Page 291 - Water and wastewater engineering

7-8 WATER AND WASTEWATER ENGINEERING

The reaction is exothermic. It yields almost 1 MJ per gram mole of lime. Because of this high
heat release, the reaction must be controlled carefully. All safety precautions for handling a
strong base should be observed. Because the chemical is purchased as lime, it is common to
speak of chemical additions as additions of “lime,” when in fact the chemical added is hy-
drated lime (calcium hydroxide). When carbonate ions must be supplied, the most common
chemical chosen is sodium carbonate (Na 2 CO 3 ). Sodium carbonate is commonly referred to as
soda ash or soda.

Softening Reactions. The softening reactions are regulated by controlling the pH. First, any
free acids are neutralized. Then the pH is raised to precipitate the CaCO 3 ; if necessary, the pH is
2
raised further to remove Mg(OH) 2 . Finally, if necessary, CO 3 is added to precipitate the non-
carbonate hardness.
Six important softening reactions are discussed below. In each case, the chemical that is
added to the water is printed in bold type. The notation (s) designates the solid form, and indi-
cates that the compound has been removed from the water. The following reactions are presented
sequentially, although in reality they occur simultaneously.

1. Neutralization of carbonic acid (H 2 CO 3 ).
In order to raise the pH, free acids must be neutralized first. CO 2 is the principal acid
present in unpolluted, naturally occurring water.* No hardness is removed in this step.
CO 2 Ca(OH) 2 CaCO (s) H O (7-12)
3
2
2. Precipitation of carbonate hardness due to calcium.
To precipitate calcium carbonate, all of the naturally occurring bicarbonate must be
converted to carbonate. The carbonate then serves as the common ion for the precipita-
tion reaction.
2

Ca 2 HCO 3 Ca(OH) 2 2 CaCO s () 2 HO (7-13)
2
3
3. Precipitation of carbonate hardness due to magnesium.
To remove carbonate hardness that results from the presence of magnesium, more
lime is added. The reaction may be considered to occur in two stages. The first stage oc-
curs when the bicarbonate in step 2 above is converted to carbonate.

2
Mg 2 HCO 3 Ca(OH) 2 MgCO 3 CaCO s () 2 HO (7-14)
2
3

The hardness of the water does not change because MgCO 3 is soluble. With the addition
of more lime, the hardness due to magnesium is removed.
2
2
Mg CO 3 Ca(OH) 2 Mg(OH) 2 ()s CaCO 3 ( ) (7-15)
s

*CO 2 and H 2 CO 3 in water are essentially the same:
CO 2 + H 2 O H CO 3
2
Thus, the number of reaction units (n) used to calculate the equivalents for CO 2 is two.