Pipeline Chlorination Simplified - Disinfection of Pipelines and Storage Facilities Field Guide - page 114

Pipelines 12 in. (300 mm) or Larger
The slug method is preferred for larger pipelines, most com-
monly larger than 12 in., but many utilities use this method for
pipelines 6 in. or larger (150 mm). This method is described in
detail in Chapter 3. Generally, a section of pipeline is filled with
100 mg/L chlorine solution, and the slug is moved slowly along
the length of the pipeline so that the contact time is at least 3 hr.
Valves and hydrants are operated as the slug passes to ensure
that they are disinfected. Again, thorough cleaning prior to
beginning the disinfection procedure is critical to success.
The slug is moved through the pipeline in one of two ways. It
is common to fill a section of pipeline that can be isolated by
valves. Many times this is about 200 ft of pipeline. The 100 mg/L
chlorine solution is held in this section for at least 3 hr. Then the
downstream valves are opened, and water is fed into the upstream
end to move the slug to the next section where it is again isolated
by valves for at least 3 hr. This process is repeated as necessary
to complete disinfection of the entire pipeline length.
A second method for moving the slug is to inject the chlorine into
the pipeline while withdrawing water from the end (Figure 8-1).
The slug is prepared as needed to ensure at least 3 hr of contact.
This slug then continues to move down the pipeline allowing
potable water to replace the 100 mg/L chlorine slug as it moves
slowly along. The rate of water withdrawal is monitored by a
flowmeter. This method is commonly used for very large diame-
ter pipelines because it uses less chlorinated water and, thus,
reduces the need for dechlorination and disposal. One negative
aspect to this procedure is that it can take many hours because of
the slow rate at which the slug must move through the pipeline.
To reduce the options and simplify the process, a 20 percent
slug volume (of the total volume of the pipeline to be disinfected)
has been selected. This is a convenient amount because the slug
fill time will be 3 hr, and the time to move the slug will always be
about 15 hr. Therefore, the entire job from set-up to flushing can
be completed in just over two shifts. These times are always the
same.