Chlorine Dioxide as Disinfectant for Pretreatment in ...

CHLORINE DIOXIDE AS DISINFECTANT FOR PRETREATMENT IN SEAWATER DESALINATION PLANTS. Enric Palacios Doñaque1; Aleix Martorell Cebrian 2; Pedro Javier Miranda Luján 3 (1)Technical Advisor Acciona Agua R&D Department; (2) Operation Manager US Desalination, O&M Department Acciona Agua; (3) International Manager Production O&M Department Acciona Agua.

Abstract - Chlorine Dioxide (ClO2) is often used as a disinfectant in water treatment plants, with the main advantage of not producing THM's, like other Chlorine (Cl2) disinfectants do. However, from Chlorine Dioxide, certain by-products can be derived such as the Chlorite Ion (ClO2-) and Chlorate Ion (ClO3-). Even though there is experience using Chlorine Dioxide for disinfection purposes in multiple water treatment plants all around the world, there is not much experience using this product in seawater desalination plants. Described in this document are results from a study with the goal of describing the disadvantages of the formation of related by-products, and basically focusing on the impact of the Chlorite Ion as it may affect the water and salts transport numbers of aromatic polyamide membranes installed in Reverse Osmosis racks. This study has been conducted in a large-scale seawater desalination facility where Chlorine Dioxide is used as a disinfectant as part of the pretreatment process. Lab-scale tests evidenced that the Chlorite Ion is not easily removed by a common reducing agent like the SBS (Sodium BiSulfite), which, in presence of metal ions such as copper (Cu(II)), can drive the catalytic formation of oxidizing sulfur compounds, which simultaneously, regenerate Chlorine Dioxide, and increase the concentration of the same Chlorite Ion from the high concentration of Chlorides in seawater. The production rate of these compounds in the seawater after the addition of SBS, depends on the pH value and principally on the salts concentration, which is higher in the last position membranes within each RO vessel in serial configuration, by the concentration factor. A worst case scenario exists when the Chlorite Ion residual has the potential to produce free Chlorine. The effect of other metal ions such as iron (Fe(III)) and manganese (Mn(II)) has been also studied, without any catalytic formation of the related compounds. The regeneration of Chlorine Dioxide and Chlorine, on the boundary layer of the RO membranes, causes significant oxidation of the aromatic polyamide, with a modification of the water and salt transport numbers. In this document, a solution is evaluated from the study made in a seawater desalination plant, in order to eliminate the possible residual concentrations of Chlorine Dioxide and Chlorite in the disinfected seawater from the pretreatment processes, before being sent to the RO membranes.

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The solution described herein was based on the dosage of Fe(II) salts, in the coagulationflocculation pretreatment stage, once the Chlorine Dioxide has already been dosed. According to available literature on the topic, the ion Fe (II), as ferrous chloride or ferrous sulfate, is effective in reducing the Ion Chlorite and Chlorine Dioxide, becoming Fe (III), which precipitates as Iron Hydroxide (Fe(OH)3). Nevertheless, in seawater, due to ionic strength, the concentration ratio of Fe(II) required is greater than mentioned in the literature (3,1 Fe(II)/1 mg ClO2-). In addition, it is concluded that the pH value influences the action of Fe(II) on the Chlorite Ion. Analytical determination for Chlorite, Chlorine Dioxide and Free Chlorine were performed by Amperometric analysis. Conversely, when there are low concentrations of Chlorite (0.1 - 0.2 mg/l approx.) to remove, the formation of Fe(OH)3 and other high charge compounds of the same ion is not sufficient to have an efficient performance of the coagulation-flocculation process. In this case, it is necessary to strengthen the process of coagulation-flocculation by adding a calculated dose of a coagulant chemical, such as FeCl3, in order to build a suitable floc for the main purpose of removing colloids and TOC from the water. The Chlorite and Chlorine Dioxide removal tests by Fe(II) were performed in a Jar-Test device (Phips&Bird) in order to observe the evolution and efficiency of coagulation process. I.

INTRODUCTION

In water treatment, disinfection provides availability of safe drinking water by removing, deactivating or killing pathogenic microorganisms. All the regulated drinking water sources are controlled by governmental agencies (e.g. Since 1974, U.S. EPA controls, under the Safe Drinking Water Act, the level of residual disinfectants in drinking water with no adverse health effects are likely to occur). There are many chemical and physical procedures for water disinfection. The introduction of water chlorination as a disinfection standard treatment technique caused a large drop in mortality from infectious disease and is considered one of the major public health advances in the 20th century. Chlorine is the most common disinfection chemical used either in gas form or in solution as sodium hypochlorite or calcium hypochlorite, in drinking water treatment processes, either in the raw water or in the final product water. The principal concern of using disinfection chemicals, is their by-products. Drinking water disinfection by-products (DBPs) are formed by the reaction of disinfectants with naturally occurring organic matter, bromide and iodide, as well as from anthropogenic pollutants. Potential health risks of DBPs from drinking water include bladder cancer, early-term miscarriage, and birth defects. It was not until 1994, when the U.S. EPA proposed the stage 1 DBP rule, in order to control the concentration of the harmful DBPs in drinking water. Nowadays, several DBPs are already regulated by governmental agencies. In many circumstances, disinfection by chlorination can be problematic for this mentioned question. Chlorine can react with naturally occurring organic compounds found in the water supply to produce harmful DBPs such as trihalomethanes (THMs) and haloacetic acids (HAAs). Future USEPA regulations will mandate increased disinfection capabilities and lower disinfection byproduct levels from water treatment plant processes. Advanced oxidants, such


as chlorine dioxide, will be needed to substitute for chlorine in pre-disinfection in order to increase disinfection capabilities and prevent the generation of THMs and HAAs. Chlorine dioxide is a faster-acting and stronger disinfectant than elemental chlorine, against bacteria, viruses, spores and algae. In terms of oxidation states, chlorine dioxide has an oxidation state of +4 against the +1 of the sodium or calcium hypochlorite. It can also be utilized in pretreatment as reducing taste and odors in raw water and distribution systems, oxidizes natural iron and manganese and reduces color. Chlorine dioxide DBPs, the ions chlorite and chlorate are under regulation to low allowable concentration levels. The ion ClO2- (chlorite) comes from the decomposition of ClO2, being the most common DBP from ClO2. However, there is an important source of ClO2- ion due to the low performance of most chlorine dioxide generators, which uses sodium chlorite (NaClO2) feedstock. Ultimately, it is difficult to get a generation of chlorine dioxide, without a drag ClO2-. The U.S.EPA rule establishes maximum residual disinfectant level goal (MRDLG) for the ClO2 and the ClO2- in drinking of 0.8 mg/L. Therefore, the chlorine dioxide dose for disinfection must be controlled by the excess of the ion chlorite, related on the performance of the chlorine dioxide generator and from the decomposition of the ClO2 itself. There are experiences of using ClO2 in drinking water treatment, but there are only a few on using this chemical as a raw water disinfectant or in pretreatment disinfection procedures in desalination water treatment. The removal of microorganisms is important to avoid the biofouling to occur in all desalination processes (e.g. in pressure driven membranes processes, distillation, ion exchange…). Biofouling or deposition of biological matter is a severe problem that can cause performance reduction on removing salts and minerals. Nowadays, pressure driven membrane processes are the most reliable and economic desalination process either for brackish or seawater. These procedures involve polymeric membranes with a high efficiency on water desalination application, but they offer poor resistance to strong oxidants such as chlorine or chlorine dioxide, causing damages on the polymeric chains. Hence, all the strong oxidants in water must be reduced before these get into the membrane system. ClO2 as a disinfectant is an actual alternative for water desalination. A study has been conducted with the goal of describing the effect of using ClO2 for seawater desalination treatment, and focused on the subsequence effects of the DBPs that might be potentially harmful for different polymeric membrane systems (e.g. Ultrafiltration, Nanofiltration or Reverse Osmosis).

II.

PROBLEM DEFINITION

Since chlorine dioxide is an actual disinfectant alternative, and because of lack of information about this chemical in water desalination treatments, the goal of this study is to present the behavior of the chlorine dioxide and its DBPs in a seawater matrix. This study has been conducted in a large-scale seawater desalination facility where chlorine dioxide is generated and dosed as a disinfectant as part of the pretreatment process. Likewise, tests were conducted adding ClO2 with artificial solutions of NaCl with distilled water to see the influence of Cl- ion concentration. 3 2015 © American Water Works Association AMTA/AWWA Membrane Technology Conference Proceedings All Rights Reserved

C.H. Barron et. at. 1966 studied the Cu(II) ion as a potential catalyst for many oxidationreduction reactions, especially with sulfur compounds such as bisulfite ions. The relationship of the chlorine dioxide and the its DBPs with bisulfite ions and Cu(II) ions are studied due to the addition of the first one in desalination treatments, as reducer for strong oxidants, and the content of the second one in the studied seawater matrix. Ion chlorite can be reduced by the oxidation of the Fe(II) into Fe(III) (Rittmann, D. Ph.D., P.E. (2003)). The addition of Ferrous Sulfate (FeSO4) is evaluated as a solution, in order to eliminate or reduce the residual concentrations of the chlorite ion in the disinfected seawater from pretreatment processes, before being sent to the polymeric membrane systems. III.

EQUIPMENTS AND REAGENTS

The analyzer instrument used to quantify the concentration of Chlorine Dioxide, Ion Chlorite and free Chlorine, is the HACH AutoCAT 9000™. This is an automated chlorine amperometric end point titration instrument based and adapted on Standard Methods for the Examination Water and Wastewater procedure 4500-ClO2 E. In addition, for measuring and adjusting the pH and the ORP, the HACH SensION+ pH3 and the HACH MTC101 probe attach to an HQ30d portable console have been used, respectively. For solutions stirring a Jar-Test device was employed. The Chlorine Dioxide was prepared by the JESCO EASYZON C generator engineered by the oxidation or acidic decomposition of the Sodium Chlorite. Hydrochloric Acid (HCl @ 33 w/w% solution) and Sodium Chlorite (NaClO2 @ 25 w/w% solution) are mixed into a reactor, where the undergo reaction process resulting in ClO2: 4

5

→ 4

5

2

Other chemicals reagents such as, Sodium Chlorite (NaClO2 80% pellets), Ferrous Sulfate (FeSO4.7H2O fine crystals), Sodium Bisulfite (HNaO3S powder) and Copper Sulfate (CuSO4.5H2O fine crystals) were used in the following tests. IV.

METHOD

Analytical method is required for differentiating between free chlorine, chlorine dioxide and chlorite in aqueous solution. E.Marco Aieta V.Roberts Paul and Margarita Hernandez, in the article Determination of Chlorine Dioxide, Chlorine, Chlorite and Chlorate in Water, published in Journal (American Water Works Association) Vol. 76, No. 1, in 1984, described an amperometric determination of iodine formed by the oxidation of iodide by chlorine dioxide, chlorine, chlorite, and chlorate, using either phenylarsine oxide or sodium thiosulfate as the titrant and adjusting the pH to differentiate among the various chlorine species. The quantification of the concentration of these analytes is performed by the following reactions of chlorine species with iodide ion along the pH value: Cl2 + 2 I- = I2 +2 ClpH: 7, 2, < 0.1 2 ClO2 +2 I- = I2 + 2 ClO2pH: 7 4 2015 © American Water Works Association AMTA/AWWA Membrane Technology Conference Proceedings All Rights Reserved

2 ClO2 +10 I- +8 H+ = 5 I2 +2 Cl- +4 H2O pH: 2 < 0.1 ClO2- +4 I- + 4 H+ = 2I2 +Cl- +2 H2O pH: 2 < 0.1 ClO3- +6 I- + 6 H+ = 3 I2 +Cl- +3 H2O pH < 0.1 Based on the equilibrium for reduction of the chlorine species by iodide is pH-dependent. The amperometric titration procedure was added to the Standard Methods for the Examination of Water and Waste Water 4500-ClO2 for the determination of chlorine dioxide, chlorite, and free chlorine in an aqueous solution. Amperometric tritration method (4500-ClO2 –E) distinguishes various chlorine compounds with good accuracy and precision. It can be applied to concentrated and diluted solutions but require specialized equipment and considerable analytical skill. The analyzer instrument HACH AutoCAT 9000™ makes a multi-step procedure involving four titrations at different pHs and using C6H5AsO (Phenyl Arsine Oxide, PAO) solution at 0.00564 N as the titrator, to determine the concentration, between 0.10 - 5.00 mg/L, of ClO2, ClO2- and free chlorine from two 200mL samples. These titrations are as follows:

Because the combining power of the titrants is pH-dependent, all calculations are based on the equivalents of reducing titrant required to react with equivalents of oxidant present. Using the Table 1 and following equations to obtain the concentration of each species. Table 1. Equivalent weights for the weight concentration calculation for chlorine species.

pH

Specie

7 2, 0.1 7,2, 0.1 2, 0.1

ClO2 ClO2 Cl2 ClO2-

Molecular Weight (mg/mol) 67,450 67,450 70,900 67,450

Electrons Transferred 1 5 2 4

Equivalent Weight (mg/eq) 67,450 13,490 35,450 16,863

[Cl2] en mg/L = [A - (B - D) / 4] · N (eq/L) · 35,450 (mg/eq) [ClO2] en mg/L = (5/4) · (B-D) · N (eq/L) · 13,490 (mg/eq) [ClO2-] en mg/L = D · N (eq/L) · 16,863 (mg/eq) Where, A, B, C and D are related on the volumes for each titrations 1, 2, 3 and 4, respectively. N is the normality of the titrator (0.00564 N) in equivalents per liter.


V.

RESULTS AND DISCUSSION

a. Seawater and Chlorine Dioxide Solution Characterization Previously at any test, the seawater, with no additives, employed was characterized. Temperature: pH: Conductivity: ORP: Turbidity:

27.0 ºC 7.81 42.2 mS/cm 106.9 mV 1.93 NTU

The amperometric determination was performed on the seawater with no additives with the result of Below the detectable limit (under 0.1 mg/L) for the selected chlorine species. Besides, every chlorine dioxide solution coming out of the onsite generator, and employed in the following tests, was analyzed for free chlorine, chlorine dioxide and chlorite ion concentration. [Cl2]: [ClO2]: [ClO2-]:

110 mg / L 833 mg / L 558 mg / L

The content of free chlorine in the solution is because the reaction carrier water and the dilution water after the generator outlet, have 4 mg/L of free chlorine. The chlorite ion content is related on reagents proportions mismatches of the onsite generator. b. ClO2 dosage testing Many tests were performed in order to identify the behavior of the chlorine dioxide in the seawater matrix presented in the previous paragraph and in artificial solutions of NaCl with distilled water. i. Seawater test Chlorine dioxide behavior was studied completing tests with 1000mL of seawater, adding 0.48mL of the chlorine dioxide solution, which represents a 0.40 mg/L of ClO2, and stirred for 30 minutes. Table 2. Results of adding 0,40 mg of ClO2/L to SW.

Parameter

Before the addition

pH ORP (mV) [Cl2] [ClO2] [ClO2-]

7.13 210.2 BDL BDL BDL

After the addition 7.16 609.4 0.053 mg/L 0.400 mg/L 0.267 mg/L

After 30 minutes of agitation at 40 rpm 7.22 270.7 BDL 0.141 mg/L 1.82 mg/L

(*) BDL: Below Detectable Limit


The free chlorine was consumed after 30 minutes, while part of the chlorine dioxide was decomposed into the ion chlorite. There is more concentration of chlorine compounds in the solution after 30 minutes. This increment is related on the formation of ClO2 and its DBPs at the expense of the chlorides present in seawater, by catalytic reactions. NaCl solutions, with distilled water, were prepared, to check the influence of the chloride concentration in the chlorine compounds studied. ii. NaCl and distilled water tests Same concentration of Chlorine Dioxide (0.40 mg/L) was added into solutions of 1,000, 5,000 and 10,000 mg/L of NaCl to perform the influence of the chloride ions on the formation of the ion chlorite. Chlorine compounds were tested before and after the samples were stirred at 40rpm for 30 minutes. Table 3. Initial and final concentration of the chlorinated compounds.

NaCl Solution mg/l

1,000

5,000

10,000

Parameter pH ORP (mV) [Cl2] [ClO2] [ClO2-] pH ORP (mV) [Cl2] [ClO2] [ClO2-] pH ORP (mV) [Cl2] [ClO2] [ClO2-]

Chlorine Dioxide Solution

After the addition

107.5 mg/L 169.5 mg/L 323 mg/L

7.01 660.0 0.253 mg/L 0.400 mg/L 0.761 mg/L 7.01 660.0 0.253 mg/L 0.400 mg/L 0.761 mg/L

-

7.24

BDL 333.5 mg/L 133.0 mg/L

BDL 0.400 mg/L 0.158 mg/L

107.5 mg/L 169.5 mg/L 323 mg/L -

After 30 minutes agitation at 40 rpm 7.03 678.6 0.251 mg/L 0.212 mg/L 0.840 mg/L 7.22 694.5 0.969 mg/L BDL 0.683 mg/L 7.24 675.0 BDL 0.479 mg/L BDL

*1,000 and 5,000 solutions used the same chlorine dioxide solution, but not the 10,000.

Figure 1. Chlorine species concentration in a 1,000 ppm NaCl solution.




The decomposition of ClO2- in an acid solution may produce ClO2. The decomposition rate is greatly influenced by experimental conditions, especially pH and the concentration of chlorides. The decomposition of sodium chlorite under acidic conditions was studied earlier by Taylor et al., 1940; White et al., 1942, wherein the authors suggested that chlorine dioxide is the main volatile product formed in a weakly acidic chlorite solution, pH < 5 and a ratio of chlorine dioxide formed to the chlorite consumed is 0.5. Sada et al., (1978) studied the absorption spectra of chlorine dioxide evolved by the reaction of chlorite and sulfuric acid. He considered chlorate and chloride as the by-products of chlorite decomposition along with chlorine dioxide as follows: (Bal Raj Deshwal et al., 2008) Stage 1. Cl- + ClO2-

2ClO - (The proportions determine the reaction)

Stage 2. ClO2- +ClO- +2H+ Stage 3. Cl2 O2 + ClO2-

H2O +Cl2O2 Cl- +2ClO2

8

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The recent studies (Adewuyi et al., 1999; Yang and Shaw, 1998; Gordan, 1982) proposed that ClO2- decomposition is catalyzed by the chlorides, which are evidently present in seawater. On the study presented has been observed that there is an effect on the formation of different chlorine species by the concentration of chlorides in water. Consequently, it is expected that in seawater, containing a high concentration of chlorides, it may be the appearance of different chlorine compounds with different distributions of concentration thereof. c. Bisulfite Ion effect. Sodium Bisulfite (NaHSO3) is the chemical compound most used in the water desalination facilities on reducing strong oxidants, mostly chlorine, before the polymeric membranes. For this reason, the effect of Sodium Bisulfite on the mentioned chlorine compounds formation has been studied, carrying several tests in a seawater matrix, adding 4.06mL of a new chlorine dioxide solution (0.40 mg/L of ClO2) and 10 mg/L of NaHSO3, with the following results. Table 4. Initial and final concentration of the chlorinated compound in a 10 mg/L of NaHSO3 in seawater.

Parameter pH ORP (mV) [Cl2] [ClO2] [ClO2-]

Chlorine Dioxide Solution 70.5 mg/L 98.5 mg/L 282.5 mg/L

Before the addition

After the addition

After 15 minutes of agitation at 40 rpm


7.16 609.4 0.286 mg/L 0.400 mg/L 1.14 mg/L

6.89 712.4 BDL 0.970 mg/L 1.272 mg/L

There is an increment on the concentration of the ClO2 and ClO2-. d. Copper Ion effect. The copper ion (Cu(II)) acts as a catalyst for many oxidation-reduction reactions. Cu(II) determinations were performed on the seawater giving a concentration average value of 20 µg/L of free copper. However, concentrations of Cu(II) ranging from 90 to 240 µg/L were found performing digestion tests with nitric acid at 50 degrees Celsius. This result indicates that Cu(II) is present in the seawater matrix tested, forming complexes with organic matter. Subsequently, measured amounts of Cu(II) were added into seawater samples, with no more additives. ClO2, ClO2- and Cl2 concentrations were measured after 15 minutes stirring at 40 rpm. Table 5. Concentration values after the addition of Cu (II) in seawater.

[Cu II] µg/L 22 66 66 111

pH 7,81 7,81 8,1 8,02

ORP mV 139,8 175,1 162,4 173,0

[ClO2] mg/L BDL BDL BDL BDL

[ClO2-] mg(L BDL BDL BDL BDL

[Cl2] mg/l BDL BDL BDL BDL

Even though the tests reveal that chlorine species are below the detectable limit (0.1 mg/L), it is possible to see a pinkish color by increasing the Cu(II) concentration, adding a DPD Free Chlorine pillow for 25mL sample. This may reveal the existence of oxidizing compounds, not 9 2015 © American Water Works Association AMTA/AWWA Membrane Technology Conference Proceedings All Rights Reserved

belonging to the family of ClO2, ClO2-, and Cl2, in the employed seawater. Likewise, it is possible, that the added Cu (II) has been combined producing oxidation reactions. 100 µg/L of Cu(II) were added to a 1000mL of seawater with 0.40 mg/L of ClO2 (4,06 mL of the chlorine dioxide solution). Table 6. Concentration values before and after the addition of the ClO2 and the Cu (II) in seawater.



Before the addition

After the addition

After 15 minutes agitation at 40 rpm


7.40 550.5 0.286 mg/L 0.400 mg/L 1.14 mg/L

7.31 738.0 0.330 mg/L BDL 2.81 mg/L

Increments on the concentration of ClO2- and Cl2 are obtained at expenses of the chlorine dioxide decomposition and by the chlorides oxidation catalytic reaction, helped by addition of Cu(II). e. Cu (II) ion effect with Sodium Bisulfite. The Cu(II) found in the seawater is complexed with organic matter content. Some of these complexes are attacked by chlorine dioxide, releasing the Cu(II). Free copper can act as a catalyst in many reactions, especially with sulfur compounds, such as the ion bisulfite (HSO3-) that can be oxidized into persulfate or peroxodisulfate anions. Stable ClO2 can be generated by the reaction of the ion chlorite with the sodium persulfate or peroxodisulfate (Patent EP 1787953 A2, Wilfred J. Hemker, Melissa A. Thompson, 2007). SO32- + Cu2+ SO3- + 1e-+ Cu2+ SO3- + 1e-+ O2 SO5-+ 1e2SO5 + 1e + SO3 SO52- + SO3- + 1eSO52- + ClClO- + SO42- (ClO-: Chlorine, Cl2)

(C.H. Barron et. at. 1966)

100 µg/L of Cu(II) were added to a 1000mL of seawater with 0.40 mg/L of ClO2 (4,06mL of the chlorine dioxide solution) and 10 mg/L of NaHSO3-. Table 7. Concentration values before and after the addition of the ClO2, NaHSO3-.and the Cu (II) in seawater.



Before the addition

After the addition



7.44 585.4 0.286 mg/L 0.400 mg/L 1.14 mg/L

7.14 752.2 BDL 1.01 mg/L 2.15 mg/L

ClO2- concentration increased by the oxidation of the chlorides. At the same time ClO2 was generated from the catalytic reaction between the ClO2- and the persulfates and peroxodisulfates, coming from the oxidation of the sodium bisulfite added. 10 2015 © American Water Works Association AMTA/AWWA Membrane Technology Conference Proceedings All Rights Reserved

f. Ion Chlorite removal tests. Fe(II) addition. High concentrations of ClO2- have been found in all the tests run with seawater. This ion comes from the initial chlorine dioxide solutions, from the decomposition of the same chlorine dioxide, or from catalytic reactions that involve the concentration of chlorides, bisulfite ions or metals ions, such as Cu(II). The Maximum Contaminant Level (MCL) for chlorite ion in drinking water is 1.0 mg/L by the U.S.EPA. Besides, the ion chlorite residual can reproduce chlorine dioxide and free chlorine, strong oxidants, which can cause irreversible damages in any polymeric membrane system. Based on laboratory testing and full plant use of chlorine dioxide, each ppm of chlorite can be reduced by 3.1 ppm of ferrous as Fe from ferrous chloride or ferrous sulfate solutions. (Rittmann, D. Ph.D., P.E. (2003)) Tests with ferrous sulfate (FeSO4) after the addition of the chlorine dioxide solution have been completed to prove the removal of the ClO2- ion by oxidizing the Fe(II) into Fe(III). The tests were carried in 1000mL seawater samples, and following the current coagulationflocculation conditions of the seawater desalination facility, adjusting the pH at 6.60 and stirring the solution in a Jar-Test for a total of 30 minutes (60 seconds at a G value of 190 s-1, 10 minutes at a G value of 33 s-1 and for 19 minutes at a G value of 25 s-1). The assumption of having 2.15 mg/L of the ion ClO2- after the stirring time, was made, in order to calculate the amount of Fe(II) to dose. 0.40 mg/L of chlorine dioxide (2.81 mL of a chlorine dioxide solution) was added and 6.6 mg of Fe (II) as FeSO4. After the 30 minutes the flocs were settled down. The sample to test the chlorine compounds was taken from the solution supernatant. Table 8. Concentration values before and after the addition of the ClO2 and the Ferrous Sulfate.

Parameter [Cl2] [ClO2] [ClO2-]


Before the addition

After the addition


BDL BDL BDL

0.086 mg/L 0.400 mg/L 0.729 mg/L

BDL BDL BDL

The ferrous ion is oxidized to the ferric ion by the all the chlorite ion (