ALGAE BACTERIA BIOFILM COOLING TOWERS COOLING WATER DISINFECTION HEAT EXCHANGERS MICROBIALS

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Abstract

Cooling water systems in electric utility plants face challenges related to corrosion, mineral scale deposition, microbiological fouling, and suspended solids accumulation. Of these challenges, microbiological control is a primary issue because it can impact the others. In addition to a direct impact on heat transfer, microbiological growth can influence corrosion, trigger mineral scale formation, and accelerate suspended solids accumulation.

This overview will discuss the common methods of microbiological control in power plants as well as discuss some emerging technologies. It will also emphasize the importance of monitoring the effectiveness of microbiological control and discuss current monitoring methodology.

This article is limited to “open” cooling systems. These are defined as those circulating water through a cooling tower or operating in a once-through fashion on a lake or river. Closed cooling, closed heating, or bearing cooling water systems are not included in this discussion. 

Introduction

There are several goals for control of microbiological growth in an open cooling water system.  These are:

• To prevent the accumulation of biofilm on the tube surfaces of the condenser or other heat exchangers. Biofilm accumulation can seriously impact condenser performance and have a negative impact on plant heat rate.
• To control the growth of pathogenic bacteria, such as Legionella pneumophila (the bacteria that causes Legionnaire’s disease). This is a safety issue and should not be ignored.
• To control the growth of algae on cooling tower surfaces, or in cooling or waste ponds. Excessive algal growth on the hot decks of cooling towers can cause structural damage and complicate microbiological control. Algal growth in cooling lakes and ponds can contribute to deposition by causing pH swings. Algal growth can also contribute suspended solids to discharge streams.

A power plant condenser cooling water system or service water system may use water recirculated through a cooling tower or use once-through water from a lake, river, or ocean.  Each of these water sources contain the nutrients necessary for microbiological growth, This can take the form of bacteria, algae, fungi, molds, or a combination of these. The key to an effective microbiological control program is maintaining adequate control but not trying to achieve a “sterile” system.  

Both chemical and mechanical methods are used for cooling water microbiological control. System metallurgy also plays a role. 

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