Chemical Technology April 2015

WASTE MANAGEMENT

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

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

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Chemical Technology • April 2015