Page 285 - Water and wastewater engineering

7-2 WATER AND WASTEWATER ENGINEERING

7 -1 HARDNESS

The term hardness is used to characterize a water that does not lather well, causes a scum in the
bath tub, and leaves hard, white, crusty deposits (scale) on coffee pots, tea kettles, and hot water
heaters. The failure to lather well and the formation of scum on bath tubs is the result of the reac-
tions of calcium and magnesium with the soap. For example:
2 soap)

(
s
Ca ( Ca soap 2 ) ( ) (7-1)
where (s) a solid precipitate
As a result of this complexation reaction, soap cannot interact with the dirt on clothing, and
the calcium-soap complex itself forms undesirable precipitates.
Hardness is defined as the sum of all polyvalent cations (in consistent units). The common
units of expression are mg/L as CaCO 3 or milliequivalents per liter (meq/L). Qualitative terms
used to describe hardness are listed in Table 7-1 . The distribution of hard waters in the United
States is shown in Figure 7-1 .
Although all polyvalent cations contribute to hardness, the predominant contributors are cal-
cium and magnesium. With the exception of a few other important polyvalent cations and natural
organic matter ( NOM), the focus of this discussion will be on calcium and magnesium.
Hardness in natural waters comes from the dissolution of minerals from geologic formations
that contain calcium and magnesium. Two common minerals are calcite (CaCO 3 ) and dolomite
[CaMg(CO 3 ) 2 ]. The natural process by which water becomes hard is shown schematically in
Figure 7-2 . As rainwater enters the topsoil, the respiration of microorganisms increases the CO 2
content of the water. As shown in Equation 6-2, the CO 2 reacts with the water to form H 2 CO 3 . Cal-
cite and dolomite react with the carbonic acid to form calcium bicarbonate [Ca(HCO 3 ) 2 ] and mag-
nesium bicarbonate [Mg(HCO 3 ) 2 ]. While CaCO 3 and CaMg(CO 3 ) 2 are not very soluble in water,
the bicarbonates are quite soluble. Calcium chloride (CaCl 2 ), gypsum (CaSO 4 ), magnesium chloride
(MgCl 2 ), and magnesium sulfate (MgSO 4 ) may also go into solution to contribute to the hardness.
Because calcium and magnesium predominate, the convention in performing softening
calculations is to define the total hardness (TH) of a water as the sum of these elements

2
TH Ca Mg 2 (7-2)

The concentrations of each element are in consistent units (mg/L as CaCO 3 or meq/L). Two

components of total hardness are: (1) that associated with the HCO 3 anion (called carbonate
hardness and abbreviated CH) and (2) that associated with other anions (called noncarbonate
hardness and abbreviated NCH). Total hardness, then, may also be defined as
TH CH NCH (7-3)

TABLE 7-1
Hard water classification
Hardness range
(mg/L as CaCO3) Description Comment
0–50 Extremely soft
50–100 Very soft
100–150 Soft to moderately hard Acceptable to most users
150–300 Hard
300 Very hard