Macrofouling Control in Power Plants

Abstract

Macrofouling organisms readily colonize artificial man-made structures, cooling water intake tunnels, culverts, pump chambers and heat exchangers. Cooling water systems if not properly treated invariably become susceptible to biofouling. The problem is particularly severe in the tropics and is site-, season-, and substratum-specific. Further, cooling systems serve as a source of macrofoul-ing organisms and breeding grounds wherein invertebrate larvae are produced and colonize equipment downstream like pipelines, valves and heat exchangers. Once-through seawater or freshwater systems encounter severe macrofouling-associated problems like flow reduction, increased pressure drop across heat exchangers and equipment breakdown. Biocidal dose and regime for cooling water systems and heat-exchangers have to be tailor-made for a power plant and should be effective in controlling microbial biofouling as well as hard foulants (barnacles, mussels, tubeworms and oysters). With regard to macrofouling control in condenser-cooling systems of power plants, chlorination has been the method of choice for fouling control over the years due to its low cost, easy availability and handling, and known degradation pathways. Increasing awareness on the toxic effects of chlorination by-products and better understanding of the biocidal action, environmental issues and higher dosages required for sanitization of surfaces has resulted in replacement of chlorine by stronger oxidizing biocides like chlorine dioxide. Experimental studies using coastal seawater in plate heat exchangers, has revealed a chlorine residual of 1.0 ppm to prevent settlement of invertebrate larvae. However, an intermittent chlorination dose of 1.2 ppm residuals at a frequency of 0.5-2 h was sufficient in controlling slime formation. Side-stream monitoring of these heat exchangers in a nuclear power plant cooling circuit revealed barnacle fouling in spite of continuous chlorination of 0.2-0.3 ppm residuals and shock doses of 0.4-0.6 ppm twice a week for 8 h. In an operational plant, continuous monitoring of the fouling situation using side-stream monitoring devices is to be practised and the biocidal dose and regime Springer Series on Biofilms, doi: 10.1007/7142_2008_14 265 265 © Springer-Verlag Berlin Heidelberg 2008 266 R. Venkatesan and P.S. Murthy altered to overcome any spikes in settlement. This is essentially because biocidal doses required to kill established fouling communities are far higher than those for inhibiting settlement. Even if killing is achieved, accumulation of dead shell biomass (barnacles and tubeworms) often results in loading on equipment surfaces and increases surface roughness, facilitating settlement of other fouling organisms.