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Water Treatment Magazine. March 2010

01.03.2010

Water Treatment Magazine. March 2010

Application of titanium coagulant for water purification and water treatment

The review uses the data of OJSC “Yarega Oil and Titanium Company”
The programme covers a wide range of issues, concerning modern trends of development in the sphere of water treatment and purification, as well as certain aspects of technology and equipment market, and application of water treatment and water purification systems in different industries.

Application of titanium coagulant increases efficiency and productivity of water conditioning stations and reduces total cost of production of purified water

The idea of creating a new and more effective reagent for drinking water production came from two properties which titanium compounds are known to possess. These are the capacity of titanium hydroxide and dioxide to sorb various metal ions on their surface, and the capacity of titanium salts to form polymeric structures as a result of hydrolysis.

Further tests of titanium coagulant and research into titanium coagulant synthesis have shown that more complex composition makes possible the creation of new types of coagulants capable of sorbing both metal ions and ionogenic organic compounds by means of collective and individual adsorption.

The new coagulants application for the removal from natural water of suspended elements with microorganisms on their surface considerably reduced the dosage of chlorine used for disinfection, or allowed abandoning preliminary chlorination completely, which in its turn helped to avoid the formation of carcinogenic organochlorines during drinking water production, as well as to simplify the further work of ion-exchange filters.

There has been conducted a large amount of tests and research into the removal of organic compounds from natural and waste water. The results have shown that the large quantity of micropores in titanium coagulant flocs allows the coagulant to sorb both ionogenic and non-ionogenic organic compounds. Titanium coagulant allows high efficiency of removal of organic compounds from natural water, and oil products from waste water.

In order to achieve a greater sorption and selective sedimentation of certain classes of compounds, the required type of coagulant can be specially obtained by means of changing the structure of the composition and the conditions of synthesis on the basis of titanium compounds.

PURIFICATION OF NATURAL WATER FOR DRINKING WATER SUPPLY TO POPULATION

Drinking water production technology

Natural water contains admixtures of natural and anthropogenic origin which may be of harm and even danger if contained in drinking water. The admixtures may be divided into several
types: suspended elements, heavy metal ions, dissolved organic substances, microorganisms, as well as phyto- and zooplankton.

Drinking water production technologies consist in the application of different methods of removing admixtures from water and of water disinfection. The principle water purification stage is the injection of reagents (coagulants) in order to remove suspended elements, microorganisms, and other admixtures. Coagulants, once injected into water, form flocs that sorb different admixtures, and precipitate. The admixtures are removed with the sediment.

The measurement of coagulants efficiency is generally based on the speed of formation and sedimentation of flocs, as well as on the efficiency of removing suspended elements.

Titanium coagulant is distinguished from traditional coagulants (which are mainly aluminum coagulants) by the formation in water of polymeric structures (large flocs) with a great number of adsorption centres on their surface, and a large amount of micropores throughout. The adsorption centres and micropores sorb different sorts of admixtures, while the large size of flocs allows faster sedimentation.

Research into the quality of purification of natural water with different degrees of turbidity has shown that with increase in water turbidity the efficiency of purification rises to 90-100%, the speed of flocs formation is around 20-30 seconds, the speed of flocs sedimentation increases (the degree of turbidity drops to zero after 30-60 minutes of sedimentation).

Bacterial contamination is known to be caused by suspended elements which are contained in water. Hence when water becomes less turbid, the total bacterial count is considerably reduced, as well as the coli index, while phyto- and zooplankton is completely removed. Titanium coagulant allows abandoning preliminary chlorination which is known to cause the formation of carcinogenic organochlorine compounds that cannot be completely removed at further purification stages.

Heavy metals are removed by means of adsorption on active adsorption centres of titanium coagulant surface. The efficiency of removal of heavy metals depends on their concentration in natural water, as well as on the ionic charge and radius. The concentration of ferrum drops by 35 times, of chrome – by 10 times, of copper – by 15 times, of silicon – by 5 times. The application of a special type of titanium coagulant reduces the percentage of arsenic in purified water by 100 times.

Depending on the composition of raw water the dosage of titanium coagulant is 2-3 times lower than the dosage of traditional aluminum coagulants.

One of the problems of the existing water treatment technologies is high concentration of residual aluminum (up to 500mcg/dm3) in purified water, which is harmful to human health. At present, according to sanitary services, the occupational exposure limit for residual aluminum does not exceed 200mcg/dm3. However traditional aluminum coagulants hardly allow achieving this level, especially during the freshets.

After the application of titanium coagulant the concentration of residual aluminum did not exceed 10-20mcg/dm3. The residual concentration of titanium ions, which is limited by sanitary requirements to 100mcg/dm3, was 8-10mcg/dm3 that is less than its concentration in natural water and foodstuff. According to World Health Organization, titanium is safe for human health as well as for the environment.

One of the most important criteria of purified water quality is concentration of organic substances (the so-called permanganate oxidizability and total organic carbon - TOC). Natural water is known to contain a wide range of various ionogenic and non-ionogenic organic compounds. Only up to 60% of organic compounds are removed by traditional coagulants, and these are mainly ionogenic compounds.

Another unique property of titanium coagulant is high efficiency of removal of organic contaminants from natural water. This property stems from effective adsorption centres which form on the surface of flocs forming as a result of coagulation, as well as from micropores in the flocs. The adsorption centres sorb ionogenic organic compounds, while non-ionogenic organic compounds are sorbed by the micropores.

Titanium coagulant removes 90% of organic compounds contained in water, while the efficiency of 2-3 times higher doses of aluminum sulfate is only 60-80%. The proportion remains the same even with seasonal variations in the composition of raw water.

Turbidity

Coagulants efficiency is calculated by means of dividing the difference between the turbidity of unpurified water and that of purified water by the turbidity of unpurified water, and is expressed in percent. The efficiency of titanium coagulant in purifying high turbidity and colour water (the Dniester, the Dnieper) is 75-80%, for low turbidity high colour water (the Neva, the Volga) the efficiency is 75-97%, whereas the efficiency of twice as much aluminum is 45-65%. It is accounted for by the fact that hydrolysis of titanium coagulant results in the formation of larger and heavier flocs. The speed of formation of titanium coagulant varies from 0.5 to 3 min. depending on conditions (temperature of water, concentration of suspended elements, opacifier). The turbidity norm, which is 1.5mg/dm3, is achieved for different types of water after 10-30 min of sedimentation, and after 20-60 min the water is purified completely.

Removal of bacterial contaminants

Bacterial contamination is known to be caused by suspended elements contained in water. Tests (the Moskva River, the Volga) have shown that, after 30-minute sedimentation and with 0.1-0.3mg/dm3 final turbidity, bacterial contamination was reduced with the coli index dropping from 25,000 to 40-80coli/dm3, and total bacterial count – from 170 to 0-1coli/ml. Similar tests conducted on samples of the Dnieper water showed the reduction of the coli index from 930 to 40coli/dm3, and total bacterial count from 900 to 11coli/ml. As to the Dniester, TBC dropped from 900 to 18coli/ml, phytoplankton – from 110,000 to 0 per dm3, and zooplankton – from 5 to 0 per dm3.

The extension of the disinfecting effect (the Dnieper River), i.e. the reduction of coli index on average by ten times within four hours, is apparently due to the duration of sedimentation, as well as to the reduction of turbidity to 0.

A special research into the reduction of bacterial contamination after the application of titanium coagulant will be introduced in a separate section.

Organochlorine contamination

Numerous research results have shown that the preliminary chlorination of natural water with a high concentration of various organic compounds causes the formation of carcinogenic organic compounds in purified water.

Special tests on the Neva water have confirmed these results (Figure 1). Chlorination of raw water causes the formation of a great number of organochlorines. Further treatment with aluminum sulfate removes only part of these compounds. Titanium coagulant removes organochlorines completely. However, we consider it more reasonable to remove organochlorines and microorganisms completely at the first stage of treatment with titanium coagulant, and then to proceed to control chlorination. It will economize on disinfecting reagents, and will help to avoid the formation of harmful organochlorines.

Figure 1. Results of tests on natural water.

Removal of heavy metals

Titanium coagulant effectively sorbs heavy metal ions. The study of sorption of heavy metals which are contained in different bodies of water and the concentration of which is restricted by norms, has shown that titanium coagulant effectively removes heavy metals from water thus allowing compliance with the SanPiN (Sanitary Norms and Regulations) 2.1.4.1074-01 and international norms.

Table 1

The efficiency of titanium coagulant application for the removal of heavy metals

Residual concentration of titanium and aluminum

One of the problems of the current water purification technology is residual aluminum, which, as international practice shows, has a harmful effect on human health. High doses of aluminum sulfate, particularly during the freshets, result in increase in residual aluminum concentration (when the dose exceeds 7mg/dm3) to more than 500mcg/dm3. In accordance with the Russian SanPiN 2.1.4.1074-01 this concentration must not exceed 500mcg/dm3, while the European standard restricts residual aluminum concentration to 200mcg/dm3. Russia is planning to impose the same norm.

Since some sorts of titanium coagulant contain aluminum which makes them cheaper, special research has been carried out in order to determine residual aluminum concentration after treatment with titanium coagulant containing aluminum sulfate and chloride.

Residual titanium concentration in purified water samples did not exceed 0.005mg/dm3.

Table 2

Residual concentration of aluminum after treatment with titanium coagulant

Titanium coagulant dose (active part), mg/dm3

Concentration of aluminum in purified water, mcg/dm3, sulfate form of the coagulant

(the Neva)

Concentration of aluminum in purified water, mcg/dm3, chloride form of the coagulant

(the Dnieper)

1

1.7

10

2

18

18

3

28

18

4

41

12

5

76

12

6

80

20

Table 3

Reduction of concentration of ferrum and silicon ions in purified water Concentration of organic substances, mcg/dm3

Raw water

Water after purification with aluminum sulfate

Water after purification with titanium coagulant

Total amount of organic compounds

34.7

6.9

3.8

Fatty acids

11.5

3.0

1.0

Alkanes

3.1

2.0

1.6

Phthalate esters

0.8

0.6

0.5

Alcohols

0.4

0.2

0.04

Permanganate oxidizability, mcg O2/dm3

27.7

5.6

2.6

Ferrum

886

280

10.5

Silicon

300

280

60

Removal of organic admixtures

A comparative study of water purification with titanium coagulant and aluminum sulfate has been conducted after determining principal classes of organic substances. The composition of organic compounds has been determined by means of chromato-mass-spectroscopy. Since many organic compounds contain ferrum and silicon ions, Table 3 also shows the reduction of concentration of these elements in purified water.

Technology of industrial waste water treatment

With industrial production growing and new plants being under construction, the anthropogenic pressure on the environment is increasing. The amount of unpurified or poorly purified water is growing. Therefore at present the development of new more efficient and cost-effective waste water treatment technologies, as well as of new reagents for these technologies is an acute issue.

Titanium coagulant with its unique properties allows solving a number of industrial waste water treatment problems.

Special sorts of titanium coagulant have been applied in order to reduce waste water treatment costs. These sorts were synthesized on the basis of intermediate products of titanium production and metallurgical industry wastes.

Special water purification tests have been carried out over the following types of water:

  • Cast iron production waste water (etching workshop waste water);
  • Steel production waste water;
  • Waste water of gas treatment facilities of titanium and magnesium production;
  • Waste water of gas treatment facilities of aluminum production (micronized suspensions of aluminum oxide);
  • Filtrates of titanium dioxide pigments production (micronized suspensions of titanium dioxide);
  • Waste water of ash disposal areas (micronized suspensions of silicon dioxide).

The above mentioned waste water contained micronized suspended solids as well as heavy metals: ferrum, chrome, manganese, nickel, copper, wolfram, vanadium, and other metals. The test purification of the waster water was carried out in comparison with traditional coagulants, based on aluminum, ferrum, and magnesium compounds, and with polyacrylamide as flocculant. The tests results have shown that:

  • The efficiency of removing suspended elements from metallurgical industry waste water by applying titanium coagulant is 80-100%, whereas for ferric chloride with polyacrylamide as flocculant it is 45-75%;
  • The efficiency of precipitation of micronized suspensions in gas purification and ash disposal areas waste water as a result of applying titanium coagulant is 90-100%, whereas for traditional coagulants it is 20-50%.

The efficiency of removing heavy metal compounds by applying titanium coagulant is:

  • for ferrum - 90-99%;
  • for chrome – 70-80%;
  • for manganese – 60-79%;
  • for nickel – 99.4-99.5%;
  • for copper – 72-75% (against 30-43% if purified by applying aluminum sulfate);
  • for mercury – 90-100%;
  • for wolfram – 60-70%;
  • for vanadium – 90-92.6%.

The optimal dose of titanium coagulant for the removal of heavy metals was 0.9-3mg/dm3 (for total amount of metals), whereas the dose of aluminum sulfate, considering the active part, was 6 mg/dm3. The dose and sort of titanium coagulant depend on the degree of waste water contamination. The removal of suspended elements and heavy metals from waste water required 20-50mg/dm3 of titanium coagulant against 40-100mg/dm3 for ferric chloride and 30-40mg/dm3 of polyacrylamide. Heavy metals concentration in purified waste water was below the occupational exposure limit.

One of the principle sources of pollution of thermal power station waste water is acid washouts from equipment which contain sulfuric acid (4-5g/dm3), trivalent ferrum, and salts of heavy metals. Such waste water is purified by means of neutralization and additional injection of activated silicic acid, aluminum sulfate, and calcium hypochlorite. The degree of removal of heavy metals does not meet the occupational exposure limit requirements. The application of titanium coagulant after preliminary oxidation of trivalent ferrum and neutralization of free silicic acid reduced the concentration of heavy metals to the occupational exposure limit.

It has become quite common of late that water bodies, including those which supply drinking water, are polluted with oil products. Application of 2-3mg/dm3 of titanium coagulant, particularly of those sorts of titanium coagulant which contain oxidizing agent, reduced oil products concentration from 35mg/dm3 to 0.1-0.12mg/dm3 with the efficiency of purification reaching 99.6-99.7%.

REMOVAL OF SUSPENDED ELEMENTS FROM INDUSTRIAL WASTE WATER

Treatment of waste water containing poorly settling micronized suspensions of different metal oxides is extremely difficult.

The test purification of waste water has shown:

  •  the efficiency of purifying waste water of gas treatment facilities of aluminum production which contains suspensions of micronized aluminum oxide can reach 85-87% against 45% when water is treated with a higher dose of aluminum sulfate;
  •  the efficiency of purifying filtrates of titanium dioxide pigments production which contain suspensions of micronized titanium dioxide can reach 100% against 42% when filtrates are treated with twice as high dose of aluminum sulfate. The resulting sediment can be used in the main production;
  •  the efficiency of purifying waste water of ash disposal areas which contains suspensions of micronized silicon dioxide also can reach 100% against 52% when water is treated with twice as high dose of aluminum sulfate;
  • waste water of gas treatment facilities of magnesium production, containing suspensions of micronized magnesium oxide can be purified completely with twice as small dose of titanium coagulant.

INDUSTRIAL WASTE WATER TREATMENT WITH NEW TYPES OF COAGULANTS BASED ON TITANIUM COMPOUNDS

A study has been conducted to compare the efficiency of coagulants based on titanium compounds with that of reagents which are traditionally used for waste water purification: polyacrylamide and ferric chloride (FeCl3).

Besides, since among the components of titanium coagulants there were metallurgical wastes, containing chrome, nickel, and copper oxides, it was important to study the possibility of secondary pollution of purified waste water with admixtures contained in the wastes.

The research used samples of waste water of non-ferrous and ferrous metals production. It was aimed at determining how much the efficiency of precipitation of micronized suspensions along with removal of heavy metals depended on the dose of titanium coagulant.

The dosage of reagents, both that of titanium coagulant and of traditional ones, varied from 10mg/l to 100mg/l of the principal substance.

The results of the research into the comparative efficiency of suspensions precipitation and heavy metals removal as a result of applying titanium coagulant (TC), ferric chloride (FeCl3), and polyacrylamide (PAA) show that:

  •  the efficiency of suspensions precipitation processes in all the samples as a result of applying titanium coagulant (optimal dose) is higher than when traditional reagents (PAA and FeCl3) are applied;
  •  the optimal dose of titanium coagulant for treatment of waste water of non-ferrous metals production is 20mg/l (with the efficiency of suspensions precipitation processes reaching 90-100%);
  •  the optimal dose of titanium coagulant for treatment of waste water of ferrous metals production is 30-40 mg/l (with the efficiency of suspensions precipitation processes reaching 70-90%);
  •  the efficiency of titanium coagulant in removing heavy metals (different metals) is 25-90% higher than that of traditional reagents.

Fail-safety and stability of heat and power plants depends a lot on the quality of feed water which is determined by whether or not it contains soluble salts of metals and organic compounds. The presence of the latter is particularly undesirable, as vaporization results in redistribution of admixtures between vapour and small quantity of the forming moisture.

A corrosive environment with high concentration of chemically active organic acids resulting from thermal destruction is created in the phase change zone in steam turbines. Active corrosion may already be observed with organic substances concentration being over 1mcg/dm3.

Therefore feed water treatment is divided into several stages: preliminary coagulation treatment, treatment with carbon filters, degasification – removal of carbon dioxide, and purification on several blocks of cation- and anion-exchange filters. Considerable costs of feed water treatment are compensated by increase in the expensive equipment lifespan and by stable functioning of power stations.

The key water treatment stage is the first stage, i.e. natural water coagulation treatment, when all principal admixtures are removed. These are suspended elements, heavy metal ions, organic compounds, and biological admixtures. Traditional reagents (aluminum and ferrum salts) remove the majority of these admixtures, but their efficiency in removing organic compounds is rather low. Organic compounds are partially removed with the help of carbon and ion-exchange filters. However, in case of organic compounds breakthrough after the first stage of the treatment the load on the filters considerably increases, and they often cannot efficiently remove organic compounds.

The new coagulant, with its unique properties, i.e. its capacity to adsorb 60-90% of heavy metals (Fe, Cr, Mn, Hg, Cd, V and others), as well as to remove 80-90% of ionogenic and non-ionogenic complex organic compounds, is an advanced reagent for heat and power plants feed water treatment. Preliminary tests of titanium coagulant on the Neva water have shown that water turbidity drops on average from 4.3mg/dm3 to 0.026mg/dm3 (against 0.14mg/dm3 when aluminum sulfate is applied), colour index is reduced from 24.4 to 2.23 degrees (against 4.8 degrees when aluminum sulfate is applied), and oxidizability falls from 9.28mgO2/dm3 to 2.12mgO2/dm3 (against 3.8mgO2/dm3 as a result of applying aluminum sulfate).

The results of industrial pilot-scale tests comparing the efficiency of titanium coagulant with that of simultaneously applied aluminum sulfate, alkali, and flocculant, are given in Table 4.

Comparative concentration of organic carbon according to TOC method is given in Table 5.

The residual concentration of titanium ions in raw and purified water was 0.003-0.005mg/dm3 and 0.002-0.007mg/dm3 respectively.

Purified water treatment with ion-exchange filter only (cation- and anion-exchange filters) almost doubles the efficiency of removal of organic compounds. Similar study of organic admixtures composition after applying the traditional scheme has shown not only a higher level of contamination but also partial breakthrough of organic compounds from ion-exchange filters.

The replacement of traditional coagulants with the new coagulant based on titanium compounds both improves the quality of preliminary water treatment and increases the efficiency and lifespan of ion-exchange filters.

Table 4

Quality of water purified with titanium coagulant compared to that treated with aluminum coagulantWater quality criteria

Units

Raw water

Water purified with titanium coagulant

Water purified with aluminum sulfate

Colour index

degrees

27-27.3

4.7-6.7

10-13

Oxidizability

mgO2/dm3

7.8-8.6

3.3-4.0

3.8-4.3

Concentration of ferrum

mcg/dm3

360-370

80-95

144-165

Residual concentration of aluminum

mcg/dm3

20-80

60-130

150-200

Table 5

Comparative concentration of organic carbon according to TOC method, mg/dm3

Points of sampling water at different stages of purification

Aluminum sulfate applying technology

Titanium coagulant applying technology

Coagulated water

6.12

4.15

After mechanical filters

6.53

4.65

After cation-exchange filters H I

6.15

4.15

After anion-exchange filters A I

3.57

1.75

After cation-exchange filters H II

3.63

1.76

After anion-exchange filters A II

3.12

1.32

APPLICATION OF TITANIUM COAGULANT FOR PURIFICATION OF WATER IN SWIMMING POOLS

There have been a number of sanitary and hygienic studies of the application of titanium coagulant in modern technologies of purification and chemical treatment of water in swimming pools.

The hygienic assessment carried out by I.I. Mechnikov State Medical Academy (Saint Petersburg, Russia) applied operating equipment for purification and chemical treatment of swimming pool water.

The research considered key factors that determine the quality of water: the quality of raw water from systems of hot and cold water supply, load per unit of volume of the pool, and coefficient of water renewal. The quality of water in the swimming pool was assessed according to the full list of criteria. These are microbiological, parasitological, and physico-chemical criteria. Tests considered water purification with titanium coagulant and without it, with different loads and the maximum time of complete passage of water through the purification system (6 hours). Special attention was paid to the capacity of titanium coagulant for hydrolysis, as well as to the risk of the formation of transformation products as a result of chlorination.

The tests have shown that:

  •  the complete hydrolysis of titanium coagulant at 20°C takes an hour; hydrolysis is catalyzed by increase in temperature, as well as by UV radiation, and ozone;
  •  titanium coagulant does not become chlorinated with time, and it does not form dangerous products of transformation;
  •  titanium coagulant has smell neither in saturated solutions nor in concentration equal to 1-10 effective doses;
  •  titanium coagulant does not colour water and does not make it opalescent;
  •  when the concentration of titanium coagulant is below 5 effective doses, the hydrolysis does not result in the formation of toxic products;
  •  titanium coagulant does not irritate the skin and the mucous membranes of the eyes. In addition, titanium coagulant has not been proved able cause allergic reactions.

The application of titanium coagulant leads to the clarification of water (turbidity decrease) as a result of removal of suspended elements and sorbed microorganisms, as well as to the discoloration of water as a result of removal of humic acids, and to a more efficient removal of heavy metals from water. At the same time titanium coagulant removes from water organic compounds, which form the basis for carcinogenic compounds, containing halogens, which result from chlorination.

The application of one effective dose of titanium coagulant brings down the concentration of saprophytic bacteria to a permissible level, and reduces total coliform bacteria and coliphages by 99.45-99.68%.

The most harmful from hygienic point of view admixtures include compounds of ferrum, which, combined with manganese, silicon, organic compounds, and microorganisms may form sediments, which cause biological fouling of pools and pipelines. Water treatment with titanium coagulant brings down the concentration of ferrum below the occupational exposure limit. The concentration of other heavy metal admixtures drops to the level which requires no further control. Thus the bio-fouling of pools decreases significantly.

Comparative study of efficiency of treatment with titanium coagulant and without it has shown that without the coagulant during the same period of time and with the same load, turbidity increased by 10 times, concentration of ammonia, chlorides and chloroform doubles, concentration of microorganisms increases by 26 times, and that of coliphages – by 3-4 times.

A hygienic certificate has been obtained on the basis of this research on “The system of purification, chemical treatment, and heating of water in swimming pools, as well as of maintenance of its level”.

The strong sorption capacity of titanium coagulant in relation to principal toxicants (heavy metals, organic compounds, radionuclides), and bacterial and viral contamination, as well as its low toxicity (Hazard Class IV) allows recommending it for treatment of water in swimming pools.

TITANIUM COAGULANT AND THE HYDROGEN ION EXPONENT (PH) OF WATER

Traditional technologies of treatment of natural water with low alkaline reserve and low pH include preliminary alkalization: injection of soda, sodium hydroxide, or lime. When applying aluminum coagulants, particularly in those cases when the treated water temperature is low, flocculant is injected at the same time in order to increase flocculation. All this complicates the technological process of water treatment and increases unit costs of purified water production.

In most cases when titanium coagulant is applied under the same conditions there is no need for additional reagents. The flexible technology of titanium coagulant synthesis produces a reagent with acid ratio which is optimal for any particular type of treated water.

Titanium coagulant with low acid ratio rapidly forms large flocs in water with a low alkalinity level (0.50-0.55mmole/dm3) and pH 7.1-7.5 and with the temperature of the treated water below 10°C, and effectively removes heavy metals, organic compounds, and microorganisms.

Titanium coagulant with a higher acid ratio is applied for the purification of natural water with pH 7.8-9.0 and with a higher alkaline reserve. The higher acid ratio reduces the unit consumption of titanium coagulant for water purification.

When treating waste water with very low pH (<5) and high ferrum concentration, we either applied special sorts of titanium coagulant, or preliminary oxidation and neutralization. This method used the sorption capacity of ferrum hydroxide and removed from water most admixtures. The injection of titanium coagulant at the final stage of treatment helps to achieve a high efficiency of removal of heavy metals, as well as to reduce the coagulant dose and water purification cost.


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