Chemical Technology April 2015

The development of an online biofouling monitor for cooling water systems 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.

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

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Chemical Technology • April 2015