Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
The present invention relates to a method of biologically purify-
ing waste water ricll in carbohydrates and protein.
In the treatment of waste water, which is heavily loaded or polluted
with carbohydrates and proteins, e.g. from sugar factories, starch factories
or preserving industries, while applying conventional purifying technology, such
as the method of using aerobically activated sludge, there arise difficultly
mastered problems pertaining to the formation of sludge with poor sedimentation
ability, so-called floating sludge. Furthermore, the process usually gives
low purification efficiency, since organic nitrogen compounds are only broken
down with difficulty in the process. Large quantities of energy are consumed
in supplying the required amount of oxygen for the destruction process. In
this known technology large quantities of biological sludge are formed, which
are only dewatered with difficulty and therefore constitute an undesirable
environmental problem.
A further problem in conjunction with the known technology is the
difficulty of arriving at a biological process having sufficiently high
stability for its easy practical performance.
The present invention has the object of providing a method whereby
the drawbacks indicated above are avoided and which can be performed in prac-
tice in a simple manner, since the biological process has very great stability
in practical operation.
According to the present invention there is provided a method ofbiologically purifying waste water rich in carbohydrates or proteins, charac-
terized in that the waste water is treated in an anaerobic stage while leading
off generated methane gas, sludge-bearing water formed being subjected to
sludge separation during formation of a first sludge concentrate which is
returned to the anaerobic stage; that the water thus treated is treated in an
aerobic stage, sludge-bearing water formed being subjected to sludge separation
during formation of a second sludge concentrate, of which at least a portion
is returned to the aerobic stage, while excess is re~urned to the anaerobic
stage, a portion of the first sludge concentrate being led off for disposal.
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The method according to the invention is based on biological purifica-
tion in two stages, a fi.rst anaerobic stage and a second aerobic stage. In
the anaerobic stage the sludge content is kept at a high level, while select-
ing the conditions so that methane fermentation with subsequent reduction of
the pollutants is obtained. Water containing sludge, formed
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in the anaerobic stage, is subsequently exposed to sludge separation while
forming a first sludge concentrate which is returned to the anaerobic stage,
i.e. it is recirculated. After said sludge separation, the water treated in
the first stage is treated in the aerobic stage, where remaining pollutants,
thanks to the conversion in the anaerobic stage, are present in such a form,
that they can easily be utilized by aerobic organisms. The sludge-bearing
water formed in the aerobic stage is subjected to sludge separation while
forming a second sludge concentrate at least a portion of which is returned
to the aerobic stage while possible surplus is recirculated to the anaerobic
stage. Sludge from the first sludge concentrate and possibly also from the
second sludge concentrate is ~rawn off and taken away for disposal.
In the method according to the present invention, the greater por-
tion of the content of organic material in the waste water is preferably
decomposed in the anaerobic first stage, while the remainder is taken care of
in the subsequent aerobic stage. Up to about 80-90 % of the total pollutant
content can thus be dealt with in the anaerobic decomposition.
In accordance with the invention, a portion of the second sludge
concentrate is returned to the aerobic stage, while the rest is recirculated
to the anaerobic stage for destruction, whereafter sludge concentrate for
disposal is only removed from the process at the first sludge concentrate
obtained from the anaerobic stage.
The present invention has the substantial advantage that in the
anaerobicstage the major portion of the nitrogen compounds are converted to
ammonia, which can be removed from
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the water comparatively easily.
In ~he treatment of protein-bearing waste water~ i.e.
water containing a large proportion of organic nitrogen com-
p~unds, it is thus a preferred step to remove the ammonium
ions formed in the anaerobic stage before the treated water
is transferred to the aerobic stage. Such removal can be
accomplished by adding an alkali and driving off the ammonia
liberated hereby. The amount of alkali is suitably adjusted
for attaining a pH Or 9-11. Liberated ammonia can be driven
off by blowing air through the water.
An alternative method of removing ammonium ions is to
pass the water through a cation exchanger, suitably a sodium
saturated cation exchanger.
As previoulsy indicated, the technique according to the
present invention signifies substantial advantages. Thus in
relation to the conventional technique, only a fraction of
the usual amount of sludge is obtained. With optimum condit-
ionsg the amount of sludge can be reduced to only about a
tenth of what is usual9 while obtaining a greater proportion
of dry substance e.g. 10 percent by weight. This is in
contradistinction to the conventional technique, whereby a
substantially lower dry substance content is obtained~ usually
about 2 percent by weight. The advantages brought about by
the technique according to the invention will thus be easily
appreciated.
Earlier in this disclosure the advantage of great stabili-
ty in the biological process has been mentioned. This stability
is primarily conditioned by the buffer action brought about
by recirculating the sludge concentrate to the anaerobic stage.
- 30 The stability in this st~ge in its turn results in substantially
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improved stability in the following aerobic stage, which is
recognised as normally being very sensitive to disturbances in
the form of variations in the composition of the incoming
materials. The total result is a process which is easy to
control~ and which works in a satisfactory way, in spite of
variations in the incoming waste water.
The method according to the present invention can to
advantage be combined with treatment of lightly loaded waste
water, which together with the treated water leaving the
anaerobic stage, can be led to the aerobic stage.
The invention will be further illustrated in the follow-
ing by means of non-limiting examples in conjunction with the
appended drawing, where Figs. 1 and 2 diagramatically show
block diagrams over the process, as respectively adapted to
the treatment of waste water having a low content and a high
content of nitrogen contaminants.
According to Fig. 1, the plant for treating waste water
which is poor in nitrogen consists of a tank 1 for complete
mixing, where the anaerobic fermentation is carried out. The
methane gas formed during this treatment is recovered at 2.
The treated water containing anaerobic sludge is transferred
to a sludge separating apparatus 3, w~ich can consist of a
lamella separator, a sedimentation basin, a decanting centri-
fuge or other suitable equipment. ~rom the separating apparat-
us 3, a proportion of the sludge is recirculated to the tank 1,while the remainder is led off for disposal.
The water thus treated is led off for treatment with
aerobically a~tivated sludge in a closed tank, an earth basin,
a concrete basin or other suitable apparatus, generally de-
0 noted by the numeral 7 allowing oxyginization. The water5
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treated in 7 is further led to a sludge separator/thickener 8,
from which separated sludge ls recirculated to the tank 1,
while the purified water is led away for disposal or other
purposes. A part of the slud~e separated in 8 is recirculated
to the activated sludge apparatus 7.
The plant in Fig. 2 is intended for treating waste water
having a high content of nitrogen pollutants, and differs from
the one illustrated in Fig. 1 only with regard to the equipment
coupled in between the anaerobic and aerobic stages, said
equipment consisting of an apparatus 4 for adding alkali, a
stripper 5 for driving off ammonia formed, and an apparatus 6
for reclaiming driven-off ammonia. The stripper 5 is at its
lower end supplied with air for removing the ammonia, while
the ammonia-bearing water is supplied at the upper end of the
stripper.
This example deals with the treatment of water from the
sugar factory of ~rtofta in Sweden. The water contains about
0.4 percent by weight of carbohydrates and has a BOD5 of about
5000 mg 02/1. (BOD5is here defined as the oxygen consumption
during 5 days of a sample kept in darkness at 20C.) The water
is treated in a plant according to Fig. 1, where the tank 1
has a volume of about 0.5 m3. The BOD load in relation to water
supplied is about 1 kg per m3 per 24 hours. In the tank there
is a temperature of about 35C, a pH of about 7 and a sludge
content of about 10 g/1.
In accordance with what is stated hereinbefore, completely
anaerobic conditions prevail in the tank.
Methane-bearing gas leaves the tank 1 in an amount of
about 0.16 m3/24 h and contains about 70 percent by volume of
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methane, the remainder being carbon dioxide. In the sludge
separating apparatus 3 3 in the liquid phase (i.e. not counting
the sludge), there is a BOD5 of about 1000 mg 02Jl, a pH of
about 7 and a temperature of about 35C~ while the surface load
is about 0.5 m/h. Sludge-bearing material returned from the
sludge separating apparatus 3 to the tank 1 is about 20 percent
by volume of the water entering tank 1. From the separator 3
is led off for disposal an amount corresponding to 1 litre ,:e,
sludge per kg of the BOD coming into the tank 1.
The apparatus 7 for further treating the water with
aerobically activated sludge consists of a tank reactor 7 with
a volue of about 0.1 m3, where complete mixing is obtained by
blowing in air. The temperature of the liquid phase in the
reactor7is about 20C and the BOD load about 1 kg per m3 per
24 hours. The water leaving tank 7 has a pH Or about 8, a
sludge content of about 3 g/1 and an oxygen content greater
than 2 mg 02~1.
The same values for temperature and pH prevail in the
sludge separat.or 8 as in the tank 7. ~he surface load is about
0.5 m/h. Sludge-bearing liquid led off from the sludge separator
8 has a volume of about 40% of the water entering the tank 1,
and of this liquid about 7/8thsis returned to the tank 7 while
about 1~8th is recirculated to the anaerobic tank 1. The
purified water leaving the sludge separator 8 has a BOD5 of
about 50 mg 02~1, and thus does not cause any disposal diffi-
culties when discharged into a recipient.
It may be added that the process described in this
example is not started by a selective culture, but is founded
on natural generation on the basis of the microorganisms to be
found in the incoming waste water It is further a~parent from
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the example that about 80% of the incoming oxygen-consuming
material taking part is broken down in the anaerobic stage
while the aerobic Stage has mainly taken care of remaining
pollutants.
Example 2
This example deals with the treat~ent of waber fram a
factory pr~ducing yeast based on mola~ses, The water contaihs
about 1 pe~Cent by weight of organic pollutants~ the major
p~rtion of which are nitrogen compounds and the remainder
mainly consisting of carbohydrates. The water has a BOD5 Or
10,000 mg 02/1 and is treated in a plant accordin~ to Fig. 2~
which, apart from the equipment 4, 5 and 6 coupled in between
both stages, completely corresponds to the plant according to
Fig. 1. The BOD load on the water supplied to the tank 1 is
about 2 kg per m3 per 24h, there being in the tank 1 a
temperature of about 35C, a pH of about 7.5 and a sludge
content of about 10 gJl.
Methane-bearing gas leaves tank 1 in an amount of about
0.3 m3/24 h, and contains about 70 percent by volume o~
methane, the remainder being carbon dioxide. In the sludge
separa~or 3 t~ere is a BOD5 o~ about 2000 mg 02/1 in the
liquid phase (i.e. not counting the sludge), a pH o~ about 7.5,
a temperature of about 37C~ and a surface load of about 0.5
m/h. Sludge-bearing material re~urned from the sludge separator
3 to the tank 1 is about 20 percent by volume of the water
entering tank 1. From the separator 3 is led off for disposal
an amount corresponding to one litre sludge per kg of the BOD
coming into the tank 1.
In the apparatus 4 ~or adding alkali, quick lime is
added to a pH of about 10.5. The apparatus 4 i8 a simple
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receptacle with an agitator, and has a volume such that a
retention time of about half an hour is obtained. About 80 %
of the ammonia set f,ree in conjunction with the addition of
alkali is driven off in the stripper 5 and reclaimed in the
apparatus 6.
In the apparatus 7, where the water is treated aerobic-
ally~ there is a BOD load of about 2 kg per m3 per 24 h, a
temperature of about 20C, a pH of about 9, a sludge content '~
of about 6 g~l and an oxygen content exceeding 2 mg/l.
The same values for temperature and pH prevail in the
sludge separator 8 as in the tank 7. The surface load is
about 0.5 m/h. Sludge-bearing liquid led off from the apparatus
8 has a v~lume of about 40% of the water entering the tank 1,
and of this liquid about 7/8ths is returned to the tank 7
while about 1/8th is recirculated to the anaerobic tank 1. The
purified water leaving the sludge separator 8 has a BOD5 of
about 200 mg 02~1 and thus does not cause any difficulties
when discharged to a municipal sewage plant or, after stora~e,
to a recipient.
The process described in this example has been started
by innoculation with activated digested sludge. As in example
1, about 80% of the incoming oxygen-consuming material taking
part is decomposed in the anaerobic stage, while the remain-
ing pollutants are decomposed in the aerobic stage.
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