What types of treatment methods can be used for external feedwater?

The correct answer involves methods specifically focused on removing hardness and impurities from water before it is introduced into a steam system. Cold lime soda, hot lime soda, and ion exchange are all established methods for softening water. Cold lime soda involves adding lime and soda ash to precipitation minerals that cause hardness, while hot lime soda does the same process at elevated temperatures, which can enhance effectiveness. Ion exchange involves exchanging sodium ions for calcium and magnesium ions in the water, effectively softening it. These methods are crucial in boiler operations to prevent scale buildup and maintain efficiency.

The other treatment methods listed serve different purposes or are typically employed in different contexts. For instance, reverse osmosis and filtration (from the first option) are primarily used for desalination or removing specific contaminants, rather than specifically addressing hardness. Chemical dosing, clarifiers, and UV treatments (from the third option) are often focused on disinfection and chemical balance, but they do not specifically target the hardness of feedwater. Lastly, steam distillation and similar methods (from the fourth option) are generally not practical for treating feedwater on a large scale in most industrial applications. They may be used in specialized situations but are not standard for managing boiler feedwater.