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Chemical Technology • April 2015
generally maintained on a planktonic (free-floating) count.
This would lead to an inaccurate representation as the
planktonic phase contains only 5-10 % of the total bacteria
present in the system. The remaining 90–95 % is found
in the biofilm (sessile phase) which was historically not
measured [11].
There have been several devices designed to monitor
the biofilm development in the cooling water system. These
include the Pedersen device and the Robbins device [12].
These devices require that the cooling water pass through
them in order for the surfaces to emulate the surfaces of
the larger system. The installation of these devices requires
systemdowntime and plant modifications to allowwater flow
through the device and back into the system. They often
have to be in active plant areas which can make sampling
a difficult task [13].
Most Eskom cooling water systems are treated with a
biocide [14,15] and a biodispersant [16,17]. The former
kills the bacteria while the latter limits their attachment to
the system surfaces. The dosing of these chemicals is not
monitored and there is little management of the contrac-
tors and no knowledge of the efficacy of the products on
the system bacteria.
In an attempt to obtain an understanding of the sessile
populations within the Eskom cooling water systems, an
alternative method of monitoring was essential. The moni-
tor had to fulfil several criteria: it had to be relatively small
(the Robbins device is 15 cm square and 2 m long), it had
to be able to evaluate the sessile population and the cor-
rosion rate within the system, it needed to be installed in
an easily accessible area on the power plant and must not
need system downtime to be installed.
Development
The need for a biological monitor led to the initial design of
a small, plastic, rectangular device which could be placed
in the cooling tower basins and easily retrieved. The device
became known as the BOMM (biological on-line microbiol-
ogy monitor); furthermore, it had nylon insets of a known
area which could be removed and the bacteria on their
WASTE MANAGEMENT