Page 306 - Water and wastewater engineering

LIME–SODA SOFTENING 7-23

Lime/soda ash

Q Mg i
Raw water Aeration
Mg Flow (1 X)Q
r

Flow bypassed XQ
First stage

Lime/soda ash

Mg f
Recarbonation Filtration Disinfection
Q

Second stage
FIGURE 7-10
Flow diagram for a two stage split-treatment lime–soda ash softening plant.

a
Hard Times water analysis
Constituent mg/L
CO 2 5.5
2
Ca 95.2
2
Mg 22.0

Na 25.8
Alkalinity b 198
Cl 67.8
2
SO 4 73
a
Assume that other ions in the water account for the lack of an ion balance.
b
mg/L as CaCO 3 .
Solution:
a. Begin by converting all the concentrations to CaCO 3 equivalents and meq.

Constituent mg/L EW EW CaCO 3 /EW ion mg/L as CaCO 3 meq/L
5.5 22.0 2.28 12.5 0.25
CO 2
Ca 2 95.2 20.0 2.50 238.0 4.76
Mg 2 22.0 12.2 4.12 90.6 1.80
Na 25.8 23.0 2.18 56.2 1.12
continued