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Chemical Technology • April 2015
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The development
of an online biofouling monitor
for cooling water systems
E
skom utilises large, open, recirculating cooling water
(CW) systems to condense the steam, used for power
generation, back to water. The CW systems range
from 48 to 128 ML in volume, depending on the age of
the plant and the size of the generating units. Eskom has
established a ‘zero liquid effluent discharge’ policy [1,2,3]
and therefore there is no water released from the site ex-
cept as steam. As a result the cooling water concentrates
as the elements in the water are not lost in the steam [4].
The CW system is comprised of several different ma-
terials including concrete, mild steel and other steels [5];
the condenser tubes are conventionally admiralty brass
although some systems, especially the newer ones, have
titanium tubes. Due to the complexity of the system there
are many areas of low flow and dead ends [5].
Although the CW system is controlled mainly on chemical
parameters, the microbiology is also closely monitored. The
development of biofilms in the CW system leads to several
problems including corrosion, blockages and decreased
heat transfer [6,7].
The biofilms form when the bacteria present in the cool-
ing water adhere to the surfaces of the system [8,9]. There
are several mechanisms whereby they may achieve this, but
mostly they secure themselves to the surfaces by flagella
and electrostatic forces. Once attached to the surfaces, they
undergo a metabolic change and begin to produce a sticky
slime as a by-product. The slime acts as a protective film
over the bacteria and other particles or bacteria in the water
may adhere to it. This increases the width of the biofilm on
the surface of the system and allows the conditions nearest
to the surface to become anaerobic.
In this anaerobic environment, the bacteria are able to
metabolise the iron in themetal andmay causemicrobiolog-
ically influenced corrosion (MIC). They excrete by-products
which harden to form a nodule over the corrosion site. Their
metabolism allows them to release hydrogen sulphide as
a by-product. When it comes into contact with water this
forms sulphuric acid which corrodes the metal and may
cause pitting or holes through the metal [10].
Unfortunately the Eskom cooling water systems are
by Kelley Reynolds-Clausen, Eskom Holdings SOC Ltd, Research, Testing and
Demonstration, Johannesburg, South Africa
The BOMM device offers a simple means of
obtaining representative sessile samples
from Eskom cooling water systems.