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Chemical Technology • April 2015
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enumerated. As the development continued, these insets
were fixed in the device and their surfaces swabbed. This
minimised the possible impact of other surfaces and any pos-
sible anaerobic zones beneath the inset were not sampled.
The requirement of corrosion rate calculations was
eliminated by including corrosion coupons on the narrow
sides of the monitor. Four coupons could be placed on each
side, each side a different metal. In order for the corrosion
coupons to be held in place, the device was made in two
parts that came apart in order to hold the coupons securely
(see Figures 1-3).
The entire device is attached to the side of the basin with
a nylon rope, which threads through the second block, so as
to not influence either the insets or the coupons.
The entire BOMM is made of plastic to ensure that there
is no cathodic corrosion between different metals on the
device. There are 6 plastic bolts that hold the two blocks
together; these are secured with plastic wing nuts to ensure
that sampling is simplified and other tools are not required.
Monitoring
The BOMMs were installed at each station, one on each
side of the cooling water system. They are placed near the
screening area to ensure that there is a water flow around
the device. The corrosion coupons are installed in the
BOMM and their exact position, weight and date of instal-
lation recorded.
Sampling occurs on a monthly basis, in an attempt to
establish a base line. The BOMM is photographed to keep
a visual record. The surface of the nylon inset is aseptically
swabbed to retrieve the attached biofilm (sessile), a cooling
water sample is collected from the same point (planktonic)
and the BOMM is taken apart and a corrosion coupon of
each metal aseptically removed.
The biofilm and cooling water sample are submitted for
microbiological analysis on aerobic and anaerobic bacteria.
The bacteria responsible for microbiologically influenced
corrosion are also enumerated.
When the corrosion coupons are removed the position
and date of removal is noted. Once the coupon has been
microbiologically analysed and cleaned, they are reweighed.
They will be referenced back to the initial installation date
and weight in order to determine the corrosion rate of the
metal in the particular system.
Figures 4 and 5 show the fouling that was detected on
the BOMM devices.
Case Study
One of the power stations that was involved with the initial
monitoring and design testing, noted that the cleanliness
of their cooling towers differed on each side, although the
same raw water is utilised for both. There were severe algal
blooms in the towers and the conventional cooling water
treatment chemicals did not seem to have any effect (Figure
6). The power station operates the system on a continuous
cycling loop and can cycle up to 30 times if there is a water
release constraint.
BOMM devices were placed in the north and south
cooling water systems in the basins of the fouled towers.
Figures 7 and 8 show the BOMM devices after a period of
one month of being submerged in the cooling water of the
north and south tower respectively.
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