Assessment of Sodium Silicofluoride as a Fluoride Source in Drinking Water Systems

Fluoridation is a process of adding fluoride to the treated water source as the fluoride present in the drinking water is beneficial in preventing dental and bone caries, especially for children.  The range for fluoride dosage concentration in drinking water systems is different from one country to another. In Malaysia, the allowable range of fluoride has been gazetted at around 0.4 to 0.6 mg/L. The source of fluoride from sodium silicofluoride (SSF) has been reportedly used due to its advantages compared to the other sources. In this work, the solubility of SSF as the fluoride source dosed at a dedicated Water Treatment Plant (WTP) was analyzed and different fluoride dosage range was evaluated from the WTP. This evaluation was done to ensure its single distribution system contains sufficient fluoride levels as recommended by the national drinking water standard. The solubility of SSF from different sources namely SSF1, SSF2, and SSF3 were studied at a different range of concentration using the gang jar test. Fluoride levels in the drinking water system were evaluated by taking the water sample from 7 sampling points, where fluoride dosages were grouped at three different ranges of fluoride concentration at the fluoridation dosing tank; (i) low (4.6 g/L), (ii) medium (7.1 g/L), (iii) high (11.8 g/L). Results show that at a high range of fluoride dosage, 10.4% of insoluble substance was recovered from the dilution of SSF3, which amount was found much lower than from SSF1 and SSF2. This finding indicates that SS3 is considered the best quality fluoridating agent compared to the other two sources to be applied in the fluoridation system. Spectroscopic analysis performed for the fluoride water samples shows that a high concentration of fluoride is required to obtain the concentration of fluoride at the allowable limit.  Meanwhile, the fluoride preparation at the dosing tank of medium range of fluoride concentration shows an average fluoride concentration level of 0.60 mg/L at all the seven (7) sampling points specified along the studied distribution system. Although the fluoride level is supposed to remain the same along the distribution system, the presence of various minerals and metals in the treated water may have caused fluoride concentration to reduce due to their reactions of them to form metal precipitates and/or complexes. Therefore, the dosage of SSF for fluoridation at WTP is recommended at the medium of-range fluoride concentration in order to obtain desired concentration at the distribution system within the allowable limit.