Manhole Menace
Of primary concern in wastewater
manholes is the threat posed by sewer gases
such as hydrogen sulfide and carbon dioxide.
Carbon dioxide gas is found in natural abundance
in the air. Hydrogen sulfide, on the other hand,
is produced in the waste stream as a result
of the decomposition of organic matter and is
released into the air when the waste stream
becomes turbulent, as often occurs in manholes.
Both gases can easily dissolve
even in the small amount of moisture present
on the walls of manholes, causing a decrease
in the pH of the concrete walls. As the pH decreases,
destructive bacteria begin to colonize –
and begin a rapid, destructive cycle, referred
to as microbial-induced corrosion.
In the presence of oxygen,
these bacteria have a nasty habit of excreting
sulfuric acid, further damaging the concrete.
As the pH becomes more and more acidic, successive
colonies of acid-producing bacteria grow and
subsequently die off, each generation replaced
by one that can tolerate a pH lower than the
previous one. The most aggressive bacteria thrive
in an environment with a pH of less than 4.
These bacteria are aptly named Thiobacillus
Concretivorous – Latin for “concrete-eating.”
Today, systems are available
to successfully combat the aggressive actions
of these menacing microbes that use coatings
or liners in combination with a quality precast
concrete manufactured with a well-designed concrete
mix.
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