WO1997034836A1

WO1997034836A1 - A method for improving the applicability of a water treatment chemical, and a coagulant

Info

Publication number: WO1997034836A1
Application number: PCT/FI1997/000173
Authority: WO
Inventors: Simo Jokinen; Eero Aitta
Original assignee: Kemira Chemicals Oy
Priority date: 1996-03-18
Filing date: 1997-03-17
Publication date: 1997-09-25
Also published as: FI102160B; FI961249A0; AU2160997A; FI102160B1; FI961249A

Abstract

The invention relates to a method for improving the applicability of a water treatment chemical as a coagulant in a water treatment process comprising a flocculation step wherein the coagulant in the form of an aqueous solution is added to the water to be purified, said water treatment chemical containing an iron and/or aluminium compound as the main component and soluble bivalent manganese Mn(II) as an impurity. According to the invention a concentrated aqueous solution of said water treatment chemical is subjected to oxidation (2) by means of an oxidant to obtain a concentrated aqueous solution to be used as said coagulant or the obtained concentrated aqueous solution is converted into a solid product, said product being subsequently converted into an aqueous solution to be used as said coagulant, said oxidant being used in an amount at least sufficient to oxidize the soluble MN(II) present in the water treatment chemical to manganese dioxide MnO2 or permanganate MnO4-, said MnO¿2? being flocculated away in the subsequent flocculation step (3), and said MnO4?-¿ acting as an oxidizer in the water treatment process.

Description

A METHOD FOR IMPROVING THE APPLICABILITY OF A WATER TREATMENT CHEMICAL, AND A COAGULANT

This invention relates to a method for improving the applicability of a water treatment chemical containing an iron and/or aluminium compound as the main component and soluble bivalent manganese Mn(II) as an impurity. The invention also relates to a coagulant in the form of a concentrated aqueous solution.

The adverse effect of soluble manganese in drinking water is not related to any specific health problem. Rather, the adverse effects associated with elevated concentrations of Mn^+ are aesthetic in nature. Water discoloration is a frequent consumer complaint. Also growth of iron and manganese-oxidizing bacteria on water mains can lead to general deterioration of the quality of the water distributed. For these reasons standards have been established for maximum concentrations of soluble manganese in drinking water. In the European Union the maximum admissible concentration is 50 μg/1 and the guide level is 20 μg/1.

Prior art knows several methods for controlling soluble manganese concentrations in water treatment facilities. Earlier the manganese control often involved chlorine alone or combined with potassium permanganate under alkaline pH conditions. In recent years application of alternative oxidants has increased. Ozone has been used to oxidize soluble manganese in water according to (Wilczak, A. et al. , Journal AWWA 85(1993)10, 98-104):

(1) Mn² + + O3 + H2O → Mnθ2(s) + O2 (aq) + 2H +

The manganese dioxide formed in the oxidation reaction is either in particulate or colloidal form. Solid manganese dioxide is difficult to capture by filtering but it can be readily removed by flocculation. For instance, a coagulant chemical like ferric chloride can be added to the water before the ozonation to form a ferric hydroxide floe which then removes the solid manganese dioxide through sedimentation.

The stoichiometry of reaction (1) predicts that 0.88 mg O3 are required to oxidize 1 mg Mn²+ . It has been reported that excessive O3 dosages or ozonation at sufficiently low pH may lead to the formation of permanganate Seby, F. et al. , Ozone Science & Engineering, vol. 17, pp. 135-147) according to the following reaction: (2) 2Mn + + 5O3 + 3H2O → 2Mnθ4" + 5O2 + 6H+

In the following the oxidation reactions are presented for KMnO HOCl (free chlorine), chlorine dioxide, and chlorates (Langlais, B. et al. , Ozone in Water Treatment, AWWA Research Fdn, 1991 , p.139)

(3) 3Mn + + 2Mnθ4 - + 2H2O → 5Mnθ2(s) + 4H +

(4) Mn^{2 +} + HOCl + H2O → Mnθ2(s) + Cl- + 3H +

(5) Mn² + + 2CIO2 + 2H2O → Mnθ2(s) + 2Clθ2" + 4H +

(6) 6Mn + + 5CIO3- + 9H2O → 5 Cl" + 6MnO₄" 4- 18H +

The various oxidants may be arranged in the order of decreasing oxidation power by means of the standard oxidation-reduction potential E^. Table A presents a list of potential values obtained from various sources (CRC Handbook of Chemistry and Physics, Langlais et al.).

Table A. Standard oxidation-reduction potentials for various oxidants

Oxidant Eθ (V)

Ozone 2.07

Hydrogen peroxide 1.78

Permanganate 1.68

Hypochlorous acid 1.49

Chlorate 1.45

Mnθ2 1.22

Chlorine dioxide 0.954

Although the values of Table A do not tell how fast a given oxidation reaction takes place it is useful in predicting which reactions are thermodynamically possible in water. For instance, Table A predicts that it is thermodynamically possible for ozone to oxidize Mnθ2 to permanganate. We also refer to the enclosed diagram (Fig. 1) showing the manganese equilibrium in water. From this figure it can be seen that a specific oxidation power is required to oxidize Mn^{2 +} into Mnθ2 depending on the pH of the aqueous system. All the above oxidants have the required oxidation power. Water treatment chemicals are typically iron or aluminium compounds manu¬ factured from various raw materials containing these metals. If the raw material from which the chemical is manufactured contains high concentration of manganese, the product chemical will also have an elevated concentration of manganese unless the manganese is removed. This is, however, difficult because of the chemical similarity of iron and manganese. As an example of this kind of raw material with high manganese content we mention ferrous sulphate which is a by-product from a titanium dioxide process.

A coagulant chemical manufactured from a raw material with elevated manganese content has a reduced market value since its use requires a manganese removal step in the purification process. If the manganese removal step already exists in the water treatment facility, as is often the case, its use requires an increased dosage of the manganese controlling oxidant at the influent water. In any case use of the manganese containing coagulant chemical introduces additional loading of soluble manganese in the water stream. An ideal solution to this problem would be a method by which the soluble manganese in the coagulant chemical could be eliminated before it is introduced into the water.

The objective of this invention is to present a method whereby the adverse effect of soluble manganese in a coagulant chemical can be reduced to a minimum. In particular, the objective of the invention is to increase the applicability of iron compounds, especially ferrous sulphate, containing manganese as an impurity as raw material in the manufacture of coagulant chemicals. The method is charac¬ terized by the features presented in the enclosed claim 1.

Thus the present invention relates to a method for improving the applicability of a water treatment chemical as a coagulant in a water tieatment process comprising a flocculation step wherein the coagulant in the form of an aqueous solution is added to the water to be purified, said water treatment chemical containing an iron and/or aluminium compound as the main component and soluble bivalent manganese

Mn(II) as an impurity, said method being characterized in that a concentrated aqueous solution of said water treatment chemical is subjected to oxidation by means of an oxidant to obtain a concentrated aqueous solution to be used as said coagulant or the obtained concentrated aqueous solution is converted into a solid product, said product being subsequently converted into an aqueous solution to be used as said coagulant, said oxidant being used in an amount at least sufficient to oxidize the soluble Mn(II) present in the water treatment chemical to manganese dioxide Mnθ2 or permanganate Mnθ4", said Mnθ2 being flocculated away in the subsequent flocculation step, and said MnC>4" acting as an oxidizer in the water treatment process.

According to the invention, before the soluble manganese present in the water treatment chemical is introduced into the influent water, it is "masked" into a form which readily flocculates together with other impurities. This masking effect is achieved by an oxidant added to a concentrated aqueous solution of the chemical. By using a suitable oxidant part of the soluble manganese may even be oxidized to a heptavalent permanganate which then acts as an oxidant in the subsequent water treatment process. Thus part of the soluble manganese may be converted into a very useful form which is beneficiated in the subsequent stages of the water treatment process.

The oxidant is added at least in an amount sufficient to oxidize the Mn ⁺ in the water treatment chemical but it is preferably added in an amount sufficient to oxidize the soluble Mn of the influent water as well.

The oxidant may be hydrogen peroxide, a permanganate like an alkali permanganate, ozone, oxygen, chlorine, chlorine dioxide, a chlorate like an alkali chlorate, a ferrate or a ferrite or a mixture thereof.

There are several ways of applying the method of the invention.

According to a first embodiment the oxidant is added to a concentrated aqueous solution of the water purification chemical to obtain a concentrated aqueous solution to be used as said coagulant in the flocculation step. In this case the oxidant may be added either in the coagulant manufacturing plant or at the water puri¬ fication plant.

According to a second embodiment the oxidant is added to a concentrated aqueous solution of the water purification chemical and then the obtained solution is granulated into a solid product, said product being dissolved at a water treatment plant to obtain an aqueous solution to be used as said coagulant in the flocculation step.

According to a third embodiment the oxidant is added to the water treatment chemical, both being in solid form, and the obtained mixture is dissolved at a water treatment plant to obtain a concentrated aqueous solution to be used as said coagulant in the flocculation step. The solid oxidant is preferably a permanganate like an alkali permanganate, a chlorate like an alkali chlorate, a ferrate or a ferrite. The water treatment chemical preferably contains ferric sulphate as the main component. A suitable aluminium containing water treatment chemical is aluminium sulphate.

In case the water tieatment chemical contains a fenic compound, the concentration of Fe in the concentrated aqueous solution to be subjected to oxidation as well as in the obtained concentrated aqueous solution is preferably about 10 to 14% by weight and more preferably about 1 1 to 12% by weight.

In case the water treatment chemical contains an aluminium compound, the concentration of Al in the concentrated aqueous solution to be subjected to oxidation as well as in the obtained concentrated aqueous solution is preferably about 6 to 9% by weight calculated as AI2O3.

In case the water treatment chemical is in solid form like e.g. ferric sulphate, it can be dissolved in water at elevated temperature. A concentrated solution (e.g. 11-12% Fe) is obtained with pH in the range -1 to 3, typically 0.5 to 1 . This solution contains soluble Mn² + . According to the invention the adverse effect of soluble bivalent manganese can be eliminated by adding a suitable oxidizing agent or several agents before this solution is used for the water purification. Bivalent manganese oxidizes into tetravalent dioxide, which remains colloidal in the solution because of the low pH. When the solution is added to the water to be purified, pH rises and manganese dioxide flocculates away together with other impurities in the water.

The oxidant may in fact be a mixture of two or more oxidants or oxidation may be performed by using several oxidants simultaneously or successively. A rec¬ ommended combination of oxidants is hydrogen peroxide and a permanganate for those who want to use the coagulant chemical as a solution.

A further objective of this invention is to provide a coagulant having a reduced content of soluble bivalent manganese Mn(II). Thus the invention also relates to a coagulant in the form of a concentrated aqueous solution containing an iron and/or alωninium compound and optionally an oxidant, and being obtainable by subjecting a concentrated aqueous solution of a water treatment chemical containing an iron and/or aluminium compound as the main component and soluble bivalent Mn(II) as an impurity to oxidation by means of an oxidant in an amount at least sufficient to oxidize the soluble Mn(II) present in the water tieatment chemical to manganese dioxide Mnθ2 or permanganate Mnθ4". The invention will be further described with reference to the enclosed drawings in which

Fig. 1 shows the diagram of manganese equilibrium in water, and

Fig. 2 presents schematically the method according to the invention incorporated with a drinking water utility.

Fig. 1 shows the potential - pH equilibrium diagram for the manganese- water system (Pourbaix, M., Atlas of Electrochemical Equilibria in Aqueous Solutions, Pergamon Press, 1966, p. 290). As seen from the figure, the domains of stability of the manganous ion Mn + ⁺ and the permanganate ion MnO'4 ^are separated by a domain of stability of Mnθ2 and other oxides depending on the pH. From the figure it can be further seen that in pH domain 0-3, which is of importance in this invention, oxidation of Mn^{+ +} may go as far as Mnθ2 or even MnO"4 in the presence of vigorous oxidizing agents.

Fig. 2 shows schematically a purification process in which solid ferric sulphate containing Mn as an impurity is used as the water treatment chemical. In the dissolution stage 1 it is dissolved in water at 80°C to yield a solution containing some soluble Mn^{2 "}*^" . In the subsequent oxidation stage 2, KMnθ4 is added to the solution in an amount sufficient to oxidize the Mn²+ from the ferric chemical into Mnθ2 and further to oxidize the soluble Mn^{2 +} in the raw water. Mnθ2 forms an insoluble particulate or colloid phase in the coagulant chemical solution.

As the coagulant chemical is added to the raw water to effect flocculation at stage

3, pH rises and solid Mnθ2 particles appear in the water. The KMnθ4 surplus now oxidizes the soluble Mn^{2 "}*^" of the raw water into solid Mnθ2- The solid manganese dioxide particles are captured by the ferric hydroxide floes and sedimentate to the bottom in the settling stage 4. Finally the solid Mnθ2 is removed along with other solid impurities from the bottom of the settling tank.

In the following, the invention is further illustrated by means of examples.

Example 1 (reference).

The water sample was river water from the Oulu river which is used as the raw water in the preparation of drinking water. This water contained 21 μg/1 Mn and its organic content expressed as the chemical oxygen demand (CODjyfn) was 1 1 mg/1. A commercial solid ferric chemical (ferric sulphate) with the trade name Ferix-3 was used as the coagulant chemical. This chemical was dissolved (dissolution time 5 h) in water at 80°C to produce the following solution: 12% Fe, 0.29% Fe^{2 +} , and 0.055% Mn. The density of this solution was 1.55 g/cm- .

The following water treatment procedure was done with the above water and the solution using a Kemira Kemi Flocculator equipment. The Ferix solution was diluted to 10/100 and 235 μl of this diluted solution was added to 1 liter of water sample. After addition the total Mn of the water was 41 μg/1. Mixing of the sample was done in two steps. First the sample was mixed for 30 seconds at a speed of rotation of 400 rpm and then for 30 minutes at 30 rpm. The temperature was maintained at 12°C and the pH during precipitation was 4.4.

After the flocculation, the water contained 19 μg/1 Mn and its CODj^n ^was -.15 mg/I. The treatment reduced the Mn concentration of the raw water by 9.5 % .

Example 2 (reference).

In this example the procedure was the same as in Example 1 except that the chemical dosage was 355 μl/1 whereby the total concentration of Mn in the water before the flocculation was 51 μg/1.

After the flocculation, the Mn concentration of the water was 27 μg/1 and its CODjyfn ²-0 ^mg/l- Thus the treatment increased the Mn concentration of the raw water by 28% .

Example 3.

A chemical solution was prepared by adding 0.5823 g of KMnθ4 at room temperature to 150 g of the ferric sulphate solution of Example 1. This solution contained 12% Fe, < 0.02% Fe + , and 0.19% Mn. The solution was used in the flocculation test which was similar to Example 1 and the dosage was 235 μl/1. After the addition, the total concentration of Mn in the water was 89 μg/1.

After the flocculation, the concentration of Mn in the water was 5 μg/1 and its CODM_Π 2.75 mg/1. Thus the treatment reduced the Mn concentration of the raw water by 76% . Example 4.

In this example the procedure was the same as in Example 3 except that the chemical dosage was 355 μl/1 whereby the total concentration of Mn in the water before the flocculation was 124 μg/1.

After the flocculation, the Mn concentration of the water was 5 μg/1 and its CODM_Π 2.0 mg/1. Thus the treatment reduced the Mn concentration of the raw water by 76% .

Example 5 (reference).

The water sample was drainage water from the peat harvesting area. This water contained 250 μg/1 Mn and its CODj jn ^was 8-3 mg/1.

The same commercial solid ferric chemical as in Example 1 was used as the coagulant chemical. This chemical was dissolved as in Example 1.

The following water treatment procedure was done with the above water and the solution using a Kemira Kemi Flocculator equipment as explained in Example 1. The ferric sulphate solution was diluted to 10/100 and 75 μl of this diluted solution was added to 1 liter of water sample. After addition the total Mn of the water was 310 μg/1. The temperature was maintained at 13°C and the pH during precipitation was 3.77.

After the flocculation the water contained 230 μg/1 Mn and its CODj in ^was 6.75 mg/1. The treatment reduced the Mn concentration of the raw water by 8% .

Example 6 (reference).

In this example the procedure was the same as in Example 5 but the chemical dosage was 110 μl/1 whereby the total concentration of Mn in the water before the flocculation was 340 μg/1.

After the flocculation, the Mn concentration of the water was 250 μg/1 and its CODj n was 6.0 mg/1. Thus the treatment did not change the Mn concentration of the raw water.

Example 7.

In this example the chemical solution (ferric sulphate + KMnθ4) of Example 3 was used in the flocculation test which was similar to Example 5 and the dosage was 75 μl/1. After the addition, the total concentration of Mn in the water was 470 μg/1-

After the flocculation, the concentration of Mn in the water was 180 μg/1 and its CODj_yjn ^was 6-75 mg/I. Thus the treatment reduced the Mn concentration of the raw water by 28% .

Example 8.

In this example the procedure was the same as in Example 7 except that the chemical dosage was 110 μl/1 whereby the total concentration of Mn in the water before the flocculation was 570 μg/1.

After the flocculation, the Mn concentration of the water was 170 μg/1 and its CODjyjn ^was 6-²^ mg/1. Thus the treatment reduced the Mn concentration of the raw water by 32% .

Table 1. Results of Examples 1-8. Examples 1 , 2 , 5, and 6 represent reference examples whereas examples 3, 4, 7, and 8 represent examples according to the invention. Column 6 of the table gives the total manganese content of the water after addition of the chemical, columns 7 and 8 give the temperature T and pH during flocculation. The last column gives the relative difference between the Mn content of the water before and after the treatment.

1) FS = ferric sulphate

²) diluted ferric sulphate solution It can be seen from the results of Table 1 that in the ferric sulphate solution where potassium permanganate was added, manganese is "masked" so that it remains in the floe. The amounts of manganese in the purified water is clearly less than in the original raw water. This is particularly clear in the case of the river water. A re- markable reduction of 76% in manganese was reached for the river water.

The worse results in the case of peat water are due to the high amount of manganese in water. This kind of water does not, however, represent the realistic case as a source of drinking water. Anyhow, the Mn content of the coagulant has been removed.

According to the results the emission of manganese caused by the ferric sulphate coagulant can be eliminated using a suitable oxidizing agent or a combination of agents before the reagent is used for the water purification. Mn^{2 +} is oxidized to Mn(IV) which remains colloidal in the reagent solution of low pH (-1 to 3, typically 0.5-1). When this solution is mixed with the water to be purified, the colloidal manganese will be flocculated away. Manganese oxidation can be done already in the manufacturing plant of the chemical solution for customers which use the chemical as a solution. The suitable oxidizing agent in that case is ozone. If the customer buys the chemical in solid form, he can oxidize manganese himself during dissolution. In this case the oxidation can be done e.g. with the combination of hydrogen peroxide and permanganate.

Claims

1. A method for improving the applicability of a water tieatment chemical as a coagulant in a water treatment process comprising a flocculation step wherein the coagulant in the form of an aqueous solution is added to the water to be purified, said water treatment chemical contaimng an iron and/or aluminium compound as the main component and soluble bivalent manganese Mn(II) as an impurity, characterized in that a concentrated aqueous solution of said water treatment chemical is subjected to oxidation by means of an oxidant to obtain a concentrated aqueous solution to be used as said coagulant or the obtained concentrated aqueous solution is converted into a solid product, said product being subsequently converted into an aqueous solution to be used as said coagulant, said oxidant being used in an amount at least sufficient to oxidize the soluble Mn(II) present in the water treatment chemical to manganese dioxide Mnθ2 or permanganate Mnθ4", said Mnθ2 being flocculated away in the subsequent flocculation step, and said Mnθ4" acting as an oxidizer in the water treatment process.

2. The method according to claim 1, characterized in that the oxidant is used in an amount sufficient to oxidize both soluble Mn(II) present in the water treatment chemical and soluble Mn(II) present in the water to be purified.

3. The method according to claim 1 or 2, characterized in that the oxidant is ozone, hydrogen peroxide, chlorine, chlorine dioxide, a chlorate, a permanganate, a feπate or a ferrite or a mixture thereof.

4. The method according to any of the preceding claims, characterized in that the oxidant is added to a concentrated aqueous solution of the water purification chemical to obtain a concentrated aqueous solution to be used as said coagulant in the flocculation step.

5. The method according to any of claims 1 to 3, characterized in that the oxidant is added to a concentrated aqueous solution of the water purification chemical and then the obtained solution is granulated into a solid product, said product being dissolved at a water treatment plant to obtain an aqueous solution to be used as said coagulant in the flocculation step.

6. The method according to claim 1 or 2, characterized in that the oxidant is added to the water tieatment chemical, both being in solid form, and the obtained mixture is dissolved at a water treatment plant to obtain a concentrated aqueous solution to be used as said coagulant in the flocculation step.

7. The method according to claim 6, characterized in that the oxidant is a chlorate, a permanganate, ferrate or ferrite.

8. The method according to any of the preceding claims, characterized in that the water treatment chemical contains ferric sulphate as the main component.

9. The method according to claim 8, characterized in that said concentrated aqueous solution of the water treatment chemical and said obtained concentrated aqueous solution both contain about 10 to 14% by weight of Fe.

10. A coagulant in the form of a concentrated aqueous solution containing an iron and/or aluminium compound and optionally an oxidant, and being obtainable by subjecting a concentrated aqueous solution of a water treatment chemical containing an iron and/or aluminium compound as the main component and soluble bivalent Mn(II) as an impurity to oxidation by means of an oxidant in an amount at least sufficient to oxidize the soluble Mn(II) present in the water treatment chemical to manganese dioxide Mnθ2 or permanganate Mnθ4~.

11. The coagulant according to claim 10, characterized in that the oxidant is ozone, hydrogen peroxide, chlorine, chlorine dioxide, a chlorate, a permanganate, a ferrate or a ferrite or a mixture thereof.

12. The coagulant according to claim 10 or 1 1 , characterized in that the coagulant contains ferric sulphate.

13. The coagulant according to claim 12, characterized in that the coagulant is in the form of a concentrated aqueous solution containing about 10 to 14% by weight of Fe.