Table of Contents
Advertisement
Waters are commonly classified in terms of the degree of hardness. There are different standards, as
listed in the following Table:
The hardness of water is derived largely from contact with the soil and rock formations. Rain water as
it falls upon the earth is incapable of dissolving the tremendous amounts of solids found in many
natural waters. The ability to dissolve is gained in the soil where carbon dioxide is released by bacterial
action. The soil water becomes highly charged with CO
carbonic acid. Under the low pH conditions that develop, basic materials, particularly limestone
formations, are dissolved. The Figure below shows where this originates and how it attacks carbonate
minerals. Since limestone is not pure carbonate but includes impurities such as sulfates, chlorides, and
silicates, these materials become exposed to the solvent action of water as the carbonates are dissolved,
and they pass into solution too.
In general, hard waters originate in areas where the topsoil is thick and limestone formations are
present. Soft waters originate in areas where the topsoil is thin and limestone formation is sparse or
absent.
Zone of intense bacterial action CO
Some action on basic compounds – CaCO
Zone of less bacterial action CO
Continued action on basic compounds – CaCO
etc.
Table: Water hardness Classification
Developed by Water Quality Association
Term
Soft
Slightly hard
Moderately hard
Hard
Very hard
Rain
TOPSOIL
in large amounts
2
+ H
3
SUBSOIL
in small amounts
2
LIMESTONE
Zone of intense chemical reaction
Ca(HCO
CaCO
+ H
CO
3
2
3
Mg(HCO
MgCO
+ H
CO
3
2
3
ppm as CaCO
3
< 17.1
17.1 – 60
60 – 120
120 –180
> 180
, which, of course, exists in equilibrium with
2
Ca(HCO
CO
)
, etc.
2
3
3
2
Ca(HCO
+ H
CO
)
,
3
2
3
3
2
)
,
3
2
)
3
2
97
Advertisement
Table of Contents
Need help?
Do you have a question about the CHC Series and is the answer not in the manual?
Questions and answers