§ Mr. Bestasked the Secretary of State for the Environment what is the method by which fluoride is added to water supplies; what safeguards are involved; what variations in the method exist in different parts of the United Kingdom; and if he will make a statement.
§ Mr. MacfarlaneAbout 95 per cent. of fluoridation plants use hexafluorosilicic acid supplied as a 20 per cent. solution in water; the remainder use disodium hexafluorosilicate supplied as a powder.
All the plants using the acid have a "day" tank which holds about one day's requirement of acid, and can be filled only once in any 24-hour period. At small waterworks the acid is diluted with water in an automatic failsafe chemical diluter. The acid or the diluted acid is them injected into the water by a dosage pump in proportion to the water flow through the works. The dosage pump is designed to work close to its maximum output. The majority of plants have an automatic continuously recording fluoride monitor which signals an alarm or stops fluoridation should the fluoride concentration in the water entering supply start to exceed a preset value of between 1.2 mg/l and 1.5 mg/l.
A few large manned works which store the water before supply rely on frequent manual sampling and analysis for fluoride.
There are thus three safeguards to ensure that consumers do not receive over-fluoridated water: the use of a "day" tank; a controlled dosing pump operating near its maximum capacity; and monitoring arrangements whether automatic or not.