Environmental Engineering Reference
In-Depth Information
•
Conductance (specific conductance)
—This is a measure of the electric
current in the water sample carried by the ionized substances; there-
fore, dissolved solids are basically related to this measure, which is
also influenced by the good conductivity of inorganic acids, bases,
and salts, as well as the poor conductivity of organic compounds.
The methods prescribed by
Standard Methods
(APHA-AWWA-WEF, 2005)
for the determination of solids include
• Total solids dried at 103°C to 105°C
• Total dissolved solids dried at 180°C
• Total suspended solids dried at 103°C to 105°C
• Fixed and volatile solids ignited at 550°C
• Settleable solids (Imhoff cone, volumetric, gravimetric)
• Total, ixed, and volatile solids in solid or semisolid samples
pH
Raw water examined for potential use as drinking water has an expected pH
value between 4 and 9, but it is more than likely that encountered values will
be between 5.5 and 8.6. What does this mean? pH is defined as the negative
log base 10 of the hydrogen ion concentration:
pH = -log
10
[H
+
]
( 7. 2)
The pH changes one unit for every power of ten change in [H
+
]; for example,
water with a pH of 3 has 100 times the amount of [H+] as that found in water
with a pH of 5. Remember that, because pH = -log
10
[H
+
], the pH will decrease
as [H+] increases.
The pH of water is controlled by the equilibrium achieved by dissolved
compounds in the system. In natural waters, the pH is primarily a function
of the carbonate system, which is composed of carbon dioxide, carbonic acid,
bicarbonate, and carbonate. Acid inputs to a water system may substantially
alter the pH. The main sources of acid include acid mine drainage and atmo-
spheric acid deposition.
Low-pH water may corrode distribution pipes in potable water plants. The
pipes may be costly to replace, and the corrosion may release metal ions such
as copper, lead, zinc, and cadmium into the treated drinking water. Ingestion
of heavy metals may pose substantial health risks to humans. According to
the Safe Drinking Water Act, the minimum and maximum allowable pH
range for potability is 6.5 to 8.5.
