, for example, Japanese Patent Publication
(KOKOKU) No. 45-22304/1970 and Japanese Patent Laid-Open Application
(KOKAI) No. 51-15567/1976.
As shown in FIG. 1, waste water 8 containing biodegradable substances, is
introduced into a rotating disc contactor 1 (i.e. a biological fixed film
reactor). In the contactor 1, the waste water 8 is contacted with
biological fixed films attached to the surface of the rotating disc 1',
whereby the biodegradable substances (B.O.D. components) contained in the
waste water 8 are biologically oxidized under conditions sufficient to
degrade the biodegradable substances. Such conditions are well known in
the art and, typically, include a BOD loading of 5 through 30 gr/m.sup.2
.multidot.day, a water residence time of 0.5 through 2 hrs and a rotation
speed of 1-5 r.p.m.
The purified waste water 8' containing sludge is then transferred into a
static inclined, screen 2 (i.e. a mechanical separator) where purified
water 9 and an aqueous concentrated sludge 10 are separated from each
other. The purified water 9 is flowed out of the waste water treatment
system, whereas the aqueous concentrated sludge 10 is continuously fed to
a digesting vessel 4 by means of a belt conveyor 3. In the digesting
vessel 4, the sludge 10 is aerobically digested by aeration with air
(oxygen) fed through an air supplying device 6 from an air supply
compressor 5. The digested liquid 11 is returned to the feed line of the
waste water 8 into the rotary disc contactor 1 by means of a pump 7. Thus,
the closed waste water purification system is completed without producing
substantially any excess waste sludge.
In the process shown in FIG. 2, two biological fixed film reactors 12, each
comprising a vessel 13, a plurality of parallel spaced and substantially
vertically disposed porous supports 14, such as plastic nets, in the
vessel 13 and an air supplying device 6' disposed under the supports 14
are used in lieu of the rotary disc contactor 1 of FIG. 1. That is, as
shown in FIG. 2, waste water 8 containing biodegradable substances is, in
series, introduced into two biological fixed film reactors 12. In the
reactors 12, the waste water 8 is contacted with biological fixed film
attached onto the surfaces of the supports 14, whereby the biodegradable
substances contained in the waste water are biologically oxidized with
aeration. The purified waste water 8' containing sludge is then
transferred into a static inclined screen 2 where aqueous concentrated
sludge 10 is separated from purified water 9. The aqueous concentrated
sludge 10 falls down into a digesting vessel 4, where the sludge is
aerobically digested by aeration with air fed through an air supplying
device 6 from an air supplying compressor 5. The digested liquid 11 is
returned to either or both of two reactors 12 by means of a pump.
Although a single biological fixed film reactor 12 can be used in the
practice of this invention, the use of the two biological fixed film
reactors is advantageous for the following reason. That is, in the first
reactor, B.O.D. components contained in the waste water 8 and the digested
liquid 11 are mainly degraded, whereas, in the second reactor, fine sludge
is converted into coarse sludge and large organisms, such as metazoa, are
grown under mild aeration conditions.
In the process shown in FIG. 3, waste water 8 containing biodegradable
substances is introduced into a trickling filter 15, wherein the
biodegradable substances contained in the waste water 8 are biologically
oxidized. The purified water 8' containing sludge is then transferred into
a floatation vessel 16. In the floatation vessel 16, the sludge contained
in the purified water 8 is floated by the action of air 17. Aqueous sludge
10 thus separated and concentrated is fed to a digesting vessel, and is
aerobically digested by aeration with air introduced through an air
supplying device 6 from an air supplying compressor 5. The digested liquid
is returned to the waste water feed line of the trickling filter 15 by
means of a pump 7.
In the process shown in FIG. 4, the aerobically digested liquid is
anaerobically digested in a digesting vessel 18. That is to say, waste
water 8 discharged from a waste water reservoir 19 is biologically treated
in a biological fixed film reactor 12, as described above with reference
to FIG. 2, and the purified water is aerobically digested in a digesting
vessel 4 after being separated and concentrated in a static inclined
screen 2. The aerobically digested liquid 11 is then introduced into
another digesting vessel 18 wherein the liquid 11 is anaerobically
digested. The digested liquid from the vessel 18 is returned to the waste
water reservoir 19 and, in turn, introduced into the biological fixed film
reactor 12.
This process is especially advantageous and effective in the case where
sludge, which is difficult to digest by only aerobic digestion, is
treated.
In the process shown in FIG. 5, the static inclined screen 2 of the process
shown in FIG. 1 is only replaced with rotating drum screens 20, where
sludge contained in the treated waste water 8' is separated from the
purified water 9 in the form of concentrated aqueous sludge 10.
As mentioned above, the biological fixed film reactor used in the practice
of the present invention can be any conventional type of biological fixed
film reactor, such as, for example, a trickling filter, a rotating disc
contactor, an immersion type contact oxidation reactor or the like.
Sludge discharged from the biological fixed film reactor is mainly
separated by supports onto which the biological fixed films are attached,
and, therefore, the sizes of clumps of the sludge are very large. Further,
this sludge easily slides down surfaces of separation elements. This is
because, since the sludge involves much water and is therefore so bulky,
the sludge has little friction against said surfaces of the separation
elements. Accordingly mechanical type separators, such as a static
inclined screen, can be advantageously used for rapidly separating an
aqueous sludge from the purified water.
The second advantage of the use of the biological fixed film reactors
resides in the fact that various organisms participate in the biological
degradation reaction in the biological fixed film reactor. That is, in the
biological fixed film system, since the organisms, which participate in
the biological degradation reaction, are attached to supports, an
anaerobical state is generated at the surfaces of the supports due to the
lack of oxygen; whereas an aerobical state is maintained at the surfaces
of the attached biological fixed films which are in contact with the waste
water, due to the fact that sufficient oxygen is supplied. Thus, since
both aerobical and anaerobical states can be simultaneously maintained in
the same reactor in the biological fixed film system (this is not the case
where a system using suspended microorganisms, such as an activated sludge
treatment, is used), many kinds of organisms are present in the reactor.
The large number of kinds of organisms also relates to a low yield of
sludge production in the method of the present invention.
The biological phases of the biological fixed films are generally
classified into the following three groups. The first group includes
microorganisms mainly containing bacteria, such as zoogloca, which are
attached onto the surfaces of the supports in the reactor in a thickness
of 2 through 10 mm. The second group includes protozoa, such as, for
example, ciliatea, flagellata, rhizopoda and the like, which are attached
to or covered over the microorganisms of the first group. The third group
includes large organisms (which are called organisms which use sludge as
food), such as, for example, nematoda, rotatoria, oligochaeta, arthropoda
and the like. These organisms form microbial food chains, that is, the
organisms of the first group are used, as food, by those of the second
group, which in turn are used, as food, by those of the third group. Thus,
these various kinds of organisms are simultaneously present in an
equilibrium state in the internal environmental conditions of the
biological fixed film reactor. These phenomena show that sludge digestion
is a kind of successive reaction, and, therefore, the production amount of
excess waste sludge in a conventional biological fixed film process is
somewhat smaller than that of the activated sludge aeration process.
We have found that the organisms of the second and third groups have a
large sludge digesting capacity and that a suprisingly high digesting
speed (which is far larger than that of conventional digestion) and an
approximately 100% digesting efficiency can be obtained by introducing the
organisms of the second and third groups into an aerobical digester after
being separated and concentrated. The output line and the dead bodies of
the organisms of the second and third groups in the aerobical digester,
which are nutrient sources of the microorganisms of the first group, are
returned to the biological fixed film reactor. Thus, microbial food chains
are formed in the present waste water treatment system.
We have also found that the line large organisms are vulnerable to the lack
of oxygen and that, when they are maintained for a few or several hours in
the state of lack of oxygen, they die and lose their activity.
Accordingly, when the large organisms are separated from the purified
water, it is necessary that they be rapidly separated to keep them alive
and fed into an aerobical digester or that they be separated under the
conditions where enough oxygen is present. For this reason, sludge
thickeners which are usually used for separating excess sludge from the
purified water in the conventional waste water treatment process cannot be
used in the present invention.
The separation and concentration devices which can be used for separating
and concentrating sludge in the practice of the present invention include
a mechanical type separator and floatation equipment. The mechanical type
separators used in the present invention include, for example:
a static inclined screen, such as a wedge wire screen composed, as a
separating element, of a plurality of parallel spaced rows of wedge wires,
each having a V-cross sectional shape;
a drum screen, such as a micro strainer;
a vibrating sieve;
a centrifugal separator;
a filtering machine and the like.
Although the separation rate of the sludge separator depends upon the
sludge concentration of the purified water discharged from the biological
fixed film reactor, the amount of water to be treated based on the unit
area of the separating element and the unit time is, preferably, within
the range of from 10 to 10,000 m.sup.3 /m.sup.2 /day, and, more
preferably, within the range of from 100 to 5,000 m.sup.3 /m.sup.2 /day.
The concentration of the sludge after the separation and concentration is
generally 5,000 mg/l or more, and preferably 20,000 mg/l or more. The
preferred separation time is 60 minutes or less.
The concentrated sludge can be transferred to the aerobical digester by
means of conventional transportation means, such as a belt conveyor, or
directly from the separator to the digester by the action of gravity.
However, it is not advisable to use a transportation means such as a pump,
which imparts a shearing force to the sludge, because the large organisms
present are killd by the action of the shearing force. The transportation
of sludge can be continuously or semi-continuously carried out. The period
of time for which the sludge is transferred into the digestor after the
separation and concentration is preferably within 4 hours, and more
preferably, within one hour.
The digester (or digesting vessel) used in the practice of the present
invention is mainly an aerobical type digester, but the addition of an
anaerobical type digester and the like, which are generally utilized in
the conventional sludge treatment cycle, can be also applied in the
present process. When the sludge concentration in the digester is high, a
good efficiency can be obtained. For this reason the sludge concentration
is preferably 5,000 mg/l or more, and more preferably, 10,000 mg/l or
more. The advantage of the present invention is that the concentration of
the aqueous concentrated sludge can be raised by using the mechanical
separator mentioned above. Therefore, the volume of the digester can be
minimized. The residence time of the aqueous concentrated sludge is
generally within the range of from 5 to 60 days, and more preferably
within the range of from 10 to 30 days. The other digesting conditions are
similar to those which are well known in the art. For example, a
temperature of 10.degree. through 40.degree. C., an aeration rate per
vessel volume of 0.3 through 3 Nm.sup.3 /m.sup.3 .multidot.hr and a pH of
3.5 through 8.5 are typically used.
The digested liquid according to the present invention can be returned to
any place which is a waste water feed portion of the biological fixed film
reactor or upstream thereof, such as, for example, a feed line of the
waste water, a raw waste water tank, a feed line of the raw waste water
tank and the like.
The digestion process of the present invention is different from the
conventional digestion processes in the following points.
(1) The sludge solubilized or finely divided in the digester is returned to
the biological fixed film reactor, wherein most parts of the sludge are
adsorbed onto or absorbed into the surfaces of the biological fixed films,
together with organic substances contained in the waste water. Thus, the
sludge is taken into the bodies of the organisms and converted to fresh
biological fixed films. This feature can be called a biological
coagulation phenomenon, in which the solubilized or finely divided sludge
in the digester is assimilated in the biological fixed film reactor.
(2) Along with the sludge recycled to the biological fixed film reacter,
the sludge which is not completely digested in the digester is again
recycled into the digester through the reactor and separator. Accordingly,
sludge which is not easily digested can be repeatedly digested in the
digester and, then, finally, completely digested. This feature can be
called a multi-cycle digestion.
(3) As mentioned above, sludge produced in the biological fixed film
reactor is partially digested by microbial food chains held in the
biological fixed films. Since the digestion in the biological fixed film
reactor is caused by organisms which are different from those in the
digester, sludge which is not digested in the digester can be sufficiently
digested in the biological fixed film reactor. This feature can be called
a multi-stage digestion.
In conventional sludge digestion processes, solubilized sludge in the
excess sludge is separated from unsolubilized sludge by washing and is
returned to raw waste or treated in another step. The unsolubilized sludge
is finally incinerated even if it contains a substantial amount of
biodegradable components. That is, the conventional process is a one-cycle
and one-stage digestion system, whereas the present invention is
multi-cycle and multi-stage digestion system.
Another feature of the present invention resides in the fact that poisonous
substances can be washed-off or degraded in the recycling system of the
present invention. That is, the poisonous substances which inhibit the
biological sludge digestion reaction are washed-off or are degraded by
bacteria in the course of the recycling system of the present invention
and, therefore, the accumulation of the poisonous substances in the
digester which causes a decrease in the aerobical digestion reaction can
be effectively avoided. This is one of the reasons that a high digesting
efficiency is obtained in the present invention.
As will be clear from the above description, according to the present
invention, microbiological food chains are completely formed in the
recycling system of the present invention and the growing environment of
organisms which is necessary for achieving the microbial food chains is
maintained in best conditions. The degradation energy which organisms have
is insufficiently utilized in conventional sludge digestion, whereas the
degradation energy is exhaustively utilized in the present invention.
Thus, a biological waste water treatment which produces no substantial
amount of excess waste sludge can be unexpectedly achieved according to
the present invention.
In accordance with another aspect of the present invention, waste water is
simultaneously treated in the activated sludge type reactor and the
biological fixed film type reactor, and a mixture of aqueous concentrated
sludge discharged from both reactors is aerobically digested. The digested
liquid can be returned to the activated sludge type reactor and/or the
biological fixed film type reactor. Thus, no substantial amount of excess
waste sludge is produced from the waste water treatment system. This
aspect of the present invention has the outstanding advantage that the
production of excess waste sludge in an already installed activated sludge
type reactor can be substantially eliminated only by combining a
relatively small size biological fixed film type reactor and a sludge
digesting vessel with any already installed activated sludge type waste
water purification plant. This drastically reduces the costs and the work
required in the conventional activated sludge system.
As shown in FIG. 6, in a conventional activated sludge type waste
purification system, waste water 21 containing biodegradable substances is
introduced into an activated sludge aeration vessel 22, wherein the
biodegradable substances are oxidized with aeration. Into the waste water
phosphoric acid 38 is added for maintaining the multiplication of organic
fungus bodies. The treated water is then transferred into a sludge
thickener 23. In the thickener 23, aqueous sludge 23 is separated from the
purified water 24, which is flown out of the system. A portion of the
aqueous concentrated sludge 25 is returned to the sludge aerator 22 as
additional activated sludge. The remainder of the aqueous concentrated
sludge (i.e. the excess waste sludge) is fed to a sludge storage tank 26.
The excess waste sludge is coagulated in a coagulation vessel 27 by the
addition of a coagulating agent and is dewatered in a centrifugal
separator 28. The centrifuged filtrate 29 is returned to the feed line of
the waste water 21. The dehydrated excess sludge is introduced into a
dryer 31 via a conveyor 30 and, thereafter is incinerated in an
incinerator 32.
Contrary to the above, in accordance with the present invention, as shown
in FIG. 7, the excess sludge treatment steps after the coagulation vessel
are eliminated and a biological fixed film reactor 33, in which a portion
of the waste water 21 is treated, and a digesting vessel 35 are added.
That is, a portion 39 of the waste water 21 is biologically treated in the
activated sludge aeration vessel 22 as in the process shown in FIG. 6. The
remainder 40 of the waste water 21 is introduced in a biological fixed
film reactor 33 and the waste water 40 is contacted with biological fixed
films in the reactor 33. Thus, biodegradable substances contained in the
waste water 40 are biologically oxidized in accordance with any
conventional technique. The purified waste water 40' containing biological
sludge is then fed into a mechanical type separator 34 (or floatation
equipment). The purified water 36 is flowed out of the system and an
aqueous concentrated sludge 41 is continuously fed to a digesting vessel
35, together with the aqueous excess sludge 25 from the thickener 23. A
portion of the aqueous excess sludge 25 is returned to the sludge aerator
22 as additional activated sludge. In the digesting vessel 35, the mixture
of the aqueous sludge 25 and 41 is aerobically digested by aeration with
air fed through an appropriate air supplying means (not shown in FIG. 7).
The sludge storage tank 26 of the installed plant shown in FIG. 6 can be
easily modified to the digesting vessel 35 in the case where an already
installed plant as shown in FIG. 6 is reconstructed into the plant as
shown in FIG. 7. The digested liquid 37 is returned to either or both of
the sludge aerator 22 and the biological fixed film reactor 33. Although
the same waste water is separately treated in the activated sludge reactor
and the biological fixed film reactor in the embodiment shown in FIG. 7 it
will be noted that different waste waters can be separately treated in
both reactors.
In stage A of this aspect of the present invention, waste water containing
biodegradable substances is contacted with flocks of suspended
microorganisms in a known manner, whereby the biodegradable substances are
oxidized.
In the stage B, waste water containing biodegradable substances is
contacted with the biological fixed films in the biological fixed film
reactor, as mentioned in the first aspect of the present invention. The
stage A operation and the stage B operation are simultaneously and
separately carried out in parallel.
In stage C, aqueous concentrated sludge discharged from stage A and stage B
is continuously or intermittently mixed together and the mixture is
aerobically digested by stirring in a known manner. Although the mixing
ratio of the sludge discharged from the stage A and the stage B can be
varied within a wide range depending upon the digesting conditions, the
mixing ratio of the excess sludge discharged from stage A to that
discharged from stage B is generally within the range of from 98:2 to
50:50, and more preferably 95:5 to 70:30, in terms of the dry weight of
the excess sludge. When the mixing ratio is below 98:2, improvement of
digestion efficiency is not clearly recognized. A mixing ratio above 50:50
can be utilized. However, since no useful purpose would be served thereby,
it is not preferable from an economical point of view.
The control of the mixing ratio of the sludge discharged from stages A and
B can be easily achieved by adjusting the feed ratio of waste water to the
stages A and B. The feed amount of the waste water to stage B (i.e. the
biological fixed film reactor) is generally within the range of from 1 to
50% by weight and, more preferably, within the range of from 3 to 30% by
weight, based on the total amount of the waste water. This means | | |
