asked the Minister for the Environment if he will state the chemicals, and their proportion, in the Dublin water supply; and the purpose these chemicals serve.
Ceisteanna—Questions. Written Answers. - Dublin Water Supply Chemicals.
545.
The water supply serving the greater Dublin area amounts to approximately 85 million gallons per day and comes from four sources:— Ballymore Eustace, 43 million gallons; Ballyboden, 4 million gallons; Leixlip, 21 million gallons; Roundwood, 17 million gallons.
The treatment of water at all four sources is different. At the first three sources the water receives full chemical treatment and rapid gravity filtration. At Roundwood the supply is treated by slow sand filtration.
The chemicals used generally in the treatment of water at the first three water works are:
Before Filtration
(a)Aluminium Sulphate of Alum — this is used for coagulation and clarification of the raw water and the amounts employed vary from 80 to 30 milligrams per litre.
(b)Polyelectrolyte — this is used as an aid to improve coagulation and subsequent sedimentation. The amounts employed vary from 0.05 to 0.35 milligrams per litre with an average dosage rate of 0.2 milligrams per litre.
Post Filtration
(c)Lime — this chemical is added to the water to correct P.H. i.e. to chemically stabilise the water so that it is neither corrosive nor scale depositing. The amounts used do not exceed 15 milligrams per litre with an average dosage rate of 9 milligrams per litre.
(d)Chlorine — gaseous chlorine is used to disinfect the water and to render it bacteriologically pure. The amounts used vary from 1.5 to 0.5 milligrams per litre.
(e)Fluorine — this chemical is added to the water supply as hydrofluosilicic acid in accordance with the Health (Fluoridation of Water Supplies) Act 1960 for the prevention of dental caries and the amount used is approximately 1 milligram per litre.
At Leixlip Treatment Works activated carbon has been used for the removal of phenolic taste and the dosage rate is 5.5 milligrams per litre for the duration of the phenol presence in the raw water.
At Roundwood where the raw water quality is of a very high standard there is no chemical treatment prior to slow sand filtration. After filtration the water is chlorinated and fluoridated in accordance with public health practice.