Alkalinity and hardness are closely related, since calcium and magnesium which cause most of the hardness of waters, are generally associated with carbonate minerals which are the principal sources of alkalinity. The equivalent CaCO3 values for the divalent Ca ++ and Mg ++ are often taken as a measure of total hardness.
Sanitary engineers categorise degrees of hardness of waters (Sawyer and McCarty, 1967; Boyd 1979):
| 0 – 75 mg CaCO3/litre | - | Soft |
| 75 – 150 " | - | Moderately hard |
| 150 – 300 " | - | Hard |
| 300 + " | - | Very hard |
(see also earlier discussion under “CO2”).
The part of total hardness chemically equivalent to total alkalinity is termed as “Carbonates hardness”, which is also referred to as ‘temporary’ hardness, as the carbonates are removed from water (scale formation) on boiling. Non-carbonate hardness is called ‘permanent’ hardness as it cannot be removed by boiling. When total alkalinity of a water exceeds its total hardness, the bicarbonate and carbonate are associated with K+ and Na+++. Likewise if total hardness is higher than total alkalinity some of Ca ++ and Mg ++ are associated with SO4= CL-, SiO3= or NO3- rather than HCO3- and CO3=.
