Note: Descriptions are shown in the official language in which they were submitted.
~7~i5
This invention is concerned with a n~w type of
material for removing suspended impurities and colour from
water by coagulation and adsorption. The new material gives
more rapid and better clarification and decolourisation than
the conventional alum treatment and uses only a small proportion
of any of the usual amount of coagulant required for such
trea-tments. In its preferred form, the invention utilizes
magnetized or magnetizable material to facilitate the settling
of the sludge and the recovery and re-use of the coagulant.
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The invention also includes methods of and apparatus
for water purification which utilize the new material.
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 were the
metal forms insoluble, positively-charged hydrolysis products.
For alumlnium sulphate ~alum), the optimum pH will range from
5 to 7, depending on the water. Negatively-charged colloids
in the feed water (eOg. bacteria, virus, clays, etc) and the
natural colouring matter in water (humic and fulvic acids)
become at~ached 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.
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In practice, the coagulation process is usually
carried out in three distinct zones. First, the coagulant,
and any acid or alkali required for pH adjustment, are
rapidly mixed with the incoming feed water for a short time
to form micro-flocs 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 se-ttling zone where the flocs are settled out.
This prior art process has several problems:-
First, the flocs are fragile and do not settle
rapidly so that a large residence time must be allowed in
the settling tank and the equipment is correspondingly
large. Even so, settling is never complete and the overflow
must be clarified by sand filtration.
Second, chemicals are expended in forming the
floc and account for -the maior cost of the process.
Third, the final sludge lS not composed solely
of the clays, etc. removed from the water but contains as
well a much larger volume of the hydrous metal hydroxide.
Not only does the disposal of this sludge create problems
but it also entrains a substantial amount of water which
is lost from the feed water.
~rhere 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.
-- 3
~7~~S
A vari~nt of the ]a-tter 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.
The principal objects of this invention include the
provision of a material and method for the clarification of
water, and the removal of colour and micro-oryanisms such as
bacteria, virus and algae, which eliminate the use of coagulant
or at least enables the use of much less than hitherto to achieve
a given degree of clarification, which produces a sludge which
for the most part is composed of the particulate matter removed
from the water and which accelerates settling of the floc.
Other o~jects include the pro~ision of such a method which
facilitates removal by magnetic means and which eliminates the
need for a sand filter, and the provision of apparatus for
carrying out the method. As hereinafter described these objects
are achieved in accordance with the invention, by using what
is believed to be a novel coagulating and decolourising
material.
We have found that three conditions must be met for
the attachment of colloids to a particulate surface.
l. The surface should carry a charge of opposite sign to tha-t
of the colloids (as measured by zeta potential),
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 potential charge
~ 7~
and the resin is very finely divided. Likewise finely divided magnetite has a
positively charged surface but will only weakly adsorb large colloids of oppo-
site charge, such as clay for example.
However, 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 adjust-
ed so that the particle surface has a positive zeta potential then the negative-
ly 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.
In accordance with one aspect of the present invention, therefore, we
provide a method for removing suspended impurities and coloured substances from
water~ which comprises con~acting the water with a particulate coagulant/-
adsorbent consisting of a finely divided particulate mineral or clay material,
the individual particles of which have a particle size of less than 10 microns
and have been treated to produce a thin hydroxylated surface layer having a pos-
itive zeta potential at the adsorption pH, and then separating the thus treated
water from the coagulant/adsorbent. The method of the invention results in a
particulate adsorbent for removing suspended impurities and coloured substances
from water by coagulation ~hereinafter referred to as a "coagulant/absorbent"),
which comprises a finely di~ided particulate mineral or clay material, the in-
dividual particles of which have a ~hin hydroxylated surface layer having a pos-
itive zeta potential at the adsorption pH ~as hereinafter defined).
As used herein the term "adsorption pH" means the pH of the water under
treatment; it must be within the range of pH where the colloidal matter in the
water retains some of its negative charge.
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In a particular embodiment, the invent-ion provides a process for the
clarification of water which comprises the steps of:
(a) contacting water with a coagulant/adsorbent consisting of a finely
divided particulate mineral or clay material, the ;ndividual particles of which
have a particle size of less than 10 microns and have a thin hydroxylated surface
layer which has a positive zeta potential at the adsorption p~l;
~ b) separating the water from the coagulant/adsorbent to obtain the
clarified water;
(c) treating a suspension of the spent coagulant/adsorbent to raise
the pH to about 10.5;
~ d) separating the coagulant/adsorbent from the resulting effluent
solutlon and recycling the coagulant/adsorbent to step (a) and where necessary
prlor to recycling the coagulant/adsorbent to step (a);
(e) regeneratlng the coagulant/adsorbent by treatment with an alka-
line solutlon;
(f) separating the regenera~ed coagulant/adsorbant from the alkaline
solutIon;
(g) washing the regenerated coagulant/adsorbent with water;
~ h) separating the regenera~ed coagulant/adsorbent from the water;
and
(i) recycling the regenerated and washed coagulant/adsorbent to
step ~a).
The inventlon further provides an apparatus for carrying out this
process, comprising:
~ a? ~irst mixing and contacting means for contacting water to be
treated with the coagulant/adsorbent;
- 5a -
.,,
~L~976~5
(b) separating means for separating the treated water from the spent
coagulant/adsorbent;
~ c) mi~ing and contacting means :Eor contacting the spent coagulant/-
adsorbent with dilute alkali for pH adjustment;
~ d) magnetic separation means for separating the coagulant/adsorbent
from the resulting e:Efluent and means for recycling the coagulant/adsorbent to
means ~a) and optionally, where necessary;
~ e) mixing and contacting means for contacting the coagulant/-
adsor~ent with alkali for regeneration of the coagulant/adsorbent;
(f) magnetic separation means for separating the regenerated
coagulant/adsor~ent from the alkali and optionally means for recycling the
latter to means ~e);
~ g) mixing and contacting means for contacting the regenerated
coagulant/adsorbent with water for washing; and
~ h) magnetic separation means for separating the regenerated
coagulant/adsorbent from the water and means for recycling the regenerated
coagulant/adsorbent to means (a) and optionally for recycling the water to
means ~c).
The invention will now be described and d~scussed in detail. Refer-
ence will be made to the accompanying
- 5b -
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drawings, in which:
Figure 1 is a diagrammatic representation showing the effect of
inorganic multivalent cations on the absorption of colloids by the hydroxy-
lated surface of the gel particle;
Figure 2 is a Elow diagram showing a preferred method of water
treatment in accordance with the invention.
Figure 3 is a diagram of a solids contact clarifier for use in
the method of the invention.
Figure 4 is a diagram showing a preferred form of apparatus
for carrying out the method of the invention.
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The coagulant/adsorbent ma-terials of the invention
may be of two notionally different types: (I) those in ~7hich
the hydroxylated layer is derived directly from the substance
of the particles; and (II) those in which the layer is derived
from another subs-tance~
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 hydroxylated 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.
In the most preferred embodiment of this invention,
the particulate material should be a magnetic or magnetisable
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 effec-tive 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 potàssium
hydro~ide or aqueous ammonia may also be used. Generally,
alkali concentrations should be at least O.OlN, preferably about
-- 7
, . . ....................... . .
,. ~ '. '
0.05N to O.lN, at which level the treatment is ef~ective after
about 10 minutes. Shorter treatment times can be achieved by
the use of elevated temperatures and/or higher alkali concent-
rations. A suggested temperature range is ~0-60C. 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 a-t about 60C for
five minutes.
Because the hydroxylated layer of the type II materials
is provided by another substance, 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 oE type
II can be provided by any of a numher 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 the colloids retain negative charge.
-- 8 --
Suitable metals with this characteristic are ferric iron,
aluminium, zirconium and thorium. Ferric hydroxide is preferred
because i-t is cheap, and exceptionally insoluble, over a wide
pH range. For example, it does not readily dissolve at high
pH, as does aluminium hydroxide.
~he preparation oE the ge-coated particle of type II
is also simply carried out, usually by suspending the particles
in wa-ter, 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 polymeriæation of the gel. Pol~merization occurs due to
elimination of water and the establishment of o~ygen ("ol")
linkages between the metal atoms:
2MOH --~ MOM + H20
This process occurs on standing, but can be accelerated
- by heating.
,h,~
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.
The invention thus offers a cheap, readily-prepared
recyclable coagulant/aclsorbent which removes colloidal particles,
micro-organisms and colouring matter from water quickly and
more economically, producing a sludge which is little different
to the impurities removed from the water and is therefore
readily disposable without environmental problems.
In another aspect the invention provides a method for
clarifying water, which comprises contacting the water with the
above-described coagulant/adsorbent and then separating the
water from the coagulant/adsorbent.
An importan-t feature of the process of the present
15 invention is that 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
20 treatments may be combined.
The gel particles may be used simply by adding them
to the water to be treated, either in a batch process or in a
continuous process by mixing them with the incoming water,
stirring them for a sufficient period to allow the colloids and
25 colouring matter to adhere to the particles and then permitting
the particles to settle. The clarified water can be removed
and the 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
30 the particles.
.
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~3Ca76~i
Furthermore, we have also discovered, in accordance with a further
aspect of this invention, that if a small quantity of inorganic multi-valent
cation having a valency of three or more ~e.g. Fe3 or A13 ) is added to the
water after the addition of the gel particles, then the amount of the cation
added is substantially less than would be required to produce water of
comparable clarity by using the cation alone, with appropriate addition of
alkali or acid, to form an absorptive Eloc as in the usual process of water
coagulation. In many situations, but not all, the optimum economic situation
will be to use the gel particles in combination with a small amount of such
a cation.
It is thought that after the gel particles have adsorbed some of
the colloidal particles, the addition of limlted amounts of the inorganic
multivalent cation suppresses, but does not comple~ely eliminate, the
negative charge on the remaining colloids so that when part of their negative
surface binds to the positive region of the gel particle, electrostatic
repulsion between neighbouring colloids on the gel particle surface is
reduced. Consequently the colloids can pack together more closely on the
gel particle surface so that less of the latter is required. ~lis is shown
diagrammatically in Figure 1. When the colloid's negative charge is
neutralized by monovalent cations such as Na (Figure la~ the colloid has
a high zeta potential and there is strong repulsion between adjacent col-
loids on the gel surface. When the colloi~ adsorbs on to the surface of
the gel particle~ some of its Na counter ions are replaced by the positive
charges on the gel surface and electrostabic binding results, no doubt
assisted by hydrogen bonding and the li~e. lf the remainder of the Na is
replaced by aluminium, or other multivalent ions, then the electrostatic
repulsion is suppressed and the particles can pack more closely together
., .
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(Figure lb).
The final sludge from this process consists essentially of the
colloids removed from the original water, but when inorganic multivalent
cations are added to facilitate the process these colloids are rejec-ted with
multivalent cations replacing most of the monovalent cations which were
present in the diffuse double layer of the original colloid. rnus the
presence of the voluminous iron or aluminium hydroxide floc in the sludge
of the conventional process is avoided and disposal of the sludge is
greatly facilitated. For example, if river water is being clarified the
sludge could be returned to the river with little environmental harm.
After desorption, the regenerated gel particles can be recycled for the
adsorption of more colloids at a lower pH. Thus the net inorganic multi- -
valent ion requirement of the process is greatly reduced.
The positively-charged nature of the coated adsorbent particles of
this invention also leads to the adsorption of acidic materials which com-
prise the natural colouring matters often present in water. Thus the appli-
cation of the present process results in substantial or complete removal
of such colour from water, often to an extent which cannot be achieved by
conven*ional alum treatment.
As indicated above, a much better purification is usual~y a-
chieved in high turbidity water if a suitable coagulant is also added to
the water. For this purpose, aluminium sulphate ~alum) is the most
convenient, but other materials such as ferric chloride may be used. This
addition is best made shortly after the
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gel particles have been added, and the pH of the water has
been adjus-ted. The mixture is stirred for a suitable time
and then the sludge allowed to settle.
There is a striking difference in behaviour when
an untreated finely divlded mineral particle (e.y. magnetite)
is added to turbicl water along with coagulant chemicals,
compared with the behaviour oE the gel particles of the
invention. In the former case -the mineral coprecipitates
with the colloids, and whilst its presence will accelerate
settling it is on,ly possible to reduce the amount of coagulant
a slight amount to achieve comparable clarification. On
- vigorous shaking the fIoc and colloids separate from the
magnetite. When the gel particles of the invention are
added, however, each particle is observed to be a separate
entity and -to be completely coated with colloids; a large
reduction in the amount of coagulant can be achieved. At
the same time the colloids are very firmly attached to the
particles and remain so even under vigorous conditions of
stirring. When the gel particles are magnetic, they are
readlly removed by magnetic means in a magnetic separator,
or by applying a magnetic field to cause the particles to
flocculate and settle rapidly; a sparkling clear supernatant
can be obtained. Under similar conditions, the agglomerate
produced by untreated magnetite tends to disperse under the
agitation conditions and a cloudy supernatant results. Thus
by using the gel particles of the invention water of high
clarity can be obtained simply by permitting the particles
to separate by sedimentation or by using a magnetic separator;
a sand filter is not rquired. If simple sedimentation is to
be used then the mineral core of the gel particle can be
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any inert material, e 7 g. sand. However, it is preferable to
use a soft ferromagnetic material (e.g., magnetite) to permit
accelerated removal of the gel particles with attached colloids
by magnetic means. In such a ca~e the gel particles should
preferably not be magnetized whilst the colloids are being
adsorbed but can be subsequently magnetized prior to separation
from the wate~. There is advantage in collecting -the particles
by passing the slurry through a magnetized wire packing to
which they adhere. Later they can be removed by switching
off the magnetic field. This mode of operation reduces abrasion
between the particles with consequent removal of the gel coat
and so facilitates their mechanical handling.
Regeneration of the gel particles is simple and merely
requires adjustment of the pH of the sludge to about pH 10,
separation of -the adsorbed material and, in the case of the
type I material, treatment o the gel particles by the same
process as was described for their preparation.
` An alternative method of operating the process is
- to use the gel particles in the orm of a filter bed through
which the feed water, at the appropriate pH and containing
the coagulant (if used) is percolated. In this case the gel
particles should be at least 100 microns in size. The colloids
attach to the gel particles during passage through the bed so
- that clear water emerges. Once the bed has become saturated
and the quality of the emerging water begins to decline beyond
a specified point, the feed water flow is stopped. The bed
is then backwashed and the contaminants removed by passing
through a flow of water at a high pE~ at a velocity which slightly
expands the bed but which avoids violent mixing and abrasion
of their surface (as in conventional sand filtration). The
76~
colloids and colour are thereby removed. The bed is then
regenerated, in the case of the type I adsorbent, by adding
sodium hydroxide solutlon to the bed and allowing contact
for sufficient time to reac-tivate the surface.
This procedure differs from conventional sand
filtration, where a Eloc is formed wi-thin the sand bed by
addition of alum and alkali with the feed water in several
crucial respects. The coagulant requirement is much less
because of re-use of the gel particles. Back-washing is
conducted so as to preserve the active gel layer on the bed
particles and permit release only of the adsorbed colloids;
conventional prac-tice aims to remove all material from the
surface of the bed particles by violent agitation, e.g. by
air scouring.
A modification of the above method is the use of
gel particles in-the filter bed which are ferro-magnetic and
which have been magnetised prior to use. This flocculates
the particles and so increases their void volume -thereby
enhancing the ability of the filter bed to retain adsorbed
particles.
While at least some of the benefits of the above-
described coagulant/adsorbent material can thus be obtained
by application of known methods and apparatus, the full
advantages are more readily reallzable with a method and
~5 apparatus therefore which we have specifically designed for
this purpose. This method and apparatus will now be described
in more detail with reference to Figures 2 to 4 of the
accompanying drawings, which relate to water treatment using
type I coagulant/adsorbents.
As illustrated in Figure 2, at Stage l feed water
" :~ _ ~; _
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to be treated is contacted with the coayulant/adsorbent in
any suitable liquid solid contactor. Alum is then added, if
necessary or desirable at stage lA, and after further mixing
the clarified water is separated from the coagulant/adsorbent
in the separation stage 2. The spent coagulan-t/adsorbent is
then treated with dilute caustic soda at stage 3 to free the
coagulant/adsorbent from the adsorbed contaminants. After
a further separation stage 4, the contaminants pass to waste
as effluent, while the coagulant/adsorbent passes to a
... .
regeneration stage 5 where it is treated with O.lN caustic
soda which is continuously recycled (with make-up when necessary).
The coagulant/adsorbent is then washed at stage 6 with water.
The washings (dilute caustic soda) are recycled to stage 3
while the regenerated coagulant /adsorbent is recycled to
stage l.
- 20
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Figure 3 shows a form of solids contact clarifier
which is suitable for the stage 1 and 2 operations
described above. This appara-tus is used in conventional
water treatment processes and is designed to ensure efficient
contact between suspended ma-tter and a reagent, such as an
alum solution, by circulating water containing the suspended
matter around a zone into which the reagent is introduced.
The apparatus consists of a long pipe 21 and a
vessel 22. The latter is generally in the form of closed
_~ .
cylinder with an integral, conical base portion 23. Within
....
the vessel 22 there is mounted an inverted cup-like housing
24 which is of generally similar shape to the vessel 22,
and serves to define an inner chamber 25. Centrally located
in the chamber 25 is a short vertical pipe 26. The pipe
21 is connected into the pipe 26 near the bottom of the
latter. An impeller 27, to aid liquid circulation in the
chamber is mounted within the bottom portion of pipe 26.
Clarified water and settled solids outlets 28, 29 are at
the top and bottom, respectively, of the vessel 22.
In typical operation, the turbid feed water
is admitted to the pipe 21, a-t such a rate that it would
take about 5 minutes to pass to the vessel 22. Acid to
ad3ust the pH to the optimum for the particular water, and
the coagulant/adsorbent are introduced at, or near to, the
inlet end of the pipe 21. Flow in the pipe 21 is turbulent
to ensure thorough mixing. The water leaving pipe 21 flows
up through the central pipe 26 into the surrounding chamber
25. Alum, if required is introduced (by means not shown)
into the pipe 26 and/or at or near the inlet end of pipe 21.
Clear water is drawn off through outlet 28 at the top of the
~ _ 7 _
~76~5
vessel 22 and the settled coagulant/adsorbent, together with
attached impurities at the bottom outlet 29. Precipitation
of the coagulant/adsorbent may be facilitated by using a
magnetizable adsorbent. The equipment can be operated
continuously, and the adsorbent regenerated in a separate
stage (as described above) by raising the pH of the slurry
to about 10.5, separating the released floc and colouring
mat-ter, and treating the adsorbent with an alkaline solution
as described above.
In using the coagulant/adsorbents of this
invention 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. When the coagulant/adsorbents are used in
conjunction with alum, it will normally be found that the
optimum pH for alum alone is suitable, typically about 5 or
above. When the adsorbent is used alone it is frequently
advantageous to lower the pH to about 4.
.
~ 25
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~ complete and preferred water treatment s~stem
is shown in Figure 4.
The incoming feedwater (A) and regenera-ted
coagulant/adsorbent (magnetite) are fed to a contactor 31,
which may be a pipe as described above (21 in Figure 3) or
any other sui-table apparatus, :Eor example a channel, trouyh
or tank provided, if necessary, with stirring equipment.
Where necessary alum ~B) is added to the mixture as it leaves
the contactor 31. The mixture then passes to a solids
clarif.er 32, which may be of the type described
above (Figure 3), or as shown may comprise a separate
clarifier 32A and solids/liquid separator 32B .
Clarified water (C) is taken off as the overflow
from the separator 32B or clarifier 32 as the case may be.
: 15 The solids underflow (D) from 32B or 32 consists
of the loaded coagulant/adsorbent, i.e. associated with the
`, colloidal and other impurities. This passes to a first
regeneration mixing stage 33, where it is mixed with recycled
dilute caustic soda (G) to free the coagulant/adsorbent from
' 20 the impurities, and thence to another magnetlc separator 34
from which emerge an effluent stream (E) containing the
impurities and partly regenerated coagulant/adsorbent (F).
The stream (F) is mixed with further dilut.e caustic soda
(L) to bring the pI~ to 10.5 and passes to the second
regeneration mixing stage 35 and thence to a further magnetic
separator 37.
The overflow stream of dilute caustic soda (G)
from 37 goes for mixing with stream (D) and the underflow
solids-containing stream (H) passes to a final regeneration
stage 38 where it is mixed with O.lN caustic soda (I). This
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mixture passes to a further magnetic separator 39 from
which the overflow stream (I) of caustic soda is recycled
for mixing with stream (H) and -the underflow solids (J)
are mixed with recycled washwater (M) and pass to the first
washing mixer 41.
From 41 the mixture passes to a further magnetic
separator 42. The overflow liquid stream (L) of dilute
caustic soda is recycled for mixing with stream (F) and the
. underflow solids stream (N) passes to the final washing mixer
43, where it is mixed with washwater (K), and thence to
the final magnetic separator 44. The overflow stream (M)
. is recycled back to first washing stage for mixing with
stream (J) and the underflow solids stream (), which now
contains regenerated and washed coagulant/adsorbent, is
returned for mixing with further feed water (A).
Where alum is used, it may be desirable to
make provision for recovery and recycling of this reagent
and this optional arrangement is also shown in Figure 4.
Here the effluent stream (E) from the first magnetic separator
34 is adjusted to pH 3.5 in a mixer 46. The mixture is
passed to a solidsjliquid separator 47 from which the stream
(P) of recovered alum solution may be recycled for mixing
with stream (B) and the solid sludge of effluen-t (Q) finally
passes to waste
The mixing stages (33, 36, 3~, 41, 43 and 46)
may be performed Ln any suitable apparatus, such as pipes
with turbulent flow, or channel, troughs or tanks with
suitable stirring or mixing equipment.
Typical contact tlmes for the main steps in
the process are:-
~c~
. .
- , . .
.
~g~
Purification - 1st stage (31) - 7 minutes
- 2nd stage (32) - 5 minutes
Regeneration - final stage (38) - 10 minutes
Similar apparatus and procedures to those described
above may be adopted when using the type II coagulant/adsorbents.
In this case, however, the regeneration staye(s) (5 in Figure
2 and 38 in Figure 4) are not required.
The following examples further illustrate the
invention which is, however, not limited by these examples.
EXAMPLE l - General Method of Preparation of the Type I
Coagulant/Adsorbent
A magnetite ore from Savage River, Tasmania,
was crushed and classified to yield l-5 micron par-ticles.
A 10 ml p~rtion of the particles was added to 200 ml of
i5 sodium hydroxide solu-tion of appropriate concentration and
at an appropriate temperature. The slurry was stirred for
a period, usually 5 to 10 minutes. The particles were
filtered and washed with water.
A number of samples were prepared under varying
conditions. These were used in following examples and were
regenerated under the conditions used initially. They are
identified according to the code letter shown in the table
below:-
.
Code Letter NaOH Conc. Temp. Contact Time
-
- 25 A 0.5N 60C5 mins
B 0.5~1 25C5 mins
C 0.05N 60C10 mins
-:
`' 7~ ~/
. .~ j . ~, _
~ .
', '
` ~' ` ,
- :
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F~r comparison, in some examples, magnetite was
treated both initially and in regeneration by slurrying in
water adjusted to pH 10.5 at 25C for five minu-tes.
It was not given any stronger sodium hydroxide
treatment as were the samples A, B and C. This sample is
identified as sample D.
EX~MPLE 2 - General Methods of Water Treatment
A. Standard Jar Test for Alum Treatment
-
_ To a lQ 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 lS stirred
rapidly (160 RPM) for 2 minutes and then continued at reduced
speed (25 r.p.m.) for another 20 mins, and the flocs which
form allowed to settle for 20 minutes. The unfiltered super-
natant 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.
B. Standard Jar Test for Magnetite in Conjunction with Alum
A lQ sample of water is contacted with 10 ml of
magnetite for 7 min at 160 RPM after addition of the optimum
amount of acid. Alum is then added and the fast stirring
continued for 2 minutes, followed by slower stirring at 75 RPM
for 5 minutes. The mlxing should be such as to keep the
magnetite fully suspended. 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 ~y the same method as described in
Example 1.
-- ~3, --
--
.
. : .' : ' '
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.
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7~
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 s-tirring is stopped
and the magnetite allowed to settle for 5 minutes. The
unf.iltered supernatant liquor is then analysed for residual
,
~ 3 _
- - '' ' ' - '
.
, ,
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turbidity and colour. 1'he magnetite is then separated by
decantation and treated by the same method as described in
Example 1.
hese condi-tions were found to be the most rapid
to bring about optimum coagulation by each of the three
methods. The to-tal process time for each method is as
follows:
Method A ~2 minutes
Method B 19 minutes
Me-thod C 20 minutes
EXAMPLE ~ - Treatment of High Turbidity River Water
In this example, four samples of water from the
Yarra River in Victoria with relatively high turbidity
ranging from 18 NTU to 68 NTU and colour ranging ~rom 55
Pt-Co units to 75 Pt-Co units were treated. Comparisons of
standard alum treatment with treatments involving samples of
magnetite ackivated according to Example 1, both with and
without alum were made. I'he results are shown in Table 1
and generally are the average of a number of cycles, which
have béen made wi-th magnetite regenerated after each
, coagulation.
EX~LE 4 - Treatment of Low Turbidity/High Colour Water
In this example, two samples of water from the
Yan Yean reservoir in Victoria were treated to remove colour.
, . .
i 25 Again, treated magnetite, with and without alum added, was
tested and compared with standard alum treatment and with
untreated magnetite. As in the previous examples, the
magnetite was generally recycled and the average results
are quoted. Results are shown in Table II.
' 30 ~y
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With this water, alum was not very effective
for colour removal whereas magnetite treated according to the
invention gave almost complete removal even without added
alum. Other magnetites were largely ineffective.
EXAMPLE 5 - Mod:iEied Regeneration of Magnetite
A modified procedure for -the regeneration of
magnetite was tes-ted in comparison with the method described
in Example 1. In the modified method, the magnetite was
washed after treatment with 0.05N sodium hydroxide and the
washings kept. These were adjusted to pH 10.5-11 and used
to treat -the magne-tite af-ter the next flocculation cycle.
Nearly all of the insoluble material was removed from the
magnetite by this step, together with much of the adsorbed
colouring material. The magnetite was separated from the
liquor, washed and then treated with the 0.05~ sodium
hydroxide solution. The remainder of the colour and insoluble
material were thus removed. The magnetite was washed (and
the washings retained for the next cycle) and then reused for
the next Elocculation. The 0.05N sodium hydroxide solution
could be used for a very large number of cycles with
occasional addition of more solution to make up for losses.
Magnetite treated by this modified procedure gave results
in clarifying and decolourising water which were indisting-
uishabl~ from those obtained with magnetite regenerated
according to Example 1. This method is particularly suitable
for continuous operation.
EXAMPLE 6
. .
Magnetites regenerated under other conditions
were tested on water from the Yarra River in comparison with
standard alum treatment. The regeneration was carried out
_ ~ _
'
s
by treating 10 ml of the magnetite with 40 ml of sodium
hydroxide solution. The samples were identified according
to the code letter shown in the table below.
CODE LETTER NaOH CONC. TEMP. CONTACT TIME
E 0.05N 60C 5 mins
F 0.05N 40C 10 mins
G 0.1N 25C 10 mins
The results obtained are shown in Table III.
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The following examples illustrate the preparation
and use of the Type II coagulant/adsorbent.
EXAMPLE 7 - 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 (ferric chloride or aluminium sulphate in this
example)f followed by sodium hydroxide solution to adjust the
p~ 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.
Three samples were prepared under the conditions
shown in Table IV.
TABLE IV
Code No. Raw materi_l chargespH for precipitation
Fe Mag 1 0.5g Fecl3-6H2o/g-Fe3o4 11.5
in 2.5 ml water
- Fe Mag 4 0.085g Fecl3~6H2o/g Fe34 9.8
in 2.5 ml water
Al Mag 2 0.5g A12(SO4)3.16H2O/g 5-5
Fe3O4 in 250 ml water
EXAMPLE 8 - Purification of River Water
Standard jar tests were carried out with Yarra
River water (turbidity 12 NTU, colour 65 Pt/Co units) whereby
aluminium sulphate (a'um3 plus coagulant/adsorbent, was mixed
_3.~ _
~976~
for a predetermined time, after which mixing was stopped.
After a further specified time for settling the turbidity
and colour of the supernatant liquid was measured. In the
experiments using the coa-ted particles the sediment was removed,
regenerated by washing witll NaOH solution at pH 10.5, rinsed
' and then recycled. After thirteen such complete cycles the
results of turbidity and colour removal for the fourteenth
cycle were taken, and are shown in Table V. The amount of
the coated particles added was 5 ml/l ~settled volume) in
all cases. An identical run using alum alone was carried
out for comparison.
TABLE VI
Alum Level5 mg/l alum 10 mg/l alum
Coagulant/adsorbent T C T C
Fe Mag 1 1.5 10 0.35 5
Fe Mag 4 5.0 30 0.85 10
Alum 9.9 50 3.6 25
T = Residual Turbidity N.T.U.; C = Residual Colour
Pt/Co Units
It can be seen that the use of the gel-coated
particles of this invention give remarkably better removal
of turbidity and colour using low levels of alum than the alum
alone. At the 10 mg/l level o-E alum, water of acceptable
quality is produced by the gel-coated particles, but not by
alum alone.
A significant advantage exhibited by the treated
^ ~ particles was the short time required for treatment and
.,
,. .
.~
., ., .~
.
. :
6~;
settling. Thus the treated particle experiments required
only 12 minutes total to achieve the clarification shown,
whereas alum required at least 35 minutes.
The treated particles aEter separation of the
clear supernatant liquid were regenerated by the addition
of a small quantity of approximately 0.5N sodium hydroxide.
The release of the a-ttached impurities was demonstrated by
plotting pH of the slurry against turbidity or apparent colour
and iron content of the liquid layer.
The results showed that as the pH is increased,
colour and turbidity increase in the water layer, corresponding
to their removal from the treated particles. This transfer
occurs mainly at pH 9 to 10. At the same time, an amount
of iron salt is removed corresponding to the amount of iron
present in the original water.
EXAMPLE 9
This example illustrates the effect of recycllng
on the performance of the treated particles. The flocculation
was carried out as described in Example 7 (except that 15
mg/l of alum was used) using a ferric hydroxide coated
particulate magnetite and one coated with aluminium hydroxide.
The results obtained are shown in Table VII.
TABLE VII
Cycle 1 2 5
T C T C T C T C T C
_ _ .. _
3.7 ml Fe
Mag 1 0.78 0 0.49 0 0.46 - 0.36 - 0.38
3.6 ml Al
Mag 1 0.59 0 0.73 0 1.2 - 1.3 - 1.3
:'~
,, ,,f ~
.
.
The results show that the iron hydro~ide treated
particles improved in their performance on recycling. On the
other hand, the performance of the particles coated with
Al(OH)3 fell off with each successive cycle, probably the
result of dissolution of the gel coat at high pH, during
regeneration.
The effectiveness of regeneration on the coated
particles was demonstrated by reusing the material after one
flocculation without treatment with alkali. In the following
cycle, normal regeneration was used. The results obtained
are shown in ~able VIII (amount of alum used in flocculation
was 5 mg/l).
TABLE VIII
Cycle _ __ _
T C T C T C
15Fe~lag 1 5 ml 1.4 10 4.6 25 1.5 10
~. . _ _
It can be seen that when -the particles are not
regenerated, the purification is poor, bu-t regeneration before
cycle 10 has restored the effectiveness.
EXAMPLE 10
This example shows the effect of increasing the
amount of gel-coated particles, while keeping the amount of
alum constant. Yarra River water was treated using a constant
dose of 5 mg/1 of alum, in conjunction with different quantities
of coagulation aid (Fe Mag 4). The results are shown in
Table I~.
TABLE IX
Amount of Amount of Residual Residual Colour
Coagulant Alum Turbidity (Pt-Co)
Aid (ml/l) (mg/l) (NTU)
Fe Mag 45 5 6.6 45
6 6 40
...... ___ .
. Alum only Nil 5 13 60
_ Nll 20 1.5 15 .
Yarra River water pH = 7.1
Turb = 14 NTU
Colour = 65 Pt~Co
The results show that the increase in Fe Mag 4
results in considerable improvement in the purity of the
trea-ted water. At four times the quan-tity, alum alone does
not have as great an effect as can be achieved with Fe Mag
4/alum.
EXAMPLE 11
The physical stability of the coated particles
is demonstrated in -this example.
A sample of coa-ted particles (Fe Mag 5) was
prepared as in Example 7 using 0.25 g FeC13.6H20/g.Fe3O4.
; Portion of this material was subjected to attrition by pumping
it as an aqueous slurry (200 ml coated par-ticles, settled
. ~ volume in total volume of 250 ml) through a peristaltic pump
at the rate of 100 ml/minute for 5 days. This material was
then tested on Yarra River Wa-ter, pH 7.3, turbidity 13 and
. colour 65-70. A sample of the original coa-ted particles was
--~6 --
, ' , ' ' ~
,
also tested for comparison. The tes-ts were carried out using
20 ml/l of the coated particles and 5 mg/l of alum. The
particulates were regenerated and recycled, and the results
obtained on the third cycle are given in Table X.
TABLE X
_locculant Turbidity Colour
Fresh l ~5
Af ter pumping 5 days 0.5 0
It can be seen that the coated particles are stable to
prolonged attrition.
EXAMPLE 12- 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
(200 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.
The particles were tested on the same water as
in Example llusing 5 ml/l of the material. The treated water
had a turbidity of only 6.4 NTU and colour of<5Pt/Co Units.
EXAMPLE 13- Separation of Chlorella Vulgaris
In this experiment~ a sample of gel-coated
magnetite (Fe Mag 5 from Example]~ was used in order to
separate a suspension of Chlorella Vulgaris. This was
achieved by contacting 50 ml of fresh Chlorella suspension
(400 mg/l suspended solids) with 8 ml of settled Fe Mag 5
for 3 minutes, during~which the flask was gently ayitated.
The suspension was allowed to settle and the supernatant
was decanted and analysed for residual suspended solids.
G
...... .
,
'~
~al976~
It was found that the residual suspended solid amounted to
only 30 mg/l, indicating that 92.5~ of the algae had been
removed.
The adsorbed algae were recovered from the
precipitated sludge by increasing -the pH of the sludge to
about ]Ø5 and agita-ting the mixture. On standing, the
heavier gel-coated magnetite particles settled readily and
the algae were removed as a suspension.
,
~&
