Influence de la préoxydation sur la coagulation par le chlorure ferrique de la matière organique The effect of preoxidation on the rernoval of organic matter by coagulation with ferric chloride

Résumé

La coagulation par le Fe(III) de substances humiques aquatiques est optimale à pH = 5,5 pour un rapport massique Fe/COT de l'ordre de 2, quant à l'élimination du COT. Le but de cet article est d'étudier l'influence d'une préoxydation sur l'élimination de la matière organique par coagulation dans ces conditions.

La préoxydation (ozone, bioxyde de chlore ou chlore) à faibles doses induit une légère dégradation des rendements de coagulation des acides fulviques. Un taux de préoxydant fort (≈ 0,5 mg oxydant par mg de carbone organique initial), pratiqué avant la coagulation, implique de mettre en oeuvre une dose de coagulant plus élevée [≈ 3 mg Fe(III) par mg de COT]. Cette augmentation est moins marquée dans le cas d'une préchloration.

Les substances humiques ne représentant qu'une partie de la matière organique des eaux naturelles, des manipulations sur des eaux brutes ont donc été entreprises. L'effet de faibles doses d'ozone conduit à une légère amélioration de la coagulation. Cependant de fortes doses d'oxydant (0,5 mg d'ozone ou de bioxyde de chlore par mg COT) nécessitent l'emploi de plus fortes doses de coagulant. De plus, le rendement d'élimination est aussi affecté.

Mots-clés:

• Acides fulviques,
• ozone,
• chlore,
• bioxyde de chlore,
• coagulation,
• fer ferrique,
• eaux brutes

Abstract

We present here some works which take place in the particular frame of the study of the drinking water treatment of reservoir waters containing organics at high concentration, mainly humic substances. Previous studies (LEFEBVRE and LEGUBE, 1990) on coagulation of fulvic acid solutions have proven that the optimal removal of organic matter was reached at pH = 5.5 with 2 mg of ferric iron per mg of organic carbon. The main question in this study is to know the impact of preoxidation (ozone, chlorine dioxide, chlorine) on the removal of organics by iron(III) coagulation.

The reconstituted waters obtained by dissolving fulvic acids (table 1) in a solution of salts in high-purity water (table 2), are defined by both a low inorganic content (as calcium, sulfate and bicarbonate ions) and a high organics concentration, main characteristics of real impounded waters (table 3). The experimental procedures of coagulation-flocculation-clarification followed the treatment lines shown in figure 1. Ozonation was carried out in a semi-continuous system (batch solution), by bubbling ozone (fig. 2). Chlorination and preoxidation with chlorine dioxide were performed in a batch reactor. The preoxidation steps were carried out at pH of reconstituted waters or of raw waters. A Dohrmann DC 80 was used for the determination of TOC. The accuracy of TOC measurement was found to be ± 0.05 mg/l C (for the range of studied concentration). Residual iron was analysed by atomic absorption on a Perkin Elmer 2380 apparatus (oxy-acetylene flame).

As shown in table 4, preozonation induced a slight decrease of Cebron fulvic acid removal by iron(III) coagulation at high ozone dose applied (= 0.5 mg O₃ per mg TOCi). In order to try to understand why ozone partially inhibits the efficiency of iron coagulation to remove fulvic acid at acidic pH, we determined the optimum dosage of coagulant required to obtain the best percent of removal on preozonated fulvic acid solutions. Figure 4 shows that preozonation at about 0.5 mg O₃ per mg TOCi appears to have shifted the region of the optimal TOC removal towards the higher Fe/TOCi mass ratios (≈ 3 mg Fe(III) per mg TOCi, for two fulvic acids).

Prechlorination (> 1 mg Cl₂/mg TOCi) induced a decrease of Cebron fulvic acid removal by iron(III) coagulation (table 5). Hence, we examined the optimum dosage of coagulant required to obtain the best percent of removal on prechlorinated fulvic acid solutions (fig. 6). The region of the optimal TOC removal was still obtained for a Fe/TOCi mass ratio of 2. However, the difference between the percent TOC remaining for inital fulvic acid of 10 and 15 mg/l was not significant (respectively 61.4 % and 61.9 %). A greater prechlorinatlon dosage (> 0.5 mg Cl2/mg TOCi) would probably give a shift of the optimal mass ratio.

Preoxidation with chlorine dioxide (even without ClO₂ residual) before a coagulation induced a decrease at TOC (table 4). So, we studied the shift of optimum dosage of coagulant required to remove the organic matter. Figure 5 shows that preoxidation (≈ 0.5 mg ClO₂/mg TOCi) shifted the region of optimal TOC removal towards higher Fe/TOCi mass ratio (≈ 3 mg per mg for Cebron fulvic acid, in figure 5).

The fulvic acids represent only an amount of TOC of a raw waters. So we carried out experiments on real impounded waters. For the study of the influence of the preozonation on the coagulation efficiency, we chose to work at the optimal coagulant dose determined from previous experiments. Several runs were conducted with different dosages of ozone between 0 and 2 mg/l O₃. Results reported in tables 6 and 7 indicate that for these two raw waters (Moulin Papon and Cebron), the preozonation slightly improved the efficiency of iron coagulation at acidic pH. These results can only be compared with Cebron fulvic acid when applied ozone dose was 0.2 mg O₃/mg TOCi.

Consequently, other experiments were carried out at a high ozone dose (- 0.5 mg O₃/mg TOCi). They showed a shift in the region of the optimal TOC removal towards the higher coagulant dose (fig. 7), as already observed with the fulvic acid solutions.

Only one experiment was made with chlorine dioxide in the case of Moulin Papou raw water. This preoxidation (≈ 0.5 mg ClO₂/mg TOCi) induced a shift to higher coagulant dosage but also a decrease of the efficiency of iron coagulation whatever the applied coagulant dosage (fig. 8).

As compared to humic chlorination literature, little information exists concerning ozonation of humics. Furthermore, in spite of small amounts of identified by-products in the literature, ozonation was usually found to increase smaller size materials in humic substances (GLOOR et al., 1981; VEENSTRA et al., 1983; FLOGSTAD and ODEGAARD, 1985; ANDERSON et al., 1986; AMY et al., 1987; LEGUBE et al., 1989). Moreover, analyses of carboxyl-group on the Cebron fuivic acid allowed us to show that the carboxyl content increased as ozone dosage increased (fig. 9).

Some works (VAN BREEMEN et al., 1979) have proven the rate of carboxyl groups in the stoichiometry of coagulation reaction of humics with iron(III) at slightly acidic pH. Consequently, it is not surprising that preozonation of fulvic acids appears to have shifted the region of TOC removal up into the higher iron dose range, according to the enhancement of the carboxyl content in the fulvic acids by ozone. This effect of preozonation was already reported by others (JEKEL, 1983; RECKHOW and SINGER, 1983).

Chlorination (GLAZE and PEYTON, 1978) or oxidation with chlorine dioxide (NORWOOD et al., 1983; COLCLOUGH Of al., 1983) led also to a decrease of the molecular weight of aquatic organic matter. The oxidative action of chlorine and chlorine dioxide gave many products like aromatic and aliphatic acids (chlorinated or not chlorinated) and probably increased the carboxyl content of humic substances. Hence, those oxidants inhibit the coagulation-flocculation of organic matter.

Keywords:

• Fulvic acids,
• ozone,
• chlorine,
• chlorine dioxide,
• coagulation,
• ferric iron,
• taw waters