https://www.collective-evolution.com/2014/02/23/97-percent-of-europe-doesnt-put-hydrofluorosilicic-acid-their-water-why-do-we
Maybe one reason is that it's classified as "Toxic Waste" thus is a fairly expensive by-product to otherwise dispose?
Unfortunately, I could find no disposal cost factors relating to Sodium Fluoride or it's analogs. However, one must assume that the material is not being "donated" to the public water systems.
Below is probably more than you ever wanted to know on the subject.
"Hexafluorosilicic acid is also commonly used for water fluoridation in several countries including the United States, the United Kingdom, and the Republic of Ireland. In the U.S., about 40,000 tons of fluorosilicic acid is recovered from phosphoric acid plants, and then used primarily in water fluoridation." These plants are otherwise known as Phosphate Fertilizer Production Plants." (https://en.wikipedia.org/wiki/Hexafluorosilicic_acid)
"About 41,200, 4,700, and 13,200 tons of byproduct fluorosilicic acid were sold for water fluoridation, AlF3 production for the aluminum industry, and for other uses, such as sodium silicofluoride production, respectively. Domestic production data for fluorosilicic acid for 2001 were developed by the U.S. Geological Survey from voluntary surveys of U.S. operations. Of the 11 fluorosilicic acid operations surveyed, 10 respondents reported production and1 respondent reported zero production (USGS 2002b)." (https://www.atsdr.cdc.gov/toxprofiles/tp11-c5.pdf)
"Information concerning the amount of sodium fluoride produced is not available. Fluorosilicic acid is a byproduct of the action of sulfuric acid on phosphate rock containing fluorides and silica or silicates." (Ibid)
"The data listed in Tables 5-2 and 5-3 are derived from the Toxics Release Inventory (TRI01 2003). Only certain types of facilities were required to report. Therefore, this is not an exhaustive list. Sodium fluoride or other fluoride salts are not listed on TRI." (Ibid)
"The chemicals most commonly used by American waterworks for water fluoridation are fluorosilicic acid, sodium silicofluoride, and sodium fluoride (Urbansky 2002). Generally, 1.5–2.2 mg of sodium fluoride is added per liter of water (0.7–1.0 mg/L as fluoride) (Mueller 1994). Data from the Centers for Disease Control’s (CDC) 1992 Fluoridation Census indicate that 25% of utilities reported using sodium fluoride; however, this corresponds to 9.2% of the U.S. population drinking fluoride-supplemented tap water
(Urbansky 2002)." (Ibid)
With respect to the disposal of sodium fluoride, "No information was found regarding the disposal of sodium fluoride. It would appear from its use that most of it is disposed of in municipal landfills or (publicly owned-treatment works) POTWs.
Publicly owned-treatment work - "A publicly owned treatment works means a sewage treatment plant that is owned, and usually operated by a U.S. government agency. It is also known as POTW. In the U.S., POTWs are typically owned by local government agencies. They are designed to treat domestic sewage, not industrial waste. The term is often seen used in the Clean Water Act, and other water pollution laws and regulations. The U.S. Environmental Protection Agency provides grants or low interest loans for establishment of POTWs. (https://definitions.uslegal.com/p/publicly-owned-treatment-works/)
"Material Safety Data Sheet - Sodium fluoride
"Danger! Causes irritation and possible burns by all routes of exposure. May be fatal if swallowed. Contact with acids liberates toxic gas. May cause lung damage. Moisture sensitive.
Target Organs: Kidneys, heart, gastrointestinal system, skeletal structures, teeth, nerves, bone.
"Potential Health Effects
"Eye: Causes eye irritation and possible burns. May cause chemical conjunctivitis and corneal damage.
Skin: Contact with skin causes irritation and possible burns, especially if the skin is wet or moist. May cause skin rash (in milder cases), and cold and clammy skin with cyanosis or pale color.
"Ingestion: May be fatal if swallowed. Ingestion of large amounts of fluoride may cause salivation, nausea, vomiting, abdominal pain, fever, labored breathing. Exposure to fluoride compounds can result in systemic toxic effects on the heart, liver, and kidneys. It may also deplete calcium levels in the body leading to hypocalcemia and death. May cause irritation of the digestive tract and possible burns. May cause respiratory paralysis and cardiac arrest." (https://fscimage.fishersci.com/msds/21230.htm)
"SODIUM FLUORIDE, SOLID SODIUM FLUORIDE
Hazard Class: 6.1" (Ibid)
"Division (Hazard Class) 6.1: Poisonous material is a material, other than a gas, which is known to be so toxic to humans as to afford a hazard to health during transportation, or which, in the absence of adequate data on human toxicity:" (https://en.wikipedia.org/wiki/HAZMAT_Class_6_Toxic_and_infectious_substances)
Additional information found here under "Sodium Fluoride"
https://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs142p2_052293.pdf
Unfortunately, I could find no disposal cost factors relating to Sodium Fluoride or it's analogs. However, one must assume that the material is not being "donated" to the public water systems.
The following is significant, "Volume of Distribution (Sodium Fluoride)
"Stored in bones and developing teeth after absorption. Fluoride is distributed into sweat, tears, hair, and saliva. Fluoride crosses the placental barrier and is distributed into breast milk.
from DrugBank (http://www.drugbank.ca/drugs/DB09325)
"During the growth phase of the skeleton, a relatively high portion of an ingested fluoride dose will be deposited in the skeleton. In infants and children with skeletal growth or individuals not consuming fluoridated drinking-water, up to 75% of the daily amount of fluoride that is absorbed may be incorporated into skeletal tissue. When a fluoride dose (e.g., a fluoride tablet or an infant formula diluted with fluoridated drinking-water) is given to infants, the retention will be strongly correlated with the absorbed fluoride dose per kilogram body weight: the higher the fluoride dose, the higher the fluoride retention. Retention of fluoride following intake of a fluoride supplement of 0.25 mg given to infants was shown to be as high as 80-90%. In a study with adults (aged 23-27 years) in which fluoride was given as a single intravenous injection, about 60% of the injected dose (3 mg fluoride as sodium fluoride) was retained.
"WHO; Environ Health Criteria 227: Fluorides (2002) Available from, as of November 15,2006:"
http://www.inchem.org/documents/ehc/ehc/ehc227.htm#6.1"
(https://pubchem.ncbi.nlm.nih.gov/compound/sodium_fluoride#section=Absorption-Distribution-and-Excretion)