This chapter focuses on water treatment operations and the various unit processes currently used to treat raw source water before it is distributed to the user. In addition, the reasons for water treatment and the basic theories associated with individual treatment unit processes are discussed. The chapter concentrates on the individual treatment unit processes that are usually found in conventional water treatment systems, corrosion control methods, and fluoridation. Two of the major chemical pretreatment processes used in treating water for potable use are iron and manganese and hardness removal. The chapter discusses disinfection as it applies to water treatment. The goal in proper disinfection in a water system is to destroy all disease-causing organisms. In water treatment, disinfection is almost always accomplished by adding chlorine or chlorine compounds after all other treatment steps, although in the United States ultraviolet light and ozone processes may be encountered.