Cooling water Biofouling

In a steam electric power plant, cooling water is utilized to absorb heat that is liberated from the steam when it is condensed to water in the condensers. The cooling water is withdrawn from a water source, passed through the system, and returned directly to the water source. Shock (intermittent) chlorination is employed in many cases to minimize the biofouling of heat transfer surfaces. Continuous chlorination is used only in special situations. Based on 308 data, approximately 65% of the existing steam electric power plants have once-through cooling water systems. 

Chlorine and hypochlorite are used to control biofouling in both once-through and recirculating cooling water systems. The addition of chlorine to the water causes the formation of toxic compounds and chlorinated organics, which may be priority pollutants. 

Oxidizing agents TBA assessment of the acute toxicity of ozone, an alternative to chlorination to control biofouling in cooling water systems of power plants, to fish larvae of three species and to D. magna. F,F,F,I (Leynen et al., 1998)... 

Ozone has been used to replace chromium-containing chemicals to control biofouling for the cooling tower water. The U.S. cooling water market for ozone is in steady growth. Approximately 500 cooling towers have installed ozone. ... 

Biofouling. Deposits formed when biological mechanisms attach and grow on the heat transfer surface. Untreated cooling water systems are particularly susceptible to biofouling. 

Treatment of cooling water to combat corrosion, scale formation and biofouling can be achieved by a suitable programme. The cost may be high and for a modest cooling water system the cost may run into tens of thousands of pounds. 

There are a number of features in the design of cooling water plant that need attention in order to reduce or even eliminate the incidence of fouling. The extent of the individual mechanisms of particulate deposition, corrosion, scale formation and biofouling, will depend on the quality of the cooling water, the temperature and the material of construction. 

In Chapter 14 the use of additives to combat potential fouling was discussed. In the use of chemical treatment for cooling water there has to be an emphasis on effective and rapid dispersion since the concentration of the additives employed must be low, i.e. a few mg/l where possible, to minimise cost and to reduce potential pollution problems. In general the additive formulation will be based on the need to limit corrosion (i.e. the use of corrosion inhibitors), scale formation (i.e. the use of crystal modifiers, dispersants or threshold chemicals or a combination) and biofouling (i.e. the use of biocides and dispersants). In many installations additives are injected on the suction side of the main pump so that turbulence within the pump will provide rapid mixing. In very large cooling systems multiple injection nozzles will be required to enhance distribution. 

Because of the widespread use of cooling water and the need to assess treatment programmes, monitors have been developed specifically for use in cooling water systems. Many of these devices are relatively simple and do not necessarily require the use of a mobile laboratory. Moore and Hays [1985] produced a survey of commercial fouling monitors that could be used in conjunction with cooling water and EPRI [1985] made a status survey of 18 biofouling detection monitoring devices. Chenoweth [1988] provides detailed information on ten such monitors. He observes that the choice of monitor depends on a number of questions, the answers to which will not only affect the cost but also the quality of the data obtained. 

Ludyanskiy, M. L., 1991. Algal fouling in cooling water systems. Biofouling, 3, pp. 13 21. 

The water in the pump suction well is chlorinated to control biofouling by the injection of sodium hypochlorite from a 10,000 gallon tank. Four vertical diffusers in each pump suction well distribute the sodium hypochlorite. RSIP item B151 (Reference 74) includes provision for adding strainer pressure instrumentation in the cooling water booster pump building, strainer bypass lines, and other improvements that will allow better operator monitoring, controls, and shutdown of pumps if the pressure limits are exceeded. 

Biofouling involves the formation of biofilm, whereby hydrated algal- or bacterial-based slimes adhere to water-wetted cooling system surfaces and often contain scales, corrosion products, or other debris embedded within a polysaccharide matrix. The role of biofilms in reducing cooling system efficiency and life span is still imperfectly understood. 

Chlorine is used to bleach all types of fabric, to disinfect relatively clean impervious surfaces, to purify water, and to control biofouling in cooling systems. It is used in the processing of meat, fish, vegetables, and fruits. It is also used in the manufacturing of synthetic rubber, plastics, pesticides, antifreeze, refrigerants, antiknock compounds, chlorinated hydrocarbons, polyvinyl chloride, and chlorinated lime. Chlorine is also used in detinning... 

Rippon [1979] suggests that it is necessary to distinguish between fresh water and esturine or sea water cooling systems in the use of chlorine as a biocide. She points out that a 2000 MW power station using inland water will require 200 - 300 tonnes per year of chlorine to control biofouling in the condensers. A similar power station using sea water for cooling will probably use 500 - 1000 tonnes per... 

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