DCOM Volume I Appendix I

From Ministry of Water DCOM Manual
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Appendix I: Measurement of Water Hardness

Hardness and its Measurement
Hardness is expressed in terms of mg/l by weight in terms of calcium carbonate. Water having hardness not exceeding 70 mg/l is termed ‗soft‘ and above that 'hard'. In public water supplies, it used to be customary to reduce carbonate hardness to 35 - 40 mg/l and total hardness to between 50 and 100 mg/l. However as indicated above this is no longer recommended unless hardness exceeds about 130 mg/l, but should still be practised for strictly industrial supplies of hard water.

Hardness = ∑ divalent cations = Ca2+ + Mg 2+ + Fe 2+ + Mn 2+ + Sr 2+.....

Principle cations causing hardness in water and major anions associated with them are as follows:

Table I.1: Principle cations and anions associated with water Hardness

Cations Anions
Ca2+ HCO-3 or CO2-3
Mg2+ SO2-4
Sr2+ cl-
Fe2+ NO-3
Mn2+ SiO-3