By Nick Pizzi1
A report published by J. M. Dunning in 1962 showed the relationship of elevated fluoride levels in children to the incidence of dental caries1. This data, and data from subsequent reports shows a minimum incidence of caries when water fluoride levels were held continuously at about 1.0 mg/L. Because of this fact, public water supplies are usually fluoridated to optimum levels. In 1985, the center for disease control stated that 130 million Americans, or 61% of those served by public water supplies, consume fluoridated water. In 1968, this figure was about 80 million.
Dental caries, the most prevalent chronic disease of human beings, is most widespread in children. Tooth decay reaches a peak in adolescence, and diminishes in adulthood. It is for this reason that fluoridated water has the greatest effect on the teeth during the formative years. Fluoride is absorbed on the teeth and bones. The softer tissues of the body do not seem to retain fluoride. When teeth are forming, an adequate source of fluoride will help to strengthen the enamel, forming a more impervious layer to the bacteria which cause decay. Caution is needed, though, in that increased levels of fluoride above the optimum can cause mottling of teeth from dental fluorosis. This condition creates brittleness in the teeth, and moderate to severe discoloration. Water purveyors must take every precaution to control the levels of fluoride ion concentration in their supply. The USEPA has set a maximum contaminant level (MCL) for fluoride at 4 mg/L to prevent crippling skeletal fluorosis.
A secondary MCL of 2 mg/L was established by USEPA to prevent dental fluorosis. These limits are reviewed every three years. It is generally agreed that a fluoride level of 0.8 to 1.3 mg/L Fluoride is most effective, dependent on temperature. (In hotter regions, people tend to drink more water, and therefore will retain more fluoride.)
Chemicals Used in Fluoridation
Fluoride is the negative ionic form of Fluorine, a gaseous element. Very abundant in nature, fluorine can be found in all soils, water supplies, plants and animals in varying amounts. Fluorine is a reactive element and therefore, it is always found in combination form, never in the free state. Fluoride-containing minerals are used by industry for fluoride compound production, the most prevalent being fluorspar, cryolite and apatite. From these minerals we get the three most common fluoride adjustment compounds: Sodium Fluoride (NaF); hydrofluosilicic acid (H2SiF6); and Sodium Silicofluoride (Na2SiF6). These three compounds are by-products of the phosphoric acid fertilizer industry, and the availability of them is dictated by agricultural production. Sales of fertilizer will have a direct effect on the manufacture and availability of fluorides for water treatment.
Sodium Fluoride (NaF) is a white, odorless powder or crystal. It has a specific gravity of 2.79, a molecular weight of 42 (Sodium = 23, Fluorine = 19), and a solubility of 4 grams/100 mls, or 4% in water. When added to water, the NaF will dissociate into Sodium and the needed Fluoride ions. NaF can be purchased in quantities which are very pure, usually near 97 to 98%. A dosage of 19 lbs. NaF into 1 million gallons of Fluoride free water will produce a 1 mg/L F- solution. The percent of available Fluoride in NaF is 45% (1942).
Calculation for an estimated dosage of 1mg/L F- when using NaF (97% purity):
19 lbs / MG NaF x (1 mg/L / 8.34 lbs/MG) x 0.97 purity = 1.0 mg/L F-
Sodium Silicofluoride (Na2SiF6) is a white odorless crystalline material. It has a specific gravity of 2.679, a molecular weight of 188 (2 Sodiums = 46, S1 = 28, 6 Fluorines = 114), and a solubility ranging from 0.44 to about 2.5%. Na2SiF6 can be purchased in purities of 98% or higher. A dosage of 14 lbs Na2SiF6 into 1 million gallons of Fluoride free water will produce a 1 mg/l F- solution. The percent available Fluoride in Na2SiF6 is about 61% (114 188).
Calculation for an estimated dosage of 1 mg/L F- when using Na2SiF6 (98% purity):
14 lbs / MG Na2SiF6 x (1 mg/L /8.34 lbs/MG) x .61 available F- x 0.98 purity = 1.0 mg/L F-
Hydrofluosilicic Acid (H2SiF6) is a straw or amber colored, transparent, fuming, corrosive liquid. It has a molecular weight of about 144 (2 Hydrogen = 2, Silicon = 28, 6 Fluorines = 114), and can be purchased as a liquid in drums or bulk at 23 to 35 percent. A dosage of 46 lbs of 23% (H2SiF6) into 1 Million gallons of Fluoride free water will produce a 1 mg/L F- solution. The percent of available Fluoride in (H2SiF6) is about 79% (114 144).
Calculation for an estimated dosage of 1 mg/L F- when using (H2SiF6) (23% purity):
46 lbs (H2SiF6) x 1 mg/L x .79 available F- x .23 Purity 8.34 lb/MG = 1.0 mg/L –
It can be noted that water has no “fluoride demand”, and therefore a contact period such as for chlorine is not needed. Some fluoride ion will, however, be lost in filtration (usually 0.1 to 0.3 mg/L), so the optimum feed point for Fluoride is in the post filtration scheme. It is prudent to perform daily checks of the Fluoride concentration on plant tap water, and to perform hourly checks of the feed system while in operation.
There are 3 accepted methods for feeding supplemental fluoride to the supply:
1. The Fluoride saturator
2. Dry Chemical Feeder
3. Chemical solution feeder, using either (H2SiF6) acid or a solution made from the dry chemicals.
The selection of feed equipment is based on many factors, including size of plant, availability and type of fluoride source, cost, and technical expertise of personnel. The larger facilities seem to prefer the liquid storage and feed systems. The operator is directed to the publication Water Fluoridation Principles and Practices, AWWA M4 by the American Water Works Association.
Fluoride chemicals are added to water as liquids, but may be measured as either liquid or solid. Solid chemicals are dissolved into solution before feeding. If so, the strength of the solution must be controlled, usually by feeding a predetermined chemical for a specific duration. Two types of dry-feed machines exist: the volumetric and the gravimetric. The volumetric delivers a predetermined volume of dry chemical within a given time, and the gravimetric delivers a predetermined weight of chemical within a given time, and is more accurate.
Fluoride Removal with Lime
Defluoridation, or removal of excess fluoride is necessary where the source of supply contains levels which are detrimental to the health and welfare of the public. Fluoride can be removed in the softening process where waters are high in magnesium content. Lime will precipitate Magnesium compounds at high pH levels. At these levels, magnesium fluoride precipitates just as does magnesium hydroxide. The sludge is settled in the sedimentation basins and discarded. This discovery was published by R. D. Scott in 1937 in the Journal AWWA in his work titles “Fluoride in Ohio Water Supplies”.
Ion exchange and adsorption on bone char and activated alumina are other methods used in the removal of fluorides.
Activate alumina is a porous inorganic adsorbent similar to the ion-exchange resins. It works well because the fluoride ion is “preferred” by the alumina over most other ions. This process is finding increased use in the home as a point of entry (POE) or point of use (POU) device.
Fluoride dusts from such chemicals as NaF and Na2SiF6 can expose operators to higher than optimal levels of fluoride, and can cause irritation of the nasal and mucous membranes. In addition, fluoride acids can burn the skin. The following precautions are advisable when handling these chemicals:
1. Avoid breathing fluoride dusts. Dust masks should be issued to all those who work with the fluoride dusts.
2. Wear rubber gloves and aprons when handling dusts.
3. Wash away any dusts from the skin immediately.
4. When handling hydrofluosilicic acid, one should wear acid proof gloves, aprons and face shields. Wash these items after use.
5. Do not allow persons with open cuts or sores handle any fluoride acids or dusts.
6. Be sure to post safety rules in the vicinity of the chemical storage and handling area.
7. Fluoride containers should be labeled as poisonous, and should never be reused for other purposes.
8. If accidental ingestion occurs, give the operator a 1% solution of calcium chloride, a glass of saturated lime (Ca(OH)2) water, or milk to drink. Flush the chemical out of the eyes with warm water. Get medical attention immediately.
Sample Dosage Scenario
A water plant treats 7.21 MGD, and wishes to keep a Fluoride ion level of 1.1 mg/L in the plant effluent. If the raw water source contains 0.36 mg/L natural Fluoride ion, how many gallons of 24.3% H2SiF6 will be needed in the month of September? Acid is 10.5 lb/gallon.
Answer: H2SiF6 has 79% available Fluoride ion
1.1 mg/L Desired – 0.36 mg/L available = 0.74 mg/L F- needed dosage
0.74 mg/L F- x (8.34 lb/MG F- / 1 mg/L) x 7.21 MGD x (30 Days / month) = 1334.92 lb F-/month
1334.92 lb F-/month (.243 x .79) = 6953.8 lb H2SiF6/month
(6953.8 lb H2SiF6/month) x (1 gal/10.5 lb) = 662.26 gal/month
1Nick Pizzi, author of the 12th edition of Hoover’s Water Supply and Treatment.
2Principles of Dental Public Health, Harvard University Press, 1962.