State of Technology Report for Force Main Rehabilitation, Final ...

Drain Rodding
Simple drain rods are an obvious low-tech alternative to water jetting for small-bore pipes. For small
diameter pipes, old-fashioned drain rods are still the most common tool for blockage clearance and can be
very effective. The limitation is that there is little control and no feedback as to whether the blockage is
adequately cleared. Another drawback is that the rods may simply be pushing the problem somewhere
else. Nevertheless, this is an effective method for removing small blockages in small diameter sewers. It
is less well-suited to force mains because they generally have higher flow rates than gravity sewers so are
less likely to have loose debris in the invert.
Removal
Pipe cleaning and blockage clearance should involve more than just passing the problem downstream,
which jetting and rodding may do. Jetters are better than rods in this respect, since they break up
accumulations of debris into finer material, which is more likely to be carried in the flow and dispersed.
Where there are greater volumes of debris that would almost certainly settle out further downstream, a
common solution is to use a combination jetter/vacuum machine to simultaneously flush the sewer and
suck out the resulting sludge. Some of the larger and more sophisticated units incorporate water filtering
and recycling systems, together with compression of the solids so they take up less space and are easier to
dispose.
Flushing
Flushing involves simply pouring a large volume of water into a sewer as quickly as possible so that the
sudden increase in flow will wash away any accumulations of debris. It is an attempt to replicate the
effect of a summer storm on a combined sewer. Today, such a procedure would be carried out using a
large-capacity tanker-jetter, possibly with recycling and filtering facilities. Flushing is seldom used since
it has been largely superseded by jetting. Jetting uses less water and is a more controllable and reliable
technique. Flushing is also not well-suited to force mains since it does little more than repeat the normal
operation of the main.
Air Scouring
Air scouring is flushing with a twist. The “twist” in air scouring is using alternating volumes of air and
water to flush the pipe. The air and water travel along the pipe in separate discrete volumes with
approximately 75% air and 25% water. The air causes the water to move with high velocity and
turbulence, scouring away sediment, soft scale deposits, and biofilm. Common air compressors provide
the air. Both fluids are fed continuously and separately into discrete volumes by themselves. Air
scouring is more aggressive than flushing and should be used with caution if the pipe’s structural
condition is poor. The AWWARF research report, Investigation of Pipe Cleaning Methods, provides
further details on the equipment setup for this method (AWWARF, 2003).
6.1.2 Cathodic Protection. Approximately 60% of ferrous force mains reportedly have some
cathodic protection applied. Steel pipes are more commonly protected with a cathodic protection system
because the electrical continuity of welded joints makes this a practical solution. It is more complicated
for DI pipe where electrical connectors (bonding) need to be placed across all bell and spigot joints.
Cathodic protection is an electrochemical method used to prevent or control corrosion of buried or
submerged metallic structures. They are active systems that rely on the application of electric current to
control corrosion. If current is interrupted, corrosion will progress at the normal rate for the
material/environment combination; if the supplied current is inadequate for complete protection,
corrosion will progress at a reduced rate. After a cathodic protection system has been installed and
adjusted to provide adequate protection, currents and potentials should remain relatively stable; changes
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