The SHA/PHE intended to pour in 200 tonnes of the stuff per annum and 2% impurities is a lot.
As I said in my previous email, we assess each application to discharge mercury individually, and we take a number of site-specific factors into account. I am sorry I cannot be of any more assistance on this issue.
Technical Advisor - Water Quality
E&B, Land and Water Quality
Environment Agency, Richard Fairclough House, Knutsford Road, Warrington, WA4 1HG
Did you know the impurities present in the fluoridation chemicals hexafluorosilicic acid? See below. The contents include mercury and even radioactive elements but as long as they are within the 2% limits no one cares including your department apparently.
Safety of water fluoridation
The UK's Water (Fluoridation) Act 1985, since incorporated into the 1991 Water Industry Act, allows hexafluorosilicic acid (H2SiF6) and disodium hexafluorosilicate (Na2SiF6) to be used to increase the fluoride content of water. The published Code of Practice on Technical Aspects of Fluoridation of Water Supplies (Department of the Environment, 1987) gives specifications for these substances and states that 'the product. ..must not contain any mineral or organic substances capable of impairing the health of those drinking water correctly treated with the product'. For H2SiF6, limits are given for a number of possible impurities, including for iron, heavy metals, sulphate, phosphate, and chloride. The specification for Na2SiF6 powder requires a minimum of 98% m/m of the pure chemical, and gives maximum limits for impurities, including heavy metals (as lead) and iron. No other substances are allowed to be used in the fluoridation process, other than an anti-caking agent (the identity of which must be disclosed) in the case of Na2SiF6. Synthetic detergents are not permitted.
Thus there is no likelihood, in normal operation, for any fluoridation plants to introduce other compounds into the drinking water supply (other than approved anti-cakinq aqents and any impurities present in the fluoridation chemicals).