Wastewater Conveyance - Wastewater Operators - page 167

wells, rivers, or lakes, the water solution is an extremely dilute liquid of
mineral salts and gases. The dissolved mineral salts are a result of water
flowing over and through the earth layers. The dissolved gases are atmo-
spheric oxygen and carbon dioxide, picked up by water-atmosphere con-
tact. Wastewater picks up corrosive materials mainly from industrial
processes and from chemicals added to the wastewater
during treatment. Several types of corrosion should be
considered in wastewater collection piping systems
(AWWA 1996):

Key Point: Materials such as
acids, caustic solutions, and
similar solutions are typical
causes of pipe corrosion.

1. Internal corrosion , caused by aggressive water flowing through the
pipes

2. external corrosion , caused by chemical and electrical conditions
in the soil

3. Bimetallic corrosion , caused when components made of dissimilar
metals are connected

4. Stray-current corrosion , caused by uncontrolled DC electrical cur-
rents flowing in the soil

4.8.5.3 Joining Metallic Pipe

Pipe joint design and selection can have a major impact on the
initial cost, long-range operating costs, and performance of the piping
system (Crocker, 2000). When determining the type of joint to be used
in connecting pipe, certain considerations must be made; for example,
initial considerations include material cost, installation labor cost, and
degree of leakage integrity required. The operator is also concerned with
periodic maintenance requirements and specific performance require-
ments. Metallic piping can be joined or connected in a number of ways.
The method used depends on: (1) the nature of the metal sections (fer-
rous, nonferrous) being joined, (2) the kind of liquid or gas to be carried
by the system, (3) pressure and temperature in the line, and (4) access
requirements. A joint is defined simply as the connection between ele-
ments in a piping system. Five major types of joints, each for a special
purpose, are used for joining metal pipe (see Figure 4.5):

1. Bell-and-spigot joints

2. Screwed or threaded joints

3. Flanged joints

4. Welded joints

5. Soldered joints

4.8.5.3.1 Bell-and-Spigot joints

The bell-and-spigot joint has been around since its development in
the late 1780s. The joint is used for connecting lengths of cast iron
water and wastewater pipe (gravity flow only). The bell is the enlarged