DCOM Volume I Appendix I

From Ministry of Water DCOM Manual

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+ + Sr2+.....

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

If hardness is too high results into precipitation of soap, scaling on pipes, boilers, cooling towers, heat exchangers. If hardness is too low, the water becomes corrosive. Hard water interferes with almost every cleaning task from laundering and dishwashing to bathing and personal grooming (IANR, Water Quality 1996). Hard water ranges between 120-250 mg/L as CaCO3 or beyond 250 mg/L as CaCO3 for very hard waters. The acceptable water hardness range is between 60-120 mg/L as CaCO3 (Dipa Dey, Amanda Herzog and Vidya Srinivasan, 2007). The scale of hardness is shown in Table xxx
(a) below: Table I.2: Scale of Hardness (classification of hard waters)

Hardness in mg/l Scale Description
15 Extremely soft
30 Very soft
45 Soft
90-110 Moderately soft
130 Hard
170 Very Hard
230 Excessively hard
250 Too hard for use

Temporary and Permanent Hardness
Hardness can be described as temporary or permanent as shown in the Table: xxxx(b) Carbonate hardness is called temporary because it precipitates readily at high temperatures since it is sensitive to heat and precipitates readily at high temperatures. Therefore, can be removed by boiling the water. Non-carbonate hardness is called permanent because it does not precipitate readily at high temperatures.

Table I.3: Compounds Producing Temporary and Permanent Hardness

Causing Temporary Hardness(Carbonate Hardness) Causing Permanent Hardness(Non-Carbonate Hardness)
Calcium bicarbonate[Ca(HCO3)2] Calcium sulphate[CaSO4]
Magnesium bicarbonate[Mg(HCO3)2] Magnesium sulphate [MgSO4]
Calcium chloride [CaCl2]
Magnesium chloride [MgCl2]

(Source: 3rd Edition Design manual, 2009)