Introduction

Disinfection of drinking water is performed to inactivate or eliminate pathogens, with the goal of achieving basic drinking water standards. Disinfection fulfills part of the primary goal of water treatment, which is to provide water free of pathogenic organisms. Fluoridation is used as a method to prevent dental cavities. Public sensitivity to fluoridation and new findings about disinfection byproducts have made both topics hot and controversial issues of the day.

Disinfection

Disinfection of drinking water has been performed for centuries. Over the past century, chlorination has become an accepted method of disinfection, and this process is considered the most important discovery in the field of drinking water treatment. However, recent concerns about disinfection byproducts produced by chlorine have provided new impetus for research into alternative disinfectants. The Safe Drinking Water Act of 1974 and its 1986 amendment have further stimulated the desire to produce safe and enjoyable drinking water. Accordingly, in addition to a review of chlorination, this section discusses alternative disinfectants such as chloramines, ozone, chlorine dioxide, potassium permanganate, and ultraviolet radiation. The advantages and disadvantages of each disinfectant are also presented.

Disinfection of drinking water is a specific treatment step designed to destroy or eliminate pathogenic organisms, and should not be confused with sterilization, which destroys or eliminates all living organisms. Although disinfection of drinking water has been practiced for centuries, the importance of this process in the treatment of drinking water was not recognized until the 1880s and the emergence of the germ theory of disease.

Pathogens (disease-causing organisms) are present in both surface and groundwater. These organisms can survive in water sources for weeks at temperatures of about 21 degrees and for months at lower temperatures under certain conditions. Destruction or elimination of these organisms is essential to provide safe drinking water. Some bacteria, viruses, protozoa, and larger organisms that enter the body through the consumption of contaminated water cause mild to fatal diseases. The disinfectants most commonly used in water treatment are oxidizing agents (halogens, halogen derivatives, ozone) and physical agents (UV radiation).

While the exact effect of oxidants on microorganisms is not well defined, some factors that affect disinfection efficiency include:

• Type and concentration of the microorganism in question
• Type and concentration of disinfectant
• Contact time of disinfectant
• Chemical quality and temperature of the water

The ability of oxidizing agents with different oxidation potentials to kill or inactivate pathogens varies. Ozone has by far the highest oxidation potential of any oxidizing agent.

Disinfection Objectives

To ensure compliance with all applicable regulations (both current and future), the specific disinfection objectives for all public water systems are as follows:

• Inactivation of 99.9% of Giardia lamblia cysts and 99.99% of viruses
• No toxicity, taste, or odor to the disinfected water
• Minimization of formation of undesirable disinfection by-products
• Achievement of the maximum pollutant content for the disinfectants used and the by-products that can be formed.

If properly designed and operated, conventional treatment, which includes coagulation, flocculation, sedimentation, and filtration, combined with disinfection, can achieve at least the required inactivation of Giardia lamblia and enteric viruses. Primary disinfection systems using free residual chlorine, ozone, and chlorine dioxide can achieve inactivation rates greater than the above values ​​for both organisms. However, such efficiencies are not achieved when chloramine is used as the primary disinfectant. When free chlorine and chloramine are used for disinfection, pilot-scale studies are required to determine the formation of disinfection byproducts and to comply with relevant regulations.