CA1118122A - Water clarification - Google Patents

Abstract

Abstract of the Disclosure A method for clarifying water in which the water is contacted with 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, characterized in that an organic polyelectrolyte is added to the water after contact and before separation of the coagulant/adsorbent from the water.

Description

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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.

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 8~7~'~

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), 2X~

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 .~ .
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 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

lli8~22 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 .22 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 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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p ' 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.

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.

:. .~.., 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.

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:-~8~22 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.

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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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Claims (9)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method for clarifying water in which the water is contacted with 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, characterized in that an organic polyelectrolyte is added to the water after contact and before separation of the coagulant/adsorbent from the water.

2. A method as claimed in claim 1, wherein the polyelectrolyte is of the cationic type.

3. A method as claimed in claim 2, wherein the polyelectrolyte is a polyamide, polyamine or polyethyleneimine.

4. A method as claimed in claim 2, wherein the polyelectrolyte is a polyacrylamide.

5. A method as claimed in claim 2, wherein the polyelectrolyte is natural cationic polymeric flocculant.

6. A method as claimed in claim 1 or 2 wherein a coagulant is also added to the water.

7. A method as claimed in claim 3, 4 or 5 wherein a coagulant is also added to the water.

8. A method as claimed in claim 1 or 2 wherein alum or a ferric salt is also added to the water as a coagulant.

9. A method as claimed in claim 3, 4 or 5 wherein alum or a ferric salt is also added to the water as a coagulant.