Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1 334543
- METHOD FOR THE TREATMENT OF SEWAGE
AND OTHER IMPURE WATER
The present invention relates to a method for
treating impure water, and more particularly to an
improved method for treating sewage to produce
treated effluent of very high quality.
It is known that color, turbidity, organic matter
and similar impurities may be removed from water by
coagulants, e.g. alum, ferric sulfate or the like.
These compounds are acidic and react with the
alkalinity in the water or with alkaline compounds,
e.g. lime or soda ash, to form voluminous insoluble
precipitates (hydrates). The precipitates have a
tremendous surface area on which the dissolved or
colloidally dispersed impurities are absorbed. The
suspended impurities are surrounded by the
gelatinous hydrates and become part of the
precipitate.
Domestic or sanitary sewage and industrial wastes
may be purified by the chemical precipitation
process, in which suitable chemicals (e.g. aluminum
sulfate, lime, iron chloride, polyelectrolytes or
combinations thereof) are added to the sewage and
the sewage passed to one or more flocculating
tanks, normally equipped with slowly rotating
agitators or paddles, in which colloidal solids are
formed into particles of size and weight that will
settle. The colloidal solids or flocs are then
separated from the liquid by being allowed to
settle in subsequent settling tanks, whereafter the
purified water is collected in a weir structure
mounted at the surface of the water, while the
sediment, consisting of flocs and sludge, is
removed, normally by means of sludge scrapers ~.
/, ~
1 334543
and/or pumps.
The prior art teaches the addition of various types
of chemicals and combinations of chemicals to
sewage and other impure water to remove various
pollutants therefrom.
There are several deficiencies in the prior art
which the present invention overcomes as indicated
below:
1. The invention, when used to treat raw sewage or
other impure water with very economical doses of
three chemicals converts a very high proportion of
the suspended, colloidal and dissolved pollutants
in the sewage or other impure water to large, dense
and stable flocs which are so resistant to shear
forces they can be settled out in a clarifier
without the aid of inclined sedimentation means,
and with an upward flow velocity of at least
eighteen to twenty meters per hour. This flow rate
is approximately ten times higher than than
recommended by those skilled in the art for
clarifiers without inclined sedimentation means.
The foregoing is a very important advantage from an
economic point of view because it allows the use of
a very much smaller clarifier, and reduces the area
of land required for a treatment plant.
2. The invention, notwithstanding the fact that
very economical doses of chemicals are used and
the floc is settled against an upward velocity
flowrate of 18-20 m/hr. without inclined
sedimentationmeans, achieves removal rates of
pollutants which heretofore have not been possible
as indicated hereunder:
1 334543
Pollutant Average % Removal
Biochemical Oxygen Demand 76%
(BOD5) Dissolved BOD5 under
0.2 microns in size 32%
BOD5 over 0.2 microns in size . 95%
Total phosphorus 97%
Turbidity 95%
Total Suspended Solids 92%
Fats, Oils and Grease 90%
Aluminum Removes all of
the aluminum
which is dosed
into the sewage
or industrial
effluent, in
addition to
approximately
70% of the small
quantity of
aluminum present
in the influent.
3. The invention is a considerable improvement
over the prior art in relation to the removal of
Biochemical Oxygen Demand (BOD5), with
approximately 95% of all BOD5 over 0.2 microns in
size being removed, and in addition, almost one
third of the BOD5 less than 0.2 microns in size
also being removed.
The implications of this fact means that the
invention can be used in many locations to treat
raw sewage to a standard that does not require
further treatment before discharge to waterways,
whereas the effluent from other chemical systems
requires additional biological treatment.
_ 4 _ 1 3 3 4 5 4 3
Furthermore, where highly polluting waste waters
are treated in accordance with this invention and
where the resulting treated effluent requires
additional biological treatment, the pollutional
load on the subsequent biological system is reduced
to a significant extent, thereby resulting in
substantial cost savings.
4. When sewage or other impure water is treated
using the methods described in this invention, the
precentage removal of suspended solids and
turbidity is significantly greater than can be
accomplished by the prior art taking into account
the dosage of chemicals and the flow rates through
the clarifier.
This is a very important improvement over the prior
art, and the eliminates the need for a subsequent
filtration process in many instances.
It also allows the use of additional processes in
many cases such as Ultraviolet Disinfection,
Reverse Osmosis, Activated Carbon and/or Ammonia
Removal using Clinoptilolite Ion Exchange Material
without the use of an intervening filtration
process.
Tests have indicated that raw sewage, after being
treated using the methods described by this
invention, and then passed directly through an
ultraviolet disinfection apparatus, was efficiently
disinfected and the resultant total coliform count
was only 10 per 100 ml.
5. A very important advantage of this invention
over the prior art is its versatility. The
invention can be used as either a Primary and/or
Secondary and/or Tertiary Treatment system, and can
1 334543
-- 5
be combined to advantage with other chemical,
physical or biological processes.
6. Another important advantage of this invention
is the overall speed with which the treatment
process takes place. While the overall retention
time required is site specific and depends on such
factors as the quality of the influent and/or the
quality of the effluent required, typically, for
sewage treatment the overall retention time is less
than thirty minutes.
The system therefore easily lends itself to
automation, which would have substantial economic
advantages such as control of chemical dosages and
reduction of labour costs.
7. The quality of the sludge produced by the use
of this invention, while being site specific, is
generally of a very high solids content and is
readily thickened in a short period of time. The
resulting thickened sludge is then readily
dewatered to a high solids content cake. This is a
very important aspect of this invention, and
distinguishes this invention over the prior art in
that the total volume of sludge to be disposed of
is lower than usual, resulting in important
economic and environmental advantages.
The invention provides a method for treating
sewage or other impure water wherein the following
three individual chemicals (but no more than two
premixed together) are added to the sewage or
other impure water in a mixing zone:
(a) an inorganic coagulant, (b) an anionic
polymer, and (c) a cationic polymer, with intimate
mixing of the added chemicals with the sewage or
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other impure water, with the proviso that (d) the
inorganic coagulant either alone or with the
anionic polymer or the cationic polymer cannot be
added last; and (e) the anionic polymer and the
cationic polymer cannot be intimately mixed and
added together, thereby to provide
chemically-treated effluent having large, compact,
firmly-bonded, substantially shear resistant and
rapidly separable flocs therein; separating the
flocs from the liquid in a separating zone; and
removing treated liquid effluent from the
separating zone.
Further scope of applicability of the present
invention will become apparent from the detailed
description given hereinafter. However, it should
be understood that the detailed description and
specific examples, while indicating preferred
embodiments of the invention, are given by way of
illustration only, since various changes and
modifications within the spirit and scope of the
invention will become apparent to those skilled in
the art from this detailed description.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Predetermined amounts of three chemicals, one from
each of the three broad generic groups namely,
Inorganic Coagulants (i.e. aluminum sulphate,
ferric chloride, Cationic Polymers, e.g.
Polyelectrolytes, and Anionic Polymers, e.g.
Polyelectrolytes are added to sewage or other
impure water. The three chemicals are intimately
mixed with the sewage or other impure water in a
mixing/flocculation zone to form large dense flocs
from the suspended, colloidal and dissolved
pollutants in the sewage or other impure water,
separating these flocs from the sewage or impure
~ 7 ~ 1 3 3 4 5 4 3
water in a separating zone, drawing of treated
effluent from the separating zone, and recycling a
predetermind amount of sludge from the separating
zone to the mixing/flocculation zone. The dosages
of chemicals, the sequence of addition, the
specific chemicals used and the amount and location
of sludge recycle are site specific and depend on
design parameters such as:
1. The quality of the influent impure water to be
treated;
2. The quality of effluent required or economic,
and/or environmental and/or health criteria.
Extensive testing has been carried out using this
process on raw sewage and on industrial-type
effluent, and it has been discovered that there are
certain combinations in which the three chemicals
at economic dosage levels can give improved and
unexpected results over the prior art, while other
combinations using the same dosage levels give most
unsatisfactory results under the same test
conditions.
The following sequence of additions of the
chemicals to the sewage of impure water are the
ones to be employed to give the desired results:
1. All three chemicals added separately in the
following sequence:
Inorganic Coagulant (A)
Anionic Polymer (C)
Cationic Polymer (B)
2) All three chemicals added separately in the
following sequence:
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Cationic Polymer (B)
Inorganic Coagulant (A)
Anionic Polymer (C)
3) All three chemicals added separatley in the
following sequence:
Anionic Polymer (C)
Inorganic Coagulant (A)
Cationic Polymer (B)
4) An inorganic coagulant (A) and a cationic
polymer (B) are mixed in the one container and then
dosed into the sewage as a single mixture,
intimately mixed with the sewage, and then anionic
polymer (C) is dosed into the sewage.
5) An inorganic coagulant (A) and an anionic
polymer (C) are mixed in the one container and then
dosed into the sewage as a single mixture,
intimately mixed with the sewage, and then cationic
polymer (B) is dosed into the sewage.
In all cases (1) to (5), the amount of inorganic
coagulant used is preferably 10 to 1000 ppm, more
preferably 10 to 300 ppm, and most preferably 30 to
200 ppm. The amount of each of the anionic polymer
and the cationic polymer is preferably 0.1 to 50
ppm, and more preferably 0.1 to 10 ppm, and most
preferably 0.1 to 5 ppm. All ppm are by weight in
relation to the impure water to be treated.
For combinations 1, 2 and 3 above, where each of
the three chemicals are added separately, the
following general procedure may be adopted.
(i) A predetermined amount of the first chemical is
dosed into the sewage or other impure water through
1 334543
g
one or more injection points at a first part of the
mixing/flocculation zone and is intimately mixed
with the said sewage or other impure water, then:
(ii) A predetermined amount of the second chemical
is dosed into the sewage or other impure water
through one or more injection points at a second
part of the mixing/flocculation zone and is
intimately mixed with the said sewage or other
impure water, and then:
(iii) A predetermined amount of the third chemical
is dosed into the sewage or other impure water
through one or more injection points at a third
part of the mixing/flocculation zone and is
intimately mixed with the sewage or other impure
water.
(iv) A predetermined amount of the sludge removed
from the solids separating zone is recycled to the
mixing/flocculation zone, and is dosed into and
intimately mixed with the sewage or other impure
water. The location of the sludge recycle point in
the mixing/flocculation zone and the quantity
recycled is site specific and depends on the design
parameters as previously described herein.
(v) The time interval between the addition of the
first chemical and the second chemical or between
the second chemical and the third chemical in the
mixing/flocculation zone is site specific and
depends on the design parameters as previously
described herein.
(vi) The time interval between the addition of the
recycled sludge and either the preceding or
subsequent chemical in the mixing/flocculation zone
is site specific and depends on the design
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parameters as previously described herein.
(vii) The degree of mixing required in the
mixing/flocculation zone is site specific and
depends on the design parameter are previously
described herein.
(viii) The total retention time in the
mixing/flocculation zone and the separating zone is
site specific, and depends on the design parameters
as previously described herein.
For the above combination 1, we have found that in
some cases it may be more beneficial to inject any
or all of the chemicals in two or more locations
into the impure water, but maintaining the
essential sequence as previously described. The
sludge recycle rate can vary from 1-20% of the
impure water flowrate, but is preferably at a
flowrate of about 10%.
The sludge can be recycled to the incoming impure
water at various locations, the best location being
found by site trials.
We have found that the total retention time (mixing
and sedimentation) of approximately 30 minutes is
satisfactory, but can be reduced below 20 minutes
if required.
The time interval between successive chemical doses
(different chemicals) can vary, e.g. from just a
few seconds up to about 8 minutes, but generally a
5 minute interval or less has been found
satisfactory.
The upward velocity in the sedimentation tank can
vary, e.g. from 10-20 metres per hour.
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For combinations 4 and 5 above, where an inorganic
coagulant is mixed in one container with one of the
polymers and then dosed into the sewage or impure
water as one homogeneous mixture and then the other
polymer is dosed into the sewage, the following
general procedure is adpoted:
(i) A predetermined amount of the inorganic
coagulant and one of the polymers is mixed in one
container and dosed as one homogeneous mixture into
the sewage or other impure water through one or
more injection points at a first part of the
mixing/flocculation zone and is intimately mixed
with the sewage or other impure water, and then
(ii) A predetermined amount of the other polymer
(i.e. of opposite charge to the polymer in Step
(i) above) is dosed into the sewage or other
impure water through one or more injection points
of a second part of the mixing/flocculation zone
and it is intimately mixed with the said sewage or
other impure water.
(iii) A predetermined amount of the sludge removed
from the separating zone is recycled to the
mixing/flocculation zone and is dosed into and
intimately mixed with the sewage or other impure
water. The location of the sludge recycle point in
the mixing/flocculation zone and the quantity
recycled is site specific and depends on the design
parameters as previously described herein.
(iv) The time interval between the addition of the
homogeneous mixture of the first two chemicals
(i.e. an inorganic coagulant and a polymer) and the
third chemical i.e. the polymer of opposite charge
to that mixed with the inorganic coagulant in the
mixing/flocculation zone is site specific and
- 12 - 1 3 3 4 5 4 3
depends on the design parameters as previously
described herein.
(v) The time interval between the addition of the
recycled sludge and either the preceding or
subsequent chemical dosage in the
mixing/flocculation zone is site specific and
depends on the design parameters as previously
described herein.
(vi) The degree of mixing required in the
mixing/flocculation zone is site specific and
depends on the design parameters as previously
described herein.
(vii) The total retention time in the
mixing/flocculation zone and the separating zone is
site specific and depends on the design parameters
as previously described herein.
The process is suitable for treating sewage or
other impure water without any further form of
treatment, but in some instances, depending on the
quality of the influent or the quality of the
effluent required, it may be necessary to adjust
the pH or the alkalinity of the influent or the
effluent by the use of methods well known in the
art.
Many types of inorganic coagulations can be used in
the application of this invention, for example,
aluminum sulphate, alum, and ferric chloride and
lime. The specific type of inorganic coagulant to
be used is site specific and depends on the design
parameters.
Many types of cationic polymers may be used, and
the following have been used with success:
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* * *
Hercofloc 885, Hercofloc 876, Hercofloc 849, all
supplied by Hercules Inc., and Percol 763 supplied
by Allied Colloids Inc, and Chemifloc 6350*and
6999.*
Many types of anionic polyelectrolytes may be used,
and the following have been used with success:
Percol 1011 supplied by Allied Colloids Inc., and
Hercofloc 831*and 847*supplied by Hercules Inc,
and Chemifloc 423 and 495.
One method for the treatment of sewage or other
impure water is disclosed wherein three chemicals
are added to the sewage in the following specific
sequence to produce treated effluent. An
inorganic coagulant, such as alum or ferric
chloride is added to the sewage and is intimately
mixed therewith to provide pretreated sewage; then
an anionic polymer is added to the pretreated
sewage and is intimately mixed therewith to provide
and interim pretreated sewage: then a cationic
polymer is added to the interim pretreated sewage
and is intimately mixed therewith to provide
chemically-treated sewage. The chemically-treated
sewage is supplied to [e.g] a separating zone
wherein the chemically-treated effluent and sludge
are separately removed. A predetermined amount of
sludge is recycled back to the mixing/flocculation
zone.
In another method, according to the present
invention, the anionic polymer is added to and
intimately mixed in the sewage to provide
pretreated sewage; then an inorganic coagulant,
such as alum is added to and intimately mixed with
the pretreated sewage to provide an interim
pretreated sewage; cationic polymer is added to and
A * Trade-marks
- 14 - 1 334543
intimately mixed with the interim pretreated sewage
to provide chemically treated effluent.
The chemically treated efluent may be supplied to
a separating zone wherein the chemically treated
effluent and sludge are separately removed. A
predetermined amount of sludge is recycled back to
the mixing/flocculation zone.
In another method according to the present
invention, high molecular weight cationic polymer
is added to and intimately mixed with the sewage to
provide pretreated sewage, then an inorganic
coagulant such as alum is added to and intimately
mixed with the pretreated sewage to provide an
interim pretreated sewage; then anionic polymer is
added to an intimately mixed with the interim
pretreated sewage to provide chemically treated
sewage. Then the chemically-treated sewage is
supplied to a separating zone wherein
chemically-treated effluent and sludge are
separately removed. A predetermined amount of
sludge is recycled back to the mixing/flocculation
zone.
In another method according to the present
invention, the inorganic coagulant (e.g. alum or
ferric chloride) is mixed in the one container with
the cationic polymer to form a homogeneous mixture
which is then added and intimately mixed with the
sewage to provide an interim pretreated sewage;
then at a later time an anionic polymer is added
and intimately mixed with the interium pretreated
sewage to provide chemically-treated sewage. The
chemically-treated sewage is supplied to a
separating zone wherein the chemically treated
effluent and sludge are separately removed. A
predetermined amount of sludge is recycled back to
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the mixing/flocculation zone.
In another method according to the present
invention, the inorganic coagulant (e.g. alum or
ferric chloride) is mixed in the one container with
the anionic polymer to form a homogeneous mixture
which is then added and intimately mixed with the
sewage to provide an interim pretreated sewage;
then at a later time a cationic polymer is added and
intimately mixed with the interim pretreated sewage
to provide chemically treated sewage. The
chemically treated sewage is supplied to, a
separating zone wherein the chemically treated
effluent and sludge are separately removed. A
predetermined amount of sludge is recycled back to
the mixing/flocculation zone.
In some cases it may be advantageous to introduce
one or more of the treatment chemicals and two or
more locations in the water to be treated, provided
that one of the essential sequences of the
invention is maintained.
The amount of predetermined sludge recycled back in
the process is typically of the order of 1 to 10%,
although rates of 20% or more can be used. This
percentage may vary depending on the quality of the
influent and the desired effluent quality. It may
be recycled to the influent or various locations,
the best location being found by site trials.
Table 1 sets out the results of numerous tests
carried out on a mixture of sewage and industrial
effluent, using an inorganic coagulant (alum),
followed by an anionic polyelectrolyte, followed by
a cationic polyelectrolyte.
These results indicate that the method of this
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invention is suitable for producing an
exceptionally high quality effluent which
heretofore was known in the field of water
treatment, considering the low overall retention
time and the speed of settling of the floc.
The method also results in a very high level of
microorganism removal. A sample of raw sewage was
found to have a total coliform bacteria count of
over 1,800,000 per 100mls, and the treated effluent
produced by the method of this invention had a
coliform count of only 5500 per 100mls,
representing a removal efficiency of over 99.7%.
The same effluent, when passed through a
commercially available ultraviolet radiation system
had the coliform count reduced from 5500 to 350 per
100mls. Other results have indicated total
coliform counts as low as 5 per 100mls after
irradiating effluent following the method cf this
invention.
This is very important advantage of the invention,
because it offers a realistic option instead of
chlorine for the disinfection of effluents, which
are known to cause the formation of chlorinated
hydrocarbons, some of which could be carcinogenic.
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TABLE 1
Inorganic Anionic Cationic Influent Effluent Removal
Coagulant Polymer Polymer Turb. Turb. Efficiency
mg/l mg/l mg/lNTU. NTU. %
196 1.00 1.4676.2 0.91 98.8
195 0.85 1.3277.2 0.87 98.9
162 1.06 1.28205.0 1.04 99.5
163 1.14 1.29126.0 0.93 99.3
169 1.05 1.2899.7 0.99 99.0
179 1.24 1.24117.5 1.01 99.1
185 1.26 1.26107.1 1.02 99.0
162 0.94 1.0159.7 1.24 97.9
163 0.94 1.0158.9 1.26 97.9
169 1.03 1.1066.8 1.05 98.4
164 0.82 1.0483.8 1.14 98.6
165 1.17 1.17174.6 1.34 99.2
170 0.98 1.20114.8 1.21 98.9
171 1.06 1.22114.2 1.50 98.7
176 1.07 1.2294.7 1.47 98.4
1 334543
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Table 1 cont/d...
Inorganic Anionic Cationic Influent Effluent Removal
Coagulant Polymer Polymer Turb. Turb. Efficiency
mg/l mg/l mg/l NTU. NTU. %
163 1.11 0.82 97.4 1.09 98.9
173 1.05 0.98 68.4 0.95 98.6
191 1.11 0.96 75.5 1.24 98.4
