Technical Operations Engineering:
Additives
After completing this training, you should be able to:You will often hear fluoride referred to as a "chemical." But the correct reference to fluoride in a water treatment facility is "additive." "Chemicals" are used in the laboratory, and "additives" are used in water treatment. These are the correct regulatory designations. However, many people continue to refer to "additives" as "chemicals." It’s best to use the correct “additive” reference for fluoride to be consistent with regulatory practice.
For water treatment, the correct designation for fluoride is ‘additive.’
Fluorine in its reduced stable ionic form is fluoride. This is what we use for water fluoridation. Fluorine is an element and is a gaseous halogen. The reactive free state of fluorine is transient in nature since it is an oxidant and readily combines with many elements. It is a pale yellow, noxious gas.
Fluorine
Fluorine is the thirteenth most abundant element in the earth’s crust. The ranking itself is not important, but it is important to remember that it is fairly common and present in all soils, plants, animals, and natural waters. It is the most electronegative element, which means that many of the fluoride compounds will be strongly bonded and quite stable.
Fluorine is present in all soils, plants, animals, and natural waters.
Precise estimates of atmospheric releases of fluoride are difficult to quantify, but scientists estimate that volcanic releases represent more than half of all atmospheric releases of fluoride, and are greater than other natural sources and anthropologic or man-made releases combined. Forest fires and wind driven erosion are the next most prevalent sources of fluoride. They represent over a quarter of all fluoride present in the atmosphere. Anthropologic releases such as coal burning for electrical energy and other minor industrial releases are less than a quarter of all fluoride sources.
Most atmospheric releases are from volcanos.
When coal is burned, natural sources and anthropologic sources are released along with small amounts of industrial emissions. These atmospheric releases eventually degrade to hydrogen fluoride and are incorporated into rain droplets. The combined atmospheric releases result in rainwater containing fluoride at approximately 0.1 mg/L.
Rainwater contains fluoride.
Groundwater can contain elevated levels of fluoride from mineral leaching from rock ores. Another geological source is faulting that extends deep underground providing a pathway for water with fluoride. As this water emerges, it can cause re-mineralization in geologic layers or mix with surface influenced groundwater. Deep faults can also provide an explanation for different fluoride content of adjacent wells. One well may be closer to a deep plutonic fault with high-fluoride water while another well may be closer to low-fluoride surface water. Sometimes it may be possible to drill another well up-gradient to obtain a lower fluoride content ground water.
Hydrochemical cycle of fluorine (Source: Pauline Smedley, British Geological Survey)
Fluoride is contained in many types of rocks. It is commonly found as a fluorosilicate in granites and sedimentary rocks, and also as a calcium fluoride. In terms of valuable ore, it can be found in minerals such as fluorspar, cryolite, and apatite. Fluorspar, a calcium fluoride is also known as fluorite, was the original source for many fluoride products and is still used for manufacturing hydrogen fluoride. Fluorite mines are now largely exhausted in the U.S. However, it is still mined in other countries and imported to the U.S. Since the early 1950’s, apatite has been the principle source of water fluoridation additives in the U.S.
Fluorspar, Cryolite, and Apatite
The principal raw material for fluoride additive production in the U.S. is the mineral apatite. Apatite is a mixture of calcium and phosphate compounds with a fluoride content of 4–7% including calcium phosphate, calcium carbonates, calcium fluorides, and fluorosilicates.
Apatite
Apatite contains 3 to 7% fluoride depending on the ore deposit. Florida, North Carolina, the Gulf Coast area, Idaho, and Mexico are all sources for apatite. China and Morocco also have large apatite quarrying.
The U.S. is the largest source for apatite.
Calcium fluoride deposits were the historical source for fluoride products before the fertilizer industry. Afterwards, apatite became the principal source. This photo shows milled apatite which is used in the manufacture of phosphate fertilizers. Shown next to it is calcium fluoride, also known as fluorite or fluorspar.
Calcium fluoride (left) and milled apatite (right)
Let’s look at the actual additives we use in water fluoridation. Theoretically, any compound that forms fluoride ions in a water solution can be used to adjust the fluoride concentration in drinking water.
Water fluoridation additives
Only three compounds are approved for use in the United States. They include sodium fluoride, sodium fluorosilicate, and fluorosilicic acid. Other fluoride products have been considered in the past as possible additives but have not been used and no standards exist for their use in drinking water facilities.
Sodium fluoride, Sodium fluorosilicate, and Fluorosilicic acid
Fluorosilicic acid (FSA) production is represented in this flowchart. Apatite is refluxed (mixed and heated) with sulfuric acid resulting in a phosphoric acid-gypsum slurry. This is the starting point for making pelletized phosphate fertilizers. The gypsum, calcium sulfate, produced from this process is the material used to manufacture drywall or sheetrock. This process also releases silica tetrafluoride and hydrogen fluoride, which are dissolved gases in the gypsum slurry. These gases are vacuum extracted along with the phosphoric acid gas. The gases are separated, and the fluoride gases are condensed into the FSA mixture used for water fluoridation.
FSA production
The silica tetrafluoride gas that otherwise would be dissolved in the gypsum slurry or carried in the phosphoric acid gas stream are recovered by gas separation and condensed. The recovered fluorosilicic acid (FSA) from this process is high purity. Some sources may tell you that FSA is a hazardous pollutant that cannot be released to the environment, but that is not true since it is either dissolved in the slurry or remains in the phosphoric acid gas. As a result, there are no regulatory requirements to capture and dispose.
Less than 5 percent of the fluorosilicic acid (FSA) used for water fluoridation is from the production of anhydrous hydrogen fluoride. Sulfuric acid added to calcium fluoride (fluorspar) releases hydrogen fluoride along with a small quantity of silica tetrafluoride gas. These gases are separated in gas separation columns as the hydrogen fluoride and silica tetrafluoride gases condense at different temperatures.
A small amount of FSA, less than 1 percent of the U.S. production, is derived from hydrogen fluoride etching of fused quartz products. The acid derived from this route is high purity.
Fluorosilicic acid (FSA) is a colorless to colored, transparent liquid. It is a fuming corrosive acid with a pungent odor. It has an irritating action on the skin. The FSA mixture includes water and free acids. The largest faction, three-quarters of the total, is water. Fluorosilicic acid is one-quarter of the mixture.
FSA contains water and free acids.
Fluorosilicic acid is not actually H2SiF6 but is actually a mixture of various polymers and oligomers compounds with an average molecular weight of H2SiF6.
If an acid has a solution concentration of 25%, what is the remaining 75%? It’s water. Because the acid contains a large quantity of water, shipping is a major component of its cost.
Shipping is a large portion of the cost for FSA.
Fluorosilicic acid (FSA) has a solution pH of 1.2 and is corrosive. Care is needed when handling it. It is the most commonly used fluoride additive. It is commercially available in solution concentrations ranging from 20% to 35% with a typical solution concentration of 23% to 25%. The density of a 25% acid is 10.1 pounds per gallon, which is more than the 8.34 pounds per gallon that water weighs. The chart shows how the density of the FSA increases as the concentration of the acid increases. Since other constituents in the water such as phosphoric acid, sulfuric acid, and other acids can affect density, it is important to use the AWWA Fluorosilicic Acid Standard method of hydrogen titration. That method converts fluorosilicates to potassium silicates, and then measures the quantity of sodium hydroxide required to neutralize the resulting hydrogen in the solution.
FSA density increases as the concentration increases.
Hydrogen fluoride (HF) is volatile, with a boiling point just below room temperature. HF gas will leave the solution if it can reach the surface. HF has an ionic dissociation relationship with SiF4. Less than 1% of FSA as delivered is in the form of free acids. This free acid fraction includes hydrofluoric acid (also known as hydrogen fluoride) along with silica tetrafluoride gas, phosphoric acid, sulfuric acid, hydrochloric acid, and other trace acids. HF is the portion of the mixture that produces the corrosive attack on materials and electronic devices.
Hydrogen fluoride causes corrosion.
Hydrogen fluoride (HF) release is slow, and this makes it possible to store the fluorosilicic acid (FSA) in tanks for long periods. When the concentration of fluoride related substances is greater than 10% of the mixture, fluorosilicic acid predominates. Consequently, as the concentration is diluted, the potential exists for silica precipitation as the fluorosilicates transition to hydrogen fluoride releasing the bound silica.
FSA can be stored in tanks for long periods.
Chemists disagree on the details of the dissociation of fluoride in drinking water. However, all agree on that only trace quantities of fluoride persist in water treated with fluoride in low concentrations. The two most plausible models based on evidence are Busey’s and Ciavatta’s models.
Busey’s and Ciavatta’s models
Each of these models uses a concentration of 1 mg of fluoride per liter of water. In Busey’s model, the maximum fluorosilicate concentration would be ten-to-the-fifteenth. In Ciavatta’s model, the maximum fluorosilicate concentration would be ten-to-the-tenth. These are much less than a part per billion and even lesser than a part per trillion.
Some fluorosilicic acid (FSA) is called "water-white" when it appears virtually colorless. However, most FSA has some coloration that results from other impurities. LCI, a distributor of FSA, conducted testing in 2007 and found an increase in the American Public Health Association (APHA) color units (measured as Pt-Co) in FSA due to increases in phosphoric acid concentration and iodine concentration. Fluorosilicic acid (FSA) used in water fluoridation has an effective dilution of more than 250,000 times (to get 0.7 mg/L of fluoride in finished water). At this level of dilution, the color in finished drinking water is below detection level.
FSA color variations
Sodium fluorosilicate, previously called sodium silicofluoride, is widely used for water fluoridation. It is a white, odorless, crystalline powder. Sodium fluorosilicate is produced by partially neutralizing fluorosilicic acid (FSA) with sodium carbonate or sodium chloride. During the process, the solution precipitates sodium fluorosilicate salt crystals. Heavy metals remain in the residual waste acidic stream and not with the precipitated sodium fluorosilicate.
Sodium fluorosilicate
Unlike sodium fluoride, which has a constant solubility, sodium fluorosilicate has a solubility that varies with temperature. This requires an extended effort over several hours to correctly dissolve the additive to saturation. When using sodium fluorosilicate, it is important to ensure the crystals dissolve before adding it to the water processing. Since there is normally a 10 minute or less period to dissolve crystals prior to addition to the water flow, this results in the need to prepare a dilute unsaturated solution of less than 50% saturation.
Sodium fluorosilicate crystals take time to fully dissolve to saturation.
Sodium fluoride was the first compound used for water fluoridation, and its physiological and health effects have been thoroughly studied. Sodium fluoride is a white odorless salt. It is the ideal additive for use with saturators for small systems because of its relatively constant solubility. It also dissolves readily and can achieve a saturated solution within 5 minutes.
Sodium fluoride
Sodium fluoride is most commonly produced by neutralizing fluorosilicic acid (FSA) with caustic soda (NaOH), but can also be produced by neutralizing hydrogen fluoride with caustic soda. It is the most expensive of the fluoride additives because of the high cost and large amount of caustic soda required to produce it. As a result of the high cost of production, sources are currently located outside the United States.
Sodium fluoride is the most expensive additive.
Water typically has both carbonate buffering mixed with silicate (sand) buffering. Fluorosilicates consumes a portion of the solution by weight. But considering the low concentration of fluorosilicate relative to other alkalinity consuming anions, the relative amount of alkalinity consumed by fluoride is quite small. Chlorine and the coagulants are typically applied at a dosage 10 to 50 times the dosage of fluoride. Sodium fluoride does not consume alkalinity buffering.
Fluoride is not a significant factor for alkalinity consumption. If fluoride has addition of 0.6 mg/L, the amount of buffer required is less than a mg/L. For some other water additives including chlorine, ferric chloride or ferric sulfate, or aluminum sulfate, the addition can be from more than 5 mg/L up to over 50 mg/L, so the amount of buffering required may be 10 to 100 times greater than that consumed by fluoride.
Usage trends of the different fluoride products shows a reduction in dry additive products with an increase in liquid acid. This is consistent with other water treatment products as "ready-made" or "pre-mixed" liquid feed in place of dry products needed to be prepared on-site for use. Sodium fluoride in saturators at small installations has only seen a small drop in usage as the acid feed may be inappropriate for some small service locations, but sodium fluorosilicate has experienced reductions in usage as many water systems desire use of liquid solution feed in place of dry salts that need handling and preparation.
| Population | Percent of population |
Systems | Percent of systems |
|
|---|---|---|---|---|
| 1993 | ||||
| Fluorosilicic acid | 80,019,175 | 63% | 5,876 | 59% |
| Sodium fluorosilicate | 36,084,896 | 28% | 1,635 | 16% |
| Sodium fluoride | 11,701,979 | 9% | 2,491 | 25% |
| 2010 | ||||
| Fluorosilicic acid | 152,501,133 | 81% | 9,125 | 75% |
| Sodium fluorosilicate | 24,224,615 | 13% | 1,208 | 10% |
| Sodium fluoride | 12,815,339 | 7% | 1,825 | 15% |
There are standards you should be aware of for fluoride additives. The American Water Works Association (AWWA) specifications cover the fluoride additives product quality as purchased. There is a separate standard for each additive, and all facilities should have the AWWA standard for the additive they use to adjust fluoride. NSF International/American National Standards Institute (NSF/ANSI) standard covers product quality for impurities and product integrity in distribution.
Fluoride Additives Standards
NSF International/American National Standards Institute (NSF/ANSI) standard 60 replaced the former Environmental Protection Agency (EPA) Water Additives Program in response to a request by the EPA in 1984. This request was for requirements for distribution and purity of products added during water treatment, thereby ensuring the public’s protection. It was developed by a consortium of associations, including NSF International, American Water Works (AWWA), ANSI, the Association of State Drinking Water Administrators, and the Conference of State Health and Environmental Managers.
A key concept of NSF/ANSI standard 60 is that each additive should not add more than the single product allowable concentration (SPAC), which is based on 10% of the Environmental Protection Agency (EPA) maximum contaminant level (MCL). As of 2012, 47 state drinking water programs require that water treatment additives have Standard 60 certification. Both NSF and Underwriters Laboratories (UL) operate programs to certify products in accordance with Standard 60.
Fluoride Additives Standards
The Food and Drug Administration (FDA) does not regulate additives to drinking water as its regulatory purview concerns only food, drugs, and cosmetic-related products. There is a 1979 Memorandum of Agreement between the Environmental Protection Agency (EPA) and FDA that gives the EPA authority for drinking water and the FDA authority for beverages. Commercially bottled water is considered a beverage, but potable drinking water is not considered a beverage.
Food and Drug Administration, Environmental Protection Agency
AWWA specifications control the quality and manufacturing of fluoride additives. Although you can and should require a supplier to provide a certified copy of these tests, facilities can run the tests themselves on each batch delivered. If you suspect the additive delivery may not conform to the specifications, then you should conduct the tests on the additive yourself and verify that the additive meets the specified criteria. The hydrogen titration for fluoride content of fluorosilicic acid is a simple procedure that is quick and allows measurement of fluoride content and can also measure other free acids.
Verification
In addition to AWWA/NSF Standard 60 water treatment additive grade, sodium fluoride can also be purchased in other grades for other uses. These other grades are not permitted to be used for drinking water adjustment of fluoride. They include:
Following is a quick assessment of the various grades.
American Water Works Association, National Sanitation Foundation, American Chemical Society, United States Pharmacopeia
47 states mandate the use of National Sanitation Foundation (NSF) certified product. Only NSF standard 60 certified products should be used for water fluoridation in drinking water. The product specification is based on what is necessary to ensure a quality product protecting consumers and meeting all EPA quality standards for drinking water.
National Sanitation Foundation
American Chemical Society (ACS) reagent grade, a high-purity sodium fluoride, is re-dissolved and then re-crystallized thereby increasing the purity for reference verification. It is suitable for use in a laboratory but does not meet verification testing requirements for use in drinking water.
American Chemical Society
United States Pharmacopeia (USP), used in pharmaceuticals, provides less protection for consumers since the testing is for non-specific heavy metals. It has no criteria on arsenic or radiological exposure. If used in water fluoridation, there is no verification testing to ensure consumer protection and impurities could potentially exceed Standard 60 grade product requirements.
United States Pharmacopeia
Some operators have speculated that water fluoridation will corrode pipes since corrosion is caused by the release of hydrogen fluoride gas from storage tanks. Hydrogen fluoride boils at slightly above standard room temperature so the characteristic smell of fluorosilicic acid is predominately hydrogen fluoride. The release rate is very small for only the hydrogen fluoride at the surface of the liquid is able to evaporate and this allows long-term storage of up to several month in a storage tank. Environmental Protection Agency (EPA) and University of Michigan (Ann Arbor) researchers have proven that at the temperatures and concentrations used for water fluoridation, fluorosilicic acid (FSA) achieves virtually complete dissociation to the product ions of fluoride, hydrogen, and silica (sand). The hydrogen fluoride release by this dissociation is neutralized by hardness and other ions in the water and becomes a stable fluoride ion. So drinking water that is fluoridated is not corrosive from the fluoride ion.
A common misconception is that fluoridation corrodes pipes.
Fluorosilicic acid (FSA) can be delivered in different sized containers. These include 20,000 gallon rail cars, 4,000 to 6,000 gallon truck tankers, 300 to 400 gallon tote tanks, 55 gallon drums, 30 gallon carboys, and even 13 gallon carboys.
Rail car delivery of FSA
Mini-bulk delivery is a recent development. The Department of Transportation (DOT) has begun recommending mini-bulk (with less than 450 gallons in a tank) as a better alternative than tote tanks for corrosive Class 8 transport. An individual authorization letter must be obtained in accordance with DOT SP 12412 using DOT specification 57 portable tanks.
DOT recommends mini-bulk containers.
Mini-bulk tanks must have testing and inspection and other hazardous materials (hazmat) requirements must be met including a positive delivery system and the ability to purge the hose and pump for transport. More information can be found in the Federal Register (Vol. 65, no. 113, Monday, June 12, 2000, page 36882).
Mini-bulk tank requirements must be met.
The preferred storage location for fluorosilicic acid (FSA) is inside a building or under cover to protect from heating by the sun. FSA can freeze at approximately 4°F. In moderate climates the tank can be outside if it is insulated and heat-wrapped, but is more commonly inside a building or under a roof structure in milder climates.
Preferred storage for FSA is inside or under cover.
Because fluorosilicic acid (FSA) is liquid it is important to have spill containment for 110% volume (double-walled tank or barrier). If concrete is used, a protective coating should be applied to minimize acid attack on the concrete in the event of a spill. Check with your coating supplier, but normally an epoxy undercoating with a top coat of urethane will be satisfactory.
Concrete used for spill containment should have a protective coating. Containment should be for 110% volume.
Be sure that you use standard operating procedures (SOPs) for filling and withdrawing from the storage tank to ensure that operators are careful and do the job right. Items in an SOP would include locks on valves and connections to prevent incorrect or unauthorized transfers, visual checks of pipes and connections, recording of volumes transferred, verification testing and certificates, and other potential issues.
Figure 40: Standard operating procedures
FSA will produce a small quantity of hydrogen fluoride gas, which is corrosive and will etch glass, attack concrete, damage electrical circuits, and stain many things upon contact. Keep fluorosilicic acid (FSA) storage containers sealed with air vented to the outside so the hydrogen fluoride gas is not released inside the storage room. Terminate the vent outside the building with an inverted “U” and an insect/bird screen.
Vent air to the outside.
Venting is an important consideration when setting up a day tank. A properly sized tank with sufficient wall thickness should be used. 55 gallon drums have a wall thickness that is too thin. The top photo shows a drum venting with a flexible hose. The middle photo is a tank not correctly vented to the outside. The bottom photograph demonstrates corrosion in the vicinity of an incorrectly vented tank. Ensure that a tank is sealed and vented to the outside and that it does not release within the building.
Correct venting to the outside
Incorrect venting that doesn’t go outside
Corrosion in an incorrectly vented tank
If a tank is poorly vented, or if there is a leak in the vent piping, there are two visual signs that will indicate that the operator should take action. The first is evidence of corrosion in the vicinity of the tank, and the second is a fine white powder covering the piping and other horizontal surfaces. This white dusting is silica resulting from decay of the silica tetrafluoride gas in fluorosilicic acid (FSA).
The venting system can be confirmed by air testing. Piping contractors typically have an air test system. The vent system can be pressurized to 2-5pounds per square inch gauge (psig) and monitored for air loss. If the system cannot maintain pressure for a 5-10 minute period, then there is a leak and corrosion will result. The bubble method should help in identifying the leak. Do not exceed 10 psig in the test as the polyethylene tank will not support high pressures. It is good practice to check for leaks at least once every 5 years.
Confirm the venting system by doing an air test.
The bulk storage container, which is the large storage tank, should not be used for additive feed supply. The system should include a day tank that is used for actual feed of fluoride additive.
A day tank should be used for feeding.
High-density polyethylene (HDPE) tanks are available in sizes from 10 to 10,000 gallons. Larger volumes are available by grouping several HDPE tanks together. Both cross-linked and linear types of HDPE have provided satisfactory service. Some facilities have reported discoloration of HDPE tanks by fluorosilicic acid (FSA). Strong acids, such as fluorosilicic acid, can have a chemical scorching of the surface layer and also discoloration resulting from high iodine content in some FSA.
HDPE tanks
An antioxidant effect called “pinking” occurs when the resin is exposed to an oxidizing environment. This causes a phenolic yellowing or gas fading. The antioxidant molecules are "activated" and can react with each other to create a color center that ranges from yellow to brown to pink. The exact color is dependent on many different factors, not the least of which is the site of attack by fluorosilicic acid (FSA). The stronger the hydrogen environment, the more the color will shift from yellow to pink.
Phenolic yellowing
The discoloration from ‘pinking’ is generally cosmetic. However, there are occasions that the resin is vulnerable to other forms of oxidative attack such as ultraviolet (UV), ozone (O3), and nitrogen oxide (NOx). Generally, the oxidative attack will not shorten the lifespan of the tank. Most tank manufacturers rate their products for a 15 to 20 year service life. If there are questions on the integrity of a tank, a technical representative of the manufacturer can make an inspection.
The service life of most tanks is 15 to 20 years.
Staining is not fluorosilicic acid specific but can be seen with any strong acid. A strong acid releases two hydrogens upon disassociation and results in a high-hydrogen environment. There are some communities that have experienced discoloration due to iodine. The iodine content of fluorosilicic acid is a derivative of the ore used for phosphate production.
Iodine can also cause discoloration.
Since debris acquired during transport and deterioration of the rubber lining of the transport tank is the most frequently observed problem with fluorosilicic acid (FSA), it is advisable to have a filter on the bulk tank feed line. Here is an installation showing the hose connection (lockable) and an in-line basket filter so if the delivery truck has debris, most of it will not enter the tank.
Bulk tank feed line
Since the delivery truck uses a pressure air system to move the fluorosilicic acid (FSA) from the truck to the tank, the feed line is cleared at the end of the delivery by air purging. This keeps the fill pipe and appurtenances including a line filter if so equipped from continued FSA exposure and in turn keeps the operator from exposure when servicing the line.
Bulk tank feed line
Fiberglass tanks provide the potential to store larger volumes than available in polyethylene. There is mixed opinion about the suitability of these tanks. Some locations have used them successfully. However, glass fibers used as reinforcement may be prone to attack by the hydrogen fluoride gas. If the gel-coat surface is intact, then the glass fibers probably have reasonable protection. When using these tanks, they should have a periodic inspection for structural integrity.
Fiberglass tanks
Fiberglass tanks intended for storage of fluorosilicic acid should have a double synthetic veil in the interior corrosion resistant barrier. They should use an MEKP (methyl-ethyl-ketone-peroxide) agent in post-production cure. The veil is a tightly-woven cloth of fine-strand fibers which is less likely to have protruding fibers from the veil. Standard production is to use a single veil, but severe service stipulates a double veil to fully contain the fibers extending from the glass reinforcement mat.
Fluorosilicic acid (FSA) will damage concrete surfaces. You should use corrosion-resistant pipe materials and provide surface protection to the concrete where leaks could occur. A dual application of epoxy undercoat with urethane topcoat normally provides suitable protection for minor leaks. Alternately, a spray on manhole rehabilitation polyurethane will also provide suitable protection to concrete.
Concrete damage from FSA
Dry additives can be delivered in 50 and 100 pound bags, in 125 to 400 pound drums, and even in 2,500 pound Super Sacks. Handling of bags requires special consideration. Use correct lifting technique to avoid personnel injury, and never tear bags open. Always use a knife to slit bag to minimize release of loose dust.
50 pound bags
Do not “bellows-out” the residual contents of a bag. Do not toss unsecured bags into a dumpster. After use, wrap the bag inside a plastic bag for disposal. Put the bag in a secure place to avoid exposure to someone else. Coordinate with the trash hauler and landfill operator so their personnel do not have exposure to fluoride dust.
Coordinate with the trash hauler and landfill operator to limit exposure.
An American Water Works Association (AWWA) survey on National Sanitation Foundation (NSF) Standard 60 compliance found that additives problems were most commonly related to delivery issues. This chart shows the common reported problems.
| FSA | Dry Additives |
|---|---|
| Breakdown and release from transfer hoses | Delivery in damaged packaging |
| Delivery in damaged containers | Improperly or inadequately equipped delivery personnel |
| Improperly equipped delivery personnel | Attempted delivery to wrong storage area |
| Attempted delivery to wrong storage area | Mixing with other chemicals |
| Transport related trash includong black particles attributed to breakdown of vehicle tank liners, plastic bags, other trash | Degradation of product during transport |
Clearly label all delivery points.
Dry additives should be kept in a separate room with secure access that is convenient to the feed location. Avoid storing other additives, lubricants, yard-care fertilizers, etc. in the same room. Do not mix additives. The room should have good ventilation in the event of dusting. It is best to have an elevated platform and keep the dry additives on pallets so they are not in contact with the floor as moisture can wick from concrete surfaces.
Dry additives should not be in contact with the floor.
Limit stacks to six bags high and make sure they’re protected from the elements. Additives cake when compressed and exposed to moisture. Do not overbuy as the additives will slowly adsorb moisture from the air and hydrate, resulting in “fish-eyes” that may never be successfully dissolved. Limit purchases to a maximum 6-month supply.
Limit the height of the stack to six bags.
For facilities that use a lot of sodium fluorosilicate, Super Sacks can be a cost-effective means of handling the additive. Super Sacks are a 2,500-pound delivery package equivalent of one pallet of bag common in the bulk handling market. They have forklift loops for lifting and transporting. Super Sacks should be stored on an elevated platform so that they are not in contact with concrete floors. They must be protected from the elements, and should not be stacked.
Super Sack
What should you do if you have a spill of your fluoride products?
If you practice good housekeeping of the facility and your dry compounds aren’t contaminated, shovel them up and use them. If you suspect contamination, then they should be properly disposed of in a landfill. Follow local and state codes and regulations. Sodium fluorosilicate is a regulated hazardous substance. While sodium fluoride is not a regulated substance, you do not want to expose a landfill operator to excessive amounts. Check with your state water fluoridation program specialist for advice.
Contact your state water fluoridation program specialist.
Liquid spills require a little more preparation and response effort than dry spills. Proper preparation starts by having a containment barrier such as a corrosion-coated concrete curbing surrounding the tank. It should be sufficient to hold 110% of the contents of the tank.
Concrete curbing
Even with good containment, it is possible for a release to occur (broken pipe, etc.). You should keep on hand spill control pillows or dams that adsorb acid to keep liquid from spreading in the event of a spill. Spill kits are commercially available and many utilities have them. Conduct an annual inspection of your spill kits to ensure that the contents are up to date. If there are multiple chemicals in the tank form, clearly label all spill kits as to their application.
Spill kit
Once a liquid spill is contained, neutralize it and then consult with authorities about disposal requirements. Avoid “flushing” to a public sewer or on-site septic tank system. Local jurisdictions have sewer use ordinances prohibiting discharge of non-domestic chemical releases, and states have septic tank regulations prohibiting unauthorized use of subsurface leach systems for chemical disposal.
Avoid ‘flushing’ liquid spills.
A good practice is to have several bags of granular lime in storage near the delivery point so that if a spill occurs it can be quickly deployed to prevent spreading. For an acid strength of 25%, you need about 0.39 pound of lime to neutralize a pound of acid. Replace the lime bags yearly so that they don’t cake with excessive moisture over time.
Lime
There is a reaction between lime and acid and almost everything formed will be calcium fluoride (CaF2) and silica (SiO2). These two products are accepted at most landfills as they are nonhazardous chemicals. However, be sure to consult with the local landfill operator employees at follow safe practice. For an acid strength of 25%, you need about 0.39 pound of lime to neutralize a pound of acid.
Limit exposure to landfill employees.
If lime is not available for neutralization, then caustic soda or soda ash can be used for fluorosilicic acid (FSA) spills. However, use of these agents will result in the formation of a combination of sodium fluorosilicate and sodium fluoride. These may be considered hazardous materials depending on the local jurisdiction and State regulations. Special caution is required to clean up these residues, and disposal may involve special licensing. Check with your state hazardous waste regulatory group and consult with the local landfill and fire chief.
Caustic soda
Congratulations! You have completed the fluoride additives section of this training. You learned what fluoride is, where it comes from, and its different forms. You also learned about the types of fluoride additives, how they’re used, their delivery and storage methods, and how to prevent and respond to fluoride spills. Possessing this knowledge will assist you in speaking about and debating the topic of community water fluoridation. It will also help you understand how fluoride additives help promote a safe and effective health benefit to communities across the United States. You’re now ready to continue to the next section of this training.
Click the ‘Close’ button below and return to TRAIN to take the ‘Water Fluoridation Principles and Practices: Additives Quiz.’ Upon successful completion of all course quizzes, you will receive your continuing education credit for this training. The next section of this training is titled ‘Water Fluoridation Principles and Practices: Equipment’ and is available in TRAIN.