Biofouling is caused by the growth of biofilm on heat exchanger tubing metal or metal alloy surfaces. Biofouling can also be caused by the accumulation of fungi and algae on the surfaces. Similar to scale formation, biofouling reduces the rate of heat transfer. It also can cause microbiologically influenced corrosion.
Cooling water systems have the environmental conditions favorable for biological growth, such as elevated temperature, mid-range pH, continuous airflow, and sunlight. The nutrients needed for biological growth are mainly from the makeup water, i.e., organic matter, nitrogen, and phosphorous. Figure 1 shows a schematic of biofilm growth process.
A traditional approach to control biomass growth in cooling systems is to maintain certain level of free chlorine residual.
When using fresh water with lower organic matter and nutrient concentrations as makeup water in cooling systems, the biomass growth is limited and relatively easy to control. However, treated municipal wastewater can have high organic matter, ammonia, and phosphate concentrations and reusing wastewater in cooling systems can make the system very conducive to microbial growth. Thus, controlling biomass growth in cooling system using treated municipal wastewater is a much more difficult task than using freshwater as makeup.
Since organic matter and ammonia are present in treated municipal wastewater, free chlorine is not easy to be maintained in cooling systems when wastewater is used as makeup. Free chlorine can react with organic matter and ammonia which reduces disinfection ability. Thus, alternative approaches to control biomass growth in cooling systems using the wastewater are often necessary.
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