Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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This invention relates to methods and reagents for water
clarification and particularly to improvements and modifications to
the methods and procedures described in our copending Canadian Patent
Application No. 277,389.
The currently standard process for the clarification and
decolourisation of turbid waters and effluents involves a coagulation
process followed by sand filtration. The water is mixed with an appropriate
amount of an aluminium salt, or a ferric salt, (the coagulant) and
adjusted to a pH where the metal forms insoluble, positively-charged
hydrolysis products. For aluminiumsulphate ~alum), the optimum pH
will range from 5 to 7, depending on the water. Negatively-charged
colloids in the feed water (e.g. bacteria, virus, clays, etc) and the
natural colouring matter in water ~humic and fulvic acids) become attached
to and entrapped within the floc and settle with it. Settling normally
takes place in a settling tank and residual floc in the overflow from
the settling tank is removed by passage through a sand filter to produce
a sparkling clear water. Once the pressure drop through the sand filter
becomes excessive the bed is backwashed to remove the deposited floc.
In practice, the coagulation process is usually carried out
in three distinct zones. First, the coagulant, and any acid or alXali
required for pH adjustmen~, are rapidly mixed with the incoming feed
water for a short time to form micro-~locs of the metal hydroxide.
These are next gently agitated with the water to promote attachment
of the colloids to the floc; excessive agitation is avoided as it disperses
the fragile flocs. Finally, the mixture passes to a settling zone
where the flocs are settled out.
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There have been a number of attempts to improve settling rates.
For example, a small amount of an appropriate linear polyelectrolyte flocculant
can be added to create bridges between the flocs and thereby facilitate settling.
An alternative is to add a finely divided solid to the feed water so that it
becomes entrapped with the floc, and by raising its density, settling is
facilitated. A variant of the latter is to use a ferromagnetic particle so that
the floc can be removed by magnetic means; however, this does not reduce
significantly the amount of coagulating chemical needed to produce satisfactory
flocs.
In our earlier application we described the preparation of new
particulate adsorbents for removing suspended impNrities and coloured substances
from water by coagulation (referred to as a "coagulant/adsorbent"), which
comprises a finely divided particulate mineral, the individual particles of
which have a particle size of 10 microns or less and have a thin hydroxylated
surface layer having a positive zeta potential at the adsorption pH, i.e.,
the pH of the water under treatment.
We also demonstrated that a much better purification is usually
achieved in high turbidity water if a suitable coagulant is added to the
water. For this purpose, aluminium sulphate (alum) was disclosed at the
most convenient, but other materials such as ferric chloride were indicated
as potentially effective.
We have now found that it is possible to obtain, in some
cases, considerable improvement in coagulation by the addition of small
quantities of polyelectrolytes, either in the absence or in the presence
of alum or other coagulants.
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It has been known for some time that colloidal suspensions,
which in natural waters are usually negatively charged, can be removed
by the use of natural or synthetic cationic flocculants in place of,
or in conjunction with the usual agents such as alum, etc. The cationic
polyelectrolytes act generally by destabilising the suspension through
a charge neutralization effect. This causes individual colloids to
collect in small aggregates or microflocs. By gently mixing, the microflocs
can be converted into large macroflocs which will settle more rapidly.
This second stage can be improved by the use of long chain non-ionic
or anionic organic flocculants. These materials act by forming long
chain bridges in between microflocs.
When suitable organic polyelectrolytes are added to the water
to be clarified, after mixing with the coagulant/adsorbent described
in our earlier application, the product water in many cases has a lower
turbidity and the rate of coagulation or of settling is considerably
faster than when the coagulant/adsorbent is used alone or in conjunction
with alum.
Accordingly, the present invention provides a method for
clarifying water which comprises contacting the water with a coagulant
adsorbent as described in copending Canadian Patent Application No. 277,389
characterized in that an organic polyelectrolyte is added to th0 water
after contact and before separation of the coagulant/adsorbent from the
water. The coagulent/adsorbent will be described in more detail below.
As most natural waters contain particles which are negatively
charged, the most useful organic polyelectrolytes for the purpose of
this invention are the strongly cationic materials. Many synthetic
materials are available, and these are generally high molecular weight
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polyamides or polyamines. The most common materials are derivatives
of polymerised acrylamide and typically molecular weights determined
on a viscosity basis are in the range of 105 - 107. Many commercial
materials are made by copolymerization of acrylamide and quaternary
ammonium polyacrylamides. Another class of cationic polyelectrolytes
are the polyethylene imines. These are generally of lower molecular
weight than polyacrylamides.
In some cases, neutral and anionic polyelectrolytes can produce
a useful effect. This is thought to be due to a bridging effect.
The most common type is also a polyacrylamide generally made by copolymerization
of acrylic acid and acrylamide or by the partial hydrolysis of polyacrylamide.
The proportion of acid groups in anionic electrolytes is generally
in the range of 5-40%.
Many synthetic polyelectrolytes are sold commercially, but
details of their structure are difficult or impossible to obtain.
In the following examples, we have used the code numbers of commercial
materials.
Natural polymeric flocculants can also be useful, particularly
those with cationic groups. Thus glue and gelatine are effective materials,
as are cationic modified starches. Other natural polymeric flocculants
are known.
Turning now to the coagulent/adsorbent, as detailed in our
copending Canadian Patent Application No. 277,389. We have found that
three conditions must be met for the attachment of colloids to a particulate
surface.
1. The surface should carry a charge of opposite sign to that of
the colloids Cas measured by zeta potential),
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2. The surface must be such that the colloid can be held by multipoint
attachment, and
3. The particle to which the colloids are to attach must be small.
For example, an anionic exchange resin, having the normal degree of
crosslinking, will not absorb clay particles on to its surface to any
significant extent even though the clay has an opposite zeta po~ential
charge and the resin is very finely divided. Likewise finely divided
magnetite has a positively charged surface but will only weakly adsorb
large colloids of opposite charge, such as clay for example.
However, as indicated in our copending Canadian Patent Application
No. 277,389 we have found that if micron size particles are treated
so as to produce a hydroxylated surface thereon (such a particle being
referred to herein as a "gel particle") and are suspended in turbid
water with the pH adjusted so that the particle surface has a positive
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zeta potential then the negatively charged colloids normally present
in natural waters and many effluents will readily attach to the surface.
Provided sufficient gel particles are present to provide an adequate
surface area then it is possible to effect substantial or even virtually
complete removal of the colloids. Accordingly, the invention described
in copending Canadian Patent Application No. 277,389 provides a particulate
adsorbent for removing suspended impurities and coloured substances
from water by coagulation (hereinafter referred to as a "coagulent/adsorbent"),
which comprises a finely divided particulate mineral or clay material,
the individual particles of which have a thin hydroxylated surface
layer having a positive zeta potential at the adsorption pH (as herein-
after defined).
As used herein the term "adsorption pH" means the pH of the
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water under treatment; it must be within the range of pH where the
colloidal matter in the water retains some of its negative charge.
The coagulant/adsorbent materials may be of two notionally
different types: (I) those in which the hydroxylated layer is derived
directly from the substance of the particles; and (II) those in which
the layer is derived from another substance.
The preferred coagulant/adsorbent materials are those of
type I and these can be derived from a wide variety of minerals and
clays provided the nature of the mineral is such as to permit the ready
formation of the hytroxylated surface. In this respect oxides and
silicates are particularly useful. Examples of such minerals include
zinc oxide, silica and siliceous materials such as sand and glass and
clay minerals such as mica, china clay and pyrophillite. This list
is not exhaustive, however, and many other minerals are suitable for
use in this invention.
Most preferably, the particulate material should be a magnetic
or magnetizable material. For this purpose iron oxides, such as gamma
iron oxide or magnetite, which are eminently suitable, or ferrites,
such as barium ferrite or spinel ferrite, can be used.
The particles should be preferably in very finely divided
form in order to be fully effective in removing colloids from solution.
The particles should be less than 10 micron in size, preferably 1 to
5 micron.
The preparation of the gel particles of type I is simply
carried out, usually by suspending the particles in alkali solution
for a short period of time, preferably in the presence of air. Sodium
hydroxide is probably most suitable, but potassium hydroxide or aqueouS
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ammonia may also be used. Generally, alkali concentrations should
be at least O.OlN, preferably about 0.05N to O.lN, at which level the
treatment is effective after about 10 minutes. Shorter treatment times
can be achieved by the use of elevated temperatures and/or higher alkali
concentrations. A suggested temperature range is 40-60C. For example,
a satisfactory material is produced using either O.lN sodium hydroxide
at room temperature ~i.e. about 20C) for ten minutes, or 0.05N sodium
hydroxide solution at about 60C for five minutes.
Because the hydroxylated layer of the type II materials
is provided by another substanceJ the range of starting materials is
broader. A wide variety of minerals and clays can be used provided
the nature of the mineral or clay is such as to permit the ready deposition
of a hydroxide gel on its surface. In this respect oxides, sulphates,
silicates and carbonates are particularly useful. Examples of such
minerals include calcium sulphate, calcium carbonate, zinc oxide, barium
sulphate, silica and siliceous materials such as sand and glass and
clay minerals such as mica, china clay and pyrophillite. This list
is not exhaustive, however, and many other minerals are suitable for
use in this invention. In some cases, pre-treatment of the surface
of the mineral may be required to produce a satisfactory deposition
of the gel. Yet another alternative is to use hollow microspheres,
e.g. of glass for the production of gel particles which can be separated
from the water, after treatment, by flotation rather than sedimentation.
The hydroxylated layer of the gel particles of type II can
be provided by any of a number of metal hydroxides, the requirements
being substantial insolubility in water, a valency preferably of three
or more, and a positive zeta potential at the adsorption pH, where
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the colloids retain negative charge. Suitable metals with this characteristic
are ferric iron, aluminium, zirconium and thorium. Ferric hydroxide
is preferred because it is cheap, and exceptionally insoluble, over
a wide pH range. For example, it does not readily dissolve at high
pHJ as does aluminium hydroxide.
The preparation of the gel-coated particle of type II is
also simply carried outS usually by suspending the particles in water,
adding a salt of a suitable metal followed by an alkaline material,
preferably in aqueous solution which will precipitate the metal hydroxide
which then forms a coating on the particle. Typically, chlorides,
sulphates, nitrates and the like are suitable salts of the metals,
thus ferric chloride or aluminium sulphate could be employed. The
alkaline material may be sodium hydroxide, calcium hydroxide ammonia
or similar soluble material. The concentration at which the preparation
is carried out is generally not critical.
In the case of where magnetite or other iron oxide materials
are used as the basis for type II particles, the metal salt which is
employed to produce the hydroxide layer may be obtained by first adding
acid to the suspension of the particles ~to give ferric and/or ferrous
salts in solution from the iron oxide) and then adding the alkaline
material.
It has been found advantageous, when forming the gel particles
of type II to provide means for increasing the degree of polymerization
of the gel. Polymerization occurs due to elimination of water and
the establishment of oxygen C'ol") linkages between the metal atoms:
2MOH ~--~ MOM + H20
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This process occurs on standing, but can be accelerated by
heating.
After preparation, it is best if the gel-coated particles
are not permitted to dry out. This can be avoided by keeping them
under water.
As indicated above, a much better purification is usually
achieved in high turbidity water if a suitable organic polyelectrolyte
is added to the water under treatment. This addition is best made
shortly after the gel par~icles have been added, and the pH of the
water has been adjusted. The mixture is stirred for a suitable time
and then the sludge allowed to settle.
More particularly, the gel particles are simply admixed with
the water to be treated, either in a batch process or in a continous
process, the organic polyelectrolyte is then added and the whole is
stirred for a sufficient period to allow the colloids and colouring
matter to adhere; thereafter the gel particles are permitted to settle
out. The clarified water can be removed and the gel particles regenerated
by the addition of a solution of a suitable alkaline material. As
mentioned earlier, the pH of the water to be treated must be adjusted
after addition of the gel particles.
The gel particles can be recycled many times. To achieve
this, the adsorbed material is removed by raising the pH of a suspension
of the adsorbent in water. In the case of type I coagulant/adsorbents,
the coagulating properties may be regenerated by treatment with alkali
solution; these two treatments may be combined.
Regeneration of the gel particles is simple and merely requires
adjustment of the pH of the sludge to about pH 10, separation of the
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adsorbed material and, in the case of the type I material, treatment
of the gel particles by the same process as was descrihed for their
preparation.
In using the coagulant/adsorbents it will be found that maximum
clarification depends on the pH of the feed water. The optimum pH
may vary from sample to sample, it is therefore recommended to test
samples and adjust the feed water pH to optimum by addition of acid
and/or alum.
The invention is illustrated, but not limited, by the following
examples.
EXAMPLE 1 - Preparation of the type I Coagulant/Adsorbent
A magnetite ore from Savage River, Tasmania, was crushed
and classified to yield 1-5 micron particles. A 10 ml portion of the
particles was added tG 200 ml of O.lN sodium hydroxide solution at
20C. The slurry was stirred for a period of 10 minutes. The particles
were filtered and washed with water.
EXAMPLE 2 - General Methods of Water Treatment
A Standard Jar Test for Alum Treatment
To a 1~ sample of water are added appropriate amounts of
alum and acid to achieve optimum pH and coagulation (these amounts
are determined in prior tests). The mixture is stirred rapidly (160
RPM) for 2 minutes and then continued at reduced speed (25 r.p.m.)
for another 20 minutes, and the flocs which form allowed to settle
for 20 minutes. The unfiltered supernatant liquor is then analyzed
for residual turbidity and colour. The turbidity was measured with
a Hach 2100 A Turbidimeter and colour measured using a Hach Colour
Measurement Kit.
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B. Standard Jar Test for Magnetite in~Conjunction with Alum
A 1~ sample of water is contacted with 10 ml of magnetite
for 2 minutes at 160 RPM after addition of the optimum amount of acid.
Alum or polyelectrolyte is then added and the fast stirring continued
for 8 minutes. The stirring is then stopped and the magnetite allowed
to settle for 5 minutes. The resulting unfiltered supernatant liquor
is then analyzed for residual turbidity and colour. The magnetite
is then separated by decantation and treated by the same method as
described in Example 1.
C. Standard Jar Test for Magnetite Alone
A lQ sample of water is contacted with 10 ml of magnetite
for 15 minutes at 160 RPM at the optimum pH ~determined in prior experiments).
The stirring is stopped and the magnetite allowed to settle for 5 minutes.
The unfiltered supernatant liquor is then analyzed for residual turbidity
and colour. The magnetite is then separated by decantation and treated
by the same method as described in Example 1.
EXAMPLE 3
In this example, a sample of water from the Yarra River in
Victoria was treated so as to compare the effectiveness of the polyelectrolytes
plus magnetite activated according to Example 1 with alum plus the
magnetite. The results are shown in Table I.
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TABLE I
Raw Water ex Yarra River, Turbidity 33 NTU
Treatment carried out by Method 2B
Coagulant Quantity FinalTurbidity of
Coagulant pHProduct Water (Nl`U)
p.p.m.
7-8101 0.5 4 0.49
7-810 0.2 4 0.74
C_5732 0 5 4 0.84
10 C-573 0.2 4 1.3
Alum 5 4 4.4
Alum 10 4 1.8
Alum 15 4 1.1
Alum 10 5 2.5
Alum3 30 5.5 1.5
Note 1 Flocculant supplied by Applied Chemicals Pty. Ltd.,
Australia; it is a liquid polyamine, strongly cationic,
synthetic polyelectrolyte.
Note 2 Flocculant supplied by American Cyanamide Coy., USA;
it is a liquid polyacrylamide, cationic, synthetic
polyelectrolyte.
Note 3 No magnetite was used in this experiment.
Example 4
In this example, water from the Yan Yean Reservoir in Victoria
was treated. The effectiveness of the combination of polyelectrolyte
and magnetite activated according to Example 1 was compared with that
of the magnetite along and the alum alone. The results obtained are
shown in Table II.
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TABLE II
Raw Water from Yan Yean Reservoir, Turbidity 3.0 NTU
Coagulant Quantity Quantity Treatment Final Turbidity of
Magnetite Coagulant Method pH Product
Water NTU
Alum - 10 mg/l A 5.5 3.0
- 10 ml/l - C 4 0.82
7-810 10 ml/l 0.2 mg/l B 4 0.22
C-573 10 ml/l 0.2 mg/l B 4 0.32
It can be seen that the synthetic polyelectrolytes together
with activated magnetite produced better quality water than alum alone
or magnetite alone.
EXAMPLE 5
Water from Mirrabooka in Western Australia was treated to
compare the effect of using alum in conjunction with magnetite and
polyelectrolyte. The results are shown in Table III.
TABLE III
Raw Water from Mirrabooka; Turbidity 16 NTU, Colour 62
Quantity Quantity CoagulantFinalProduct Water
20Magnetite Alum pHTurbidity Colour
10 ml/l - 81012,2mg/l 4 O.9 23
10 ml/l 40mg/l - 5 2.6 10
lO ml/l 40mg/1 81012,2mg/1 5 0.9 9
1 Measured spectrophotometrically at 400 nm after millipore filtration.
2 Flocculant supplied by Catoleum Pty. Ltd.
Other polyelectrolytes which we have tested and found to
work effectively are as follows:-
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Name or Manufacturer or Type
Code No. Supplier
D Magnafloc~LT22S Allied Colloids Cationic; polyacrylamides
" 177 " Cationic; polyacrylamides
Superfloc C521 Cyanamid Cationic
" C573 " Cationic; polyacrylamide
" C577 " Cationic; polyacrylamide
" C581 " Cationic; polyacrylamide
" NlOOS " Anionic; polyacrylamide
" C460 " Cationic
Alfloc~ 6361 ICI Ltd. Cationic
" 8101 " Cationic
Wisproflo~P Scholten's Cationic; starch-based
Chemische Product
Fabrieken NV
The following example illustrate the preparation of the
Type II coagulant/adsorbent.
EXAMPLE 6 - Preparation of the Gel Particles on Magnetite
A magnetite ore from Savage River, Tasmania, was crushed
and classified to yield 1 - 10 micron particles. These were slurried
in water to which a hydrolysable metal salt was added Cferric chloride
or aluminium sulphate in this example), followed by sodium hydroxide
solution to adjust the pH to the desired level. After the precipitation
of the hydroxide coating was complete, the mixture was heated to increase
the polymerization of the coating - 1 hour at boiling point for a ferric
hydroxide coating, 40 minutes at 80C for an aluminium hydroxide coating.
The supernatant liquor was then decanted off and the coated particles
thoroughly washed by decantation with cold water.
0 ~
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EXAMPLE 7 - Preparation of Gel Particles from Titanium Dioxide
Gel particles were prepared by slurrying titanium dioxide
~RMS, 20 g, particle size 10-20 micron) in water C200 ml) and adding
ferric chloride solution ~60%, 5 ml) followed by sufficient dilute
sodium hydroxide solution to bring the pH to 11.5. The mixture was
boiled for 1 hour.
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